HomeMy WebLinkAboutSEPA E99-0017 - KIEROUZ JIHAD / JAK INC - NEW SINGLE FAMILY RESIDENCEJIHAD KIEROUZ, J.A.K. INC
CONSTRUCTION OF
SINGLE-FAMILY RESIDENCE
13031 33RD AVE. S.
E99-0017
A F F I D. A V I T 0-E- D I S' T.RI B U T I O.N
•
•
Natice of Public Hearing
.Notice of Public -Meeting
Board of Adjustment.Agenda
Packet -
Hoard.. a f Appeals. Agenda.
Packet:
Planning Commission Agenda
Packet.
Short Subdivision Agenda
Packet
hereby declare: that.:
Notice of. Application far
Shoreline Management Permit
Shoreline Management Permit
•
giKtermination of Nom, /((o -444
/C�
significance -.
Mitigated..D.eterinination of_.
Nonsignificance
jDeternination_. of,_ Significance
.and Scop i ng. Notice
Notice of Action
Official Notice
Other
Other
was mailed to each of the following addresses an
Name of Project S i creature
File Number/'� 7359/00'��73
VoM •
CITY OF TUKWILA
DETERMINATION OF NONSIGNIFICANCE (DNS)
DESCRIPTION OF PROPOSAL:
REASONABLE USE EXCEPTION TO FILL A TYPE 3
WETLAND AND PIPE A TYPE 3 WATERCOURSE TO
CONSTRUCT A SINGLE FAMILY RESIDENCE.
PROPONENT: JIHAD KEIROUZ
LOCATION OF PROPOSAL, INCLUDING STREET ADDRESS. IF ANY:
ADDRESS: 13041 33 AV S
PARCEL NO: 735960-0473
SEC/TWN/RNG:
LEAD AGENCY: CITY OF TUKWILA FILE NO: E99-0017
The City has determined that the proposal does not have a probable
significant adverse impact on the environment. An environmental
impact statement (EIS) is not required under RCW 43.21c.030(2)(c).
This decision was made after review of a completed environmental
checklist and other information on file with the lead agency. This
information is available to the public on request.
*•k***** *****************************•******** k*•***** *** k•k*****•k*************
This determination is final and signed this _11214day of
1994L.
Stev_e`C'ancaster, Responsible Official
City of Tukwila. (206) 431-3670
6300 Southcenter Boulevard
Tukwila, WA 98188
Copies of the procedures for SEPA appeals are available with the
Department of Community Development.
CHECKLIST: ENVIRONMENTAL REVIEW/SHORELINE PERMIT MAILINGS
( ) U.S. ARMY CORPS OF ENGINEERS
( ) FEDERAL HIGHWAY ADMINISTRATION
( ) DEPT OF FISH & WILDLIFE
OFFICE OF ARCHAEOLOGY
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DEPT NATURAL RESOURCES
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DEPT OF COMM. TRADE & ECONOMIC DEV.
DEPT OF FISHERIES & WILDLIFE
FEDERAL AGENCIES
( ) U.S. ENVIRONMENTAL PROTECTION AGENCY
( ) U.S. DEPT OF H.U.D.
WASHINGTON STATE AGENCIES
K.C. PLANNING & COMMUNITY DEV.
) BOUNDARY REVIEW BOARD
FIRE DISTRICT #11
FIRE DISTRICT #2
) K.C. WATER POLLUTION CNTRL SEPA OFFCL
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) TUKWILA LIBRARIES
RENTON LIBRARY
KENT LIBRARY
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U S WEST
SEATTLE CITY LIGHT
PUGET SOUND ENERGY
HIGHLINE WATER DISTRICT
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TCI CABLEVISION
OLYMPIC PIPELINE
KENT PLANNING DEPT
TUKWILA CITY DEPARTMENTS:
PUBLIC WORKS
POLICE
PLANNING
PARKS & REC.
CITY CLERK
( )
( )
( )
( )
FIRE
FINANCE
BUILDING
MAYOR
PUGET SOUND REGIONAL COUNCIL
P.S. AIR POLLUTION CONTROL AGENCY
SW K C CHAMBER OF COMMERCE
MUCKLESHOOT INDIAN TRIBE
DUWAMISH INDIAN TRIBE
(� SEATTLE TIMES
07/09/98 C:WP51DATA\CHKLIST
( ) DEPT OF SOCIAL & HEALTH SERV.
) DEPT OF ECOLOGY, SHORELIND DIV
DEPT OF ECOLOGY, SEPA DIVISION*
( ) OFFICE OF ATTORNEY GENERAL
* SEND CHKLIST W/ DETERMINATIONS
* SEND SITE MAPS WITH DECISION
KING COUNTY AGENCIES
) K.C. DEPT OF PARKS
HEALTH DEPT
PORT OF SEATTLE
K.C.DEV & ENVIR SERVICES-SEPA INFO CNTR
K.C. TRANSIT DIVISION - SEPA OFFICIAL
SCHOOLS/LIBRARIES
HIGHLINE SCHOOL DISTRICT
K C PUBLIC LIBRARY
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SEATTLE SCHOOL DISTRICT
RENTON SCHOOL DISTRICT
UTILITIES
PUGET SOUND POWER & LIGHT
VAL-VUE SEWER DISTRICT
WATER DISTRICT #20
WATER DISTRICT #125
CITY OF RENTON PUBLIC WORKS
RAINIER VISTA
SKYWAY
CITY AGENCIES
RENTON PLANNING DEPT
CITY OF SEA -TAC
CITY OF BURIEN
TUKWILA PLANNING COMMISSION MEMBERS
TUKWILA CITY COUNCIL MEMBERS
CITY OF SEATTLE - SEPA INFO CENTER - DCLU
SEATTLE OFFICE OF MGMNT & PLANNING*
* NOTICE OF ALL SEATTLE RELATED PLNG PROJ.
OTHER LOCAL AGENCIES
METRO ENVIRONMENTAL PLANNING DIV.
OFFICE/INDUSTRIAL 5,000 GSF OR MORE
RESIDENTIAL 50 UNITS OR MORE
RETAIL 30,000 GSF OR MORE
MEDIA
-� Ave 'z.i po u Z
AFFIDAVIT
5 -Tri
Notice of Public Hearing
['Notice of Public Meeting
Board of Adjustment. Agenda
Packet
ElBoard of Appeals Acenda
Packet
Planning Commission Agenda
Packet.
Short Subdivision Agenda
Packet
OF DISTRIBUTION
hereby declare that:
Q Notice of Application far
Shoreline Management Permit
QShoreline Management Permit
\igt
Determination of Non-...
significance
Mitigated .Determination af..
Nans i gni ficance
J Determi nation of Significance
and Scoping -Notice
Notice of Action
Official Notice
Other
Other
was_ e& to .ate of the following addresses
\i Name of Project ccd ,
File Number 7— cot -7
on
City of Tukwila John W. Rants, Mayor
Department of Community Development Steve Lancaster, Director
MEMORANDUM
To: Steve Lancaster
From: Michael Jenkins
Date: November 16, 1999
Re: E99-0017, SEPA review for a Reasonable Use Exception — 13031 — 33rd
Ave. S.
Project Description:
This SEPA review is for a Reasonable Use Exception for the construction of a new
Single Family House at 13031 — 33' Ave S. The request for Reasonable Use Exception
is based on the piping of a regulated watercourse and the filling of related wetlands.
Agencies with jurisdiction:
None
Summary of Primary Impacts:
• Earth
The site is sloped through approximately 1/2 the site, up to a 20% grade. The soil
includes gravelly sandy loam and topsoils. There is seepage along the west and
south perimeters of the parcel. Grading and filling will occur to create a developable
lot. The activities will include the piping of a Type 3 watercourse and filling of a
related Type 3 wetland. Both the wetland and watercourse were rated for the
purposes of this application and were not included in either the May 1990 Water
Resource Study or the October 29, 1990 Watercourse Rating Study prepared for the
City of Tukwila. The applicant has submitted a June 9, 1999 statement by a
Wetland Biologist (copy attached) indicating that the entire property is a wetland.
The project will likely result in approximately 2,250 square feet of impervious
surfaces. The applicant has submitted Grading and Drainage Plans as well as an
Erosion Control Plan. All earth movement must comply with City Ordinances as well
as the King County Surface Water Design Manual requirements as they relate to
earth movement. A Geotechnical Report has also been submitted with this
application.
Air
Negligible vehicle emissions will occur during construction.
6300 Southcenter Boulevard Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 a Fax (206) 431-3665
E99-0017
SEPA Checklist — Keirouz Reasonable Use Exception
13031 — 33rd Ave S.
November 16, 1999
• Water
A Class 3 watercourse, per TMC 18.45, is located along the southern edge of the
property. The watercourse is part of the Riverton Creek Drainage Basin. The
applicant prepared a Drainage Assessment and Downstream Analysis. A Class 3
wetland is located on the property, related to the watercourse. The applicant has
proposed piping of the watercourse along the southern portion of the property and
filling of the related wetland.
The applicant has submitted a wetland mitigation plan for replanting of areas
disturbed by construction and relocation. Due to activities related to this application,
the applicant must comply with the Small Site Targeted Drainage requirements of
the King County Surface Water Design Manual as well as City Ordinances.
• Plants
A mitigation plan was submitted in conjunction with this plan, due to the filling and
relocation of the wetland and piping of the watercourse. The mitigation plan must be
approved by the Director of Community Development as a Type 2 decision.
• Animals
No threatened or endangered species known in area.
• Energy/Natural Resources
No impacts.
• Environmental Health
No significant noise will be generated by project. Any noise generated during
construction or operation of the facility must comply with Tukwila's Noise Ordinance
(TMC 8.22).
• Land/Shoreline Use
Proposed development is located in the Low Density Residential (LDR) zone. The
Single Family Residence is a permitted use generally, however the proposed piping
of a watercourse and filling of a related wetland require approval of a Reasonable
Use Exception, as indicated in TMC 18.45. The parcel is a pre-existing legal lot of
record, as it is less than the 6,500 square foot minimum lot size in the LDR zone.
The parcel was annexed under Ordinance 1574. TMC 18.70, Nonconforming Lots,
Uses and Structures govern development of pre-existing legal lots of record. The
properties surrounding the proposed development are also zoned LDR. No
demolition of structures is proposed.
E99-0017
SEPA Checklist — Keirouz Reasonable Use Exception
13031 — 33rd Ave S.
November 16, 1999
• •
• Housing
If approved, the Reasonable Use will result in an additional dwelling unit.
• Aesthetics
The tallest structure on the parcel will be approximately 25 feet tall. The proposed
house will obstruct no view.
• Light and Glare
The development will not produce significant Tight or glare. No off-site light or glare
will be produced.
• Recreation
No impact.
• Historic/Cultural Preservation
No known places or landmarks.
• Transportation
The project will not result in any measurable impacts to the transportation system.
• Public Services
Proposed development will not impact or require specific public services.
• Utilities
The facility will be served by electric and telephone services.
Conclusion:
Implementation of existing Tukwila ordinances and standards will result in avoidance or
mitigation of significant adverse environmental impacts.
Recommendations:
DNS
Michael Jenkins - Re: public comment - 410,1Keirouz
Page 1
From: Jill Mosqueda
To: Michael Jenkins
Date: 10/4/99 12:20PM
Subject: Re: public comment - Jihad Keirouz
we asked the KPFF Engineer to supply drainage and sizing calculations for the piping. We can then
determine if the pipe sizing is correct. I am figuring the woman is talking about the water at the southwest
corner or jihads' lot. The piping will collect and direct the water. There is a good drop from the upper
property to jihads'. it would take the kind of flooding the Arc was built for to back up the hill. Probably no
increased risk of backup into her water fall. If there is backup, it will come out of the two catch basins and
flood jihads' property and the downstream neighbors before it ever reached her. Is this the woman who
was in here about Looney clearing and the two lots uphill from jihads'?
»> Michael Jenkins 10/04/99 12:07PM »>
I received a phone call today from a Mabel Bloomfield whose house is at 13208 - 32 Ave S. She lives to
the south and west up the hill from Jihad. She is concerned that the waterfall that she has on her property
wil be adversly effected by the piping of the watercourse. She did not go into any detail, but I am
assuming she has fears that the system will not be able to handle the piping and will result in flooding on
her property. What are the issues here?
1
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199810652
14200
199810821
14110
199810850
14400
199900021
13000
199900019
14400
199900554
14110
199900889
13200
199901002
4100
199901264
14300
199901656
12606
199902113
14200
199902185
12822
199902207
13800
199902274
14000
199903305
Date:
TIB
Date:
TIB
Date:
TIB
Date:
40
Date:
TIB
Date:
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Date:
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Date:
140
Date:
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12/14/98
12/19/98
12/20/98
1/ 1/99
1/ 1/99
1/19/99
2/ 2/99
2/ 5/99
2/16/99
2/28/99
3/16/99
Date: 3/18/99
EMW
Date: 3/19/99
TIB
Date: 3/21/99
42
Date: 4/24/99
S 144 ST/42 AV S
37
RD
Apt
RD
290
290
Apt 103
RD 290
Apt
RD 290
Apt
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Apt
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Apt
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Apt
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Apt
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Apt
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Apt
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Apt
RD 290
Apt
RD 290
Apt
ID: 106
ID: 105
ID: 097
ID: 095
ID: 097
ID: 116
ID: 105
ID:
124
ID: 090
ID: 096
ID: 136
ID: 128
ID: 137
ID: 105
ID: 096
i
City of Tukwila
Department of Community Development
September 22, 1999
Jihad Keirouz
JAK, Inc.
13407 — 51st Ave W.
Edmonds, WA 98062
Re: Reasonable Use Exception, 13031 — 33`d Ave S. (L99-0049, E99-0017)
Dear Jihad:
John W. Rants, Mayor
Steve Lancaster, Director
Your application for a Reasonable Use Exception to construction -a Single Family Residence, as
referenced, was determined to be complete on. September 10, 1999..While we were able to
determine your application complete, there are a number of outstanding substantive issues that
have not been fully addressed.
As indicated in the August 2, 1999 Notice of Incomplete Application, we requested that you
provide a conceptual drainage plan designed to meet King County Surface Water Design Manual
(KCSWM) requirements, specifically those related to Small Site Drainage Review and Targeted
Drainage Review requirements, Category 1, from said manual. Accordingly,.a Grading and
Drainage Plan was submitted on September 1, 1999, along with other materials in response to the
August 2, letter.
On review of the Public Works Department, Core Requirements #2, 3, 4 and 6 from the Small
Site Review were not adequately addressed in your recent submittal. This review must include:
• An on site flow analysis and supporting calculations
• An offsite analysis including a Level One downstream analysis
• Indications on how water will be controlled during construction
• How the proposed system will be maintained after construction.
To assist you, copies of the pertinent sections from the referenced -manual are attached.
The following is a review of additional information that must be submitted to continue our
review:
• You submitted a Mitigation -Plan with your initial application that is not consistent with the
materials from your September 1 submittal.. Specifically, the Mitigation Plan appears to call
for open stream and outfall area while the later materials call for the entire watercourse to be
piped. The technical basis for the piping is also not well explained in either the Geotechnical
Report or in the plans provided. This must be corrected.
6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665
September 22, 1999 •
Jihad Keirouz
Re: L99-0049 / E99-0017, 13033 - 33`d Ave S.
On review of the Geotechnical Report, there is a significant lack of detail -regarding the
potential construction in relation to the existing watercourse. For example, there,does not
appear to be any recommendations regarding mitigation of existing or potential
surface/ground water during construction. The report must include, at a minimum, an
explanation of how this situation will be addressed.
• Staff visited the site in July, 1999 regarding an issue concerning' the properties immediately
to the west that abut your property. At that time, staff noted that the neighboring- site was
cleared: At that time, we noticed that the Red Alder and Cottonwood listed on the Mitigation
Plan did not appear to be on your property, although the plan calls for their retention. This
discrepancy should be further explained. If these features were.not on your property, they
should be removed from the plans.
• In your September 1 submittal you provided a Grading and Drainage Plan, Erosion and
Sediment Control Plan and a Survey. Please provide an 8 ''/z" x-11" reduction for each of the
plans you submitted on September 1. These reductions are used for archival purposes.
Finally, staff has some concerns over the overall validity of your application. Your application
for a Reasonable Use Exception is to seek relief from requirements of the Sensitive Areas
Ordinance (TMC 18.45) in order to allow you to alter an existing watercourse and fill a related
wetland covering a significant portion of the property. Since it appears that you obtained the
property after the adoption of TMC 18.45 and had previous knowledge of the drainage problems
related to the site -as a whole, including the two adjacent lots that you purchased and have
developed, you may not be eligible for a Reasonable Use Exception.
It is our intent to take this application to a hearing before the Planning Commission. Before this
can be taken to a hearing, the issues detailed in this letter must be resolved and additional plans
submitted in their entirety. In addition, we feel it is necessary to discuss our potential
recommendation to the Planning Commission. We would like to schedule a meeting with you to
discuss this matter more fully. Please contact me at (206) 431-3685 to set up a meeting. We
would like to meet as soon as possible, preferably the week of September 27, 1999.
Under TMC 18.104.130, the City of Tukwila has 120 days from "the time the applicant is
notified the application is complete" to issue a permit. Under subsection 1, this time period may
be put on hold when additional studies or information is required to conduct a substantive
review. Accordingly, the 120 day review period is stopped until the additional information
requested in this letter is received and reviewed for completeness.
Sincerely,
Michael Jenkins
Associate Planner
/Fe- 0043
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City of Tukwila
John W. Rants, Mayor
Department of Community Devel pment Steve Lancaster, Director
141\\)1(16'L 0
MEMORANDUM
TO: Jim Morrow, Public Works Director
FROM: Gary Schulz, Urban Environmentalist
DATE: September 15, 1999
RE: Americon, Inc. (Looney Prop.) D99-0224 & D99-0225.
