1. 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. 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