Chehalis River Basin Hydraulic Modeling and Flood Relief Alternatives Evaluation

1. Chehalis River Basin Hydraulic Modeling and Flood Relief Alternatives Evaluation 15 March 2012 WATERSHED Science & Engineering WEST Consultants

2. Overview of Today’s Discussion Summary of model development efforts •     main stem including, extension to Pe Ell •     main stem extension to Aberdeen •     tributaries •     hydrology Current Status – under review by state tech team and UASCE Potential flood relief projects •     upstream retention •     Corps Twin Cities Project • WSDOT I-5 protection project •     other potential flood relief projects Questions and next steps

3. Model Development

4. Model Development Main stem Grand Mound to Porter WEST work under contract to Flood Authority • New channel survey data collected (MGS, Nov 2011) • 2002 LiDAR data for overbank areas (PSLC) • Based on PIE UNET Model (PIE 2001) • HEC-RAS model developed and calibrated • Attached as downstream extension on Twin Cities model

5. Model Development Main stem Porter to Montesano WEST work under contract to Corps • New channel survey data collected (MGS, Dec 2011) • 2002 LiDAR data for overbank areas (PSLC) • New HEC-RAS model developed and calibrated • Attached as downstream extension on Chehalis River model

6. Model Development Main stem Montesano to Aberdeen WSE work under contract to Flood Authority • New channel survey data collected (PGS, Dec 2011) • 2009 LiDAR data for overbank areas (PSLC) • New HEC-RAS model developed and calibrated • Attached as downstream extension on Chehalis River model

7. Model Development Main stem Doty to Pe Ell WEST work under contract to Corps • New channel survey data collected (MGS, Nov 2011) • 2006 LiDAR data for overbank areas (PSLC) • New HEC-RAS model developed and calibrated • Attached as upstream extension on Twin Cities model

8. Model Development Skookumchuck River WSE and WEST work under contract to Flood Authority • New channel survey data collected (PGS, Feb 2012) • 2002 LiDAR data for overbank areas (PSLC) • 2001 PIE Model geo-referenced by WEST • WEST HEC-RAS Model refined by WSE and calibrated • Replaces Skookumchuck River Branch on Twin Cities Model

9. Model Development Newaukum River WSE work under contract to Flood Authority • No new channel survey • 2002 LiDAR data for overbank areas (PSLC) • 2001 NHC Model geo-referenced, refined, and calibrated by WSE • Extends Newaukum River Branch on Twin Cities Model from RM 4.1 to RM 10.6

10. Model Development Satsop River WEST work under contract to Corps • Added 2001 HEC-RAS model as tributary to Chehalis River model WSE work under contract to Flood Authority • New channel and bed survey (PGS, Feb 2012) • Data compared to 2001 WEST HEC-RAS Model and cross section surveys • Comparison shows significant changes in channel plan and cross sections • New model recommended for Satsop River investigations (not imperative for Chehalis)

11. Model Development Hydrology WEST work under contract to Corps • New Flood Frequency Analyses • 17 gaged sites • 66 ungaged sites • Data for February 1996, Dec 2007, and Jan 2009 Calibration Events • Design Flood Hydrology (peaks, timing, and volumes) for 1.5- through 500-year events • Ecologically Significant (Low-flow) hydrology Hydrology has been reviewed by USACE and State Agencies

12. Current Model Status Update Preliminary (USACE) Model Distributed – February 14, 2012 Modeling Meeting with State team – February 23, 2012 Distribution of Refined Model – March 7, 2012 Currently under review by State Tech Team and USACE Remaining tasks: • Insert WEST/USACE Design Event hydrology • Connect WEST Wynoochee Model to Main Stem

13. Flood Relief Alternatives Upstream retention WSE task under contract to Flood Authority • Use Revised WSE/WEST Model • Use new WEST/USACE Hydrologic data • Evaluate effect of Main Stem dam on Water levels throughout main stem from Doty to Aberdeen • Compare to prior NHC and Anchor evaluations

14. Flood Relief Alternatives Corps Twin Cities Project WEST task under contract to USACE • Use Revised WSE/WEST Model • Use new WEST/USACE Hydrologic data • Evaluate effect of Twin Cities Levee and Skookumchuck Dam Project on Water levels from Chehalis to Aberdeen WSE task under contract to Flood Authority • Compare to other Flood Relief Alternatives (not in Corps scope)

15. Flood Relief Alternatives WSDOT I-5 protection project Work to be conducted by NHC under contract to WSDOT • Use Revised WSE/WEST Model • Use new WEST/USACE Hydrologic data • Evaluate effect of I-5 isolation (via levee or floodwalls) • Consider with and without Airport levee improvements WSE under contract to Flood Authority • Compare downstream effects to other Flood Relief Alternatives

16. Flood Relief Alternatives Other potential flood relief projects • Bridge Removal or Modification: • Mellen Street, Sickman Ford, Galvin Street, S. Montesano • In isolation or in combination • Skookumchuck Railroad trestle • Land use • Fill in floodplain • Impact of impervious surfaces • Impact of other structures • I-5 fill/levee • Railroad fill/levee • Other levees • Conceptual level high flow bypass • Sediment management (“dredging”)

17. Questions and next steps I-5 at Hwy 507 Interchange Looking South – December 4, 2007