I have the following comments regarding the above building permit applications. I have not
reviewed any plans; however, my input is related to the clearing violation and potential wetland
area on the property. The applications are for two vacant lots at 13028 & 13030 32nd Avenue S.
Recent clearing activity was observed on the subject lots in July, 1999. A previously identified
wetland drainage area exists directly east of the southern lot (13030 32nd )
In accordance with Sensitive Area Ordinance (TMC 18.45) standards, a wetland study is required to
identify and delineate any wetland area located on the subject lots. The potential for regulated
wetland is most likely located at the southeast corner of the southern lot. Natural groundwater
appears to be crossing the site from the wetland system to the west of 32nd Avenue S.
The wetland study shall be conducted by a qualified professional and include_ delineation on the site
if wetland area is present. Due to the recent clearing, some restoration effort may be required.
Please contact me if you want to discuss the project or existing conditions further.
Cc: Steve Lancaster, DCD Director
Duane Griffin, -Building Official
6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665
•.
City of Tukwila
Department of Community Development . Steve Lancaster, Director
John W. Rants, Mayor
MEMORANDUM
TO: Gary Shultz
- FROM: Michael Jenkins -
DATE: September 10, 1999
RE: Reasonable Use Exception — Jihad Keirouz (L99-0049)
We determined Jihad's application complete today. Now that the application is complete, Public
Works has provided me with initial comments concerning the content of the Drainage Plan.
Apparently, the plan does not meet the requirements of the King County Surface Water Design
Manual and must be revised. In conjunction with this, we must also have a definitive statement
of the type of Watercourse and Wetland that is on the subject property. The watercourse and
wetland is not indicated on our sensitive area map, and presumably, in the supporting studies.
Accordingly, I need to have the wetland and watercourse rated for this application. The case is
tentatively scheduled for a hearing.on October 28, 1999. Depending on when the materials are
revised to meet Public Works concerns, this date may be changed to November or December.
However, the rating of the property should occur as soon as possible. I will be out of the office
on Sept 13 and 14. We should talk after that date
6300 Southcenter Boulevard Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665
•
City of Tukwila - • John W. Rants, Mayor
Department of Community Development Steve Lancaster Director
NOTICE OF COMPLETE APPLICATION
September 10, 1999
Jihad Keirouz
JAK Incorporated
13407 — 51st Ave West
Edmonds, WA 98062
RE: Application for a Reasonable Use Exception, 13031 — 33rd Ave S. (L99-0049, E99-0017)
Dear Jihad:
Your application for a Reasonable Use Exception to construct a Single Family Residence at 13031 — 33rd
Ave S. has found to be complete on September 10, 1999 for the purposes of meeting state mandated time
requirements. The project has been assigned to me and is tentatively scheduled for a public hearing before
the Planning Commission on October 28, 1999.
The next step is for you to install the notice board on the site within 14 days of the date of this letter. The
sign should be posted in a location that is easily accessible to people using the site. You received
information on how to install the sign with your application packet. If you need another set of those
instructions, you may obtain them at the Department of Community Development (DCD). Also, you must
obtain a laminated copy of the Notice of Application to post on the board. This notice is also available at
DCD. After installing the sign with the laminated notice, .you need to return the signed Affidavit of Posting
to our office.
This determination of complete application does not preclude the ability of -the City to require that you
submit additional plans or information, if in our estimation such information is necessary to ensure the
project meets the substantive requirements of the City or to complete the review process. Based upon your
submittals, a follow up letter detailing specific concerns about your proposal will be sent within the next
week. I will be contacting you soon to discuss this project. If you wish to speak to me sooner, feel free to
call me at 206-431-3685.
Sincerely,
Michael Jenkins
Associate Planner
cc: Reviewing City Departments
Lance Erickson, J.S. Jones
6300 Southcenter Boulevard Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665
.Hug 01 99 11:52a
JIM' FINLEY
•
GEOTECH
CONSULTANTS. INC.
13256 NE 20th Street. quite 16
Bell, vue. WA ')8005
(425)747-561r
FAX (42.5) 747-x561
J.A.K. Inc.
13520 Linden Avenue North
Seattle, Washington 98133
Attention: Jihad Keirouz
Subject: Geotechnical Considerations
Proposed Residence
130XX — 33' Avenue South
Tukwila, Washington
Dear Mr. Keirouz:
14251741-Eibb1 to. 1
•
August 31, 1999
JN 8128-1
eiriVAILA
179 4
,20
Pamir 66,FP
In September of 1998, the undersigned Associate of Geotech Consultants, Inc. visited lots in
Tukwila that had just been excavated for proposed residences. We understand that you propose to
construct another residence adjacent to the southem side of those residential Tots. The purpose of
this letter is to discuss the likely near -surface soil and groundwater conditions, as well as foundation
parameters, on the proposed residence lot based on the conditions seen in the earlier, adjacent
excavations.
Baed on the Grading and Drainage Plan for the project dated May 1, 1999, we understand that the
proposed residence will be located near the middle of the property with a driveway along the
northeastern side of the property. Cuts of up to approximately 4 feet are proposed on the westem
side of the residence and garage. An approximate 4 -foot rockery is proposed west and south of the
residence; a cut will be made to install this rockery. A catch basin and tighttine pipe will be
constructed to control surface water that enters the southwestern side of the property. The pipe will
uttimately discharge the surface water to an existing ditch on the eastem side of the property. The
project subsurface drainage system will include typical footing drains.
The subject property is located on the upslope, westem side of 33nd Avenue South. It slopes
upward above the street with a topographic relief of about 15 feet. The property is undeveloped
and heavily overgrown with vegetation. Some surface water is currently channeled diagonally to
the northeastern side of the property to the existing ditch.
Excavations in the range of 3 to 5 feet were made for the two residences to the north. We
observed that the upper, approximate 3 feet of soil was loose. It was underlain by competent, firm
silty sand, silt, and sand. Groundwater was observed perched on these competent soils.
Conclusions and Recommendations
The competent soils that we observed in the adjacent excavation are very suitable
to support the building toads. Therefore we believe that competent soil should be
revealed on the subject site at relatively shallow depths. We recommend that
continuous and individual spread footings have minimum widths of 16 and 24
inches, respectively. They shouldbe bottomed at least 18 inches below the lowest
_Drug .31 99 11:52a
J.A.K. Inc.
August 31, 1999
tyFINLEY
•25)747-8561
JN 8128-1
Page 2
adjacent finish ground surface. The local building codes should be reviewed to determine if
different footing widths or embedment depths are required. An allowable bearing pressure of 2,000
pounds per square foot (psf) is appropriate for footings supported on the competent, native soil.
Because the native soil is somewhat sensitive to moisture, we recommend that any loose surficial
soil in the footing areas we removed pnor to pouring the foundation. Another option would be to
cover footing areas with a mat of imported, granular fill at the time the initial excavation is done. A
one-third increase in this design bearing pressure may be used when considering short-term wind
or seismic loads. Lateral loads due to wind or seismic forces may be resisted by friction between
the foundation and the bearing soil, or by passive earth pressure acting on the vertical, embedded
portions of the foundation. For the latter condition, the foundation must be either poured directly
against relatively level, undisturbed soil or be surrounded by level structural fill. We recommend
using the following design values for the foundation's resistance to lateral loading:
Parameter
Design.. Value
Coefficient of Friction 0.40
Passive Earth Pressure I 300 pcf
Where: til pet is r,cup►ds per cubic foot, and tit) passive earth
pressure is computed using the equivalent fluid density.
If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will
not be appropriate. We recommend a safety factor of at least 1.5 for the foundation's resistance to
lateral loading, when using the above design values.
We observed groundwater perched at or near the level of the competent native soil on the adjacent
sites. We believe that a similar groundwater conditions will be found on the subject property. The
amount of groundwater seen on the adjacent sites, as well as the density of the competent soil,
indicate that the soil has a very low permeability (not suitable for percolation). We expect that
groundwater will be encountered in the excavation of the proposed residence. It is our professional
opinion that the amount of groundwater flow will be readily handled by the project's subsurface
drainage system. We agree with the system that is shown on the Grading and Drainage Plan, but
recommend that another footing drain be added behind the western and southern garage walls.
The drainage system shown on the plan, and recommended around the garage, is very similar to
the system we recommended earlier for the adjacent residences. All footing drains should consist
of a perforated pipe that is surrounded with washed rock and filter fabric.
p.2
_Ouc .31 99 11:52a
J.A.K. Inc.
August 31, 1999
OM FINLEY
#25)747-8561
p.3
JN 8128-1
Page 3
We trust that this report meets your immediate needs for the proposed development. Please
contact us if we can be of further service.
1
Respectfully submitted,
GEOTECH CONSULTANTS, INC.
4
30
D. Robert Ward, P.E.
Associate
August 2,.1999
•
City of Tukwila John W. Rants, Mayor
Department of Community Development Steve Lancaster Director
Jihad Keirouz
JAK, Inc.
13407 — 5151 Ave West
Edmonds, WA 98062
NOTICE OF INCOMPLETE APPLICATION
Re: Application for Reasonable Use Permit, 13031 — 33`d Ave S. (L99-0049, E99-0017)
Dear Mr. Keirouz:
Your application for a Reasonable Use Exception from the Sensitive Areas Ordinance for the construction of
a single-family residence has been found incomplete. In order for this application to be determined as
complete, you must submit the following information to the Department of Community Development, based
on the comments detailed below:
• A site plan, as described on the application checklist, including all proposed setbacks
• A plan documenting both existing and proposed conditions of the property, including the proposed
mitigation plan
• A Geotechnical Report with Hydrological Assessment, including impacts to surrounding areas
• A conceptual drainage plan designed to meet current Small Site Drainage Review and Targeted Drainage
Review category 1 from the King County Surface Water Design Manual
• A temporary erosion prevention plan
• A temporary sediment control plan
In addition, the following features should be included in the materials you submit so that technical review of
your proposal can be adequately done:
• The existing and proposed contours should be shown on the site plan at two (2) foot intervals for a
minimum of 100 feet off site, in support of TMC 16.54.110. A copy of the code is included
Upon receipt, the City will re -review the materials you submit for completeness and will mail you written
notification of completeness or incompleteness within 14 days. This application will expire if we do not
receive the additional information within ninety (90) days of the date of this letter unless an extension is
granted pursuant to Section 18.105.070(E). If you have any questions, my telephone number is 431-3685.
Sincerely,
Michael Jenkins
Associate Planner
6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665
•
A. BACKGROUND
Control No. qj -
Epic File No.
Fee $325 Receipt No.
ENVIRONMENTAL CHECKLIST
1. Name of proposed project, if applicable: JAK Single-family Residence, 33rd Ave S. Tukwilla, Lot 12, Block 5,
"Robbins Springbrook Addition to Riverton", Vol 16 of Plats, Page 67, Records of King County
2. Name of applicant: Jihad Kierouz, J.A.K. Inc.
3. Address and phone number of applicant and contact person: Phone: (206) 300-6874
Address: 13407 5151 Avenue West Edmonds, Washington 98062;
4. Date checklist prepared: June 29, 1999
5. Agency requesting checklist: City of Tukwila
6. Proposed timing or schedule (including phasing, if applicable):
The applicant wants to construct as soon as possible, while the intermittent stream is dry.
7. Do you have any plans for future additions, expansion, or further activity related to or connected with this
proposal? If yes, explain. No
8. List any environmental information you know about that has been prepared, or will be prepared, directly related
to this proposal. A wetland investigation was performed by J. S. Jones and Associates, Inc., dated June 9, 1999.
9. Do you know whether applications are pending for governmental approvals of other proposals directly affecting
the property covered by your proposal? If yes, explain. Yes, Single -Family Permit No. D98-0215
EGEVE
JUL 0 6 1999
NITY
DEVEL PME T
0)
2
• •
10. List any government approvals or permits that will be needed for your proposal. Single -Family Residence
Permit and Reasonable Use Exception
1. Give brief, complete description of your proposal, including the proposed uses and the size of the project and
site. There are several questions later in this checklist that ask you to describe certain aspects of your proposal.
You do not need to repeat those answers on this page. Section E requires a complete description of the
objectives and alternatives of your proposal and should not be summarized here.
The applicant proposes to construct a 1,300 square foot house on a 5,733 square foot lot. The proposal includes
enhancement of the stream channel and remaining wetlands.
12. Location of the proposal. Give sufficient information for a person to understand the precise location of your
proposed project, including a street address, if any, and section, township, and range, if known. If a proposal
would occur over a range of area, provide the range or boundaries of the site(s). Provide a legal description,
site plan, vicinity map, and topographic map, if reasonably available. While you should submit any plans
required by the agency, you are not required to duplicate maps or detailed plans submitted with any permit
applications related to this checklist. The site is located immediately south of 13031 33`d Avenue South in
Tukwila, Washington. The site is Lot 12 of Robbins Block 5, "Robbins Springbrook Addition to Riverton",
According to the Plat Thereof Recorded in Volume 16 of Plats, Page 67, Records of King County, Washington.
The site is further described as located in the NW 1 /4 of the NW 1/4 of Section 15, Township 23 North, Range 4
East, of the Willamette Meridian.
2. Does the proposal lie within an area designated on the City's Comprehensive Land Use Policy Map as
environmentally sensitive?
No
3
TO BE COMPLETED BY APPLICANT
Evaluation for
Agency Use Only
B. ENVIRONMENTAL ELEMENTS
1. Earth
a. General description of the site (circle one): Flat, rolling, hilly steep slopes, mountainous, other
East -facing hillside
b. What is the steepest slope on the site (approximate percent slope)? Approximately 20%
c. What general types of soils are found on the site (for example, clay, sand, gravel, peat, muck)? If you know
the classification of agricultural soils, specify them and note any prime farmland. The soil on site is Norma
gravelly sandy loam, described in the Soil Survey of King County Area Washington (Snyder, 1973).
d. Are there surface indications or history of unstable soils in the immediate vicinity? If so, describe.
A seepage spring originates southwest of the site. There are no indications of unstable soils
e. Describe the purpose, type, and approximate quantities of any filling or grading proposed. Indicate source
of fill. The fill is for the construction of a single-family residence. Fill is not proposed.
f. Could erosion occur as a result of clearing, construction, or use? If so, generally describe.
Erosion as a result of clearing and construction is a potential problem. A seasonal stream flows along the
south property line.
g. About what percent of the site will be covered with impervious surfaces after project construction (for
example, asphalt or buildings)? 2,250 square feet
h. Proposed measures to reduce or control erosion, or other impacts to the earth, if any: Filter fencing will
prevent the movement of suspended solids off-site.
4
• •
2. Air
Evaluation for
Agency Use Only
a. What types of emissions to the air would result from the proposal (i.e. dust, automobile, odors, industrial
wood smoke) during construction and when the project is completed? If any, generally describe and give
approximate quantities if known. There will be the normal dust, equipment and automobile odors
associated with the construction of a single-family residence.
b. Are there any off-site sources of emissions or odor that may affect your proposal? If so, generally describe.
No
c. Proposed measures to reduce or control emissions or other impacts to air, if any: None proposed
3. Water
a. Surface:
1) Is there any surface water body on or in the immediate vicinity of the site (including year-round and
seasonal streams, saltwater, lakes, ponds, wetlands)? If yes, describe type and provide names. If
appropriate, state what stream or river it flow into. Yes, an unnamed seasonal stream flows along the
south property line.
2) Will the project require any work over, in or adjacent to (within 200 feet) the described waters? If yes,
please describe and attach available plans. Yes, an existing pipeline will be replaced and the stream will
and remaining stream buffer will be reconstructed and enhanced with native vegetation.
3) Estimate the amount of fill and dredge material that would be placed in or removed from surface water or
wetlands and indicate the area of the site that would be affected. Indicate the source of fill material.
11.1 cu. yds of stream substrate and rock.
5
• •
Evaluation for
Agency Use Only
4j Will the proposal require surface water withdrawals or diversions? Give general description, purpose, and
approximate quantities if known. No
5) Does the proposal lie within a 100 -year floodplain? If so, note location on the site plan. No
6) Does the proposal involve any discharges of waste materials to surface waters? If so, describe the type of
waste and anticipated volume of discharge. No
b. Ground:
1) Will ground water be withdrawn, or will water be discharged to ground water? Give general description,
purpose, and approximate quantities if known. No
2) Describe waste material that will be discharged into the ground from septic tanks or other sources, if any (for
example: Domestic sewage; industrial, containing the following chemicals ...; agricultural; etc.). Describe the
general size of the system, the number of such systems, the number of housed to be served (if applicable), or the
number of animals or humans the system(s) are expected to serve. None
c. Water Runoff (including storm water):
1) Describe the source of runoff (including storm water) and method of collection and disposal, if any (include
quantities, if know). Where will this water flow? Will this water flow into other waters? If so, describe. The
source of runoff will be roof drains and the driveway. Runoff will flow into the road ditch that the stream flows
into.
2) Could waste materials enter ground or surface waters? If so, generally describe. Yes, pollutants from the
street and driveway may enter surface waters.
6
Evaluation for
Agency Use Only
d. Proposed measures to reduce or control surface, ground, and runoff water impacts, if any: Vegetative
enhancement of the stream buffer, substrate and rock in channel to prevent erosion.
