Loading...
HomeMy WebLinkAboutTIC 2019-01-23 Item 2D - Report - Tukwila 205 Levee Certification / Phase II Draft ReportAllan Ekberg, Mayor Public Works Department - Henry Hash, Director INFOR ATIONAL E ORANDU TO: Transportation and Infrastructure Committee FROM: Henry Hash, Public Works Director /1Af. BY: Ryan Larson, Senior Program Manager CC: Mayor Ekberg DATE: January 18, 2019 SUBJECT: Surface Water Fund — Tukwila 205 Levee Certification Project No. 91341203, Contract No. 14-164 Phase II - Draft Report ISSUE Provide an update on the Tukwila 205 Levee Certification effort and the Phase 2 draft report. BACKGROUND The Tukwila 205 Levee is located on the left bank of the Green River between S 196th St and 1-405 and provides flood protection to the Tukwila Urban Center. The Corps of Engineers (Corps) notified the City that due to a change in their policy, they would no longer provide levee certification for this or any federal levee. Levee certification expired in August 2013. The City hired NHC to perform the first phase of the levee certification effort that included an engineering analysis in accordance with FEMA requirements. This analysis looked at the entire levee system to determine which segments did not meet FEMA requirements. The results of the Phase 1 study revealed that the levee does not meet minimum free board requirements in 14 segments and that embankment, foundation, and stability issues are present. Phase II of this certification effort is near completion and looked at each of the deficient areas identified in Phase 1, collected additional site information, developed alternatives for correcting the deficiencies, and developed costs estimates. DISCUSSION The Phase 2 report identified ten separate segments totaling 17,900 feet of levee or approximately 75% of the total length that do not meet federal requirements for levee safety. These segments have been identified as needing large scale improvements to primarily address steep slopes and foundation issues. To address steep slopes, levee setbacks that require additional levee easements or the use of floodwalls are required. NHC has provided conceptual designs for levee repairs. All proposed designs are only to bring the levee to a 100-year level of protection to allow the levee to be certified. It is recommended that all future levee projects be constructed to the 500-year level of protection given the nature of the area being protected. FISCAL IMPACT Final cost estimates are still being developed, but the total cost to correct all deficiencies are anticipated to exceed $100M. Staff is working with the Flood District to add these projects to the Flood District's Capital Improvement list so that they can be prioritized with the other needed projects throughout King County. RECOMMENDATION Informational item only. ATTACHMENTS • Tukwila Levee Accreditation, Phase 2 Levee Segment Assessment 37 W:\PW Eng\ PROJECTSlA- DR Projects \Tukwila 205\205 Certification (91341203)\Phase 2\ Info Memo Status Update.docx TUK ILA LEVEE ACCREDITATION PHASE 2 - LEVEE SEG ENT ASSESS ENT AND PROPOSED SEQUENCING FOR LARGE SCALE I PROVE ENTS INTER1 REPORT Prepared for: City of Tukwila Public Works Tukwila, WA Prepared by: Northwest Hydraulic Consultants Inc. Tukwila, WA October 17, 2018 NHC Ref. No. 2000098 38 DISCLAIMER This document has been prepared byNorthwest Hydraulic Consultants Inc. inaccordance with generally accepted engineering practices and is intended for the exclusive use and benefit of City of Tukwila and their authorized representatives for specific application to the Tukwila 205-Green River Levee Accreditation in Tukwila, WA, USA. The contents of this document are not to be relied upon or used, in whole or in part by orfor the benefit of others without specific written authorization from Northwest Hydraulic Consultants Inc. No other warranty, expressed or implied, is made. Northwest Hydraulic Consultants Inc. and its officers, directors, employees, and agents assume no responsibility for the reliance upon this document or any of its contents by any parties other than City of 39 EXECUTIVE SUMMARY Northwest Hydraulic Consultants |nc,and their subconsu|tant Wood (formerly AmecFoster Wheeler Environment& Infrastructure, Inc.) and KPG, are providing professional engineering services toevaluate and certify the Tukwila 205 Levee in accordance with Federal Emergency Management Agency (FEMA) requirements. The 4.5-mile long levee is located on the left bank of the Green River between Interstate- 405andSouth196thStreet.ThisreportdocumentsPhaseI,the—A|ternative»Ana|y»isforLargeSca|e Improvements ofthe project. The purpose of the prior Phase 1 was to evaluate the levee condition relative to Code of Federal Regulations Title 44 Chapter |, Subchapter B, Part 65, Section 10criteria on a project reach scale, determine if the levee could be certified in the existing condition and, when not, identify the deficiencies in meeting the criteria. Tabulated below are the findings for each criteria evaluated. Criteria Freeboard Results � feet total of levee are below minimum elevation, but typically order ofinches. Phase 1Evaluation Less than I000 only onthe Closures Noflood closures inthe system tocertify. Embankment Protection Existing rock and vegetated banks are sufficient to certify that there will be no appreciable anticipated erosion of the levee banks during the 100'yearf|uod. Embankment and Foundation Stability Steep slopes significantly limit levee certification and the addition of predicted scour increases failure risk. Settling No significant levee settlement issues that will negatively impact freeboard. Interior Drainage Little significant interior flooding landward of the levee for the simulated 100- yearcondition. The Phase 1 general reach analysis found that steep levee side slopes result in significant lengths of levee failing to meet the required factor of safety for several geotechnical criteria. Including estimated river channel scour further decreases the factors of safety. A review of historic records shows the levee was purposely