Flood Risk Management on the Elwha: Case Study of the Federal Levee Modification Project

Transcription

1 Flood Risk Management on the Elwha: Case Study of the Federal Levee Modification Project US Army Corps of Engineers Zac Corum, PE, Hydraulic Engineer Travis Ball, PE, CFM, Hydraulic Engineer Seattle District U.S. Army Corps of Engineers ASFPM 2014 Conference 03-June, 2014

2 Overview of talk Elwha River Basin Overview Current Status on dam removals Project purpose Background on the Corps role in restoring the river Unique technical challenges associated with Lower Elwha Klallam Tribe Federal Levee modification project Lessons Learned July 2005 July 2013 photo credit: Ian Miller, WA Sea Grant

3 Elwha River Basin Federal/Tribal Levee You are here

4 Elwha River Basin Birds Eye View You are here Federal/Tribal Levee Glines Canyon Dam site about 20 feet left to go as of spring 2014 Elwha Dam site dam completely removed as of 2013

5 Elwha Basin Overview - Flood Hydrology Nov Dec Annual Exceedance Probability (AEP) % 1% 2% 5% 10% 20% 50%

6 Project Purpose: Ensure downstream flood risks to people, property, and water infrastructure not worsened by largest controlled release of sediment in history. Glines Canyon Dam removal in progress April 2014 (NPS)

7 Source of the issue #1 - Former Lake Mills Reservoir April , (NPS/USBR/USGS)

8 Source of the issue #1 - Former Lake Mills Reservoir April , (NPS/USBR/USGS)

9 Corps Role in Elwha River Restoration Conducted under Interagency Agreements with NPS in close coordination with Dept of Interior Floodplain studies on the Elwha Identify at risk properties and infrastructure Estimate change in flood risk Propose mitigation options to NPS Corps and NPS implement flood risk mitigation measures (levees, hatchery upgrades, well upgrades, private home elevation)

10 Corps Role in Elwha River Restoration Federal levee (raised, extended, new pump station) Elwha Water Facilities levees (new) Westside levee (raised)

11 Federal Levee Modification: Originally authorized and built in 1980s to provide 200-year level of protection with 4 feet of freeboard. Protects Reservation from riverine flooding, but not coastal flooding feet long, average increase of 3 feet. Certified to provide 95% CNEP of overtopping in 0.5% AEP event maintained after dam removal. Levee extended to prevent increased risk of flanking. New pump station for interior drainage Designed and constructed by USACE. Total cost of $8.7m.

12 Primary Technical Challenges associated with Federal levee modification project 1. How much do we need to raise the levee? 2. How far do we need to extend the levee? 3. How do we protect the levee from erosion and scour?

13 Factors affecting flood levels along Federal Levee 1. Tides, waves, and delta morphodynamics 2. Floods 3. Channel conditions 1. Cross section, roughness, planform, slope 2. Morphodynamics (River migration and avulsion) 4. Dam removal 1. Sediment supply, rate, type 2. Wood 5. Human intervention levees, roads, ELJs These are all inter-related, variable, uncertain, and time dependent when it comes to predicting flood elevations, modeling is required to understand relative contributions of these factors and their uncertainties.

14 How do we address complex flood problem? Risk and Uncertainty (R&U) Framework 1. What is existing reliability against flooding caused by overtopping? 2. Does reliability against overtopping after dam removal decrease? 3. If so, how much? 4. Elevate until existing reliability against overtopping achieved 5. For background:

15 USACE method for addressing flood impacts 1. Modify hydraulic models to reflect best guess and conservative sedimentation scenarios 2. Evaluate sensitivity of flood stage to different scenarios 3. Perform statistical analysis of stage uncertainty, also accounting for hydrologic uncertainty

16 Real world complications: Delta morpho-dynamics and effect on river flood levels 400 ft wide mouth in flood event 800 ft wide mouth in 2008