4. Plants
a. Check or circle types of vegetation found on the site:
deciduous tree: alder, maple, aspen, other
evergreen tree: fir, cedar, pine, other
7/shrubs
✓grass
pasture
crop or grain
wet soil plants: cattail, buttercup, bulrush, skunk cabbage, other
other types of vegetation
b. What kind and amount of vegetation will be removed or altered? Reed canarygrass, Himalayan blackberry, red
alder, western red cedar, and willow.
c. List threatened or endangered species known to be on or near the site. None
d. Proposed landscaping, use of native plants, or other measures to presence or enhance vegetation on the site, if
any: See Mitigation Plan, J. S. Jones and Associates, Inc., June 30, 1999
5. Animals
a. Circle any birds and animals which have been observed on or near the site or are know to be on or near the site:
birds: hawk, heron, eagl , on_bir� , ether:
mammals: deer, bear, elk, beaver, other:
fish: bass, salmon, trout, herring, shellfish, other:
b. List any threatened or endangered species known to be on or near the site. None
7
c. Is the site part of a migration route? If so, explain. Yes, Pacific Flyway
•
EVALUATION FOR
AGENCY USE ONLY
d. Proposed measures to preserve or enhance wildlife, if any: Vegetative enhancement of wetland, stream, and
buffers.
6. Energy and Natural Resources
a. What kinds of energy (electric, natural gas, oil, wood stove, solar) will be used to meet the completed project's
energy needs? Describe whether it will be used for heating, manufacturing, etc. Electricity for heating
b. Would your project affect the potential use of solar energy by adjacent properties? If so, generally describe.
No
c. What kinds of energy conservation features are included in the plans of this proposal? List other proposed
measures to reduce or control energy impact, if any: No
7. Environmental Health
a. Are there any environmental health hazards, including exposure to toxic chemicals, risk of fire and explosion,
spill, or hazardous waste, that could occur as a result of this proposal? If so, describe. No
1) Describe especial emergency services that might be required. Fire and emergency medical
2) Proposed measures to reduce or control environmental health hazards, if any: None
8
• .
EVALUATION FOR
AGENCY USE ONLY
b. Noise
1) What types of noise exist in the area which may affect your project (for example: traffic, equipment, operation,
other)? Aircraft
2) What types and levels of noise would be created by or associated, with the project on a short-term or a
long-term basis (for example: traffic, construction, operation, other)? Indicate what hours noise would come from
the site. Construction noise between the hours of 7 am and 5 pm on weekdays
3) Proposed measures to reduce or control noise impacts, if any: None
8. Land and Shoreline Use
a. What is the current use of the site and adjacent properties? The site is currently a vacant lot. Adjacent properties
are single-family residences.
b. Has the site been used for agriculture? If so, describe. No
c. Describe any structures on the site. None
d. Will any structures be demolished? If so, what? No
e. What is the current zoning classification of the site? LDR
f. What is the current comprehensive plan designation of the site? LDR
9
• .
EVALUATION FOR
AGENCY USE ONLY
g. If applicable, what is the current shoreline master program designation of the site?
Not Applicable
h. Has any part of the site been classified as an "environmentally sensitive" area? Is so, specify. Yes, the entire site
is wetland with a stream present.
i. Approximately how many people would reside or work in the completed project? Between one and five
j. Approximately how many people would the completed project displace? None
k. Proposed measures to avoid or reduce displacement impacts, if any: Not applicable
1. Proposed measures to ensure the proposal is compatible with existing and projected land uses and plans, if any:
None
9. Housing
a. Approximately how many units would be provided, if any? Indicate whether high, middle, or low-income
housing. One
b. Approximately how many units, if any, would be eliminated? Indicate whether high, middle, or low-income
housing. None
c. Proposed measures to reduce or control housing impacts, if any: None
10. Aesthetics
a. What is the tallest height of any proposed structure(s), not including antennas; what is the principal exterior
building material(s) proposed? Twenty-five feet
b. What views in the immediate vicinity would be altered or obstructed? None
c. Proposed measures to reduce or control aesthetic impacts, if any: None
10
• •
TO BE COMPLETED BY APPLICANT EVALUATION FOR
AGENCY USE ONLY
11. Light and Glare
a. What type of light or glare will the proposal produce? What time of day would it mainly occur? None
b. Could light or glare from the finished project be a safety hazard or interfere with views? None
c. What existing off-site sources of light or glare may affect your proposal? None
d. Proposed measures to reduce or control light and glare impacts, if any: None
12. Recreation
a. What designated and informal recreational opportunities are in the immediate vicinity? None
b. Would the proposed project displace any existing recreational uses? If so, describe. No
c. Proposed measures to reduce or control impacts on recreation, including recreation opportunities to be provided
by the project or applicant, if any: None
13. Historic and Cultural Preservation
a. Are there any places or objects listed on, or proposed for, national, state, or local preservation registers known to
be on or next to the site? If so, generally describe. No
b. Generally describe any landmarks or evidence of historic, archaeological, scientific, or cultural importance
known to be on or next to the site. None
c. Proposed measures to reduce or control impacts, if any: None
14. Transportation
a. Identify public streets and highways serving the site, and describe proposed access to the existing street system.
Show on the site plans, if any. The site is located between Military Road and Hwy. 99, accessed from S.132nd St.
and 33`d Ave. S.
11
• •
TO BE COMPLETED BY APPLICANT EVALUATION FOR
AGENCY USE ONLY
b. Is the site currently served by public transit? If not, what is the approximate distance to the nearest transit stop?
The nearest bus stop is at Highline Riverton Community Hospital
c. How many parking spaces would the completed project have? How many would the project eliminate? The
project will have two parking spaces outside the garage and will not displace any parking spaces.
d. Will the proposal require any new roads or streets, or improvements to existing roads or streets, not including
driveways? If so, generally describe (indicate whether public or private). No
e. Will the project use (or occur in the immediate vicinity of) water, rail, or air transportation? If so, generally
describe. No
f. How many vehicular trips per day would be generated by the completed project? If known, indicate when peak
volumes would occur. 2-6
g. Proposed measures to reduce or control transportation impacts, if any: No
15. Public Services
a. Would the project result in an increased need for public services (for example: fire protection, police protection,
health care, schools, other)? If so, generally describe. No
b. Proposed measures to reduce or control direct impacts on public services, if any. No
16. Utilities
a. Circle utilities currently available at the site: electricity, natural gas, water, refuse service, telephone, sanitary
sewer, septic system, other.
b. Describe the utilities that are proposed for the project, the utility providing the service and the general
construction activities on the site or in the immediate vicinity which might be needed. Seattle City Light, U. S.
West, Seattle Public Utilities
C. Signature
The above answers are true and complete to the best of my knowledge. I understand that the lead agency is relying
on them to make its decision. A
Signature:
Date Submitted:
?/1 /99
12
J S JONES AND ASSOC:
1 206 7253606
• •
CO 13E COMPLETED BY APPLICANT
Evaluation for
Agency Use Only
E. SUPPLEMENTAL SHEET FOR ALL PROJECT AND NON PROJECT
PROPOSALS
The objectives and the alternative means of reaching the objectives for a proposal
will be helpful in reviewing the aforegoing items of the Environmental Checklist.
This information provides a general overall perspective of the proposed action in
the context of the environmental information provided and the submitted plans,
documents, supportive information, studies, etc.
I. What are the objective(s) of the proposal? The objective of the proposal is to
obtain a sin:le-£anvil residence •ermi and corn•l with the sensitive area
e ulations to the greatest extent ossible.
2. What are the alternative means of accomplishing these objectives? There are
no alternatives, because the applicant has requested the reasonable use of the
property with the minimum impact possible to wetland. Compensation for
impacts is limited to vegetative enhancement of existing wetlands and
restoration of the stream channel. Since the entire site is wetland there is no
area that wetlands can be created in. Existin wetlands are dominated b
invasive non-native plants in the understory and would benefit from
enhancement. The stream channel is an eroded ditch. Restoration of the
stream will provide an esthetically pleasing and stable channel.
3. Please compare the alternative means and indicate the preferred course of
action: Denying the applicant a single-family residence based on the presence
of wetlands would constitute a taking of property and may le • all re uire
com•cnsation. he minimum reasonable use of the site is for one sinle-
family residence, with minimum setbacks to the street and side of lot, and
minimum yard area adjacent to the proposed house. Although mitigation in
the form of creating wetland for wetland impacts is not possible, the site will
benefit from vegetative enhancement of remaining wetlands and stream
restoration. These measures are the only possible mitigation. The preferred
course of action is minimal impact by selecting the least impact location for
the residence, and stream, restoration and vegetative enhancement of
retraining wetlands.
t3
4. Does the
Use Polic
J S JONES AND ASSOC
•
ro osal conflict with
Plan? If so what •olicies of the
the Tukwila Corn rehensive Land Use Polic Plan.
with zonin and in character with the nei hborhood.
•
1 206 7253606
•
P.03
Evaluation for
Agency Use Only
olicies of the T kwila .o •rehensive L- _d
lan? There is no conflict with
ro•osal is -consistent
The
•
Proposed measures to avoid or reduce the conflict(s) are: None
14
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CITY OFEIVED T KWILA
1 1r
PEgMIT OEHTER
66
li
0
J. S. Jones and Associates, Inc.
June 9, 1999
Mr. Jihad Kierouz
J.A.K. Inc.
13407 51st Avenue West
Edmonds, Washington 98062
RE: Lot 12, Robbins Block 6, Located Immediately South of 13031 33rd Avenue South,
Tukwila, Washington
Dear Mr. Kierouz:
The property is entirely wetland, except for a 2 to 5 -foot fringe along the south property
line where old (ill was placed for the residence to the south. The northeastern third of the
site has been recently graded. The undisturbed portion of the site is dominated by a
hydrophytic plant community. Dominant species include western red cedar (7ht ja
plicala), giant horsetail (Equisetum telmateia), creeping buttercup (Ranunculus repens),
reed canarygrass (Ranunculus repens) (see attached data sheets). Soils are black gravelly
sandy loam from 0 to 20 inches. Soils were inundated or saturated to the surface. In the
graded portion of the site, vegetation had been cleared. Soils are gleyed at ten inches.
Wetland hydrology was not present a the time of the June 8, 1999 site visit, but was
present on a previous site visit.
According to the City of Tukwila Sensitive Areas Overlay from the Tukwila Municipal
Code, the wetland is an isolated Type 3 wetland. "Type 3 wetlands (are) those wetlands
which are equal to or less than one acre in size and that have two or fewer wetland
classes" (Tukwila Municipal Code, 1997). A Type 3 watercourse crosses the property
from west to east. Tukwila requires a 25 -foot buffer for Type 3 wetlands and a 1 5 -foot
buffer for Type 3 watercourses. An additional 10 -foot building setback is required for
residential developments (Tukwila Municipal code, 1997).
The only way to it appears you can achieve the intended use for this property is to apply
for a reasonable use exception. Section 18.45.115 of the Tukwila Municipal Code,
explaining exceptions to Tukwila wetland regulations, is attached.
J. S. Jones and Associates, Inc. does not guarantee that any intended use may be
achieved. Wetlands are subject to seasonal and annual variation. Use is dependent upon
approval of all regulating jurisdictions.
Very Truly
I 1
Charles Repath
Wetland Biologist
3408 52nd Place, N.E.
TACOMA, WASHINGTON 98422
253-942-7131 / FAX 253-942 -7132
MITIGATI❑N PLAN
Typical Strom Cross Section
North
Scale: 1" = 10'
Tree.ed Sbroh Stake Det ij .
PLANT H D T E
me« 000140,... yl., 00a SU w.
* vmeasua. +ar.ad • w. I
O Te +e7 as
Q 14a lam Roam 5 9 11
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490 Rol a (lama reral
TO. (flechampliacm9919199)
aNorttomi
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AREA OF ROCK MATERIA. FORSTREAM E.RIIAUCFMENT
TOTAL AREA: ILII rt.
usMS10
y isBaer* Slaw.
Truical Outflow to Stream Channel
VICINITY MAP
0
MITIGATION PLAN
u
ut
J. S. Jones and Associ
Environmental Consultants
5
1
9
oR
Appl. No.
D98-0215
SHEET W-1
SECTION 1.1 DRAINAGE REVIEW •
99-
ElbE
/131-_ le? - o/10,-
`
���.���������� '��,����_'
'` �/ �
Small Site
Drainage
Review
Targeted
Drainage
Review
Full
Drainage
Review
Large Site
Drainage
Review
Single
family
residential
projects
that add
2,000 to
10,000 sf
of new
impervious
surface
AND clear
<2acres
or < 35% of
the site,
whichever
is greater.
Small Site projects or other small
projects that are notaubjectoFuUor
Large Site Drainage Review as deter-
mined in Sections 1.1.2.3 (p. 1-13) and
1.1.2.4 (p. 1-13), AND which have the
characteristics of one or more of the
foliowing categories of projects:
1. Projects containing or adjacent to
floodplains/sensitive areas; or projects
within a Landslide HazarDrainage
Area (LHDA) or Critical Drainage Area;
or rural zoned projects subjecto areal
clearing limits per KCC1G.82.150(n)
2. Projects proposing to tct or
modify a drainage pipe/ditch that
12" or Iarger or receives runoff from a
12" or Iarger drainage
3. Redevelont pects proposing
�$1OO.D0Uinimprovements too
high'uoende.m
All projects,
indudingredevel-
opment projects,
that add 5.000
of within
a LH of new
impervious sur-
face but do not
qualify for Small
Site Draine
Review, OR
redevelopment
projects
$500,000 that
create 5,000 sf
of contiguous
PGIS from new
and/or replaced
impervious
surface.
UPDs, OR
projects that
result in 50
acres of new
impervious
surface
within a
subbasin or
multiple
subbasins
that are
hydraulically
connected,
OR projects
on sites 50
acres within
the recharge
area of a
sole -source
aquifer.
Category
1
Category
2
Category
3
SMALL SITE REQUIREMENTS
67)
CORE REQUIREMENT #1
Discharge n1No�odLocation
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Offsite Analysis
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(1) Category 3 projects that install oil controls which construct or modify a 12 -inch pipe/ditch are also Category 2 projects.
(2) May be applied by DDES based on project or site-specific conditions.
«a These requirements have exemptions or thresholds which may preclude or limit their application to a specific project.
9/1/98*
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1998 Surface Water Design Manual
•
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1:1:2 DRAINAGEIII/IEW TYPES AND REQUIREMENTS
1.1.2.1 SMALL SITE DRAINAGE REVIEW
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Small Site Drainage Review.is:a simplified_altemative to Full Drainage Review for small residential -
building and subdivision projects adding less than 10,000 square feet,of new impervious surface and
restricting site clearing to less than 2 acres or less than.35% of. the site, whichever is greater. .The core and
special requirements applied under Full Drainage Review are replaced with simplified-Si/all-site
requirements which can be applied by anon -engineer' These requirements include flow control Best
ManagementPractices (BMPs) such as setting aside open space to limit future site clearing, and -using •
simple measures such as splash,Clocks and- gravel trenches to disperse- orinfiltrate runoff from impervious
areas. Also included are simple BMPs for erosion and sediment control (Esc).rormal water qualitytreatment is not necessary. • This alternative to Full Drainage Review acknowledges that drainage impacts _
for many small development proposals can be effectively mitigated without construction of cosily flow
control and water quality facilities: :
The Small Site Drainage Review process minimizes the time andeffort required to design, submit, revieW,
and approve drainage facilities for these proposals. In post cases, the requirements can be met with
engineer.subrnittal.
s prepared by contractors, architects, or homeowners without the involvement of aficensed civil
Threshold
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Small Site Drainage Review is allowed for any project that is subject to drainage review as determined in
Section 1.1.1 (p. 1-6) and that meets all of the following criteria:
• The projedis a single family residential project,' AND
• The project adds 2,000 to 10,000 square feet8 of new impervious surface, AND
• The p_roject clears less than 2 acres or less than 35% of the site, whichever is greater.
Note: Some projects qualifying for Small Site Drainage Review may also require Targeted Drainage
Review if they meet any of the threshold criteria in Section 1.1.2.2 (p._I-10).
Any potential small site proposal may elect to go through Full Drainage Review described in Section
1.112.3 (p. 143).
• •
Scope of Requirements
. . _-. .
IF Small Site Drainage Review is allowed, THEN -the applicant may apply the simplified small site
su. se II tand .draina• e- des. tom -. -.- III - II detailed in Small Site Drainage Requirements adopted as
A-Ap erp jscth ocC this manual (detached) and available as a separate booklet from DNR or DDES. These
requirements include simplified BMPs for flow control and erosion and sediment control. Note: An open
space tract. or covenant may be required to preserve'uncleared areas.
- ---- . .
Exemption from Core and Special Requirements
The simplified drainage requirements applied under Small Site Drainage Review are considered sufficient
to meet the overall intent of the core and special requirements in Sections 1.2 and 1.3, except under certain
conditions when a proposed project has characteristics that trigger Targeted Drainage Review (see the
threshold for Targeted Drainage Review in Section 1.1.2.2, p. 1-10) and may require the involvement of a
licensed civil engineer. Therefore, any proposed project that qualifies for Small Site Drainage Review as
determined above and complies with the small site drainage requirements detailed in Appendix C is
considered exempt frotri-all core and special requirements in Sections 1.2 and 1.3 except those which would
apply to the project if it is subject to Targeted Drainage Review as specified in Section 1.1.2.2 (p. 1-10).
7 Single family residential project is defined on page 1-4. --. -
8 The threshold of 10,000 Square feet of new irnpervious surface shall be applied by threshold discharge area and shall
include all impervious surface that will ultimately result from the proposed project (e.g., impervious -surface that will result from
" future homes within a plat.or short plet)..-
1998 Surface Water Design.Manual
1-9
9/1/98
SECTION 1.1 DRAINAGE REVIEW.
