CHAPTER 3 STORMWATER HYDROLOGY Commented [RC1]: Page Numbers to be updated at Table of Contents 100% SECTION 3.1 METHODS FOR ESTIMATING STORMWATER RUNOFF 3.1.1 Introduction to Hydrologic Methods ..................................................3.1-1 3.1.2 Symbols and Definitions .....................................................................3.1-3 3.1.3 Rainfall Estimation ................................................................................3.1-5 3.1.4 Rational Method ....................................................................................3.1-8 3.1.4.1 Introduction ................................................................................3.1-8 3.1.4.2 Application .................................................................................3.1-8 3.4.4.3 Equations ..................................................................................3.1-8 3.1.4.4 Time of Concentration ...............................................................3.1-9 3.1.4.5 Rainfall Intensity ........................................................................3.1-10 3.1.4.6 Runoff Coefficient ......................................................................3.1-10 3.1.4.7 Example Problem ......................................................................3.1-11 3.1.5 SCS NRCS TR-55 Hydrologic Method ................................................3.1-17 Commented [RC2]: SC32 (Typ.) 3.1.5.1 Introduction ................................................................................3.1-17 3.1.5.2 Application .................................................................................3.1-17 3.1.5.3 Equations and Concepts ...........................................................3.1-17 3.1.5.4 Runoff Factor.............................................................................3.1-19 3.1.5.5 Urban Modifications of the SCS NRCS TR-55 Method ............3.1-21 3.1.5.6 Travel Time Estimation .............................................................3.1-24 3.1.5.7 Simplified SCS NRCS TR-55 Peak Runoff Rate Estimation ....3.1-28 3.1.5.8 Example Problem 1 ...................................................................3.1-32 3.1.5.9 Hydrograph Generation ............................................................3.1-33 3.1.5.10 Example Problem 2 .................................................................3.1-37 3.1.6 U.S. Geological Survey Peak Flow and Hydrograph Method .........3.1-39 3.1.6.1 Introduction ................................................................................3.1-39 3.1.6.2 Application .................................................................................3.1-39 3.1.6.3 Peak Discharge Equations........................................................3.1-39 3.1.6.4 Peak Discharge Limitations for Urban Basins ..........................3.1-39 3.1.6.5 Hydrographs ..............................................................................3.1-42 3.1.6.6 Hydrograph Limitations .............................................................3.1-43 3.1.6.7 Example Problem ......................................................................3.1-45 3.1.7 Water Quality Volume and Peak Flow ................................................3.1-47 3.1.7.1 Water Quality Volume Calculation ............................................3.1-47 3.1.7.2 Water Quality Volume Peak Flow Calculation ..........................3.1-47 3.1.7.3 Example Problem ......................................................................3.1-48 Volume 2 (Technical Handbook) Georgia Stormwater Management Manual 3.1-1 3.1.8 Water Balance Calculations ................................................................3.1-49 3.1.8.1 Introduction ................................................................................3.1-49 3.1.8.2 Basic Equations .......................................................................3.1-49 3.1.8.3 Example Problem ......................................................................3.1-52 3.1.9 Downstream Hydrologic Assessment ...............................................3.1-54 3.1.9.1 Reasons for Downstream Problems .........................................3.1-54 3.1.9.2 The Ten-Percent Rule ..............................................................3.1-55 3.1.9.3 Example Problem .....................................................................3.1-56 References ............................................................................................3.1-58 SECTION 3.2 METHODS FOR ESTIMATING STORMWATER VOLUME REDUCTION – DESIGN WORKSHEET 3.2.1 Volume Reduction DesignIntroduction ..............................................3.2-1 3.2.1.1 Introduction ................................................................................3.2-1 3.2.2 XXXXXXXXXX ........................................................................................3.2-5 3.2.3 XXXXXXXXX ...........................................................................................3.2-5 SECTION 3.3 STORAGE DESIGN 3.3.1 General Storage Concepts ...................................................................3.3-1 3.3.1.1 Introduction ................................................................................3.3-1 3.3.1.2 Storage Classification ...............................................................3.3-2 3.3.1.3 Stage-Storage Relationship ......................................................3.3-2 3.3.1.4 Stage-Discharge Relationship ..................................................3.3-4 3.3.2 Symbols and Definitions ......................................................................3.3-5 3.3.3 General Storage Design Procedures ..................................................3.3-5 3.3.3.1 Introduction ................................................................................3.3-5 3.3.3.2 Data Needs ...............................................................................3.3-6 3.3.3.3 Design Procedure .....................................................................3.3-6 3.3.4 Preliminary Detention Calculations ....................................................3.3-7 3.3.4.1 Introduction ................................................................................3.3-7 3.3.4.2 Storage Volume ........................................................................3.3-7 3.3.4.3 Alternative Method ...................................................................3.3-8 3.3.4.4 Peak Flow Reduction ................................................................3.3-8 3.3.5 Channel Protection Volume Estimation .............................................3.3-9 3.3.5.1 Introduction ................................................................................3.3-9 3.3.5.2 Basic Approach .........................................................................3.3-9 3.3.5.3 Example Problem ......................................................................3.3-9 3.3.6 The Modified Rational Method ............................................................3.3-12 3.3.6.1 Introduction ................................................................................3.3-12 3.3.6.2 Design Equations ......................................................................3.3-13 3.3.6.3 Example Problem ......................................................................3.3-13 References ......................................................................................................3.3-16 SECTION 3.4 OUTLET STRUCTURES 3.4.1 Symbols and Definitions ......................................................................3.4-1 3.4.2 Primary Outlets .....................................................................................3.4-1 3.4.2.1 Introduction ................................................................................3.4-1 3.4.2.2 Outlet Structure Types ..............................................................3.4-2 3.4.2.3 Orifices ......................................................................................3.4-3 3.4.2.4 Perforated Risers ......................................................................3.4-5 3.4.2.5 Pipes and Culverts ....................................................................3.4-5 3.1-2 Georgia Stormwater Management Manual Volume 2 (Technical Handbook) 3.4.2.6 Sharp-Crested Weirs.................................................................3.4-6 3.4.2.7 Broad-Crested Weirs.................................................................3.4-7 3.4.2.8 V-Notch Weirs ...........................................................................3.4-8 3.4.2.9 Proportional Weirs .....................................................................3.4-9 3.4.2.10 Combination Outlets ...............................................................3.4-10 3.4.3 Extended Detention (Water Quality and Channel Protection) Outlet Design .........................................................................................3.4-11 3.4.3.1 Introduction ................................................................................3.4-11 3.4.3.2 Method 1: Maximum Hydraulic Head with Routing ..................3.4-11 3.4.3.3 Method 2: Average Hydraulic Head and Average Discharge ..3.4-12 3.4.4 Multi-Stage Outlet Design ....................................................................3.4-12 3.4.4.1 Introduction ................................................................................3.4-12 3.4.4.2 Multi-Stage Outlet Design Procedure .......................................3.4-13 3.4.5 Extended