17 Real world complications: Delta morpho-dynamics and effect on river flood levels Large reduction in flood backwater from mouth widening results in justifiably lower levee elevations

18 Real world complications: Sediment fate and delta growth PREDICTION 2009 VS. REALITY 2014 Image credit: CWI & Roordaaerial.com

19 Putting it all together PDF? Flood elevation Likelihood of occurrence

20 Issue 2 Levee extent LiDAR saves the day! LiDAR identifies unknown paleo channel and terrace south of levee Updated hydraulic models indicate river can activate these channels during 50-year return interval flood, allowing water to flank levee Depending on aggradation scenario flows can change by 2 orders of magnitude Sensitivity analysis and risk based levee design used to set crest height. BUILDING STRONG

21 Issue 2 Levee extent south end Heavy vegetation obscured topography on high terrace during previous floodplain modeling efforts. No extension anticipated.

22 Issue 2 Levee extent south end 2000 LiDAR flight results in discovery of several paleo channels on high terrace and realization that flood protection needed to be extended to prevent levee flanking during 200-year design flood

23 Issue 2 Levee extent check your work 1. Paleo channel discovery resulted in decision to extend levee south to cutoff potential overflow path. River channel flood levels used for design. 2. Major problem identified with levee elevation at south end during final floodplain map revisions, using new data set and updated models. 3. Levee modified using emergency contract and now provides continuous 200-year level of protection Original design crest > 3 above main channel water level

24 Issue 2 Levee extent check your work 1. Paleo channel discovery resulted in decision to extend levee south to cutoff potential overflow path. River channel flood levels used for design. 2. Major problem identified with levee elevation at south end during final floodplain map revisions, using new data set and updated models. 3. Levee modified using emergency contract and now provides continuous 200-year level of protection Predicted design flood level in side channel from split flow analysis

25 Issue 2 Levee extent check your work 1. Paleo channel discovery resulted in decision to extend levee south to cutoff potential overflow path. River channel flood levels used for design. 2. Major problem identified with levee elevation at south end during final floodplain map revisions, using new data set and updated models. 3. Levee modified using emergency contract and now provides continuous 200-year level of protection New crest accounting for split flows and uncertainty

26 Issue 3 Levee slope and scour protection Risk Informed Revetment Design 1. River was actively migrating and avulsing under pre-dam removal conditions 2. CMZ informed by LiDAR and HEC-RAS modeling of existing and future conditions 3. Risk evaluation of setback levee segments 4. Risk based design to limit costs

27 Issue 3 Levee slope and scour protection

28 Issue 3 Levee slope and scour protection Severe risk slope protection Moderate-high risk slope protection

29 Lessons Learned Flood risk management in a complex, dynamic ecosystem requires good communication, creative thinking and risk informed decision making or much better models Restoration requires mitigation with traditional hard engineering approaches to make floodplain property owners whole Modification of a Federal levees is difficult and requires close coordination with Corps of Engineers and potentially imposes a significant technical and financial burden on proponent

30 Lessons Learned Creative thinking required, but traditional methods and tools are flexible enough to apply to unprecedented problems Corps H&H modeling and statistical approaches are technically robust and powerful way to demonstrate Federal Government is meeting its responsibilities when it comes to flood risk management Buying out properties reduces unknowns and reduces risks. It also simplifies the project and reduces need for due diligence during planning, design and implementation phases.

31 Unofficial Lessons Learned

32 Thank You Project Partners: Seattle District USACE, NPS, USBR, USGS, Lower Elwha Klallam Tribe, City of Port Angeles, Clallam County, URS, HDR, Watts-Delhur, PBS&J, JW Fowler, Fed Con Special thanks: Travis Ball, Brian Winter, Jeff Bohman, Tim Randle, Jennifer Bountry, Chris Magirl, Roordaaerial.com, Ian More info flood-protection.htm Zac Corum (206) zachary.p.corum@usace.army.mil