Thurston County FLOOD HAZARD MITIGATION PLAN FINAL JANUARY 2013 Prepared for: Thurston County Planning Department 2000 Lakeridge Drive SW Olympia, WA 98502-6045 Prepared by: Project #135-12410-12002 Thurston County Flood Hazard Mitigation Plan TABLE OF CONTENTS Executive Summary .............................................................................................................. ES-1 PART 1 — PLANNING PROCESS AND PROJECT BACKGROUND Chapter 1. Introduction ........................................................................................................... 1-1 1.1 Why Prepare This Plan? ................................................................................................................... 1-1 1.2 Guidelines for Flood planning ......................................................................................................... 1-1 1.2.1 CRS Steps for Comprehensive Floodplain Management Plan ............................................ 1-2 1.2.2 FCAAP Requirements for Comprehensive Flood Control Management Plan .................... 1-2 1.3 How to Use This Plan ...................................................................................................................... 1-3 Chapter 2. Plan Development Methodology .......................................................................... 2-1 2.1 Formation of the Planning Team ...................................................................................................... 2-1 2.2 Defining the Planning Area .............................................................................................................. 2-1 2.3 The Steering Committee .................................................................................................................. 2-2 2.4 Coordination with Other Agencies ................................................................................................... 2-2 2.5 Review of Existing Programs ........................................................................................................... 2-3 2.6 Public Involvement .......................................................................................................................... 2-4 2.6.1 Strategy ............................................................................................................................... 2-4 2.6.2 Public Involvement Results ............................................................................................... 2-12 2.7 Plan Development Chronology/Milestones ................................................................................... 2-12 Chapter 3. Thurston County Profile ....................................................................................... 3-1 3.1 Historical Overview ......................................................................................................................... 3-1 3.2 Physical Setting ................................................................................................................................ 3-3 3.2.1 Topography ......................................................................................................................... 3-3 3.2.2 Geology and Soils ............................................................................................................... 3-3 3.2.3 Drainage .............................................................................................................................. 3-4 3.2.4 Climate ................................................................................................................................ 3-6 3.3 Critical Facilities and Infrastructure ................................................................................................. 3-6 3.4 Demographics .................................................................................................................................. 3-9 3.4.1 Population Characteristics ................................................................................................. 3-10 3.4.2 Income ............................................................................................................................... 3-11 3.4.3 Age Distribution ................................................................................................................ 3-11 3.4.4 Race, Ethnicity and Language ........................................................................................... 3-12 3.4.5 Disabled Populations ......................................................................................................... 3-13 3.5 Economy ........................................................................................................................................ 3-13 3.5.1 Industry, Businesses and Institutions ................................................................................ 3-13 3.5.2 Employment Trends and Occupations .............................................................................. 3-14 Chapter 4. Relevant Programs and Regulations ................................................................... 4-1 4.1 Federal .............................................................................................................................................. 4-1 4.1.1 Disaster Mitigation Act of 2000 .......................................................................................... 4-1 4.1.2 National Flood Insurance Program ..................................................................................... 4-1 4.1.3 The Community Rating System .......................................................................................... 4-2 i Thurston County Flood Hazard Mitigation Plan 4.1.4 Endangered Species Act ...................................................................................................... 4-2 4.1.5 The Clean Water Act ........................................................................................................... 4-4 4.2 State .................................................................................................................................................. 4-4 4.2.1 Washington State Floodplain Management Law ................................................................ 4-4 4.2.2 Flood Control Assistance Account Program ....................................................................... 4-5 4.2.3 Shoreline Management Act ................................................................................................. 4-5 4.2.4 Growth Management Act .................................................................................................... 4-5 4.2.5 Washington State Building Code ........................................................................................ 4-6 4.2.6 Comprehensive Emergency Management Planning ........................................................... 4-6 4.2.7 Watershed Management Act ............................................................................................... 4-7 4.3 Local ................................................................................................................................................. 4-7 4.3.1 Comprehensive Plans .......................................................................................................... 4-7 4.3.2 Emergency Management Plan ............................................................................................. 4-7 4.3.3 Critical Areas Ordinance ..................................................................................................... 4-7 4.3.4 Shoreline Master Program ................................................................................................... 4-8 4.3.5 WRIA Planning ................................................................................................................... 4-8 4.3.6 Capability Assessment ........................................................................................................ 4-8 PART 2 — RISK ASSESSMENT Chapter 5. Risk Assessment Methodology ........................................................................... 5-1 5.1 Purpose of Risk Assessment ............................................................................................................ 5-1 5.2 Risk Assessment Approach .............................................................................................................. 5-1 5.2.1 FEMA’s HAZUS-MH Software ......................................................................................... 5-1 5.2.2 Application for This Plan .................................................................................................... 5-2 5.2.3 Limitations .......................................................................................................................... 5-3 Chapter 6. Thurston County Flood Hazard Profile ............................................................... 6-1 6.1 General Concepts ............................................................................................................................. 6-1 6.1.1 Measuring Floods and Floodplains ..................................................................................... 6-1 6.1.2 Floodplain Ecosystems........................................................................................................ 6-2 6.1.3 Effects of Human Activities ................................................................................................ 6-2 6.2 Principal Flooding Sources in Thurston County .............................................................................