constructed at these steep slopes; however, those designs do not now pass the required design criteria. FEMA requires that levees be certified in their entirety— no segmentation of contiguous levee system isallowed sothe levee cannot beaccredited inits current condition. The Phase 2 approach has taken a more detailed look at site specific information, including additional Xeoteohnica|work, toinvestigate ifmore accurate data and refined methods may result in segments of levee meeting certification criteria. Work has included dividing the levee reach into segments that can and can't beaccredited. Detailed work during Phase 2 of the project has shown that roughly 20% of the levee can becertified inits existing condition. For those segments ofthe levee that could not be certified in the current state, Phase Z has considered large scale improvement options for certifying Tukwila Levee Accreditation Phase 2 — Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 40 levee segments, developed conceptual level plans and initial cost estimates, and proposed a prioritization for constructing large scale improvements. Once Phase Zisfinalized, the construction repair for each deficient segment of the levee will then move into final design, permitting, and construction in Phase 3. Phase 4will provide the documentation necessary to certify the Tukwila 205 Levee. Tabulated below are a description of the levee segments, which of these segments are anticipated to be certified in their existing condition, and for the remaining levee segments, proposed sequencing for large scale improvements to obtain levee accreditation. Levee Segment Stationing Beginning � -- ---' U � (feet) Ending ____����'=��!v�a�.m 600 Anticipated Certifiable of — � � , Sequencing Construction Improvements Certification Total Length � for � of �Levee Description . (feet) ,--� � N/A — (feet) 0 pa�eUon �'�� � 66mAvenue lapproach road, |-405 and tie intohigh topographic ground. 600 1600 68mAvenue S}ChhstensenRoad. Low 700 1600 2100 Wide left (south side ofriver) overbankdownstream nfthe pedestrian/bicydebrid8e. N/A O 2108 3200 Riverview Plaza Business Park. O /� 3200 3580 Bicentennial Park. N/A O 3500 5300 Business parks and related parking from upstream ofS1rander Boulevard tojust beyond railroad bridge crossing. N/A O 5300 6500 Business parks and related parking along Christensen Road capital project inthe Green RiverSVV|F. 0 High 1200 6500 7700 Tukwila Pump Station. N/A O 7700 10150 Sperry Drive and parking lots near [ostcoand Home Depot. O ' 10150 10250 S1O[P^Street crossing. N/A O 10250 12200 From S18Oth bridge crossing upstream including Lily Point and the Rato|o Levee Green River SVV|Fproject area. O High 1950 12200 13200 Outside ofbend along S18Om. O ' Tukwila Levee Accreditation Phase 2 — Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 2 41 Levee Segment Stationing (feet) Length of Anticipated Certifiable Construction Improvements for Certification Description of Land Features Levee Segment Proposed Total Length Beginning Ending Approximately Paralleling Segment (feet) 13200 14300 15900 18500 22750 14300 15900 18500 22750 23800 Business parks and associated parking upstream of S 180th. Area of 2008 Corps levee repair. Business parks and associated parking upstream of 2008 Corps levee repair. Corps' Gaco-Mitchell Levee being designed. Segale property. Cross -levee. 0 0 0 0 Sequencing (feet) N/A 0 2600 Very High Low 4250 1050 Total Length: 5900 17900 Tukwila Levee Accreditation Phase 2 — Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 17, 2018 Interim Report 42 4 � 1 INTRODUCTION 6 I BACKGROUND 6 3 PURPOSE 8 4 SITE SPECIFIC ENGINEERING ANALYSES 8 4.1 Levee Accreditation 11 S ALTERNATIVES ANALYSIS — LARGE SCALE PROJECTS 19 5.1 Construction Fixes 19 6 PROPOSED PRIORITIZATION 34 7 PHASE ]PREVIEW 35 Q REFERENCES 35 APPENDIX AConcept Designs Tukwila Levee Accreditation Phase 2 — Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 4 43 Ll � � Table 1. Tukwila 2OSLevee Segments. 10 Table 2. Code ofFederal Regulations levee certification criteria and Phase l assessment 11 Table 3. Summary by cross-section of additional Phase 2 geotechnical slope stability assessments. 18 Table 4. Planning level cost estimate for final design and construction of levee certification improvements 20 Table 5. Summary by cross-section of Phase 1 geotechnical slope stability assessments21 Table 6. Proposed improvement prioritization based on risk and opportunity 34 ,0�: Figure 1. Project Location 7 Fi0ureI. Levee Condition 9 FiQure3. Tukwila Levee segment where stationing Uto6U0can becertified a»is. 12 Figure 4. Tukwila Levee segment 600 through 1600 where existing portion 600 through 900 can be certified 13 Figure5. Tukwila Levee segment where stationing 1600 through 2100 can be certified. 14 Figure 6. Tukwila Levee segment where stationing 3200 through 3500 can be certified. 14 Figure7. Tukwila Levee segment where existing section 3SOUtoS3O0can becertified. 15 FiQure8. Tukwila Levee segment where existing section 6500 to 7700 can be certified. 16 Figure9. Tukwila Levee segment where existing section l3Z0Otol43U0can becertified. 17 Fi8ureI0. Tukwila Levee segment 600 through 1600 where existing portion 900 through 1600 can't be certified ZZ Figure11. Tukwila Levee segment 2100 through 3200 that can't be certified in its existing condition 23 Figurel2. Tukwila Levee segment 5300 through 6500 that can't be certified in its existing condition 24 Fi0urel3. Tukwila Levee segment 7700 through 10150 that can't be certified in its existing condition25 FiKure14. Tukwila Levee segment 10250 through 12200 that can't be certified in its existing condition. 26 FigureI5. Tukwila Levee segment 12200 through 13200 that can't be certified in its existing condition. 27 FiQure16. Tukwila Levee segment 14300 through 15900 that can't be certified in its existing condition. 28 Figure 17. Tukwila Levee segment 15900 through 18500 that can't be certified in its existing condition. 29 Figurel8. Tukwila Levee segment 18500 through 22750 (downstream portion) that can't be certified in its existing condition. 