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Home My WebLink AboutTIS 2022-04-04 Item 2F - Discussion - Tukwila 205 Levee Ceritification: Introduction to LeveesCity of Tukwila Allan Ekberg, Mayor Public Works Department - Hari Ponnekanti, Director/City,Engineer INFORMATIONAL MEMORANDUM TO: Transportation and Infrastructure Services Committee FROM: Hari Ponnekanti, Public Works Director/ City Engineer BY: Sherry Edquid, Project Manager CC: Mayor Allan Ekberg DATE: April 1, 2022 SUBJECT: Tukwila 205 Levee Certification Project Project No. 91341203 Introduction to Levees ISSUE Provide the Council with a general overview of levees within the City and an introduction to the Tukwila 205 Levee Certification Project. BACKGROUND Levees are manmade structures typically built parallel to a river designed to contain, control and/or divert floodwaters thereby reducing the risk from temporary flooding on the landward side. However, they do not eliminate flood risk altogether. In the Green-Duwamish watershed, an integral part in minimizing flood risk is the Howard A. Hanson Dam, which is located in the upper watershed, 21 miles upstream from the City of Auburn. The dam's primary purpose is to provide flood risk reduction in the winter and provide additional flow for fish in the summer and early fall. Flood risk reduction is accomplished by capturing water runoff from the upper watershed and releasing it under controlled conditions. Both the dam and the levee system work in conjunction to reduce flooding in the lower Green River to a fraction of what it once was. The dam was formerly thought to control water up to a 500 -year flood event but recently the Army Corps of Engineers (Corps) revised the dam capacity to just 140 -year flood event. DISCUSSION There are several levees along the Green River and within the City that provide various levels of flood protection. The King County Flood Control District (District), as a special purpose government, assists the City in managing, maintaining, funding, and oversight of these levees. The levee certification process is expected to lead to additional levee repair projects that are required to meet the certification criteria. Certification is valid for a 10 -year period. A levee assessment has been completed for the Tukwila 205 Levee. The study found approximately 80% of the levee could not be certified in its current condition and will require repair or reconstruction. The City and the District are working jointly to maintain Federal Emergency Management Agency (FEMA) certification along with making levee improvements to withstand 500 -year flood events, all while integrating habitat restoration, flood storage, and recreation. 101 The City has petitioned the King County Flood Control District to make levee improvements to upgrade the 205 Levee to provide a minimum of 100 -year level of protection, and where construction is needed, construct to a 500 -year level of protection. KCFCD has agreed to begin design of several projects in 2020, which will take approximately 15 years to complete and will require extensive work with the City of Tukwila and property owners. FINANCIAL IMPACT The overall construction costs of the Tukwila 205 Levee is estimated to be around $200 million and the majority of that is funded by the King County Flood Control District (KCFCD). RECOMMENDATION Discussion only. Attachments: 2021 CIP pg. 86 Levees Presentation Tukwila Levees 102 Introduction to Levees Presentation April 4, 2022 Agenda • Watershed Overview • History • Howard A. Hanson Dam • Levee Vocabulary • Flood Hazard Management • Flood Control Basics • Levees in Tukwila • Future Floodplain Management • Flooding Notification 1 0.1 Green-Duwamish River Watershed Duwamish Estuary Subwatershed URIEN ti RENTS NORM PAR DY SEATAC Middle Green River Subwaters hed WASHINGTON STATE DER WAY Marine Nearshore Subwatershed Lower Green River Subwatershed ACK DIAMOND' F. R Howard Hanson Dam • Harvard Horton Rt envatr River Major Road WRIA 9 Subwatershed Boundary WRIA 9 Boundary 1 Open Water Unincorporated King County Upper Green River Su bw aters h ed 0 2 4 6 Miles AAarch 2009 King County Department of Natural Resources and Parks 0903_W9MAPpptai LPRE z.:mew V 14401:4.14- I.4 Historical Flooding November 22 & 23, 1959 Flood Event Howard A. Hanson Dam 107 Definition of a Levee Manmade structure(s) typically built parallel to a river designed to contain, control and/or divert floodwaters thereby reducing the risk from temporary flooding on the landward side. However, they do not eliminate flood risk altogether. 