City of Coos Bay COOS COUNTY, OREGON STORMWATER MASTER PLAN VOLUME A March 2006 City of Coos Bay Stormwater Master Plan Table of Contents 1.0 Executive Summary ....................................................................................................................... 1-1 1.1 Introduction .............................................................................................................................. 1-1 1.2 Existing System ........................................................................................................................ 1-1 1.3 Identification of Deficiencies and Development of Improvement Alternatives ....................... 1-2 1.4 Recommended Plan .................................................................................................................. 1-2 Table 1.4.1 – Storm Drain System Project Prioritization Summary .................................................. 1-3 1.5 Plan Implementation ................................................................................................................. 1-3 1.6 Potential Financing Options ..................................................................................................... 1-3 1.7 Potential Impacts to Rate Payers .............................................................................................. 1-4 2.0 Introduction, Purpose and Need ..................................................................................................... 2-1 2.1 Background ............................................................................................................................... 2-1 2.2 Previous Planning Efforts ......................................................................................................... 2-1 2.3 Purpose and Need ..................................................................................................................... 2-1 2.4 Scope of Engineering Services ................................................................................................. 2-2 2.5 Authorization ............................................................................................................................ 2-2 2.6 Acknowledgements .................................................................................................................. 2-3 3.0 Study Area Characteristics ............................................................................................................. 3-1 3.1 Study Area ................................................................................................................................ 3-1 3.2 Physical Environment ............................................................................................................... 3-1 3.2.1 Climate ................................................................................................................................ 3-1 Figure 3-1 – Project Location ............................................................................................................ 3-2 Figure 3-2 – Study Area Map ............................................................................................................ 3-3 Figure 3-3 – Monthly Mean Precipitation .......................................................................................... 3-4 Figure 3-4 – Average Annual Precipitation ....................................................................................... 3-4 3.2.2 Soils ..................................................................................................................................... 3-5 3.2.3 Geologic Hazards ................................................................................................................ 3-5 Figure 3-5 – Soils Map ...................................................................................................................... 3-6 3.2.4 Water Resources ................................................................................................................. 3-7 3.2.5 Flora and Fauna ................................................................................................................... 3-7 3.2.6 Air Quality and Noise ......................................................................................................... 3-8 3.2.7 Environmentally Sensitive Areas ........................................................................................ 3-8 3.3 Socio-Economic Environment .................................................................................................. 3-8 3.3.1 Economic Conditions and Trends ....................................................................................... 3-8 3.3.2 Population ........................................................................................................................... 3-8 3.4 Land Use ................................................................................................................................... 3-9 Figure 3-6 – Land Use Designation Map ......................................................................................... 3-10 Table 3.4 – Land Use Designations ................................................................................................. 3-11 3.4.1 Development Density, Impervious Area, and Runoff ....................................................... 3-11 Table 3.4.1 – Drainage Basin Characteristics .................................................................................. 3-12 3.4.2 Equivalent Dwelling Unit (EDU) Methodology ............................................................... 3-12 Table 3.4.2 – Existing EDU’s per Drainage Basin .......................................................................... 3-13 3.4.3 Future EDU’s .................................................................................................................... 3-13 Table 3.4.3 – Future EDU’s per Drainage Basin ............................................................................. 3-14 4.0 Existing Stormwater Facilities ....................................................................................................... 4-1 4.1 General ..................................................................................................................................... 4-1 4.2 Information Gathering and Sources .......................................................................................... 4-1 4.3 Storm Drainage System Overview ........................................................................................... 4-1 Figure 4-1 Overall Basin Map ........................................................................................................... 4-2 4.4 Existing Storm Drain System Inventory ................................................................................... 4-4 HBH Consulting Engineers Page i City of Coos Bay Stormwater Master Plan Table 4.4A – Storm Drain Structure Inventory .................................................................................. 4-4 Table 4.4B – Storm Drain Pipe Inventory ......................................................................................... 4-5 4.4.1 Storm Drain Outfalls ........................................................................................................... 4-5 Table 4.4.1 – Storm Drain Outfall Summary ................................................................................... 4-17 4.4.2 Culverts ............................................................................................................................. 4-17 Table 4.4.2 – Culvert Summary ....................................................................................................... 4-29 4.4.3 Pump Stations ................................................................................................................... 4-30 5.0 Hydrologic Analysis ...................................................................................................................... 5-1 5.1 Rational Method ....................................................................................................................... 5-1 5.2 SCS Rainfall Runoff Relationship ............................................................................................ 5-2 5.2.1 Factors Affecting Runoff Volume ...................................................................................... 5-2 5.2.2 SCS Rainfall – Runoff Equation ......................................................................................... 5-2 5.2.3 Runoff Curve Numbers ....................................................................................................... 5-3 Table 5.2.3 – SCS Curve Numbers for Identified Land Uses ............................................................ 5-4 5.2.4 Time of Concentration ........................................................................................................ 5-4 Table 5.2.4A – Land Cover Coefficients ........................................................................................... 5-5 Table 5.2.4B – Manning’s n for Partially Full Pipes and Open Channels ......................................... 5-6 5.2.5 Rainfall ................................................................................................................................ 5-6 Figure 5-1 – Geographic Areas for SCS Rainfall Distributions......................................................... 5-7 Table 5.2.5 – Design Storm Rainfall Totals for Coos Bay ...............................................................