31 Figune19. Tukwila Levee segment 18500 through 22750 (upstream portion) that can't be certified in its existing condition 32 FiguneZO. Tukwila Levee segment 22750 through 23800 that can't be certified in its existing condition. 33 Tukwila Levee Accreditation Phase 2 — Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 5 44 1 The City of Tukwila (City) Department of Public Works has selected an engineering consultant team led by Northwest Hydraulic Consultants Inc. (NHC) to evaluate the Tukwila 205 Levee in accordance with the Federal Emergency Management Agency (FEMA) levee accreditation requirements described in Code of Federal Regulations Title 44,Chapter |, Subchapter B,Part 6S,Section 10(44[FRSection 6S.1O). NHC is the prime consultant for this project and is responsible for analyzing hydraulic aspects of the project including freeboard, embankment protection, sour potential, and interior drainage, as well as project management. The team subconsultants are Wood Group (Wood) responsible for geotechnical analyses of the levee, and KPG, responsible for general civil including surveying services. Wood staff nn the first phase of this project were then working under the company name of Amec Foster Wheeler Environment & Infrastructure, Inc. (Amec). � � The Green River flows from the Cascade Mountains generally westward until the city of Auburn, and then northward to Elliott Bay in Seattle, WA, passing through the cities ofKent, Tukwila, and then Seattle. The Tukwila 205 Levee is located on the left bank (looking downstream) of the Green River within the City ofTukwila, WA. The 4.S'mi|elong levee extends from |'405upstream past bridge crossings at Strander Boulevard and South 180t» Street to a cut-off segment that extends to the west valley wall just downstream of the South 2 00th Street bridge. Figure 1 shows an overview of the overall Green River system downstream of Howard Hanson Dam, operated by the U.S. Army Corps of Engineers (USA[E),and the Tukwila IU5Levee project area. Portions of the Tukwila Levee were first constructed to limit Green River flooding of agricultural farmlands (a general chronology of levee construction and repairs can be found in Amec, 2015). After construction of the Howard Hanson Dam in the 1960s, the USACE controlled releases above the 2-year level (50% annual exceeclance probability, approximately 9,200 cfs) by detaining flood waters within the Howard Hanson reservoir and releasing them over a period of days to weeks (NHC, 2007). The United States Geological Survey (USGS) stream gage at Auburn is used by the USACE for its regulation targeting. The dam initiated a development boom in the Green River valley, with extensive levee and revetment improvements constructed during the l9G8s. Tukwila 205 Levee was initially certified by USACE as providing protection up to the 100-year event. The USACE has since generally discontinued their levee certification services, so levees must be re -certified byothers to remain recognized by FEMAas providing flood protection. The City was notified by FEMAof the need for re -certification onJuly lU,ZOlZ. Tukwila Levee Accreditation Phase 2— Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 6 45 Legend AAuburn Flow Gage Howard Hanson Dam Tukwila 205 Levee Green River City Boundaries /Auburn, Bellevue ;_ Newcastle° ngton Samrdamish City of Tukwila no rtitty st hydrautic consultants SCALE-1:150.000 N 0 2 3 A Mites Coordinate System: NAD 1983 STATEPLANE WASHINGTON NORTH FIPS 4801 FEET Job: 2000098 Date: 08-Aug-2015 TUKWILA LEVEE ACCREDITATION ° SITE OVERVIEW Figure 1. Project Location 0) The ultimate purpose of the project is to certify the Tukwila 205 Levee to FEMA standards, Aawork was started on the project, it became clear that much of the levee system could not be certified under current design standards. Due to the costs and complexity of reconstructing thousands of feet of levee inanurban area, the project was broken into phases. Phase 1, "Engineering Analyses and Improvement Identification" (NHC, 2015), work included data collection, performing engineering analyses required under 44 CFR 65.10, and identifying levee deficiencies that must beaddressed for accreditation. That work documented the hydrologic, hydraulic, and geotechnical engineering conducted to evaluate whether the existing levee satisfies FEMA's accreditation requirements, and where and what deficiencies were found. One of the most significant findings from that work was that steep bank slopes throughout the reach prevent the ability to certify the levee as'is. Aasuch, Phase 2work, "Alternatives Analysis for Large Scale Improvements", documented in this report, had two main tasks. The first was tm perform more detailed, site specific assessments at locations where the existing levee might be certified. This included discretizing the project reach based on common characteristics. This initial Phase 2task looks at finer scale with more site specific analyses, versus the generalized reach wide Phase l approach. The second task ofPhase Ievaluates alternative options, develops conceptual level construction plans, and proposes a prioritization for those segments of the Tukwila Levee that must be modified to meet certification standards. Phase 3will take concepts through final design for the Phase 2 construction improvements, including providing permitting and construction support. Phase 4,once all deficiencies have been addressed, would then provide the documentation necessary to certify the Tukwila Levee,including developing an Operations and Maintenance Manual, of which the basis will be the existing USACE approved manual, but also will adopt other recent policies and procedures, if appropriate, such asthose from King Count/s ongoing System —Wide Improvement Framework (SVV|F) assessment for the Green River. Once Phase 2 is completed, the specific tasks to be completed in Phases 3 through 4 can be refined and a cost estimate prepared tocomplete the work. The project reach was discretized based on common physical characteristics