1. 18 October 2012 WATERSHED Science & Engineering WEST Consultants Chehalis River Basin Hydraulic Modeling and Flood Relief Alternatives Evaluation

2. Overview Accomplishments under WSE Flood Authority contract model development evaluation of flood relief projects coordination and communication Summary of model development efforts for Flood Authority (WSE and WEST) for USACE (WEST) Evaluation of potential flood relief projects 15 separate projects modeled 10 alternatives (combinations of projects) Reporting (WSE report and OFM report) & Meetings Questions

3. Project Milestones WSE-WEST Team Selected – September 2011 Work Plan Finalized – October 2011 Initial Data Collection and Basin Reconnaissance – October 2011 New Channel Surveys – November /December 2011 Preliminary Hydraulic Model Distributed – March 2012 Review by State Tech Team and USACE – March and April 2012 Final Existing Conditions Model Distributed - May 3, 2012 Preliminary With-Project Runs – May 9, 2012 Modeling of Alternatives–May 31, 2012 Draft Reporting and Presentation of Results – June 2012 Refined Alternatives and Analysis – June/July 2012 Final Analysis and Reporting – July 2012

4. Model Development

5. Evaluation of Flood Relief Projects Upstream retention on mainstem Chehalis USACE Twin Cities Project Skookumchuck Levees Airport Levee WSDOT I-5 flood protection project Mellen Street Bypass Scheuber Road Bypass Channel Dredging (Mellen Street to Lincoln Creek Confluence) Complete Bridge Replacement Option Specific Bridge Replacements Highway 6 Mellen Street Galvin Road Sickman Ford S. Elma (Wakefield) Road Porter Creek Road

6. Results Reported for All Projects for Four Flood Events

8. Preliminary Alternatives 1. Mainstem dam and airport levee improvements 2. WSDOT floodwalls/berms and airport levee improvements 3. Scheuber bypass, Mellen Street bypass, and airport levee improvements 4. Dam, airport levee improvements, WSDOT floodwalls/berms 5. Dam, airport levee improvements, WSDOT floodwalls/berms, downstream bridge removals 6. Scheuber bypass, Mellen Street bypass, airport levee improvements, WSDOT floodwalls/berms, downstream bridge removals 7. Scheuber bypass, Mellen Street bypass, airport levee improvements, small floodwall along I-5 near Dillenbaugh Creek

9. OFM Alternatives A. Mainstem Dam on the Chehalis River, Airport levee improvements, small floodwall along I-5 near Dillenbaugh Creek, Skookumchuck Levees, Sickman Ford Bridge modification, and Wakefield Road (South Elma) Bridge Modification. B. Same as Alternative A with the addition of WSDOTs proposed I-5 berms and floodwall protection project. C. WSDOTs I-5 berms and floodwalls, Airport levee improvements, Mellen Street and Scheuber Road Bypasses, Skookumchuck Levees, Sickman Ford Bridge modification, and Wakefield Road (South Elma) Bridge Modification.

10. Evaluation of Alternatives

12. Twin Cities Area – Change in Water Surface Elevation

13. Evaluation of Alternatives Twin Cities Area – Change in Water Surface Elevation

14. Evaluation of Alternatives Twin Cities Area – Change in Water Surface Elevation

15. Reporting Stand Alone Report – included as Appendix in Ruckelshaus Center Report Model also documented in USACE Report by WEST

16. Presentation of Results Flood Authority Meetings (April and May 2012) Meeting with WSDOT (May 2012) Chehalis Basin Technical Workshop (May 2012) Chehalis Basin Policy Workshop (June 2012) Lower Chehalis Basin Meeting (June 2012) Various Discussions (July – September 2012)

17. Questions What can we do to get back to pre-1990 flood conditions in the lower basin? Which projects reduce flooding throughout the basin? Why doesnt the WSDOT I-5 Project show downstream water level increases (like the USACE project)? What about other projects (e.g. levee setbacks, floodplain storage easements, etc.)? Other questions?

18. What can we do to get back to pre-1990 flood conditions?

19. Which projects reduce flooding throughout the basin?

20. Why doesnt the WSDOT I-5 Project show downstream water level increases (like the USACE project)? It does, but the increases are not as large WSDOT project does not include all of the levees that were in the USACE Project In particular, WSDOT Project does not include Galvin Road Levee Modeling approach for complex hydraulics in Twin Cities may affect results at very fine resolutions

21. What about other projects (e.g. levee setbacks, additional floodplain storage, etc.)? For 100-year event: Peak Storage in HEC-RAS Model = 584,000 Acre-ft Storage along main stem Chehalis = 458,000 Acre-ft For comparison: Flood storage in proposed retention facility = 80,000 Acre-ft Flood storage in Twin Cities reach = 60,000 Acre-ft

22. Other Questions