Skagit River Flood Risk Management General Investigation Skagit County, Washington Draft Feasibility Report and Environmental Impact Statement Appendix B – Hydraulics and Hydrology May 2014 Skagit River Flood Risk Management Draft Feasibility Report and Environmental Impact Statement Appendix B – Hydraulics and Hydrology Hydraulics and Hydrology Appendix 1. Hydraulic Analysis, Final Report, August 2013 2. Hydraulic Technical Documentation, Final Report, August 2013 3. Hydrology Technical Documentation, Final Report, August 2013 4. Sediment Budget and Fluvial Geomorphology, June 2008 1 U.S. ARMY CORPS OF ENGINEERS SEATTLE DISTRICT Downtown Mount Vernon October 2003 Flood fighting October 2003 SKAGIT RIVER BASIN GENERAL INVESTIGATION FLOOD RISK REDUCTION – HYDRAULIC ANALYSIS FINAL STUDY REPORT AUGUST 2013 SKAGIT RIVER BASIN GENERAL INVESTIGATION FLOOD RISK REDUCTION – HYDRAULIC ANALYSIS FINAL STUDY REPORT Prepared for: US Army Corps of Engineers Seattle District 4735 East Marginal Way South Seattle, WA 98134 Prepared by: Northwest Hydraulic Consultants Inc. 16300 Christensen Road, Suite 350 Seattle, WA 98188 August 2013 NHC project #200074 Table of Contents 1 Introduction ................................................................................................................................. 1 1.1 Datum............................................................................................................................................ 1 1.2 River Stationing ............................................................................................................................. 2 2 BNSF Bridge Hydraulic Modeling .................................................................................................. 3 2.1 Bridge Survey and Reassessment of HEC‐RAS Model Calibration ................................................ 4 2.1.1 Bridge Survey ........................................................................................................................ 4 2.1.2 1995 High Water Data and Model Calibration ...................................................................... 4 2.2 Investigation of HECS‐RA Bridge Computational Methods ........................................................... 5 2.3 Sensitivity Analysis ........................................................................................................................ 7 2.3.1 Model Sensitivity to Debris Blockage .................................................................................... 8 2.3.2 Model Sensitivity to Contraction and Expansion Coefficients .............................................. 8 2.3.3 Model Sensitivity to Right Bank Station ................................................................................ 9 2.3.4 Steady State vs. Unsteady Flow Modeling ............................................................................ 9 2.4 Effects of Bed Scour on Bridge Hydraulics .................................................................................. 10 2.5 Further Refinements to HEC‐RAS Model Representation of BNSF Bridge .................................. 11 2.6 Conclusions and Recommendations ........................................................................................... 11 3 Early Season Flood Regulation .................................................................................................... 14 3.1 Reservoir Record Analysis ........................................................................................................... 15 3.1.1 Upper Baker ........................................................................................................................ 15 3.1.2 Ross ..................................................................................................................................... 16 3.1.3 Analysis of Reservoir Elevation and Storage Data .............................................................. 16 3.2 Impact of Existing Flood Storage Requirements on Regulated Peak Flows ................................ 17 3.3 Impact of Increased Early Season Flood Storage Requirements on Regulated Peak Flows ....... 19 3.4 Conclusions ................................................................................................................................. 19 4 Lower Baker Dam Flood Regulation ............................................................................................ 21 4.1 Lower Baker Reservoir Flood Regulation Plan ............................................................................ 21 4.1.1 Lower Baker Dam Project Features and Spillway Gate Regulation Schedule ..................... 21 4.1.2 Flood Control Regulation .................................................................................................... 22 4.2 Impact of Flood Control Storage at Lower Baker Dam ............................................................... 24 4.3 Conclusions ................................................................................................................................. 26 Skagit River Basin General Investigation Final Study Report Flood Risk Reduction Hydraulic Analysis ii August 2013 5 Hydraulic Modeling for Economic Flood Damage Analysis .......................................................... 28 5.1 Economic Damage Reaches ........................................................................................................ 28 5.2 Levee Failure Data and Hydraulic Modeling Approach for Flood Damage Analysis ................... 29 5.3 Hydrologic Inputs and Discharge Uncertainty ............................................................................ 31 5.3.1 Hydrologic Inputs ................................................................................................................ 31 5.3.2 Discharge Uncertainty and Equivalent Record Length ....................................................... 31 5.4 Hydraulic Modeling for Existing Conditions ................................................................................ 32 5.4.1 No‐Breach Analysis ............................................................................................................. 32 5.4.2 With‐Breach Analysis .......................................................................................................... 33 5.5 Hydraulic Modeling with Additional Early Season Flood Regulation Storage ............................ 34 5.5.1 No‐Breach Analysis ............................................................................................................. 34 5.5.2 With‐Breach Analysis .......................................................................................................... 35 5.6 Hydraulic Modeling with Improved Levees ................................................................................ 35 5.6.1 No‐Breach Analysis ............................................................................................................. 35 5.6.2 With‐Breach Analysis .......................................................................................................... 36 5.7 Stage‐Discharge Uncertainty ...................................................................................................... 36 5.7.1 Model Calibration and Measurement Uncertainty ............................................................. 37 5.7.2 Hydraulic Model Parameter Uncertainty ............................................................................ 38 5.7.3 Estimation of Total Stage‐Discharge Uncertainty ............................................................... 40 6 Initial Hydraulic Design of Flood Risk Reduction Alternatives ...................................................... 41 6.1 Setback Levees ............................................................................................................................ 41 6.1.1 Primary Design Criteria ....................................................................................................... 41 6.1.2 Modeling Methods .............................................................................................................. 42 6.1.3 Project Elements for the Preferred Configuration .............................................................. 42 6.1.4 Other Configurations Analyzed ........................................................................................... 43 6.1.5 Project Performance ........................................................................................................... 43 6.1.6 Hydraulic Modeling for Economic Flood Damage Analysis ................................................. 44 6.2 Joe Leary Slough Flood Bypass .................................................................................................... 45 6.2.1 Primary Design Criteria ....................................................................................................... 45 6.2.2 Modeling Methods .............................................................................................................. 45 6.2.3 Project Elements ................................................................................................................. 45 6.2.4 Project Performance ........................................................................................................... 48 Skagit River Basin General Investigation Final Study Report Flood Risk Reduction Hydraulic Analysis iii August 2013 6.2.5 Hydraulic Modeling for Economic Flood Damage Analysis ................................................. 49