1.1.2.2 TARGETED DRAINAGE REVIEW.
Targeted -Drainage Review (TDR) is an abbreviated evaluation by DDES permit review staff of a proposed
project's compliance with selected core and special requirements. Projects subject to this type of drainage
review aretypically small -site proposals or other small projects that have site-specific or project -specific
drainage concerns that must be addressed by a licensed civil engineer,or DDES engineering, review staff:
Under Targeted Drainage Review, engineering costs -associated with drainage design and review are kept.
to a minimum because the.review includes only those requirements that,would apply to the particular
project.• --
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Targeted Drainage Review is_required-for those projects subject to drainage review as determined in
Section 1.1.1 (p. 1-6), AND which are not subject to Full or Large Site Drainage Review as determined in
Sections 1.1.2.3 (p. 1-13) and 1-.1.2.4 (p. 1-13), AND which have the characteristics of one or more of the
following project categories' _
• TDR Project Category #1: Projects that contain or are adjacent toa floodplain, stream; -lake,
wetland, closed-depression;_or other sensitive areaas defined by the Sensitive Areas Ordinance
(codified in KCC 21A.24).excluding seismic, coal mining, and volcanic hazard areas; OR projects
located within a Landslide Hazard Drainage Areas or a Critical Drainage Areal°; OR projects located
within a rural zoned. areal'. subject to areal clearing limits -under KCC 16.82.150(c),and which clear
more than 7,000 square feet or 35% of the site, whichever is greater.
• TDR Project. Category #2: Projects that propose to construct or modifyt 2 a drainage pipe/ditch that is
12 inches or more in size/depth or receives surface and storm water runoff from a drainage pipe/ditch
that is, 12 inches or more in size/depth.
• TDR Project Category #3: Redevelopment projects that propose $100,000 or more of. improvements
to an existing high -use site.13
Scope of Requirements
IF Targeted Drainage Review is required, THEN the applicant must demonstrate that the proposed project
complies with the selected core and special requirements corresponding to the project category or
categories that best match the -proposed project. The project categories and applicable requirements for
each are described below and summarized in Table 1.1.2.A (p. 1-8).
Note: If theproposed project has the characteristics of more than one project category, the requirements
of each applicable category shall apply.
Compliance with these requirements requires submittal of engineering plans and/or calculations stamped
by a licensed civil engineer registered in the state of Washington,unless deemed unnecessary by DDES.
The engineer need only demonstrate compliance withthose core -and special -requirements that have been.—_.--
predetermiined to be applicable based on specific project characteristics as detailed below and summarized
in Table 1.i.2.A (p. 1-8). The procedures and requirements for submittal of engineering plans and
calculations can be found in Section 2.3. -
9 Landslide'Haiard Drainage Areas are delineated on a map adopted with this manual (see map pocket inside of back cover).
10 See Reference Section 3 for a list of Critical Drainage Areas:; -
11 See Reference Section 1 for a list of rural zoned areas where this threshold applies... 0
12 Construct and modify is defined on page 1 3.
13 See the full definition of high -use site on page 1-12.
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1998 Surface Water Design Manual
•1':1".2 DRAINAGE TYPES AND REQUIREMENTS
TDR Project: Category #1
_ _.-This category includes prdjects.that.are too small to trigger application of most core requirements, but
may be.subject to site-specific floodplain or sensitive area requirements; or other area -specific
drainage requirements adopted by the County. Such projects primarily include single family
residential projects in Small Site:Drainage Review.
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IF the proposed project meets the characteristics of TDR Project Category #1, THEN the applicant
must demonstrate that the project complies with the following five requirements:
• Core Requirement #5: Erosion and Sediment Control, Section 1.2.5 (p. 1-43)
• Special Requirement #1 Other Adopted Area -Specific Requirements, Section 1.3.1 (p. 1-59)
•
Special Requirement. #2: Floodplain/Floodway Analysis, Section 1.3.2 (p. 1-60)
• Special Requirement #3:. Flood Protection Facilities, Section 1.3.3"(p. 1-61)
• Special Requirement #4: Source Control, Section 1.3.4 (p. 1-61).
In addition, DDES- may require the applicant to demonstrate compliance with any one or more of the
remaining seven_ corerequirements in Section 1.2 based on project or site-specific. conditions. For
example; if the proposedproject contains or is adjacent to a SAO -defined landslide -or steep slope
hazard -area, DDES may require compliance with"Core Requirement #1: Discharge at the Natural
Location" -(Section 1.2.1, p. 1-17). This may in turn require compliance with "Core Requirement #2:.
Offsite Analysis" (Section 1.2.2, p. 1-19) if a tightline is required by Core Requirement #1:. If a
tightline is found to be unfeasible, DDES may instead require a flow.'control facility per "Core
Requirement #3: Flow Control" (Section 1.23, p. 1-25). If a tightline is feasible, "CoreRequirement.
#4: Conveyance System" (Section 1.2.4, p. 1-38) would be required to ensure proper size:and design.
Any required flow control facility or tightline system may also trigger compliance with -"Core
Requirement #6: Maintenance and Operations" (Section 1.2.6, p. 1=46), "Core Requirement -#7:-- -
Financial Guarantees and Liability" (Section 1.2.7; p. 1-47), and possibly "Core Requirement #8,
Water Quality" (Section 1.2.8,•p. 1-49) if runoff from pollution -generating impervious surfaces is
collected.
The applicant may also have to address compliance with any applicable sensitive areas requirements
in KCC 21A.24 as determined.by DDES. - -
TDR Project Category #2.
This category is intended to apply selected core and special requirements to those projects that
propose to construct or modify a drainage system.ofspecified size, but are not adding sufficient
impervioussurface to trigger Full Drainage Review or Large Site Drainage Review. -
IF the proposed project meets the characteristics of TDR Project Category #2, THEN th-e applicant.
must demonstrate that the proposed project complies with the following requirements. "
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• Core Requirement #1: Discharge at the Natural Location, Section 1.2.1 (p. 1-17)
• Core Requirement #2: Offsite Analysis, Section 1.2.2 (p. 1-19)
•• Core -Requirement #4: Conveyance System, Section 1.2.4 (p. 1-38)
•. Core Requirement.#5: Erosion and Sediment Control, Section 1.2.5-(p. 1-43)
• Core Requirement #6: Maintenance and Operations, Section 1:2.6(p. 1-46)
• Core -Requirement #7: Financial. Guarantees and Liability, Section 1.2.7 (p. 1-47)
• Special Requirement #4: Source Control, Section 1.3.4 (p. 1-61).
1998 Surface Water Design Manual
9/1/98
L2.2 CORE i AREMENT #2: OFFSITE ANALYSIS.
1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS
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All proposed projects must submit an offsite analysis report that assesses potential offsite drainage impacts
associated with development of the project site and proposes appropriate mitigations of those impacts.
The initial permit submittal shall include, at minimum, a Level 1 downstream analysis as described in
Section 1.2.2.1 below.
Intent: To identify and evaluate offsite drainage problems that may be created or aggravated by the
proposed project; and to determine appropriate.measures for preventing aggravation of those problems in
accordance with the requirements of this manual.
The primary component of an offsite analysis report is the downstream analysis, which examines the
drainage system within one-quarter mile downstream of the project site or farther as described in Section
1.2.2.1; below. It is intended to identify' existing or potential/predictable downstream problems so that
appropriate mitigationas.specified in Section 1.2.2.2 (p. 1-22), can beprovided to prevent aggravation. of
these problems. A secondary' componentof the offsite analysis report is an evaluation: of the upstream
drainage system to verify and document thatimpacts will not occur as a result of the proposed project.
The evaluation `Must extend upstream to a point where any backwater effects created by the project cease.
EXEMPTION FROM CORE. REQUIREMENT #2
A proposed project is exempt from Core Requirement #2 if any orie ofthe following _is_true:
1. DDES determines there is sufficient information for them to conclude that the project will not have a
significant adverse impact on the downstream and/or upstream drainage system, -.OR:.
2. The project -adds less than 5,000 square feet of new impervious surface, AND does not construct or -
modify a drainage pipe/ditch that is 12 inches or more in size/depth or that receives "runoff from a
drainage pipe/ditch that is 12 inches or more in size/depth, AND does-not-contain-orlie-adjacent to a
SAO -defined landslide, steep slope, or erosion hazard area, OR .
3. - The project does not change the rate, -volume, duration, or location of discharges to and from the ..
project site (e.g., where existing impervious surface is replaced with other impervious surface having
similar runoff -generating characteristics, or where pipe/ditch modifications do -not change existing
discharge characteristics).
1.2.2.1 DOWNSTREAM ANALYSIS
The downstream analysis must consider the existing conveyance system(s) for a minimum flowpath
distance downstream of one-quarter mile and beyond that as needed to reacha point where the -project -site
area constitutes less than 15% of the tributary -area. This minimum distance may be increased as follows:
• Task -2 of aLeveLl_downstream analysis (described in detail in Section-2:3.T4i-a_review of all_
availableinformation on the downstream area and is intended to identify existing drainage problems.
In.all cases, this: information review shall extend one mile downstream .of_ the.project site. The
existence of flooding, erosion, or nuisance problems may extend the one -quarter -mile minimum
distance for other tasks. to allow evaluation of impacts from the proposed development to the
identified problems.
If a project's impacts to flooding, erosion, or nuisance problems are mitigated by improvements to the k.
downstream conveyance system, the downstream analysis will extend a minimum of one-quarter mile
beyond the improvement. This is necessary because many such improvements result in a reduction of.
stormwater storage or an increase in peak flows from the problem site.
• At their discretion, DDES may extend the downstream analysis beyond the minimum distance -�
specified above on the reasonable expectation of impacts.
1998 Surface Water Design Manual 9/1/98
1-19
SECTION 1.2. COREREQUIREMEN40
The Level -1 -downstream analysis is aqualitative survey- of each downstream system midis the_first step
in identifying flooding, erosion, or nuisance problems as defined below under "Downstream Problems
Requiring.Special Attention." Each Level_1-analysis is. composed of four tasks at a minimum:
• Task 1: Define and map the study area
• Task 2: Review all available information ori the study_ area
• Task 3: Field inspect the study area
• Task 4: Describe the drainage system, and its existing and predicted problems.
Upon'review of the Level 1 analysis, DDES may require a Level 2 or 3. downstream analysis, depending
on the -presence of existing or predicted flooding, erosion, or nuisance problems identified in the Level 1
analysis.`
Levels 2 and 3 downstream analysis quantify downstream problems by providing information on the
severity and frequency of an existing problem or the likelihood of creating a new problem. A Level 2 -
analysis is a rough quantitative analysis (non -survey field data, uniform flow analysis). Level 3 is a more
precise analysis (survey field data, backwater analysis). of significant problems. If conditions warrant,
additional, more detailed analysis may be required. beyond Level 3.
A detailed description of offsite analysis scope and submittal requirements is providedin Section 2.3.1.11 --
Hydrologic analysis methods and requirements for Levels 2 and 3 downstream analysis are contained in..
Chapter 3; hydraulic analysis methods are contained in Chapter 4
❑ DOWNSTREAM PROBLEMS REQUIRING SPECIAL ATTENTION
While the basic flow control. standards in Core Requirement #3. serve to minimize the creation and
aggravation of many types of downstream drainage problems, there are some types that are more. sensitive _.
to aggravation than others depending on the nature or severity of the problem and which basic -flow control—_
standard is being applied. In particular, there are three types of downstream problems where the County-
has
ountyhas determined that the nature and/or severity of the problem warrants additional attention through the
downstream analysis and possibly additional mitigation to ensure no aggravation:
1. Conveyance system nuisance problems
2. Severe erosion problems
3. Severe flooding problems.
Conveyance system nuisance problems are minor but chronic flooding or erosion problems that result from,
the overflow of a constructed conveyance system that is substandard or has become too small due to •
upstream development. Such problems -warrant additional attention because of their chronic natureand_..–
because they result from the failure of a conveyance system to provide a minimum. acceptable level.of:
protection (see definition below). Severe flooding and erosion problems as defined belo.w also warrant.-__ . _
additional attention because they either pose a significant -threat to health and safety or can cause significant
damage to public or private property.
•
Conveyance System Nuisance Problems (Type 1)
Nuisance problems in general are' defined as any existing or predicted flooding or erosion which does not
constitute a severe flooding or erosion problem as defined below. Conveyance system nuisance problems
are defined as any nuisance flooding or erosion that results from the overflow of a constructed conveyance -
system for runoff events less than or equal to a 10 -year event. Examples include inundation of a shoulder
or lane of a roadway, overflows collecting in yards or pastures, shallow flows across driveways, minor
flooding of crawl spaces or unheated garages/outbuildings, and minor erosion.
If a conveyance system nuisance problem is identified or predicted downstream, the need for additional
mitigation must be evaluated as specified in Section 1.2.2.2 under "Problem -Specific Mitigation
9/1/98-
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1998 Surface Water Design -Manual
•
1.2.2 CORE #2 OFFSITE ANAi:YSIS=�',
Requirements"(p. 1-23): This may entail additional onsite flow control -or -other measures as needed to
prevent creation or significant aggravation of the problem.
For any other nuisance problem which- may be identified downstream, this -manual -does hot require
mitigation beyondthe basic flow control standard applied in Core Requirement #3. This is because to •
prevent aggravation of such problems (e.g., those caused by the elevated water surfaces of -ponds, lakes,
wetlands, and closed depressions or those involving downstream.erosion) can -require two to three times as
much onsite detention volume, which is considered unwarranted for addressing nuisance problems.
However; if under some unusual circumstance, -the aggravation of such a nuisance problem is determined
by DDESto be a significant adverse impact, additional mitigation may be required.
Severe Erosion- Problems (Type 2) •
Severe erosion problems are defined as downstream channels, ravines, or slopes-withevidence of or
potential for erosion/incision sufficient to pose.a sedimentation hazard to downstream conveyance systems`
or pose a landslide hazard by undercutting adjacent slopes. Severe erosion problems, do not include
roadway shoulder rifling or minor ditch erosion:
If a severe erosion problem is identified or predicted downstream, additional mitigation must be
considered as specified in Section 1.2.2.2 under "Problem-Specific•Mitigation Requirements" (p. 1-23).
This may entail additional onsiteflow control:or:.other measures as needed to prevent creation or -- --
aggravation of the problem.
Severe. Flooding Problems (Type 3)
Severe flooding problems can be caused by conveyance system overflows or the elevated water surfaces of
ponds, lakes, wetlands, or closed depressions. Severe flooding problems are defined as.follows:
• Flooding of the finished area23 of a habitable building,24 or the electrical/heating system of a habitable
building for runoff events less. than -or- equal to a 100 -year event: Examples include flooding. of
finished floors of homes and commercial or industrial buildings, or flooding of electrical/heating
system components in the crawl space or garage of a home. Such.problems are referred to in this
manual as "severe building flooding problems."
• Flooding over all lanes of a roadway° or severely impacting a sole access driveway26 for runoff
events less than or equal to the 100 -year event. Such problems -are -referred to in this manual as
"severe roadway flooding problems."
ff a severe flooding problem is identified or predicted downstream, the need for additional mitigation must -
be evaluated as specified in Section 1.2.2.2 under "Problem -Specific Mitigation Requirements" (p. 1-23).
This may entail consideration of additional onsite flow control or other measures as needed to prevent _
creation or significant aggravation of the problem.
23 Finished area, for the purposes of this definition, means any enclosed area of a building that is designed to be served by the
building's. permanent heating'or cooling system.
24'Habitable building means any residential, commercial, or industrial building that is equipped with a permanent heating or
cooling system and an electrical system.
25 Roadway, for the purposes of this definition, means the traveled portion of any public or private road or street classified as
such in the King County Road Standards.
26 Sole access driveway means there is no other unobstructed, flood -free route for emergency access to a habitable building.
Severely impacting means the flooding overtops a culverted section of the driveway, posing a.threat of washout or unsafe - •
access conditions due to indiscemible driveway edges, or the flooding is deeper than 6 inches on the driveway, posing a
severe impediment to emergency access. `-_
1998 Surface Water Design Manual
1-21.
*9/1/98
SECTION 1.2 • CORE.REQUIREMENTS•
1,2.2.2 IMPACT- MITIGATION
A proposed project must not significantly aggravate existing downstream problems or create new
problems as a result of developing the site. This manual does not require development proposals to fix
or otherwise reduce the severity of existing downstream drainage problems, although doing so maybe an
acceptable mitigation.
❑. PRINCIPLES OF IMPACT MITIGATION
Aggravation of an existing downstream problem means increasing the frequency of occurrence
and/or severity of the problem. Increasing peak flows at the site of a_problem caused by conveyance
system overflows can increase the frequency of the problem's occurrence. Increasing durations of flows.at
or abovethe overflow return frequency can increase the severity of the problem by: increasing the depth. __.
and_duration of_ flooding. Controlling peaks and durations through onsite detention can prevent --
aggravation of such problems by releasing the increased volumes due to. development only. at return..
frequencies -below the conveyance overflow return frequency, with the net result of causing the
conveyance system to flow full for a longer period of time:.
When a problem is caused by high water -surface elevations -of a. volume, sensitive water body; such as. a
lake, wetland, or closed depression, aggravation means the same as for problems caused by conveyance
overflows: --Increasing the volume of flows to a volume -sensitive water.body can increase the frequency of
the problem's occurrence. Increasing the duration of flows fora range of return frequencies both above:-- ---
and below the problem return frequency can increase the severity of the, problem; mitigating these impacts
requires control of flow durations for a range of return frequencies both above and below the problem
return frequency. The net effect of this duration control is to release the_increased volumes. due to
development only at water surface elevations below that causing the problem, which in turn can cause an
increase in these lower,.but more frequently occurring, water surface.elevations. This underscores an
unavoidable impact of development upstream of volume -sensitive water bodies: the increased volumes.
generated by the development will cause some range of increase in water surface elevations, no matter
what detention standard is applied. _
Creating a new problem means increasing peak flows and/or volumes such that after development,
the frequency of conveyance overflows or water surface elevations exceeds the thresholds for the
various problem types discussed in Section 1.2.2.1. For example, application of the Level 1 flow control
standard requires matching predeveloped and developed 2- and 10 -year peak flows. The 100 -year peak
flow is only partially attenuated; and the flow increase may be enough to cause a "severe flooding problem
as described on page 1-21. Ttie'potential for causing a new problem is often identified during the Level 1 -
downstream analysis, where the observation of a reduction in downstream pipe sizes; for example, may be
enough to predict creation of a new problem. A Level 2 or 3 analysis will typically be required to verify the
capacity of the system and determine whether 100 -year flows can be safely conveyed.
