Skagit River Flood Risk Management and Ecosystem Restoration Feasibility Study Skagit County, Washington
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U.S. Army Corps of Engineers Seattle District SKAGIT RIVER FLOOD RISK MANAGEMENT AND ECOSYSTEM RESTORATION FEASIBILITY STUDY SKAGIT COUNTY, WASHINGTON Feasibility Scoping Meeting Read‐Ahead Report August 2009 SKAGIT RIVER BASIN SKAGIT RIVER FLOOD RISK MANAGEMENT AND ECOSYSTEM RESTORATION FEASIBILITY STUDY SKAGIT COUNTY, WASHINGTON Feasibility Scoping Meeting Read‐Ahead Report US Army Corps of Engineers Seattle District August 2009 i ii TABLE OF CONTENTS 1.0 INTRODUCTION......................................................................................................................................... 1 1.1 Feasibility Study Authority ......................................................................................................... 1 1.2 Feasibility Study Background...................................................................................................... 2 1.3 Study Area .................................................................................................................................. 2 1.4 Study Partnership....................................................................................................................... 3 1.5 Study Stakeholders..................................................................................................................... 3 1.6 References.................................................................................................................................. 4 2.0 MAJOR FEDERAL AND STATE REGULATIONS ............................................................................................ 5 2.1 Federal Regulations and Local Implications ............................................................................... 5 2.2 State Regulations .......................................................................................................... 8 2.3 References ................................................................................................................ 10 3.0 PRIOR STUDIES, REPORTS, AND WATER PRODUCTS................................................................ 11 3.1 Prior Corps Studies and Project Authorizations.......................................................... 11 3.2 Skagit County Reports................................................................................................. 14 3.3 Federal Water Resources Projects in Study Area ....................................................... 14 3.4 Non Federal Water Resources Projects in Study Area................................................ 16 3.5 Non Federal Flood Control Planning........................................................................... 18 3.6 References ................................................................................................................ 19 4.0 FEASIBILITY STUDY PURPOSE, SCOPE, AND STATUS ................................................................ 20 4.1 Purpose ....................................................................................................................... 20 4.2 Scope...........................................................................................................................20 4.3 Feasibility Study Status ............................................................................................... 28 5.0 WITHOUT PROJECT CONDITIONS............................................................................................. 35 5.1 Hydrology and Hydraulics ........................................................................................... 35 5.2 Geomorphology and Sedimentation........................................................................... 45 5.3 Economics ................................................................................................................... 47 5.4 Environmental............................................................................................................. 56 5.5 References .................................................................................................................. 72 6.0 PLAN FORMULATION ............................................................................................................... 76 6.1 Problems and Opportunities....................................................................................... 76 6.2 Planning Objectives..................................................................................................... 77 6.3 Planning Constraints ................................................................................................... 78 6.4 Planning Assumptions................................................................................................. 78 6.5 Measures Screening.................................................................................................... 79 iii 6.6 Alternatives Formulation ............................................................................................ 83 6.7 References .................................................................................................................. 83 7.0 NEXT STEPS ............................................................................................................................. 85 7.1 Feasibility Scoping Meeting Compliance .................................................................... 85 7.2 Technical Updates and Revisions................................................................................ 85 7.3 Integration of Ecosystem Restoration Measures........................................................ 85 7.4 Engineering Design...................................................................................................... 85 7.5 Plan Formulation......................................................................................................... 85 7.6 Feasibility Report/Decision Document ....................................................................... 86 8.0 PLATES ............................................................................................................................. 88 iv Tables 3‐1 Prior Corps Reports, Skagit River Basin ................................................................................... 13 4‐1 Sensitive Policy Areas Specific to Flood Damage Reduction ................................................... 28 4‐2 Sensitive Policy Area Specific to Ecosystem Restoration......................................................... 29 4‐3 Agency Technical Review Status.............................................................................................. 31 4‐4 Project Milestones ................................................................................................................ 32 5‐1 Summary of Streamflow Data in CFS....................................................................................... 36 5‐2 Peak Flows ................................................................................................................ 39 5‐3 Skagit River Project Reaches.................................................................................................... 41 5‐4 Skagit River Rating Stage Discharge Function by Event and Reach......................................... 42 5‐5 Index Locations ................................................................................................................ 45 5‐6 Residential Structures & Content Values................................................................................. 51 5‐7 Nonresidential Structure & Content Values ............................................................................ 51 5‐8 Total Residential Inundation Damages by Event ..................................................................... 53 5‐9 Total Nonresidential Inundation Damages by Event ............................................................... 53 5‐10 HEC‐FDA Expected Annual Damages by Reach...................................................................... 54 5‐11 Project Performance by Damage Reach for the Without Project Condition......................... 55 5‐12 Summary Table of Salmonids Found in the Lower and Upper Skagit Basins ........................ 62 5‐13 Listed Species Potentially Occurring in the Project Area....................................................... 65 6‐1 Problems and Opportunities.................................................................................................... 77 6‐2 Planning Objectives ................................................................................................................ 77 6‐3 Planning Constraints................................................................................................................ 78 6‐4 Planning Assumptions.............................................................................................................. 79 6‐5 Eliminated Flood Risk Management Measures for Initial Screening....................................... 80 6‐6 Remaining Measures to be Evaluated – by Type..................................................................... 81 i Plates Plate 1 – Study Area Plate 2 – Lower Basin Detail Plate 3 – Diking Districts Plate 4 – Drainage Districts Plate 5 – Flood Control Infrastructure Plate 6 – Skagit River Dams Plate 7 – 50 year Floodplain Plate 8 – 100 year Floodplain Plate 9 – 500 year Floodplain Plate 10 – Downstream Study Reaches Plate 11 – Upstream Study Reaches Plate 12 – Urban Reach 1A Plate 13 – Urban Reach 2A Plate 14 – Urban Reach 4A Plate 15 – Urban Reach 5A Plate 16 – Urban Reach