and relative complexity of levee reconstruction alternatives needed to achieve certification (Figure 2) and then detailed analysis conducted to determine if existing levee conditions for each segment could be certified to provide flood protection. Table 1 provides a brief description of the individual segments. To try and provide more detailed site information and maximize the levee lengths that could be certified, Wood collected additional soil characteristics through cone penetration test (CPT) probes (Wood, 2018) and additional bathymetricdata were surveyed for the entire study reach. Tukwila Levee Accreditation Phase 2— Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 8 47 Legend • River Mile x Levee Stationing - 1000ft Levee Stationing - 10oft Levee Condition Improvement Needed Certifiable DATA 5U RCES ESRI EFTAIIHA renown/ 20..f7 CGx,'t'Siitr` Sim NAG 1983 HA N STAFEEFLANE WAa7 INGTON PASE7H PIPS #6P+ FEET Iota 2000038 Date 10Oct-2078 TUKWILA LEVEE ACCREDITATION Figure 2. Levee Condition co Table 1' Tukwila 2O5Levee Segments. Beginning Station Ending Station 600 | (feet) O (feet) Description of Land Features Approximately Paralleling Segment � Hightopographicsectiunnorthof|'4O5,|-405,and66mAvenueS.approachroad. 600 1600 68mAvenue S./ChristensenRoad. 1600 2100 Section of undeveloped wide left (south side of river) overbank downstream of the pedestrian/bicydebridge. I100 3200 Riverview Plaza Business Park including a2UO'long parking lot section. ]ZOU ]SOU Bicentennial Park. 3500 3500 StranderBou|evard 3600 5300 Business parks and related parking from upstream of Strander Boulevard to just beyond railroad bridge crossing. 5300 6500 Business parks and related parking along outside river bend. Referred toas Christensen Road capital project inthe Green River SVV|F. 6500 7700 Tukwila Pump Station. 7700 10150 Sperry Drive including [ostcoand Home Depot parking lots. 10150 10250 51O(Y»Street crossing. 10250 12200 From S18(Y»bridge crossing upstream. Includes Lily Point and the Rato|oLevee OP project area discussed inthe Green River 5VV|F. lZZUO 13200 Outside ofbend along Sl80th. 13200 14300 Business parks and associated parking upstream of S 1801h and through the area of the Z0O8Corps levee repair. 14300 15800 Business parks and associated parking upstream of the 2008 Corps levee repair 15900 18500 Corps Gaco'MitcheULevee Design Site including outside bend. Referred tnasthe Sega|e'Greenand Gacu-Nesternproject area inthe Green River SVV|F. 18500 22750 Sega|epnoperty. 22750 23800 Cross -levee. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 10 49 4.1 Levee Accreditation There are several components toaccredit alevee following the Code ofFederal Regulations Title 44 Chapter |, Subchapter B,Part 65,Section l0criteria. Phase 1ufthis project evaluated the levee ona reach scale and of these criteria (Table 2), embankment and foundation stability were found to be the most limiting factor, with much of the levee failing to meet these criteria. Table 2. Code of Federal Regulations levee certification criteria and Phase 1 assessment. Criteria elevation, but typically Phase 1Evaluation Results Less than Z00Ofeet total of levee are below min/mum only onthe order nfinches. Freeboard Closures Noflood closures inthe system tocertifv. Embankment Protection Existing rock and vegetated banks are sufficient to certify that there will be no appreciable anticipated erosion of the levee banks during the 100-year flood. Embankment and Foundation Stability Steep slopes significantly limit levee certification and the addition of predicted scour increases failure risk. Settling No significant levee settlement issues that will negatively impact freeboard. Interior Drainage Little significant interior flooding landward of the levee for the simulated 100- yearcondition. Work as part of Phase 2 took a more detailed look at site specific information to determine if more accurate data collected and refined methods could result insegments oflevee meeting certification criteria. Based on this work, of the 4.5 miles of levee, it's anticipated that approximately 20% of the levee can becertified inits existing condition. This includes segments that have stable slopes when accounting for potential scour, segments that fail levee stability criteria, but that when under the most likely failed scenario have at least 11 feet of remaining levee width at or above the Base Flood Elevation (BIFE), as well as the three bridge crossings where as -built drawings and other documentation is assumed tobesufficient tosupport certification. ° Levee stationing 0to6O0. This segment includes the 66thAvenue 3approach road tothe bridge crossing |'4US,|'40S,and ground tothe north of|'48S(Figure 3). For this segment, should the levee fail, there is sufficient width of ground landward, that is at or above the BFIE (27'), to provide flood protection. • Levee stationing 600to900. 68mAvenue S./[hristensenRoad (Figure 4).Approximately 300feet ofthis segment could becertified. Simulated geotechnica|failures would leave I1'ormore of existing ground (at 3O')atorabove the 8FE(27'). • Levee stationing 16O0toJl00. The levee isapproximately 10Ofeet ormore away from the channel edge behind a forested floodplain bench (Figure 4), and further than any potential point of slope failure. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 11 50 ° Levee stationing 32OOtu 3500Bicentennial Park (Figure 6). Slope failure may reach landward of the levee; however, the ground elevation (28) is still greater than the BFE (27.7'). ° Levee stationing 35OOto530O StnanderBoulevard and upstream section through the business parks and related parking have stable slopes (Figure 7). ° Levee stationing 65OOto77O0. Portions ofthe levee along the Tukwila Pump Station and a portion ofthe business park downstream ofthe pump station (FigureO) have stable slopes. ° Levee stationing 1Ol5Otu1U25O. It's assumed sufficient documentation exists tocertify this approximate 100long existing 3 180th Street bridge crossing section. • Levee stationing 1320Oto1430O. Business parks and associated parking (Figure 9)along the Corps 2OO8levee repair have stable bank slopes. Results of the 8eotechnica| slope stability analysis for additional cross -sections evaluated in Phase 2 are presented inTable 3. |norder tobecertified onthe embankment and foundation criteria, the representative cross-section for the levee segment must have a computed factor ofsafety greater than a USA[E minimum value for all cases (e.8. evaluations at river miles 13.055, 13.11and 13219 define stable slopes for levee segment 360UtoS3OO,evaluations atriver miles 13.6Z4and 13.892define stable slopes for levee segment 6500 to 7700, and the evaluation at river mile 14.934 defines stable slopes for levee segment 13ZU0tol4]OO). Levee Condition - Improvement Needed �~—oertifiable Figure 3. Tukwila Levee segment where stationing 0 to 600 can be certified as is. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 12 51 nhc :4tk)slof , r „ tjuu A 1110441440' . , ‘N. Levee Condition Improvement Needed Certifiable 0 50 100 200 NENFeet Figure 4. Tukwila Levee segment 600 through 1600 where existing portion 600 through 900 can be certified. Tukwila Levee Accreditation Phase 2 - Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 17, 2018 Interim Report 13 52 nhc Levee Condition Improvement Needed Certifiable 0 50 100 200 Feet Figure 5. Tukwila Levee segment where stationing 1600 through 2100 can be certified. Levee Condition *Improvement Needed - Certifiable 0 50 100 200 momisim Feet Figure 6. Tukwila Levee segment where stationing 3200 through 3500 can be certified. Tukwila Levee Accreditation Phase 2 - Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 17, 2018 Interim Report 14 53 nhc Levee Condition Improvement Needed Certifiable 0 50 100 200 Feet Figure 7. Tukwila Levee segment where existing section 3500 to 5300 can be certified. Tukwila Levee Accreditation Phase 2 - Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 17, 2018 Interim Report 15 54 nhc gatilln,!kAtit„":3) 1 6300 • MINKLER SLVD Legend • River Mile X Levee Stationing Levee Condition Improvement Needed - • • - • Certifiable 0 50 100 200 IIIIMM=E Feet II /III' I I trfkktroitrwoo,t,:i, 7700 13.'75 *o-r:••143 Figure 8. Tukwila Levee segment where existing section 6500 to 7700 can be certified. Tukwila Levee Accreditation Phase 2 - Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 17, 2018 Interim Report 16 55 ������ �� ������� Legend 0 River Mile X Levee Stationing Levee Condition ,improvement Needed I Certifiable o on 100 am Feet Figure 9. Tukwila Levee segment where existing section 13200 to 14300 can be certified. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 1/ 56 Table 3. Summary bycross-section ofadditional Phase 2 geotechnicm|slope stability assessments. Location Evaluated (River Mile) 12.412 13.055 13.11 13219 13.6243 13�B235 13.892 14.534 Case End of Construction (Existing [aseU Rapid Conditions) Drawdown � � 0.53 Case III Steady State Seepage 127 121 1.39 l5l 152993 W':N' "\ R, USA[E minimum Factor of Case IV Earthquake Post EQ kh=0 liquefied 0.57 OS 0.73 0.38 0.5 0.56 0.38 Case IV Earthquake kh=ODg � \� 1% 1.3 1 14 1.2 1 kh~ kyie|d ~ Case IV Earthquake kh @ k yield 0.14 019 0.18 0.11 0.2 0.24 0.11 0.2 0.24 0.11 0.15 Case IV Earthquake Approximate deformation <1inch < 1 inch <1inch ~1inch <linch <1 inch ~linch <linch <linch ~linch <1inch Meets USACELevee Design Manual minimum factor nfsafety (FS) Does not meet USA[Eminimum F5incurrent configuration. Meets USA[Eminimum FSbut does not meet King County minimum P5(~1.4) Does not meet minimum USACE minimum FS, but predicted deformation is minor, therefore acceptable. lateral earthquake design load (0.10g was considered the 100-year earthquake) lateral earthquake load that produces anon-liquefied F5~1.O Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 18 57 5 kU Potential alternative construction fixes that will lead to the levee being certified were identified for each levee segment that could not currently becertified. For each levee section characterized, alternatives wereeva|uatedandvverediscussedwiththeOtyduringanin'personmeetin0unSeptember6,ZU1O. 5.1 Construction Fixes Alternative construction fixes were considered for the segments that were deemed non -certifiable in their existing condition. One or more of three alternatives were considered; re -grading and flattening the levee, raising and widening the levee, and installing aflondvvaU. Concept designs for three alternatives are provided in Appendix A. Cost estimates to design and construct the alternatives are provided in Table 4. These are initial, planning level estimates that were determined in conjunction with KPG and are based on implementing typical sections over a 1,000-foot stream reach to develop a construction cost per linear foot. This approach assumes consistent grading quantities throughout the reach; however, this could vary significantly along any reach and should be refined as design details evolve for specific levee segments. Costs assume right nfway ispurchased for the levee setback and widening alternatives, but not for f|oodwaUs. Design, permitting, and construction management were estimated at 40% of the construction cost in anticipation of a fairly extensive permit process. Table 5 shows the results of the various geotechnical slope stability assessmentsfrom the initial Phase 1 reach assessment. These evaluations were made at various river cross -sections along the Tukwila 205 Levee, and a representative location was chosen when assessing each levee segment. Yellow shading in this table indicates where USACE factors of safety are not met (and therefore the levee segment is not certifiable). For consistency with the Green RiverSVV|F (King County, 2016), conceptual future levee fixes were assumed to provide a 500-year level of protection (i.e. 500-year water surface elevation plus three feet). This is a peak Green River flow of 18,800 cb based on a 2013 Corps report that downgraded the level of protection provided by the upstream Howard Hanson Dam to a 140'yearevent. This is a much higher level than historically the lower Green River Valley has been developed for, as it has generally been assumed that the upstream Howard Hanson Dam provided protection for 500-yearevent. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 19 58 Table 4. Planning level cost estimate for final design and construction of levee certification improvements. Retaining WaII/Floodwall $4,100,000 $4,100,000 1 000'005`9$ < 000'00Z`E$ aanai PaualWA/PaqeM 000'008`E$ V/N 1.0 < 000`00VS$ Re-Grade/Flatten Levee $5,100,000 < d d < d 0 < Description 68th Avenue S./Christensen Road. Riverview Plaza Business Park. Business parks and related parking along Christensen Road capitol project in the Green River SWIF. Sperry Drive and parking lots near Costco and Home Depot. From S 180th bridge crossing upstream including Lily Point and the Ratolo Levee Green River SWIF project area. Outside of bend along S 180th. Business parks and associated parking upstream of 2008 Corps levee repair. Segale property. Cross -levee. 10150 13200 22750 24000 o � C � 0 m 0 � 10250 r1j 14300 06 22750 Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 0 59 Table 5. Summary bycross-section ofPhase 1 geoiechn|ca|slope stability assessments. Location Evaluated (River Mile)_ [ase|-Endof Construction (Existing Condit-kon-x) [aseU-Rap|d Dravdnvn CaseU|- Steady State Seepage [aseK/- Earthquake Nx=O.IOg non -liquefied Case IV Earthquake - Approximate deformation 12.61 12.98 13.11 13S3 14.02 14.72 14.82 15.49 15.86 16.07 16.47 1�5' 0.82 1�23 0.73 1�34 < 1-2inohen 1.1 0.72 112 0.87 8'12 inches 1.03 0.82 0,Q1 0.81 94 2-4 inches l 1.1� U.74 U.UU O.81 114 1K08 0.73 0.92 O.82 USACE minimum Factor of Safety 1.3 1 1,4 1 The following text describes the alternatives considered for each segment as well as approximate costs todevelop final design and build the project. Segment stationing 600 through 1600 parallels 68 1h Avenue South/Christensen Road (Figure 10). Here steep slopes exacerbated by anticipated river channel scour could lead to a levee bank failure (as evaluated at cross-section 12.61 in Table 5) that includes portions of the roadway and the Green River trail. As discussed in prior Section 4.1, the segment of this levee from stationing 600to9OUcan be certified as is, as there is sufficient existing ground remaining after the predicted failure, that is at or above the BFE, to prohibit floodwaters from reaching landward infrastructure. Similarly, from station 900 to 1600, a slope failure would not flood landward of the levee (the ground elevation at the point of extent of failure is very close to, but above, the 27' BFE); however, a portion of Christensen Road, as well as nearby parking and potentially the closest buildings, would be lost. Slope stability in this reach is relatively less of concern at this segment compared to other portions of the levee, based on the computed factor of safety from the geotechnical analysis of this segment (Table 5). Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 21 60 ������ �� ������� o =� _=�_.- ��� Levee Condition — ` -Improvement Neeeu o no 100 200 Feet| Figure1O Tukwila Levee segment 600 through 1600 where existing portion 900 through 1600 can't be certified. Aconstruction fix here toprotect the roadway, trail, and buildings uptothe 5U0iearlevel ofprotection includes either alevee setback, raising the levee, nraf|oodvvaU. Table 4provides aplanning level cost estimate for these alternatives. For all concepts, the construction fix was assumed tooccur for the entire length of the segment, so that the same level of protection would be continuously provided from stationing 6UOthrough I6UO. For the levee setback and raising options, aretaining wall would likely be placed on the landward side ofthe levee, where existing buildings would limit ability tnconstruct a 2:1 backs|ope. This alternative would require relocation of any utilities and raising of Christensen Road to provide access tothe Holiday Inn constructed inZO18. [ukwi|aLevee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 23 61 ������ �� ������� Segment 2100 through 3200 parallels the Riverview Plaza business park, which is built up to the levee edge (Figure 11). Estimated scour depths here and the short distance between river edge and the business park, preclude any realistic fix other than sheet pile walls. Table 4provides acost estimate for a construction fix at this segment. There is a short reach from approximately 2400 to 2700 that has an open parking lot, where the levee potentially could be set back or a design other than a sheet wall could be considered as part of the final design. Slope stability in this reach is relatively moderate compared to other segments, based on the computed factor of safety from the geotechnical analysis of this segment (as evaluated atriver mile 12.9OinTable S). ~�-~cemnau� Figure 11. Tukwila Levee segment 2100 through 3200 that can't be certified in its existing condition. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 23 62 ������ �� ������� Segment 5300 through 6500 (Figure 12) is in the outside of a river bend and includes the Green River SVV|FChristensen Road capital project. Similar toother levee segments, unstable channel slopes under anticipated potential channel scour create conditions that can't becertified (as evaluated at river mile l3.53inTable S). Failure risk here, where scour is greater in the outside of a channel bend, is some of the greatest inthe system. Levee setback, levee raising, and floodwall alternatives were considered for this site (cost estimates provided inTable 4). The levee setback and raising would take land currently used for business parking. � � � V 50 100 200 No Figure 12. Tukwila Levee segment 5300 through 6500 that can't be certified in its existing condition. Segment 7700 through 10150 parallels Sperry Drive and parking for large box stores (Figure 13). Slope stability in this reach is relatively moderate compared to other segments, based on the computed factor ofsafety from the geotechnica| analysis of this segment (as evaluated at river mile 14.02 in Table 5). Cost estimates to design and construct both a levee setback and a MoudwaU for the entire segment are Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20IDInterim Report 24 63 ������ �� ������� provided inTable 4. Note that the levee setback and levee raising concepts would modify land currently used for business parking, though parking lots through this segment are relatively wide. A simple site specific concept through just the non -certifiable section of the Tukwila Pump Station Pond, stationing 770Othrough O2O0,was also considered. The cost ofbringing inadditional levee material tuprovide protection to the 500-year water surface plus three feet, for this short 500-foot section, is estimated to beapproximately $I3million. 