❑ SIGNIFICANCE OF IMPACTS TO EXISTING PROBLEMS
The determination of whether -additional onsite mitigation or other measures are needed to address an
existing downstream problem depends on the significance of the proposed project's predicted impact on
that problem: For some identified problems, DDES will make the determination as to whether the project's
impact is significant enough to require additional mitigation. For the downstream problems defined on
pages 1-20 and 1-21, this threshold -of significant impact or aggravationis defined below.
For conveyance system nuisance problems, the problem is considered significantly aggravated if there
is any increase in the project's contribution to the frequency of occurrence and/or severity of the problem
for runoff events less than or equal to the 10 -year event. Note: Increases in the project's contribution to
this type of problem are considered to be prevented if sufficientonsite flow control and/or offsite
improvements are provided as specified in Table 1.2.3.A (p: 1-26)...
9/1/98*
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—1998 -Surface Water Design Manual
1.2.2 CORE. #2: OFFSITE ANALYSIS=
For severe erosion problems; the -problem -is -considered- significantly:aggravated if there is any increase: in, •
project's contribution to the flow duration27 of peak flows ranging from 50% of the 2 -year peak flow up• to" -_
the full -50 -year peak flow atthe-eroded area,: Note: Increases in the project's contribution -to this type- of .
problem are considered to'be prevented if Level -2 flow control or offsite improvements are provided as
specified. in Table 1.2.3.A (p..1-26).
For severe building flooding problems, the problem is considered significantly aggravated if there is any
increase in the project's contribution28 to the frequency, depth, and/or duration of -the .problem for runoff
events less than or equal to..the.100-year event.
For severe roadway flooding problems, the problem is considered.significantly aggravated if any ofthe_
following thresholds are exceeded and there is any increase in the project's contribution28 to the frequency,
depth,-and%or duration of the problem for runoff events less than or equal to the 100=year:event:
• The existing flooding29. over all lanes of a roadway or overtopping the culverted section of a "sole
access driveway" is predicted to increase in.depth more than aquarter-inch or 1:0% _(whichever is
greater) for the 100 -year runoff event
• The "existing flooding", overall lanes of a roadway or "severely impactinga sole access driveway"
• i•
s more than 6 inches deep or faster than .$.- feet per second for runoff events less than. or.equal to the
100 -year event. •
• The "existing flooding": over all lanes of a sole access roadway30 is more than 3 inches deep or faster
: tlian.5 feet per second for runoff events: less_than or equal_ to the 100 -year -event, oris at any depth for
runoff events less than or. equal to the 10 -year event. .
❑ PROBLEM -SPECIFIC MIIIGAT1ON REQUIREMENTS
1. IF a proposed project or threshold discharge area within a project drains to one or more of the three
;— — types of downstream drainage problems defined in Section 1.2.2.1, (pages 1-20 and 1-21) as identified
through a downstream analysis, THEN the applicant must do one of the following:::>
a)-.. Submit a Level 2 or Level 3 downstream analysis per Section 2.3.1 demonstrating that the
proposed project will not create or significantly aggravate the identified downstream problem(s),
OR
b) Show that the natural discharge area or threshold discharge. area draining to the identified
problem(s) qualifies for an exemption from Core Requirement #3: Flow Control, OR
c). Document,that the basic area -specific flow control standard required in Core. Requirement #3 -is:•
adequate to prevent creation orsignificant aggravation of the identified downstream problem(s) as -.
indicated in Table 1.2.3.A (p. 1-26) with,the phrase, "No additional flow control needed," OR _.
d) Provide additional onsite flow control necessary to preventcreation or significant aggravation of..
the downstream problems) as specified,in Table 1.2.3.A (p. 1-26) and further detailed in
Section 3.3.5, OR
27 Flow duration means the aggregate time that peak flows are at or above a particular flow rate of interest (e.g., the amount of
time over the last 40 years that -peak flows were at or above the 2 -year flow rate). -
Z8 Increases in the projects contribution are considered to be prevented if sufficient onsite flow control and/or offsite
improvements are provided as specified for 'severe flooding problems' in Table 1-:2:3.A (p. 1-26). For 'severe flooding
problems' located within the mapped 100 -year floodplain of a'major receiving water' (see Table 1.2.3.B; p. 1-29) or the
mapped 100 -year floodplain of.a major stream for which there is an adopted basin plan, increases in the project's contribution
are considered negligible (zero) regardless of the flow control standard being applied, unless DDES determines there is.a
potential for increased floodingseparate from that associated with the existing 100 -year floodplain.
29 Existing flooding; for the purposes of this definition, means flooding over all lanes of the roadway or driveway has occurred in
the -past and.can be verified. by County records, County personnel, photographs, or other physical evidence.
30 Sole access .roadway means there is no other flood -free route for emergency access to one or more dwelling units.
1998 Surface: Water Design -Manual .- -- -
1-23
*9/1/98
SECTION 1.2 CORE REQUIREMENTS./
•
•
e) Prov-ide offsite improyetnents necessary -to -prevent creation or significant aggravation of the
identified downstream problem(s) as detailed in Chapter 3 unless identified as not necessary in
- Table 1.2.3.A (p. 1-26), OR
f) Provide a combination of additional onsite flow control and OffSite improvements sufficient to
prevent creation or significant aggravation of the downstream problem(s) as demonstrated by a
Level 2 or Level 3 downstream analysis. --"
2. IF it is identified that the manner of discharge from a proposed project may create a significant
adverse impact as described in Core Requirement #1, THEN DDES may require the applicant to
implement additional measures or demonstrate the impact will not occur.
, •
Intent: To ensure provisions are made (if necessary) to prevent creation or significant aggravation of the
three typesof downstream problems requiring special attention by this manual, and to ensure compliance
with the discharge requirements of Core Requirement #1. •
In addressing downstream problems per Problem -Specific Mitigation Requirement 1 above; the easiest of
the provisions to implement will often be that of additional onsite flow control. This involves designing
the required onsite flow control facility to meet an additional set of performance criteria targeted to
prevent significant aggravation of specific downstream problems. To save time and analysis, a set of
predetermined flow control performance criteria corresponding to each of the three types of downstream
problems is provided in Table -1.2.3.A (p. 1-26) and described in more detail in Chapter 3.
Note that in some das-es the basic area -specific flow controlstandard applicable_to:the proposed project per ---
Section 1.2.3.1 (p. 1-31) is already sufficient to prevent significant aggravation of many of the defined
downstreain prOblem types. .Such situations are noted in Table 1.2.3.A (p. 1-26) as not needing additional
onsite flow control or offsite improvements. For example, if the Level 2 flow control standard is required
by Section 1.2.3.1 (p. 1-32), and a -"conveyance system nuisance problem" is identified through offsite
analysis per Core Requirement #2, no additional onsite flow control is needed, and no offsite
improvements are necessary.
•
9/1/98* 1998 Surface Water Design Manual
1-24
• 1.2.3 COIREQUIREMENT #3: FLOW CONTROL.
1.2.3 CORE REQUIREMENT #3: FLOW CONTROL
R
4
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T
All proposed projects, including redevelopment projects, must provide onsite flow control facilities to
mitigate the impacts of increased storm and surface water runoff generated by the addition of new .
impervious surface and any related land cover conversion. These facilities shall, at a minimum, meet the
performance criteria for one of the area -specific flow control standards described in Section 1.2.3.1
(p. 1-31) and be implemented according to the applicable flow control implementation requirements in
Section 1.2.3.2 (p. 1-35).
Intent: To ensure the minimum level of control needed to protect downstream properties and resources
from increases in peak, duration, and volume of runoff generated by new development. The level,of control.
varies depending on location and downstream conditions identified under Core Requirement #2.
Guide to'Applying?,Care: Requirement`#3:.
Core: Requirement:#3: requiresthat:onsite on'•a
detentindior•infil•tration_facilities. be constructed- to' control:
runoff discharges tromahie proleccstte These facilities mustmeera minimumflo;v,control:pertormance--
tandard-as set forth. irr.Secdorc t 2 T 1;,Area-SPecifc.Elow: Control"Standards" (page:1-31), and,may
need to be even larger• to ensurediardownsuearirproblems are not created` orsigniticaritFy aggravated as
• set forth in.:Section_1.2.2: Eroblein.Speciffc'Mitigation;Requirements (p 1-23) .Table 1.2:3 `A. s
(.1-26) provide . a quickguide fodselectingthe flow contro1.performancecriteria-necessary to.meet.
p.
both of these requirements:.: --
;,_Area-specific.flow. control, standards, target. the level of flow control pertormance.to the protection. ..,.
needs:of:specific regions or areas ofthe ..ountv These_areas•are called flow control>areas-;:and:there are-
•three such areasdepictedon:the.Flow:Control,Applications Map adopted.with this:, manual (see:p ;
pocket on:inside of back -cover).. Eacit flow control area has a basic. Clow: control standard that is=specific,
to that area. The perforrnance-critena.of_thatbasic standard may need to -be increased.to address!u:
specific.dow•nstream:drainage:problemas explained•in Step.4 below: t
Flow control. implemeritation:requirements:are the minimum requirements;;for analyzing;:designing;..
and: maintaining,tlow, contro (facilities,
For etficient application of Core-Requirement#3, the following steps are recommended ---
1` Use the: Flow Contr'ol'P pplications:Map. to determtne the flow control area in which your project is
located:.: If;.this.determinatiorrcannot bernadefrom the map,_a more-derniled.delineationyot flow
control:lareas: is available on:Ki4g County's Geographic Information System. (GIS):
_Check:the list of exemptions -beginning on page 1-27 to. determine ifand/or which'portions-ot;)our
project must provide -flow control- facilities per Core Requirement#3..
:-If. flowacontrol facilities: are: required,:determine:(for the: flow control. area-identitiedabove):which.
area -specific Clow control standardapplies to yourprojectby consulting,the detailed threshold
..information: in-Section,1 2 3 1 The: applicable flow control standard. will determinethe minimum
flow' control.: pertormancereqwred tor your proposed:project:
4... If downstream.problems:were identified, through_ offsiteanalysis: per.: Core. Requirement.#2 and.*
proposed to be addressed:through onsiteflow: control; use Table 1.2.3:A'(p. 1-26) to determine if
::;and what additional: flow control Pertornrance:is necessary to mitigateimpacts-(i.e:; to. prevent
;_creation:or-aggravation-of the,identified problems):.
Use: Section, 1.2.3•:2 (p:.'1-35) to:determinethe.minimumrequirements:forimplementing flow..
controls.
1998 Surface Water Design Manual
1-25
9/1/98
SECtION 1.2 CORE REQUIREMENO
•
9/1/98
1-26
1998 Surface Water Design Manual--
Downstream Problems
Identified through
Offsite Analysis per
Core Requirement #2
AREA -SPECIFIC STANDARD ,
Level 1 Row Control
Level 2Flow Control
Level 3Flow Control
•
Noproblem idendfied.
.`
App|ybaainu�odard
performance criteria.
��u�h2�n&1O'yrpeako
' ...
/Wotchdura�onafor 5O�,of
2-yr through 50 -yr peaks �
��atohdurat�nofor 5O��of
2 -yr though 50 -yr peaks
AND match 1 OO -year peaks
Type 1
Conveyance System
Nuisance Problem
Additional FIow Control
No additional flow control or
other mitigation is needed
No additiona/ fl�w control or
other /nitioa�on/nneeded
-
Hold 1O'yrpeak tooverflow T,.'
peak�)m)
Type 2 `
Severe Erosion
Problem
Additional Flow Control
No additional flow control is
needed, but other mitigation
may be rquired4
No additionaflow control is
needed, but other mitigation
may be requirecl4)
ApplyLeYe| 2flow uontrn|w(v
Type3 �Additional~
Severe Flong
Problem
Flow Control
Additional Flow Control
Additional Flow.Contro
Apply Leve 3 flow controL
If flooding is from conveyance
system overflow, Level 3 may
be modifiedto match durations
above the overflow T,. peak
rather than 5O%ofthe 2'yr
'peak. If flooding is from a
cosed depression, make
design adjustrnents as needed
to meetdhe "special provision
to
for closed depressions5
Apply Level 3 flow contrql.
If flooding 15 from a dosed
depression, make design
adjustments as needed t�
meet the "special provision
for closed depnaauiona"/aX5>
.-
)fflooding. iofrom aclosed
depression, make design
adjustments as needed to
meet the "special provision
for closed depressions' (35)
Notes:
m More than one set of problem -specific perforrnance criteria may apply if two or more.downstream
identified through offsite analysis per Core Requirement #2. If this happens, the performance
appUcable probtem-specific criteria must be met. This can requir extensive, time-consuming
implement multiple sets of outflow performance criteria if additional onsite flow control
mitigating impacts to these problems. In these cases, it may be easier and more pfudent
3 flow control standard in place of the otherwise required area -specific standard. Use
control standard satisfies the specified performance criteria for all the area -specific and
requirements except if adjustments are required per the special,provision for dosed
below in Note 5.
(2) -
Overflow T,. is the retum period of conveyance system overflow. To determine T,. requires
downstream analysis as detalled in Section 2.3.1.1. To avoid this analysis, a 7',. of 2
m'-
Offsite improvements may beimp|ome�ed in Iieuof or in combination with add�onalOowcontrol
Section 1.2.2.2 (p. 1'22) and detailed in Section 3.3.5.
(4) A tightline system may be required regardless of the flow control standard being applied
discharge requirements of Core Requirement #1 (p. 1-17) or the outfall requirements
(p. 1-41), or is deemed necessary by DDES where the risk of severe damage is high:
(5) Spec a|9rovioionforC|000dDepreooiona�bho�enenaF|oodingProb|onn�
IF the.proposed project discharges by overland flow or conveyance system to a closed
a "severe flooding problem" AND the amount of impervious surface area proposed by
orequal to1O%of the 100 -year water surface area of the closed depression, THEN
analysis technique" described in Section 3.3.6 to verify that water surface levels are
frequencies at which flooding occurs, up to and including the 100 -year frequency. If
onsite flow control performance to prevent increases. Note: The "point of compliance
field measurements taken directly at the closed depression (e.g., soils tests, topography,
enter private property for such measurements is denied, DDES may waive this provision
flow,contrn/otandandmithamandutn/y20Y& safety factor on the storage volume.
•
problems are
goals of each
analysis to
is the only viable option for
to implement the Level
of the Level 3 flow
problem -specific
depressions described •
-'
a minimum Level 2
years may be assumed.
-
oad|owda'in -
if needed to meet the•
of Core Requirement #4
- '
depression experiencing
the project is greater -than
use the "point of compliance
not increasing for the return
necessary, iteratively adjust
analysis" relies on certain
etc.). 1! perrnission to
and apply the Level 3
9/1/98
1-26
1998 Surface Water Design Manual--
• 1.2.3 CO)QUIREMENT #3: FLOW CONTROL.
❑ EXEMPTIONS FROM CORE REQUIREMENT #3
There are eight possible- exemptions from the requirement to provide a formal flow control facility per
Core Requirement #3. The intent of these exemptions is to provide for situations where a facility may not
be practical or needed, where other alternatives to a facility can be just as effective, or where it makes
.sense to provide incentives for retaining native vegetation or for- maximizing use of existing developed
areas.
1. -Impervious Surface Exemption
A proposed projector any threshold discharge area within a project is exempt if less than 5,000
square feet of new impervious surface will be added and the project or threshold discharge area is not
within a Landslide Hazard Drainage Area.31 If the project or threshold discharge area is located
within a Landslide Hazard Drainage Area, this exemption only applies to new impervious surface less
than 2,000 square feet.
.2. Impervious.Surface Exemption Using Flow Control BMPs
Any threshold discharge area within a proposed project is exempt if less than 10,000 square feet of
new impervious surface will be added; AND -all of the following criteria are met:
a) The area cleared to accommodate the proposed project must be less than 35% or less than 2 acres
of the threshold discharge area (whichever is greater), AND
b) If the project is a single family residential project, flow control BMPs must be applied within
the threshold discharge area as specified in Small Site Drainage Requirements (detached
Appendix C), AND
c) For projects other than single family residential projects, the new impervious surface within..
the threshold discharge area must be comprised of either non -pollution -generating roofs that
comply with the roof downspout controls in Section 5.1, OR roads, trails, or driveways that
comply with the rural roadway dispersion requirements in Section 5.2.1, AND
d) The manner in which runoff is discharged from the project site must not create a significant
adverse impact per Core Requirement #1.
3. Peak Flow Exemption Using Flow Control BMPs
Any threshold discharge area within a proposed project is exempt if the project improvements within
the threshold discharge area generate less than a 0.1 cfs increase in the existing site conditions32 100 -
year peak flow rate, AND all of the following criteria are met:
a) If the project is a redevelopment project, flow control BMPs must be applied as specified in
Section 5.2, and the project improvements must not significantly impact a "severe erosion
problem" or "severe flooding problem" (see -page -1-21), and must not be located within a
Landslide Hazard Drainage Area, AND
b) If the project is a single family residential project, the runoff from impervious surfaces must be
infiltrated or dispersed using flow control BMPs specified in Appendix C, and any areas of native
31 Landslide Hazard Drainage Areas are delineated on a map adoptedrwith this manual (see map pocket on inside of back
cover).
Existing site conditions depend on what, if any, land conversion activity has occurred on the site since May 1979 when King
County first required flow control on developments adding more than 5,000 square feet of new impervious surface. IF a
drainage plan has been approved by the County since May 1979 for any land conversion activity which includes the addition of
more than 5,000 square feet of new impervious surface, THEN existing site conditions are those created by the site
improvements and drainage facilities constructed per the approved engineering plans. OTHERWISE, existing site conditions
are those that existed prior to May 1979 as determined from aerial photographs and, if necessary, on knowledge of individuals
familiarwith the area. The intent is -to mitigate unaddressed impacts created: by site alterations.or improvements, such as
clearing, which have occurred since May 1979.