5C4 YR. .1, 1±u CL 100 YR WSEL 4' EARTH BERM OIERRLIILD--_ °SCALLOP` I°1.4OG MEAN DALLY N1SEL 1C.S' '[ APPROXIMATE Ormiro,I; h'I3EL 12.4' LAULICHABLE ROCK TOE PROTECT1CIt PAVED WALKING RATH EXISTING GRADE if kou.TIE R!PAIR IAll H.4BaITAT PA 7.I',E RECRCAT1014 - `- - - �'-E,ARTIE1J LEVEE TERRESTRIAL HABITAT PASSIVE RECREATION, .PPRC?RI"•1ATE i2 01;1111 lfr' ".1:111T. CDRRIPDR 108 How do we measure floods on the Green River? • i Flood Phase Thresholds for Green River Phase Green River at Auburn Gage (RM 32) * � Flood Frequency 1 5,000 cfs Routine wet season high flow 2** 7,000 cfs Peak flow exceeded 15 of last 20 years 3 9,000 cfs Median estimate of 2 -year flood 4 12,000 cfs 10- to 100 -year flood (2006 and 2009 floods) * Auburn - US Geological Services Gauge https://waterdata.usgs.gov/monitoring- location/12113000/#parameterCode=00060&period=P7D ** King County Flood Warning Center Opens 109 Levee Failure Rates Over 30 -Year Period * Likelihood of Levee PailurePloodingOver a 30 -Year Residential Mortgage LEVE. OF FLOOD RROTECTION 100 year 200 yea• 300 ye 400!rt+1r 500 year 5% I 15% 20% 25% SOURCE NatLona f Committee on Lease Saifit°a= Figure 1. Likelihood of flooding from a mu to 500 year -flood event during a3o-year period. (From the National Committee on Levee Safety). 110 Why do levees fail? Height deficiency Erosion Seepage Stability 42 miles toss! of Lower Green River shcreI ne "kw. 1P" Weta? le+c:E .�rrsLn7 "6019 {iyFa Ar mn ri i0i.1G 4 nom. e>r 91Ecrraor =I9 iL &imam Scheel P.gh! r144.1.4.141,17.0P lmg sulci dm& !',J9-17.114 LEMRr Muria- Hv11 aj !4!Mn,I Re1hi PALI.7q Upper Ilimeelb• S raiwi NUR, Nth. ElApit.0.21-111. OP 1Sharp 13415-1L27 Gell Wok ihiPla141-14 leznasians West > harwalseie IlBesd t.hl A di_°M1-IA 91 113 1 n STs fiHalektg.4k29.i6U thrigitra Irh ®elk y•m11 Green River Flood Facilities #F 1 • Tukwila 205 Levee • 4.6 Mile levee along the left bank • South 196th to 1-405 • Levee last built/modified in 1964 • Levee is currently not certified nor accredited by FEMA due to: • Embankment and foundation stability • Steep riverward slopes • Not meeting freeboard elevation 112 Desimone Levee February 7, 2020 Damage Occurred —11,700 CFS Seepage occurring and water pooling on landward side of the levee. Lower Green River Corridor Flood Hazard Management Plan • 500 Year Flood Protection / 18,800 CFS + 3 feet of freeboard • FEMA Certification and Accreditation • Multi -Benefit Approach • System -Wide Approach Be Prepared FI https://green2.kingcounty.goy/floodalertsystem/ or download the Flood Warning App BE FLOOD READY YOUR GUIDE TO FLOOD PREPAREDNESS IN KING COUNTY https://your.kingcounty.goy/dnrp/library/water-and- land/flooding/Flood-Brochures/2021-2022-flood- brochures/KCFCD Mailer2021 County Final.pdf L41 King County King County Flood Warning Center and Information Line: Connect to a representative during a flood and year-round access to automated flood phase information, river gage readings, and emergency sandbag distribution locations. 206-296-8200 or 800-945-9263 115 • r een/Duwamish River ;w \ 5.5y A GD1 -2017 Levee/Revetment 1/10 River Mile King County Outfall City Boundary h 0 400 I I I Feet LOA King County Department of Natural Resources and Parks For Official Use Only 0 Ta o €: a 2. 0) 0 0 S '108th -St l "Tukw1a 111 11�111�11111�11 �, doori 1111 1 fS111th 0) U -o tyU co ,`, f`� r1 p1r f v;`a Mb) N • \kW\6.1 S 110th St• f alb c AQ,P 6.2 N �Q ,� reen$River Opas Qi Sor 4 A tine I...r.. ik ' i Irl rl.. 6.9 • ver Tri GD 1-2017.ai/2017GreenRiverFloodMaps/GreenRiver/1709_6 •08m_FloodPatrolBooks • 6.8 way Gate � Lowest N N • 6.9 w • * S 115th St S 115th St 7.1 Interurba• n South S 11 a 1 s. 4 J°r • S 119th B rg 41. 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