0 River Mile X Levee Stationing Levee Condition ' Improvement Needed Certifiable ' Figure 13. Tukwila Levee segment 7700 through 10150 that can't 6ecertified inits existing condition. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20IDInterim Report 25 64 ������ �� ������� Segment 10250 through 12200 (Figure 14) has a history of slope instability and was repaired most recently in2008(Amec,IO1S). Slope stability inthis reach isrelatively moderate tohigh, depending on location within the segment, compared to other segments (as evaluated at river mile 14.534 in Table 3 and atriver mile 147ZinTable 5). Both alevee setback, levee raising, and f|oodwaUwere evaluated as potential construction fixes at this segment (Table 4 provides the estimated costs). ` � .Wy VP, LLJ IL Levee Condition Improvement Needed --~ -Cem*wmv Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 26 65 ������ �� ������� For segment 12Z00through 13J0O the proximity ofS18OmStreet severely limits construction options given how close the roadway istothe existing levee (Figure 15). |naddition, the levee being inthe outside of the river bend has relatively more scour compared to straight river sections, leading to less stable slopes (as evaluated at river mile 14.82 inTable 5). Only sheet pile fluodwaUswere considered for this site. Table 4 provides a cost estimate tn design and construct this project. Levee Condition Improvement Neede Figure 15' Tukwila Levee segment 12200 through 13200 that can't be certified in its existing condition. Segment 14300 through 15900 (Figure 16) is the site of an existing repair, like the downstream levee segment; however, unlike the downstream section, steep slopes and soil conditions for this segment indicate that this section can't be certified in its existing condition (e.0. as evaluated at river mile 15.1 in Table 3. This is one section that may warrant additional soil testing (e.g. CPT) tofurther classify soils and thus potentially beable tocertify the levee segment. Alevee setback, levee raising, and af|oodwaU were considered for this site (cost estimate provided inTable 4). USACE is managing the design and construction of the Gaco-Mitchell Levee repair from station 15900 to l85UO(approximate river mile 15.4to1S.9)(Figure l7). This has some ofthe greatest failure risk within the entire levee reach (as evaluated atriver mile 15.86inTable 5). The new levee along the Green River left bank will provide 500-year level of protection plus 3 feet of freeboard per the SWIF. There isa detailed analysis and design being completed for that project, so therefore no additional conceptual level designs and planning level cost estimates were conducted for this Phase 2 work. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20IDInterim Report 27 66 ������ �� ������� Levee Condition Improvement Needed Figure1G' Tukwila Levee segment 14300 through 15900 that can't be certified in its existing condition. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20IDInterim Report 28 67 ������ �� ������� Levee Condition -improvement Needed Figure 17. Tukwila Levee segment 15900 through 18500 that can't be certified in its existing condition. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 29 I 68 Thegeotechnica| analyses (evaluated at river mile 16.07and 16.47 inTable 5) indicate that a potential slope failure could occur along the Segale property from station 18500 to 22750 (Figure 18 and Figure 19). However, with the grade landward of any slope stability failure at or above BFE, the section could be certified as is no infrastructure is threatened by a slope failure. This is the only segment of the Tukwila 205 Levee reach, and one of the few remaining in the Lower Green River, where there is currently no development, though site grading has occurred in preparation for construction. As future development plans are uncertain, alternatives were developed for this segment including developing setback levees, raising the levees and constructing sheet pile walls for 500-year protection. Cost estimates for these fixes are provided inTable 4. For the fill option, a site specific cost estimate was made based on actual ground elevations (from 2014) versus computing by linear foot. |naddition, acost was estimated for filling the entire undeveloped area paralleling levee segment 18500 to 22000 (a majority of which is shown in Figure 18) to a uniform elevation at the SOO'year level of protection. This is estimated to be approximately 523 million assuming fill quality suitable for construction (e.g. free from contamination, compacted and prepared for foundation, etc.) and does not include the cost for purchasing land. Assuming a purchase price of $10 per square foot for undeveloped property, and approximately 1.7 million square feet of area, land costs are estimated to be another $17 million. In this segment, work could be completed in phases, with the first phase acquiring land (before development occurs) and then constructing levee improvements (e.g. levee setback) later. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 30 69 nhc oOt „Adl0,4,04° lot Ati:$4k Figure 18. Tukwila Levee segment 18500 through 22750 (downstream portion) that can't be certified in its existing condition. Tukwila Levee Accreditation Phase 2 - Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 17, 2018 Interim Report 31 70 ������ �� ������� 1044C��` " Figure19. Tukwila Levee segment 18500 through 22750 (upstream portion) that can't be certified in its existing condition. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 32 71 ������ �� ������� Levee segment 22750throu0h I3800 is a cross -levee section that spans from the Green River to high ground onthe east side ofthe valley (Figure Z0). The BFEhere isapproximately 3S.6feet and the levee low point is approximately 38.1; therefore, the levee elevation needs to be raised a few inches to be certified. The 500-year elevation plus 3 feet of freeboard is 38.7 feet; therefore, the recommended fix would betobring this levee uptnthe SOO-yearlevel ofprotection. Table 4provides acost estimate for a construction fix at this segment. Ink Figure 20. Tukwila Levee segment 22750 through 23800 that can't be certified in its existing condition. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 33 72 6 PROPOSED PRIORITIZATION Of those levee segments that could not be certified, the proposed construction sequencing based on risk and apparent opportunity is tabulated in Table 6. Table 6. Proposed improvement prioritization based on risk and opportunity. Levee Segment Stationing (feet) Beginning Ending 600 2100 5300 7700 10250 12200 14300 15900 18500 22750 1600 3200 6500 10150 12200 13200 15900 18500 22750 23800 Basis for Sequencing 68th Avenue S./Christensen Road. Has a lower risk of slope failure and 300 feet of this could be certified in its current condition. Riverview Plaza Business Park. Has a moderate risk of slope failure. Business parks and related parking along Christensen Road capital project in the Green River SWIF. Has some of the highest deviation from slope stability factors of safety. Sperry Drive and parking lots near large box stores. Has a moderate risk of slope failure. From S 180th bridge crossing upstream including the Ratolo Levee Green River SWIF project area. One of highest scour areas and potential opportunity to work with landowner. Outside of bend along S 180th. Has a moderate to high risk of slope failure. Business parks and associated parking upstream of 2008 Corps levee repair. Has a moderate risk of slope failure. Corps' Gaco-Mitchell Levee being designed. One of the greatest levee failure risks in the levee reach based on comparison with computed factors of safety. Segale property. This is one of the last undeveloped overbank areas in the lower Green River. At a minimum land acquisition should occur as soon as possible. Cross -levee likely will be in contact with floods during the highest of events. 1000 1200 1950 2600 Construction Improvements for Certification Proposed Total Length Sequencing (feet) Low High High High Low Tukwila Levee Accreditation Phase 2 - Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 17, 2018 Interim Report 34 73 Phase 1 conducted a reach wide assessment of the levees using generalized information and identified deficiencies for levee certification. In Phase 2,the project reach was discretized based on common characteristics and the deficiencies in each of these segments addressed in one of two ways. First, each segment was evaluated to determine if a more site specific analysis, versus the generalized reach wide Phase 1 approach, could potentially show that the current condition is certifiable. For each ofthe segments identified as such, a site specific analysis was conducted including collecting additional data and conducting additional or refining existing analyses to determine if that portion of the levee could be certified as is. For those segments of the levee that couldn't be certified in their current condition, costs were estimated for one or more construction alternatives that would lead to the levee being certified. Phase 3 will provide civil design and construction support of the preferred design determined in Phase 2 for |ar8e*ca|e,complex physical deficiencies. Meetings will beheld asneeded. The scope ofwork for Phase 3 will be refined at the completion of Phase 2, and a cost estimate prepared. Once all deficiencies have been identified and corrected, the Tukwila Levee Certification report will be prepared for submittal. The Phase 4scope ofwork will berefined atthe completion ofPhase land a cost estimate prepared. � � Amec Foster Wheeler, 2015. Preliminary Embankment and Foundation Stability Analysis, Tukwila 205 Levee Certification. Report dated October 1,ZO15. King County, 2016.0ng County System Wide Improvement Framework, Green River, King County, Washington, Interim Report. February 2016. NHC,J0U7. HydrologyforGreenRiverFloodpkzin4nolysisondLeveeRiskAssossnnent.P,eparedforKinQ County Water and Land Resources Division, Department ofNatural Resources and Parks, Seattle, WA, April ZU07. NHC, 2015. Tukwila Levee Accreditation — Phase 1 — Engineering Analyses and Improvement Identification. Prepared for City of Tukwila Public Works Department, Tukwila, WA, October 6, 2015. VVood,ZOl8. Preliminary Embankment and Foundation Stability Analysis Update, Tukwila 205 Levee Certification. Unpublished Draft, 2U18. Tukwila Levee Accreditation Phase Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October l7'20I8Interim Report 35 74 Tukwila Levee Accreditation Phase 2 - Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 15, 2018 Interim Report 75 CITY OF TUKWILA Job:21 Rev: 1 Chk L TH 6 Da7e:©o5ept8 SKETCHES QR LEVEE REPAIR OPTIONS RE-GRADE/FLATTEN LEVEE IALT A Tukwila Levee Accreditation Phase 2 - Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 15, 2018Interim Report Station 10` 2O 40' CITY OF TUKWILA d; 2 Rev, 1 D .MA Chkd: TH® erate:45Sept€ SKETCHES OF LEVEE REPAIR OPTIONS RAISED WIDENED LEVEE IALT Tukwila Levee Accreditation Phase 2 - Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 15, 2018Interim Report CITY OF TUKWILA Ork-MA© Chkp; TH® Defe:06SepB SKETCHES OF LEVEE REPAIR OPTIONS RETAINING WALL /FL00 WALL `ALT C Tukwila Levee Accreditation Phase 2 - Levee Segment Assessment and Proposed Sequencing for Large Scale Improvements October 15, 2018Interim Report