32
1998 Surface Water Design Manual- 9/1/98
1-27
SECTION -1.2 CORE REQUIREME
vegetation assumed not to be cleared for the purposes of computing the increase in -100 -year peak -
flow must be preserved within a tract or by covenant as described in Appendix C, AND -
c) For projects other than redevelopment projects and single family residential projects; the
new impervious surface within the threshold discharge area must be comprised of either non-
pollution -generating roofs that comply with the roof downspout controls in Section 5.1, OR roads,
trails, or driveways that comply with the rural roadway dispersion requirements in Section 5.2.1,
AND
d) The manner in which runoff is discharged from the project site must not create a significant
adverse impact per Core Requirement #1.
4. Peak Flow Exemption for Urban Redevelopment Projects
Any natural discharge area of a redevelopment project located within the Urban Growth Area is
exempt if.the:project improvements within the natural discharge area generate less than a 0.1 cfs
increase in the existing site conditions 100 -year peak flow, AND all of the following criteria are met:
a) The application of this exemption to natural discharge areas within a proposed project must not
result in more than a 0.4 cfs increase in the existing site conditions 100 -year peak flow rate for any
threshold discharge.area of the project, AND = -.•
b) Flow control BMPs must be applied to the runoff from new impervious surfaces as. specified in
Section 5.2.1, AND
c) The.project improvements within the natural discharge area must not be located within a
Landslide Hazard Drainage Area and must not significantly impact a "severe erosion problem"
or "severe flooding problem" (see page 1-21), AND
d) The manner in which runoff is discharged from the project site must not create a significant
adverse impact per Core Requirement #1.
5. Forested Open Space Exemption for Rural Residential Projects
Any natural discharge area within a proposed rural residential project (zoned RA -2.5, RA -5, RA -10,
or RA -20) is exempt if all of the following criteria are met:
a) At least 65% of the unsubmerged portion33 of the natural discharge area will be set aside as
forested open space as specified in Section 5.2.1; AND
b) The runoff from new impervious surfaces within the natural discharge area will be dispersed over
native vegetation using the flow control BMPs detailed in Section 5.2.1, AND
c) The manner in which runoff is discharged from the project site will not create a significant
adverse impact per Core Requirement #1.
6. Direct Discharge Exemption--
Any
xemptionAny natural discharge area within a proposed project is exempt if it -drains to one of the "major
receiving waters" listed in Table 1.2.3.B, AND meets'all of the following criteria for direct
discharge34 to that receiving water:
a) The flowpath from the project site discharge point to the edge of the 100 -year floodplain of the
major receiving water shall be no longer than a quarter mile, except for discharges to Lake
Sammamish, Lake Washington, and Puget Sound, AND
b) The conveyance system between the project site and the ordinary high water line of the major
receiving water shall be comprised of manmade conveyance elements (pipes, ditches„outfall
33 Unsubmerged portion means any portion outside the ordinary high water line of streams, lakes, and wetlands.
34 Direct discharge means undetained discharge from a proposed project to a "major receiving water.'
9/1/98 - - 1998 Surface Water Design Manual
1-28
1.2.3 CORLOQ6REMENT #3: FLOW CONTROL
protection, etc.) and shallbe.within_public right-of-way or a public -or private drainage easement,
AND
c) The conveyance system shall have adequate capacity per Core Requirement #4, Conveyance
System, for the entire contributing drainage area, assuming build -out conditions to current
zoning for the "equivalent area" portion (defined in Figure 1.2.3.A, below) and existing
conditions for the remaining area, AND
d) The conveyance system will be adequately stabilized to prevent erosion; assuming the same
basin conditions as assumed in Criteria (c) above, AND
e) The direct discharge proposal will not divert flows from or increase flows to an existing wetland
or stream sufficient to cause.a significant adverse impact.
• Cedar River
• Green/Duwamiish River below River Mile 6
- (S. Boeing Access Road) and above SR 18
• Snoqualmie River (includes the North, South, and
Middle Forks)
• Sammamish River
• White/Stuck River
• Skykomish River
• Tolt River -
• Lake Meridian
• Lake Sawyer
• Lake Sammamish
• Lake Washington
• Puget Sound
Note: Major Receiving Waters" do not include side channels, spring- or groundwater -fed streams, or
wetland habitats that provide salmonid spawning or rearing habitat that may be connected or adjacent
to major rivers.
FIGURE 1.2.3.A EQUIVALENT AREA DEFINITION AND ILLUSTRATION
Equivalent area: The area tributary to a direct discharge conveyance system that is contained within
an arc formed by the.shortest, straight line distance from the conveyance system discharge point to the
furthermost point of the proposed project.
Existing
Conveyance
System
Discharge
Point
Basin
Boundary
Major
Receiving
Water.
Equivalent
Area-
(shaded)
rea(shaded)
Basin
Boundary
1998 Surface Water Design Manual
1-29
*9/1/98
SECTION 1:1 CORE REQUIREMENTS • •
7. Peak Flow Exemption for Urban Residential infill Projects
Any single family residential project located within the Urban Growth Area is exempt if the total
project improvements (within a single threshold discharge area) will generate less than a 0.4 cfs
increase in the existing site conditions 100 -year peak flow, AND all of the following criteria are met:
a) The surrounding area within 1/4 mile of the project site must be over 75%- built-out3 to the
zoned density as of the year 1998, AND
The project must be within a Level 1 Flow Control Area as indicated on the Flow Control
Applications Map adopted with this manual or otherwise subject to Level 1 flow control (see
page 1-31), AND
c) The proposed project must not drain to a "severe flooding problem" or "severe erosion.
problem" as defined on page 1-21, AND
d) The runoff from new impervious surfaces must be infiltrated or dispersed using flow control
BMPs specified in Appendix C, and any areas of native vegetation assumed not to be cleared for
the purposes of computing the increase in 100 -year peak flow must be preserved within a tract or
by covenant as described in Appendix C, AND
e) The manner in which runoff is discharged from the project site must not create a significant
adverse impact per Core Requirement #1.
8. Discretionary Exemption for Infill Projects
Using the procedures detailed in Sections 1.4.3 and 1.4.4 of the adjustment process, the DDES Land
Use Services Division Manager/designee or Building Services Division Manager/designee may grant
an exemption fromthe flow control requirements in Core Requirement #3 provided all of the
following criteria are met:
a) The catchment (defined as the tributary area to a point where the project site comprises 15% of
the tributary area, or 1/4 mile downstream, whichever is greatest) is over 90% built -out to the
zoned density, AND
b) Eighty percent of the existing development within the catchment was constructed prior to 1979
(as determined from aerial photos) or is otherwise without formal flow control, AND
c) There are iii Class 1 or 2 streams with salmonids within 1/2 mile downstream of the project site
(except streams designated as major receiving waters), AND
d) There are no Class 1 wetlands, within 1/2 mile downstream of the project site, AND
e) There are no "severe building flooding problems" (see page 1-21) within 1 mile downstream of
the project site, AND
f) Undetained-flows from the proposed project_wilt generate less than a 10% increase in the 10 -year
peak flows to a downstream "conveyance system nuisance problem" (see page 1-20).
3e -Percent build -out is calculated by dividing the number of existing residential dwelling units (including existing multifamily units)
by the total potential number of residential dwelling units as determined from current base zoning. The total potential number
of residential dwelling units is defined as the sum of (1) existing residential dwelling units, (2) existing vacant non-subdividable
single family residential lots, (3) potential single family residential lots (net buildable area of subdividable parcels multiplied by
the base zoning, and subtracting out any lots with existing residential dwelling units), and (4) potential multifamily dwelling
units on vacant or subdividable multifamily -zoned parcels. Permanent open space areas (e.g., sensitive areas and buffers,
recreational tracts) and those properties that are zoned commercial or industrial, or are publicly -owned (e.g., parks, schools,
arterial roadways, stormwater tracts) shall be excluded from these calculations. -
9/1/98* ----- - - - 1998 Surface Water Design Manual
1-30
•
• 1.2.3 COQUIREMENT #3: FLOW CONTROL
1.2.3.1 AREA -SPECIFIC FLOW CONTROL STANDARDS
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Projects subject. to Core Requirement #3 must, at a minimum, comply with one of the three area -
specific flow control standards: Level 1, Level 2, or Level 3, whichever applies per the threshold
information detailed in this section:
❑ LEVEL 1 FLOW CONTROL
Level 1 flow control is a peak -matching performance standard primarily applied in areas where ---
maintaining peak flows is sufficient to protect the natural and constructed conveyance systems that are not
sensitive to development -induced increases in runoff volumes and flow durations. King County designates
these areas as Level 1 Flow Control Areas. Most Level 1 Flow Control Areas are delineated on the Flow
Control Applications Map adopted with this manual (see map pocket on inside of back cover). Any urban -
zoned areas of unincorporated King County not shown on this map shall also be considered Level -1 Flow
Control Areas.
Threshold
The Level 1 flow control standard shall be applied to the design of required flow control facilities for
any proposed project which meets one of the following criteria:
• The project is located within a Level 1 Flow Control Area as defined above, OR
The project is located within a Level 2 Flow Control Area as defined on page 1-32, but does not meet
the threshold for application of the Level 2 flow control standard (see p. 1-33).
Performance Criteria
Level 1 Flow Control:. Match the developed peak discharge rates to the existing site conditions36 peak
discharge rates for 2- and 10 -year return periods.
Reduced Level 1 Flow Control: A modified version of this standard, controlling only the 10 -year
frequency peak flow rate, is allowed if the applicant demonstrates both of the following:
• The proposed project site discharges to a conveyance system not subject to erosion that extends from
the project discharge point to one of the major receiving waters listed in Table 1.2.3.B (p. 1-29), AND
There is no evidence of capacity problems along this conveyance system as determined by offsite
analysis per Core Requirement #2, or such problems will be resolved prior to project construction.
• .
-intent
Level 1 flow control is intended to protect flow -carrying capacity and limit increased erosion within the
downstream conveyance system for runoff events less than or equal to the 10 -year event. Matching the. 2 -
and 10 -year peakaflows is intended -to prevent increases in return -frequency peak flows less than or equal to
the 10 -year peak flow down to the 2 -year peak flow. This level of control is also intended to prevent
creation of new "conveyance system nuisance problems" as defined in Section 1.2.2 (p. 1:20).
Effectiveness in Addressing, Downstream Problems
While the Level 1 flow control standard provides reasonable protection from many development -induced
conveyance problems (up to the 10 -year event), it does not prevent increases in runoff volumesor, flow
durations that tend to aggravate the three types of downstream problems described in Section 1.2.2.1.
Consequently, if one or more of these problems are identified through offsite analysis per Core
Requirement #2, additional onsite flow control and/or offsite improvements will likely be required (see
"Problem -Specific Mitigation Requirements" in Section 1.2.2.2, p. 1-23).
36 Existing site conditions is definedin footnote 32 on page 1-27.
1998 Surface Water Design Manual
1-31
*9/1/98
SECTION 1.2 CORE REQUIREMENTS •
0 LEVEL 2 FLOW CONTROL
Level -2 flow control is a duration -matching performance standard which is effective in preventing
increases in existing erosion rates. The standard is applied in areas where the County has determined that
a greater level of control is needed and will be effective in preventing severe erosionand sedimentation
damage caused by development -induced increasesinflow durations.37 Such areas include those draining
through SAO -defined erosion hazard areas or to salmonid -bearing streams considered sensitive to
increased flow durations based on County studies or resource assessments. These areas are designated by
king County as Level 2 Flow Control Areas, and they collectively include the following five types of
special defined drainage areas and/or basin plan subbasins:
L `Basin Plan Stream Protection Areas: These are subbasins in adopted basin plans where the County
has determined through hydrologic modeling that increases in flow durations from future development
will cause erosion and sedimentation damage to salmonid -bearing streams. They are identified as
requiring increased onsite detention to prevent acceleration of in -stream channel erosion as well as
sediment -generating erosion in the stream's tributary areas.
2. Rural Stream. Protection. Areas:. These are areas not covered by basin plans that drain to relatively
. undisturbed high-value resource streams on the rural side of GMA urban growth boundaries. There
are nine such areas originating from a group of 17 basins identified by King County as having the
highest valuehabitat and aquatic resources from among the county's 72 basins. -The 17 basins were
identified through a.county-wide assessment of habitat/resource values conducted in 1994 as part of
the Waterways 2000 Program. Although extensive modeling has not been done to confirm the
sensitivity of these streams to increased flow durations, there is a high probability they are sensitive
based on County modeling of similar streams in adopted basin plans. Given this high probability and
the high value of the resource, application of Level 2 flow control in these areas is warranted. The
rural portions of the following nine stream basins are designated as Rural Stream Protection Areas:
• Tokel Creek
• Harris Creek
• Griffin Creek
• Patterson Creek
• Tolt River
• Raging River
• Middle Green River
• White River
• Snoqualmie River
In addition to the above nine basins, any rural zoned areas of the County not shown on the Flow
Control Applications Map are also considered Rural Stream Protection Areas.
3. Sensitive Slope Protection Areas: These are areas outside of stream protection areas that drain.to..
those SAO -defined "erosion hazard areas" that are on slopes steeper than 15% (a delineation of all
known SAO erosion hazard areas can be found in King County's Sensitive Areas Map Folio) and.
where the potential for future severe erosion is high based on the amount of upstream area -yet to be—
developed. These areas require Level 2 flow control to_ prevent creation or aggravation of severe
erosion problems.- - -
4. Landslide Hazard Drainage Areas: These are areas both inside and outside of adopted basin plans
which are mapped on the Landslide Hazard Drainage Areas Map adopted with this manual (see map
pocket on inside of back cover) and which drain to SAO -defined "landslide hazard areas" that are on
slopes steeper than 15% (a delineation of known SAO landslide hazard areas can be found in King
County's Sensitive Areas Map Folio). Because these hazard areas pose a significant threat to health
and safety, Level 2 flow control is the basic area -specific standard unless a tightline is provided per
Core Requirement #1. If a tightline is provided, then the basic standard defaults to that required for
whatever other drainage or flow control area the proposed project may occupy. For example, if the
•
37 Flow duration means the aggregate time that peak flows are at or above a particular flow rate of interest (e.g., the amount of
time over the last 40 years that peak flows were at or above the 2 -year flow rate).
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• 1.2.3 COR)QUIREMENT #3: FLOW CONTROL
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project is located_within-a Basin Plan Stream Protection Area or. a Rural Stream Protection_Area, as .
defined above, then Level 2 flow control would still be the basic standard.
In cases where. a.tightline is not provided to convey project flows through the landslide hazard area;
Level 2 flow control must be implemented in a manner which infiltrates as much•runoff as is feasible
to prevent significant disturbance of the landslide hazard area by overland flows (see "Facility •
Requirement in Landslide Hazard Drainage Areas," Section 1.2.3.2, p. 1-37).
5. Forest Production Zone: These areas are typically steeper in slope and often drain to the County's
most pristine, streams. Level 2 flow control is therefore required to prevent creation or aggravation of
severe erosion problems.
Most Level 2 Flow Control. Areas are delineated on the Flow. Control Applications Map adopted with
this Manual (see map pocket on inside of back cover). Any forest production zone or rural -zoned areas of
unincorporated King County not shown on this map shall also be considered Level 2 Flow Control Areas.
Note: A more detaileddelineation of Level 2 Flow. Control Areas, including the five component areas
described above, is available on King County's Geographic Information System (GIS).
Threshold -•
The Level 2 flow control standard shall be applied to the design of required flow control facilities for
any proposed project which is located within a Level 2 Flow Control Area as defined above, AND which
is confirmed to meet one of the following criteria for application of the Level 2 flow control standard: •
• The project is located within a.Basin Plan Stream Protection Area as defined above and confirmed by
detaileddelineation information in the applicable basin plan, OR
• The project is located within a Rural Stream Protection Area as defined above and, in fact, drains to a
natural stream within that area, OR
• The project is located within a Sensitive Slope Protection Area as defined above and, in fact, ultimately --------
drains over the erodible soils of a SAO -defined "erosion hazard area" with slopes steeper than:15%,
OR
• The project is located within a Landslide Hazard Drainage Area as defined above and, in fact, ultimately
drains over the erodable soils of a SAO -defined "landslide hazard area" with slopes steeper than 15%,
OR
• The project is located within a designated Forest Production Zone.
Note: If the proposed project does not meet the above threshold criteria, then the Level 1 flow control
standard shall apply as detailed on page 1-31.
Performance Criteria
Level 2 Flow Control: Match developed discharge durations to predeveloped durations for the range of
predeveloped discharge rates from 50% of the 2 -year peak flow up to the full 50 -year peak flow, assuming
existing site conditions (see Footnote 32,-p. 1-27) as the predeveloped condition: Note: The peak -
matching criteria of Level 1 flow control must also be met.
Intent
Level 2 flow control is intended to prevent initiation or aggravation of erosion or stream channel instability
by maintaining existing erosion rates. This is accomplished by maintaining at predevelopment levels the
aggregate time that developed flows exceed an erosion -causing threshold (i.e., 50% of the 2 -year peak
flow). Maintaining existing erosion rates within streams and -their tributary areas is important for
preventing increases in channel erosion and sediment loading detrimental to fish habitat and production.
Maintaining existing erosion rates on sensitive slopes is important for preventing initiation and/or
aggravation of severe erosion problems.
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*9/1/98
SECTION 1.2 CORE REQUIREMENTS •
---- Effectiveness in Addressing Downstream Problems -
While the Level 2 flow control standard provides an excellent level of protection for preventing most
development -induced problems; it does not necessarily prevent increases in 100 -year peak flows which
can aggravate "severe flooding problems" as defined in Core Requirement #2 (see page 1-21), nor does it
necessarily prevent aggravation of all "severe erosion problems." Consequently, if one or more of these
problems are identified through offsite analysis per Core Requirement #2, additional onsite flow control
and/or offsite improvements will likely be required (see "Problem -Specific Mitigation Requirements" in
Section 1.2.2.2, p.-1.-23).
❑ LEVEL 3 FLOW CONTROL
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Level 3 flow control is a duration -matching and peak -matching performance standard which is .
effective in preventing significant increases in water surface levels of lakes, wetlands, and closed.
depressions. The standard is primarily applied in areas that drain to certain lakes, wetlands, or closed
depressions where the County has determined that a higher average level of flow -control is needed to
prevent.aggravation of existing documented floodingproblems; the County has designated such. areas as
Level 3 Flow Control Areas. Note that these areas are not specifically delineated on the Flow Control _ _
Applications Map (located inside the back cover of this manual), but they are listed on the map by name of
lake, wetland code number (from the King County Wetlands Inventory), or approximate address.
Threshold
The Level 3 flow control standard shall be appliedto the design of required flow control facilities for
any proposed project which is located within the contributing drainage area of one of the County -
inventoried wetlands or lakes listed on the Flow Control Applications (FCA) Map.
Note: If the proposed project does not meet the above threshold criteria, then apply the area -specific
standard for the flow control area in which the project is located as indicated on the FCA map.
Performance Criteria
Level 3 Flow Control: Apply the Level 2 flow control standard AND match the developed 100 -year peak
discharge rate to the 100 -year peak discharge rate for existing site conditions. Note: The peak -matching
criteria of Level 1 flow control must also be met.
Intent
Level 3 flow control is intended to prevent significant increases in existing water surface levels for 2 -year
through 100 -year return frequencies. Such increases are expected to occur as the volume of runoff
discharging to the water body is increased by upstream development. Because inflow rates to these water
bodies are typically much higher than the outflow rates, increased runoff volumes from upstream
development are, in -effect, stacked on top of existing volumes in the water body, resulting in higher water
surface levels. The duration -matching and 100 -year peak -matching criteria of the -Level 3 flow control
standard counteract this stacking effect by slowing the arrival of additional runoff volumes.
Effectiveness in Addressing Downstream Problems
If the Level 3 flow control standard is implemented onsite, no additional measures are required to prevent
aggravation of the three types of downstream problems defined in Core Requirement #2. The one
exception is when the wetland or lake is a closed depression with a "severe flooding problem," and the
proposed project is adding impervious surface area amounting to more. than 10% of the 100 -year water
surface area of the closed depression. In this case, additional onsite flow control or offsite improvements
may be necessary as determined by a "point of compliance analysis" (see "Special Provision for Closed
Depressions" in Table 1.2.3.A (p. 1-26), and see Section 3.3.6, "Point of Compliance Analysis').
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• 1.2.3 COROQUIREMENT #3: FLOW CONTROL_
1.2.3.2 FLOW CONTROL IMPLEMENTATION REQUIREMENTS
Onsite vs. Offsite Implementation
All required flow control must be implemented onsite except where the below requirements can be met for
direct dischargeto a regional or shared facility constructed to provide flow control for the proposed -
project. Regional facilities are typically constructed as part of a basin plan or master drainage plan.
Shared facilities may be constructed under a County -developed shared facility drainage plan or under an
agreement between two or more private developers.
1. The regional or shared facility must be of adequate size and design to meet the current flow control •
requirements for the proposed project's increased surface and storm water runoff. Note: the current flow
control requirements are those specified by Core Requirement #3 of this manual unless superceded by
other adopted area -specific flow control requirements per Special Requirement #1 (see Section 1.3.1).
In some cases where the current flow control requirements differ from those used to originally design the
regional or shared facility, additional analysis and possible retrofitting of the_facility may be required to
ensure adequate size and design. In other cases where the current flow control requirements are not
significantly different or are less stringent, adequate size and design may already be documented by an
adopted King County basin plan or master drainage plan, an approved shared facility drainage plan, or a
detailed drainage analysis approved by the County for a separate permitted development. -
2 The regional or shared facility must be fully operational at the time of construction of the proposed
project. In the case of a shared facility, the proposed project must comply with the terms and conditions
of all contracts, agreements, and permits associated with the shared facility.
3. The conveyance system between the project site and the regional facility must meet the same criteria
specified for direct discharge to_a major receiving water except for Criterion (a) (see "Direct Discharge
Exemption" on page 1-28). In the case of a shared facility, the criteria are the same, except the
conveyance system need only have adequate capacity and erosion protection for buildout of the
participating portion38 of the contributing drainage area.
Methods of Analysis and Design
Flow control facilities must be analyzed and designed using a continuous flow.simulation method such as
HSPF (Hydrologic Simulation Program FORTRAN) or the simplified HSPF-based runoff files method.
Specifications for use of the runoff files method and associated computer program, KCRTS, are found in
Chapter 3. Detailed design specifications for flow control facilities are found in Chapter 5.
Land Cover Assumptions
Land cover assumptions for designing flow control facilities are detailed in Chapter 3. For residential
development (plats, short plats, and large single family projects), flow control facilities must be sized for
the ultimate potential development of the site; this assumes that all forest -and -shrub cover (outside of
proposed impervious surface areas) will be converted to grass unless protected by an open space tract or
covenant. For rural residential developments, all forest/shrub cover outside of proposed impervious
surface areas will be assumed to be converted to 50% pasture and 50% grass, unless likewise protected.
Roof Downspout Controls,in Subdivisions
All proposed single family residential subdivision projects must, on a lot -specific basis, provide for or
implement one of three types of roof downspout controls in the order of preference specified in Section
5.1. These include downspout infiltration, dispersion, or a perforated stub -out connection.
38 The participating portion includes those properties that have agreements for use of the shared facility.
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SECTION 1.2 CORE REQUIREMENTS
Sizing Credits for Roof Downspout -Controls
When sizing flow control facilities serving single family residential subdivisions, the following credits
may be applied:
• • Where roof runoff is infiltrated_according to the requirements of Section 5:1:1; the roof area may be
discounted from the net impervious area used for sizing flow control facilities.
• Where roof runoff is dispersed according to the requirements of Section 5.1.2 on lots 22,000 square
feet or larger, and the vegetated flowpath of the roof runoff is 50 feet or longer, the roof area may be
modeled as grass surface rather than impervious surface when sizing flow control facilities.
Note: These credits do not apply when determining eligibility for exemptions from Core Requirement #3.
Onsite Runoff Bypass
"Proposed project runoff may bypass proposed onsite flow control facilities provided that all of the
following are true:
1. Runoff from both the bypass area and the flow control facility converges .within a quarter -mile
downstream of the project site discharge point, AND
2 • The flow control facility is designed to compensate for the uncontrolled bypass area such that the net
effect at the point of convergence downstream is the same with or without bypass, AND
3. The 100 -year peak discharge from the bypass area will not exceed 0.4 cfs, AND
4. Runoff from the bypass area will not create a significant adverse impact to downstream drainage
systems or properties, AND
5:.. Water quality requirements applicable to the bypass area are met.
Offsite Bypass- Requirement
IF the existing 100 -year peak flow rate from any upstream offsite area is greater than 50% of the 100 -year
developed peak flow rate (undetained) for the project site, THEN the runoff from the offsite area must
bypass onsite flowcontrol facilities. The bypass of offsite runoff must be designed so as to achieve all of
the following:
1. Any existing contribution of flows to an onsite wetland must be maintained, AND
2. Offsite flows that are naturally attenuated by the project site under predeveloped conditions must
remain attenuated,.either by natural means or by providing additional onsite detention so that peak
flows do not increase, AND
3. Offsite flows that are dispersed or unconcentrated on the project site under predeveloped conditions
must be discharged in a safe manner as described in Core Requirement #1 under "Discharge
Requirements" (p:-1-17). -
Manifold Detention Facilities
A manifold detention facility is a single detention facility designed to take the place of two or more
otherwise required detention facilities. It combines the runoff from two or more onsite drainage areas -
having separate natural discharge points, and redistributes the runoff back to the natural discharge points
following detention. Because manifold detention facilities divert flows from one natural discharge point to
another and then back, they are not allowed except by an approved adjustment (see Section 1.4,
"Adjustment Process").
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1.2.3 CORIUIREMENI'#3: FLOW CONTROL
Facility Requirement in Landslide Hazard Drainage Areas -
Proposed projects subject to Discharge Requirement 2 in Core Requirement #1 (see p. 1-18) must provide
a tightline system unless the 100 -year runoff from the project site can be feasibly infiltrated or one of the
other exceptions listed on page 1-18 apply. For infiltration to be used as an alternative to the tightline
requirement, it must be feasible per the facility design requirements and limitations specified in Section
5.4. When evaluating the feasibility of. infiltration, multiple facility locations scattered throughout the
project site shall. be considered and used where feasible and practical to avoid concentrating infiltrated
water in one location. If multiple facilities are not feasible or practical, then a single infiltration facility
meeting the minimum setback requirements in Section 5.4 may be used.
Where infiltration isnot feasible, a proposed project may still qualify for one of the other exceptions to the
tightline requirement specified in Core Requirement #1 (p. 1.18). If such a project is subject to Core
Requirement #3, then the required flow control facility must be a detention pond sized to meet Level 2 -
flow control performance with a safety factor of 20% applied to -the storage volume. The detention pond
must be sited and designed so as to maximize the opportunity for infiltration in the pond. To accomplish
this, all of the following design requirements must be met:
1. The detention -pond must be preceded by either a water quality treatment facility per Core
Requirement #8 or a presettling basin per Section 5.4, AND
2. All detention pond side slopes must be 3H:1 V or flatter and must be earthen, AND
3. Detention pond liners which impede infiltration shall not be used, AND
4. The pond bottom shall be at or above the seasonal high groundwater table, AND
5.. The detention pond outflow must meet the discharge dispersal requirements specified in Discharge
Requirement 1 of Core Requirement #1 (p. 1-17).
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SECTION I.2 CORE REQUIREMENT •
1.2.4 CORE REQUIREMENT #4: CONVEYANCE SYSTEM
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All engineered conveyance system elements for proposed projects must be analyzed, designed, and
constructed to provide a minimum level of protection against overtopping, flooding, erosion, and structural
failure as specified in the following groups of requirements:
• "Conveyance Requirements for New Systems," Section 1.2.4.1 (below)
• "Conveyance Requirements for Existing Systems," Section 1.2.4.2 (p. 1-39)
• "Conveyance System Implementation Requirements," Section 1.2.4.3 (p. 1-40)
Intent: To ensure proper design and construction of engineered conveyance system elements:
Conveyance systems are natural andengineered drainage facilities that collect, contain, and provide for the
flow of surface and storm water: Thiscore requirement applies to the engineered elements of conveyance
systems—primarily pipes, culverts, and ditches/channels.
1.2.4.1 CONVEYANCE REQUIREMENTS FOR NEW SYSTEMS
All new conveyance system elements,39 both onsite and offsite, shall be analyzed, designed, and
constructed according to the following requirements.
Pipe Systems
1. New pipe systems shall be designed with sufficient capacity to convey and contain (at rriinimum) the
25 -year peak flow, assuming developed conditions for onsite tributary, areas and existing conditions
for any offsite tributary areas.
Pipe system structures may. overtop for runoff events that exceed the 25 -year design capacity,
provided the overflow from a 100 -year runoff event does not create or aggravate a "severe flooding
problem" or severe erosion problem" as defined in Core Requirement #2; Section 1.2.2 (p. 1-31).
Any overflow occurring onsite for runoff events up to and including the 100 -year event must
discharge at the natural location for the project site. In residential subdivisions, such overflow must
be contained within an onsite drainage easement, tract, covenant, or public right-of-way.
3. The upstream end of a pipe system that receives runoff from an open drainage feature (pond, ditch,
etc.) shall be analyzed and sized as a culvert as described below.
Culverts
1. New culverts shall be designed with sufficient capacity to meet the headwater requirements in Section
4.3.1• and convey (at minimum) the 25 -year peak flow, assuming developed conditions for onsite
tributary areas and existing conditions for any offsite tributary areas.
2. New culverts must also convey as much of the 100 -year peak flow as is necessary to preclude -dealing
or aggravating a "severe flooding problem" or "severe erosion problem" as defined in Core
Requirement #2, Section 1.2.2 (p. 1-31). Any overflow occurring onsite for runoff events up to and
including the 100 -year event must discharge at.the natural location. for the project site. In residential
subdivisions, such overflow must be contained within an onsite drainage easement, tract, covenant, or
public right-of-way.
3. New culverts proposed in Class 1 streams or Class 2 streams with salmonids shall be designed to
provide for fish passage as detailed in Section 4.3.2. Note: The SAO or the state Department of Fish
and Wildlife may require a bridge to facilitate fish passage.
39 New conveyance system elements are -those that are proposed •to be constructed where there are no existing constructed.
conveyance elements.
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1.2.4 CORE REQ#4: CONVEYANCE SYSTEM
Ditches/Channels
1. New ditches/channels shall be designed with sufficient capacity to convey and contain, at minimum,
the 25 -year peak flow, assuming developed conditions for onsite tributary areas and existing
conditions for any offsite tributary areas.
2. New ditches/channels must also convey as much of the 100 -year peakflow as is necessary to preclude
creating or aggravating a "severe flooding problem" or "severe erosion problem" as defined in Core
Requirement 2, Section 1.2.2 (p. 1-31). Any overflow occurring onsite for runoff events up to and
including the.100-year eventmust discharge at thenatural location for the project site. In -residential
subdivisions, such overflow. must be contained within an onsite drainage easement, tract, covenant, or
public right-of-way.
Tightline Systems Traversing Steep Slopes
New tightline conveyance systems traversing slopes that are steeper than 15% and greater than 20 feet in
height, or are a "sensitive area steep slope," shall be designed with sufficient capacity to convey and
contain (at minimum) the 100 -year peak flow, assuming full build -out conditions40 for all.tributary areas,
both onsite and offsite. Tightline systems shall be designed as detailed in Section 4.2.2.
Bridges
New bridges shall be designed to: pass the 100 -year peak flow with clearance as specified in -Section 4.3.3.
1.2:4.2 CONVEYANCE REQUIREMENTS FOR EXISTING SYSTEMS -
The following conveyance requirements for existing systems are less rigorous than those for new systems
to allow some salvaging of existing systems that are in useable condition. Existing systems may. be
utilized if they are capable of providing a minimum level of protection as -is or with minor modifications.
Existing Onsite Conveyance Systems
No Change in Flow Characteristics: Existing onsite conveyance systems that will not experience a
change in flow characteristics (e.g., peak flows or volume of flows) as a result of the proposed project
need not be analyzed for conveyance capacity.
Change in Flow Characteristics: Existing onsite conveyance systems that will experience a change in
flow characteristics as a result of the proposed project must comply with the following conveyance
requirements:
1. The existing system must be analyzed and shown to have sufficient capacity to convey and contain (at
minimum) the 10 -year peak flow assuming developed conditions for onsite tributary areas and existing
conditions_ for any offsite tributary areas.
2. Theapplicant must demonstrate that the 100 -year peak flow to the existing system will not create or
aggravate a "severe flooding problem" or "severe erosion problem" as defined in Core Requirement
#2, Section 1.2.2 (p. 1-31). -
3. Minor modifications may be made to the conveyance system to achieve the required capacity stated
above. Examples of minor modifications include raising a catch -basin rim, replacing or relaying a
section of pipe to match the capacity of other pipes in the system, improving a pipe inlet, or enlarging
a short, constricted reach of ditch or channel.
4. Modifications to an existing conveyance system or element which acts to attenuate peak flows due to
the presence of upstream detention storage shall be made in a manner that does not significantly
4° Full build -out conditions means the tributary area is developed to its full zoning potential except where there are existing
sensitive areas, open space tracts, and/or native growth protection easements/covenants.
1998 Surface Water Design Manual
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:. _ *9/1/98._ ___
SECTION 1.2 CORE REQUIREMENT* •
increase peak flows downstream. For example, if water is detained in a pond upstream- of a restrictive
road culvert, then installing an overflow system for the culvert should -prevent overtopping of the road
without significantly reducing existing detention storage.
Existing Offsite Conveyance Systems
1. Existing offsite conveyance -systems need not be analyzed for conveyance capacity except as required
by Core Requirement #2, or if offsite improvements or direct discharge are proposed per Core
Requirement #3.
2. Improvements made to existing offsite conveyance systems to address the problem -specific mitigation
requirements in Section 1.2.2.2 (p. 1-23) need only change existing conveyance capacity sufficient to
prevent aggravation of the drainage problem(s) being addressed.
3. Existing offsite conveyance systems proposed to be used for direct discharge to a major receiving
water per Core Requirement #3 (p. 1-28) shall meet the same conveyance requirements specified in
Section 1.2.4.1 (p. 1-38) for new systems.
1.2.4.3 CONVEYANCE SYSTEM IMPLEMENTATION REQUIREMENTS
Methods of Analysis and Design
Properly -sized conveyance elements provide sufficient hydraulic capacity to convey peak flows of the
return frequencies indicated in Sections 1.2.4.1 and 1.2.4.2. Conveyance capacity shall be demonstrated
using the methods of analysis detailed in Chapter 4. Design flows for sizing conveyance systems shall be
determined using the appropriate runoff computation method specified in Section 3.2.
Spill Control Provisions
Projects proposing to construct or replace onsite conveyance system elements that receive runoff from
non -roof -top pollution -generating impervious surface' must provide a spill control device as detailed in
Section 4.2.1.prior to discharge from the project site or into a natural onsite drainage feature.42 More
specifically, this requirement applies whenever a proposed project does either of the following:
• Constructs a new onsite conveyance system that receives runoff from non -roof -top pollution -
generating impervious surface, OR
• Removes and replaces an existing onsite conveyance system element that receives runoff -from 5,000
square feet or more of non -roof -top pollution -generating impervious surface onsite.
The intent of this device is to temporarily detain oil or other floatable pollutants before they enter the
downstream drainage system in the event of an accidental spill or illegal dumping. It may consist of a tee
section in a manhole or catch basin, or another alternative as specified in Section 4.2.1. Note: Spill control
devices were referred to as "oiUwater separation. devices" in previous editions of this manual.
Composition
Where feasible, conveyance systems shall be constructed of vegetation -lined channels, as opposed to pipe
systems. Vegetative channels shall -generally be considered feasible if all of the following conditions are
present:
1. The channel gradient generally does not exceed five percent, AND
41 Pollution -generating impervious surface means an impervious surface considered to be a significant source of pollutants in
storrnwater runoff. Such surfaces include those which are subject to vehicular use or storage of erodible or leachable
materials, wastes, or chemicals, and which receive direct rainfall or the run-on or blow-in of rainfall (for more details, see
page 1-50). Metal roofs are also considered to be pollution -generating impervious surface unless they are treated to prevent
leaching.
42 Natural onsite drainage feature means a natural swale, channel, stream, closed depression, wetland, or lake.
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• 1.2.4 CORE REQUllIENT #4: CONVEYANCE SYSTEM
2.__ No modifications to currently adopted standard roadway cross sections in the_King_ County Road
Standards are necessitated by the channel, AND
3. The channel will be accessible for maintenance (see Section 1.2.6), AND -
4. The channel will not be subject to erosion.
Exceptions: The following are exceptions to the requirement for vegetative channels:
• Conveyance systems proposed under roadways, driveways, or parking areas
• Conveyance systems proposed between houses -in urban -zoned plats and short plats
• Conveyance systems conveying roof runoff only.
Outfalls
An outfall is defined as a point where collected and concentrated surface.and storm water runoff is
discharged from a pipe system or culvert.
Energy Dissipation: At a minimum, rock erosion protection is required at outfalls from all drainage
systems and elements except where-DDES determines that erosion protection is being provided by other
means or is not needed. Details on outfall structures are included in Section 4.2.2.
New Point Discharges Over Steep Slopes: Proposed outfalls that will discharge runoff in a.location
where the natural (existing) discharge is unconcentrated over a slope steeper than 15% and greater than 20
feet in height, or over a SAO -defined steep slope hazard area, must meet the following criteria:
1. IF the 100 -year peak discharge is less than or equal to 0.2 cfs43 under existing conditions and -will
remain less than or equal to 0.2 cfs under developed conditions, THEN outfall runoff maybe
discharged onto a rock pad shaped in a manner so as to disperse flow. The outfall and rock pad must
be located upstream from any landslide or steep slope buffer and no less than 50 feet from the top of a
SAO -defined steep slope unless -otherwise approved by DDES based on an evaluation/report by a
geotechnical engineer. -
2. IF the 100 -year peak discharge is greater than 0.2 cfs but less than or equal -to 0.5 cfs under existing
conditions and will remain less than or equal to 0.5 cfs under developed conditions, THEN runoff must
be conveyed to a dispersal trench Cr other dispersal system. The dispersal trench or system must be
located upstream from any landslide or steep slope buffer and no less than 50 feet from the top of a ..
SAO -defined steep slope unless otherwise approved by DDES based on an evaluation/report by a
geotechnical. engineer.
3. IF the 100 -year peak discharge is greater than 0.5 cfs for either existing or developed conditions,
THEN a tightline conveyance system must be constructed to convey the runoff to the bottom of the
slope unless other measures are approved.by DDES based on an evaluation/report by a geotechnical -
engineer. Tightline systems must be designed such that existing baseflow conditions are not
significantly changed and adequate energy -dissipation is provided at the bottom of the slope.
Outfalls to the Green River
New stormwater outfalls or modifications to existing stormwater outfalls discharging to the Green River
between River Mile 6 (South Boeing Access Road) and SR 18 are allowed only through the adjustment
process. These outfalls must comply with requirements of the Green River Pump Operations Procedure
Plan, which establishes storage volumes and release rate criteria for developments proposing to construct
or modify outfalls. Copies of the plan are available from DNR.
43 Peak discharges shall be as computed using KCRTS as detailed in Chapter 3.
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SECTION 1.2 CORE REQUIREMENI110
interflow and Interception _-
Interflow is near -surface groundwater that moves laterally through the soil horizon following" the hydraulic
gradient of underlying relatively impermeable soils. When interflow:is expressed on the surface, it is
termed a spring or seepage. Any significant springs or seepage areas that impact a roadway or structure
proposed by the project must be intercepted and directed into a conveyance system. Where_ roadways may
impede the passage of interflow to downstream -wetlands or streams, provision for passage of
unconcentrated flows must be made.
Pump Systems
Pump systems may be used to convey water from one location or elevation to another within the project
site provided they meet the design criteria specified.for such systems in Section 4.2.3 and. will be privately
owned and maintained. -
Pump systems that discharge flows from the project.site that would not have discharged by gravity flow
under existing site conditions will require an approved adjustment to Core Requirement #1 (see.Section
1.4, "Adjustment Process"). These pump systems will, be considered. only_ when they are the sole "
alternative to solving a flooding or erosion problem as defined in Section 1.2.2. Typical conditions of
approval for these systems are available in Reference Section 8-J under "Adjustment Application Form
and Process Guidelines."
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1-42
SECTION 1.2. CORE REQUIREMENT •
1.2.6 CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS
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Maintenance and operation -of all drainage facilities is the responsibility of the applicant or property
owner, except those facilities for which King County is granted an easement, tract, or right-of-way and
officially assumes maintenance -and operation as. described below. Drainage facilities must be maintained
and operated in compliance with King County maintenance standards.
Intent: To ensure that the maintenance responsibility for drainage facilities is clearly assigned and that
these facilities will be properly maintained and operated in perpetuity.
Drainage Facilities to be Maintained by King County
King County will assume maintenance and operation45 of the flow control and water quality facilities and
the conveyance system within improved public road right-of-way for any residential subdivision with two
ormore lots, and anysimilar development where at least two-thirds of the developed contributing area is
from single family or duplex, Jesidential structures on individual lots, except where such facilities are
approved by King County to be maintained by the homeowners association. Note: King County may - -
assume maintenance of such facilities serving any mix of developments as part of a shared facilities plan.
King County will assume maintenance and operation of these facilities two years after final.•
construction approval by DDES and an inspection by the County to ensure the facilities have been
properly maintained and are operating. as designed-.
Flow control and water quality facilities to be maintained and operated by King County must be located
in a tractor right-of-way dedicated to King•County. Access roads serving these facilities must also be
located in the tract or right-of-way and must be connected to an improved public road right-of-way.
Underground flow control or water quality facilities (tanks or vaults) may be allowed in private rights-of-
way or roads if the easement includes provisions for facility access and maintenance.
Conveyance systems to be maintained and operated by King County must be located in a drainage
easement, tract, or right-of-way granted to King County. Note: King County does not normally assume
maintenance responsibility for conveyance systems which are outside of improved public road right-of-
way.
Drainage Facilities to be Maintained by Private Parties
All privately -maintained drainage facilities must be maintained as specified in Appendix A, "Maintenance
Requirements for Privately Maintained Drainage Facilities," and as further prescribed in_Chapter 6 for
water quality facilities. A copy of the Operation and Maintenance Manual submitted as part of the
permit application (see Section 2.3.1) shall be retained on site and shall be transferred with the property to
the new owner. A log of maintenance activity indicating when cleaning occurred and where waste was
disposed of shall also be kept by the owner and be available for inspection by the County. King County
may inspect all privately -maintained drainage facilities for compliance with these requirements. If
property owner(s) fail to maintain their facilities to the acceptable standards, the County may issue a
written notice specifying the required actions. If these actions are not performed in a timely manner, the
County may enter the property to perform the actions needed and bill the property owner(s) for the cost of
the actions. In the event a hazard to public safety exists, written notice may not be required.
If the proposed project is a commercial, industrial, or multifamily development or redevelopment, or a
single family residential building permit, a "Declaration of Covenant" (see Reference Section 8-F) must -
be recorded at the King County Office of Records and Elections prior to engineering plan approval.
If the proposed project is a residential subdivision development, all privately maintained conveyance
systems or other drainage facilities which convey flows through private property must be located in a
45 King County does riot assume maintenance of lot drainage systems or drainage stub -outs serving single family residential lot
downspout, footing, or yard drains, nor does King County assume maintenance of those water quality facilities installed and
integrated into site landscaping. -
9/1/98 -- - -- 1998 Surface Water Design Manual
1-46
i1.2.7 FINAL GUARANTEES AND LIABILITY
drainage easement dedicated to convey surface and storm water. Individual owners of the properties
containing such easements must maintain the drainage facilities through their property. The legal instrument
creating drainage easements on private property must contain language that requires a private property
owner to obtain written approval from King County prior to removing vegetation (except by routine
mowing) from any drainage easement containing open, vegetated drainage facilities (such as swales,
channels, ditches, ponds, etc.). See "Drainage Easements" in Reference Section 8-H.
1.2.7 CORE REQUIREMENT #7:
FINANCIAL GUARANTEES AND LIABILITY
4
M
T
All drainage facilities constructed or modified for projects (except downspout infiltration and dispersion
systems) must comply with the financial guarantee requirements in King County Ordinance 12020 and the.
liability requirements of King County Code 9.04.100. There are two types of financial guarantees for
projects constructing or modifying drainage facilities: the drainage facilities restoration and site
stabilization guarantee, and the drainage defect and maintenance guarantee.
Intent: To ensure financial guarantees are posted to sufficiently cover the cost of correcting, if necessary,
incomplete or. substandard drainage facility construction work, and to warrant for two years the - _..
satisfactory performance and maintenance of those newly -constructed drainage facilities; to be assumed by
King County for maintenance and operation. Core Requirement #7 is also intended to ensure that a
liability policy is provided which protects the proponent and the County from any damages relating to the
construction or maintenance of required drainage facilities by private parties.
Drainage Facilities Restoration and Site Stabilization Financial Guarantee
Prior to commencing construction, the applicant required to construct drainage facilities.pursuant to the
drainage requirements in this manual and-KCC 9.04.050 must post a drainage facilities restoration and site
stabilization financial guarantee. This guarantee must be an amount sufficient to cover the cost of
corrective work on or off the site performed specifically for the given project. .Note:.DDES may waive the
requirement of this guarantee on projects proposing -only minor modifications or improvements to the
drainage system (e.g., catch basin inserts, spill control devices, pipe replacements, etc.). In addition, this
guarantee may be combined with other required guarantees as allowed.in Ordinance 12020.
Before King County will release the project's drainage facilities restoration and site stabilization financial
guarantee, the applicant must do the following: -
1. Construct the drainage facilities
2. Receive final construction approval from DDES
3. Pay'all required fees.
Drainage Defect and Maintenance Financial Guarantee
For any constructed or modified drainage facilities to be maintained and operated by King County, the
applicant must do the following:
1. Post a drainage defect and maintenance financial guarantee for a period of two years (see Reference
Section 8-E, "Maintenance and Defect Agreement").
2. Maintain the drainage facilities (per the maintenance standards in Appendix A) during the two-year
period following posting of the drainage defect- and maintenance financial guarantee.
Before King County will release the drainage defect and maintenance financial guarantee and assume
maintenanee'and operation of drainage facilities, the applicant must do the following:
1. For plats; record the final plat.
1998 Surface Water Design Manual
1-47
9/1/98
20709'
EDGE or/45F W I' MKMENT
DESIGN CEN7EmP or RNiq-Or-MAY
ELLE Or 45PINLT PA1EW.T -.
SURVEYOR'S' CERTIFICATE
SURVEYORS & . PLANNERS
945 N. CENTRAL STE. 1104. KENT. WA 98032
(253)852-4880 (local) br 1—(800)251-0189 (loll free) (253)852-4955 (fm)
E-MAIL' ealOcramemr.eom
DRAWN BY:JAC. ` • •
JOB NO.:99038T SCALE: 1'IO' SHEET: I of 1 , DATE:Tue.. Feb. 23, 1999
FOR JIHAD ICISROUE
LOCATED IN THE MW. 1/4, OF THS MW. 1/4,
OF SECTION 16, ' TOWNSHIP 23 NORTH, RANGE 4 EAST,
KING COUNTY, WASHINGTON
THIS MAP CORRECTLY REPRESENTS A SURVEY MADE BY ME OR
UNDER MY DIRECTION IN CONFORMANCE WITH THE REQUIREMENTS '
OF THE SURVEY RECORDING ACT AT THE REQUEST OF
•- • 'JIHAD KIEROOTUZ • IN - Feb. 1992 -
CERTIFICATE NO. 18898
F"'" cows
LLTOAITA1IFABNC16MX
Ydu C►tfllLFD 1111-.
08IC 01 EGU.I. .
BIER FA8011,'.
M 12E001 FILED
-11101 CRAWL 8A01 -
SECTION A -A
FILTER FABRIC FENCE DETAIL
irrs.. - . .
Lf.( AREA:. 5.882± 50. F7.\•.:
I.EGEND & ABBREVIA11ON1
k ROCK CN151RUCITON ENTRANCE MALL BE CONSTRUCTED WT.(TO -:..
• 6' RCM. RACE FLIER FABRICREI.01/ ROO( PA0. -
._ B. MARK CLEARING EDITS 161H 818061LY COLORED ORAQIE TAPE - ....
TAPE SHALL BE SUPPORTED 8Y STAKES' AND SHNL BE -310 6 FEET
NKDL EQUIPMENT OPERATORS SHALL BE BFORIIED CF MEAS THAT - -
.gtR(WTIIRE TtivTRT TABLP
'1;:11/1•lPc. • .:M•:WI. UIiWP'!MYy^�
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5 86. - ..unl: :71{LY::it•".
SITE •PLAN a n °:,iJ�,UE "
C. MULCH SHALL BE APPLL-D ON DISTURBED AREAS LEFT MMORIOAD F06
MORE THAN- 7 DAYS (2 DAYS IN THE NET SEAS00.
D. OVERS ON DIMES d SFDDIE T HWS 70 BE LOCATED mat TO EXCAVATION.
C. NET SEASON DATES ARE FROZE OCIOLSR 1 TO 4108. 30. •„• ..
••• FILTER FABRIC F174:E •
Om'OPTUKWIA
OCT 2 0 1999
PERMR CENTER
DRAWN BY DATE
AS 10/13/99
DESIGNED BY
TKS 10/13/99
NO.
DATE
:BY
CHD.
APPR.
REVISION 1;
1 mm ww9t 1
SCALE:
HORIZ.. 1'=10'
VERT.
JOB NO.:
98430
SHEET
J.A.K. INC
(206) 300-6874
PARCEL 1 735960-0473-07
EROSION- AND SEDIMENT
CONTROL PLAN
C2.
liee. fat he 12 ebb oneembil
CRAWL SPACE ORA t AGE SECTION
.:.. - HIS - ,
(2 .., ..m - 22o-,00 4.,r -0n
• a aw s..`y r r ,aA ..r be V., :.
441.7111;=:!,
City ' o
Tukwila
IF D & ABBREVIATIONS
PROPOSED'
A. COMMIE) ALUMINUM PPE AND COUPLING BANDS SHAH.
MEET THE. REQUIRDIEHTS ff AASNTO N 196 AND Y 197.
& - THE DACHAU. SHALL BE RACED EQUALLY ON 8016 SIDES CF_
THE PPEORPIPE ARCH IN LAYERS 11111H A LOOSE AVERAGE
DEPTH OF 6 INCHES, MA1mAW DEPTH 8 INCHES, THOROUGHLY TAMPING
EACH LAYER THESE COMPACTED .LAYERS MUST EXTEND. FOR
ONE DIAMETER ON. EACH SCE OF .THE PPE OR TO THE SDE CF ...
THE TRENCH
C. GALVANI2D STEEL Cu 9HALL. MEET THE (EQVI9YEN1S OF
• AA91T0 DF.9CNA1I011 M-36, IYPE.1 AND TYPE 2. - PIPE SHALL
HAVE ASPHALT TREATMENT 1 CA BETTER..
D. CATCH BASIN FRAMES AND CRATES SHALL BE OLYMPIC . FOUNDRY
MODEL- 5435, 5435A 0R 50503A LACKING TYPE OR EQUAL
MODEL 5435A 5 RUERRE00 TO A5 A ')@OUCH CURB RILEY - CH ..
THE .PUN. MCOEL 50503A 5 REFERRED TO AS A. 'RCIED
GRA1E DUEY IN TIE PUN.
T PAVD/ENT
HIED PVC PPE
• E .ALL ,N@PERFORATED- METAL PPE SHALL HAVE NECPRDNE • GASKETS:
AT TIE JOINTS O-RING GASICEIS MAY BE USED -FOR TYPE F
C JPLDIC BAND.: -
- F..DOUBLE-WALLED (5710016 INTERIOR) CORRUGATED PCLYETHYLI NE
PPE 9HALL MEET 111E.REOU EMENT5 of AAS4T0 It 252 IN
8 -DIC H S12E ARM-AA9410,9 2945 DI 92E5 12-I4CH DROWN 36-0104
SITE PLAN
DRAWN BY DATE
1xs ' 10/13/99
DESIGNED BY.
TKS 10/13/99.
NO.
DATE
BY
CHD.
APPR.
REVISION
SCALE
HORIZ.. 1'=10'
VERT.
JOB NO.:
98430 a .
(206) 300-6874
PARCEL # 735960-0473-07.
L141 Mari
'IT oon
SHEET
GRADING AND .
DRAINAGE.PLAN.