CHAPTER 3 STORMWATER HYDROLOGY

1 Volume 2 (Technical Handbook) Georgia STORMWATER Management Manual CHAPTER 3 STORMWATER HYDROLOGY Table of Contents SECTION METHODS FOR ESTIMATING STORMWATER RUNOFF Introduction to Hydrologic Methods .. Symbols and Definitions .. Rainfall Estimation .. Rational Method .. Introduction .. Application .. Equations .. Time of Concentration .. Rainfall Intensity .. Runoff Coefficient .. Example Problem .. SCS NRCS TR-55 Hydrologic Method .. Introduction .. Application .. Equations and Concepts .. Runoff Urban Modifications of the SCS NRCS TR-55 Method .. Travel Time Estimation .. Simplified SCS NRCS TR-55 Peak Runoff Rate Estimation.

2 Example Problem 1 .. Hydrograph Generation .. Example Problem 2 .. Geological Survey Peak Flow and Hydrograph Method .. Introduction .. Application .. Peak Discharge Peak Discharge Limitations for Urban Basins .. Hydrographs .. Hydrograph Limitations .. Example Problem .. Water Quality Volume and Peak Flow .. Water Quality Volume Calculation .. Water Quality Volume Peak Flow Calculation .. Example Problem .. Commented [RC1]: Page Numbers to be updated at 100% Commented [RC2]: SC32 (Typ.) Georgia STORMWATER Management Manual Volume 2 (Technical Handbook) Water Balance Calculations .. Introduction.

3 Basic Equations .. Example Problem .. Downstream Hydrologic Assessment .. Reasons for Downstream Problems .. The Ten-Percent Rule .. Example Problem .. References .. SECTION METHODS FOR ESTIMATING STORMWATER VOLUME REDUCTION DESIGN WORKSHEET Volume Reduction DesignIntroduction .. Introduction .. XXXXXXXXXX .. XXXXXXXXX .. SECTION STORAGE DESIGN General Storage Concepts .. Introduction .. Storage Classification .. Stage-Storage Relationship .. Stage-Discharge Relationship .. Symbols and Definitions .. General Storage Design Procedures .. Introduction .. Data Needs .. Design Procedure.

4 Preliminary Detention Calculations .. Introduction .. Storage Volume .. Alternative Method .. Peak Flow Reduction .. Channel Protection Volume Estimation .. Introduction .. Basic Approach .. Example Problem .. The Modified Rational Method .. Introduction .. Design Equations .. Example Problem .. References .. SECTION OUTLET STRUCTURES Symbols and Definitions .. Primary Outlets .. Introduction .. Outlet Structure Types .. Orifices .. Perforated Risers .. Pipes and Culverts .. Volume 2 (Technical Handbook) Georgia STORMWATER Management Manual Sharp-Crested Broad-Crested V-Notch Weirs.

5 Proportional Weirs .. Combination Outlets .. Extended Detention (Water Quality and Channel Protection) Outlet Design .. Introduction .. Method 1: Maximum Hydraulic Head with Routing .. Method 2: Average Hydraulic Head and Average Discharge .. Multi-Stage Outlet Design .. Introduction .. Multi-Stage Outlet Design Procedure .. Extended Detention Outlet Protection .. Trash Racks and Safety Grates .. Introduction .. Trash Rack Design .. Secondary Outlets .. Introduction .. Emergency Spillway Design .. References .. List of Tables Table Applications of the Recommended Hydrologic Methods .. Table Constraints on Using Recommended Hydrologic Methods.

6 Table Symbols and Definitions .. Table Frequency Factors for Rational Formula .. Table Recommended Runoff Coefficient Values .. Table Runoff Curve Table Roughness Coefficients (Manning s n) for Sheet Flow .. Table Ia Values for Runoff Curve Numbers .. Table Dimensionless Unit Hydrographs .. Table USGS Peak Flow Regression Equations .. Table Dimensionless USGS Table USGS Rural Peak Equations .. Table Flood Hydrograph .. Table Water Balance Values for Atlanta, Georgia .. Table Saturated Hydraulic Conductivity .. Table Evaporation Monthly Distribution .. Table Summary Information for Austin Acres .. Table Symbols and Definitions .. Table Rainfall Factors a and b for the Modified Rational Method (1-year through 100-year return periods).

7 Table Symbols and Definitions .. Table Circular Perforation Sizing .. Table Broad-Crested Weir Coefficient (C) Values .. Commented [RC3]: Table list to be updated at 100% Georgia STORMWATER Management Manual Volume 2 (Technical Handbook) List of Figures Figure Location of Rainfall Data Sites .. Figure Example IDF Curve (Athens, Georgia) .. Figure Rainfall Isohyetal Lines (10-year, 24-hour values) .. Figure Rational Formula Overland Time of Flow Nomograph .. Figure Manning s Equation Nomograph .. Figure Kinetic Wave Nomograph .. Figure Approximate Geographic Boundaries for SCS NRCS TR-55 Rainfall Distributions Figure SCS NRCS TR-55 Solution of the Runoff Equation.

8 Figure Composite CN with Connected Impervious Areas .. Figure Composite CN with Unconnected Impervious Areas .. Figure Average Velocities Shallow Concentrated Flow .. Figure SCSNRCS TR-55 Type II Unit Peak Discharge Graph .. Figure SCSNRCS TR-55 Type III Unit Peak Discharge Graph .. Figure Dimensionless Unit Hydrographs for Peaking Factors of 484 and 300 .. Figure USGS Hydrologic Regions in Georgia .. Figure Average Annual Free Water Surface Evaporation (in inches) .. Figure Augusta Precipitation Information .. Figure Detention Timing Example .. Figure Effect of Increased Post-Development Runoff Volume with Detention on a Downstream Hydrograph .. Figure Example of the Ten-Percent Rule.

9 Figure Examples of Typical STORMWATER Storage Facilities .. Figure On-line versus Off-line Storage .. Figure Stage-Storage Curve .. Figure Double-end Area Method .. Figure Stage-Discharge Curve .. Figure Triangular-Shaped Hydrographs .. Figure Detention Time vs. Discharge Ratios .. Figure Approximate Detention Basin Routing for Rainfall Types 1, 1A, II, and III .. Figure Modified Rational Definitions .. Figure Typical Primary Outlets .. Figure Orifice Definitions .. Figure Perforated Riser .. Figure Schematic of Orifice Plate Outlet Structure .. Figure Sharp-Crested Weir .. Figure Broad-Crested Weir .. Figure V-Notch Weir .. Figure Proportional Weir Dimensions .. Figure Schematic of Combination Outlet Structure.

10 Figure Composite Stage-Discharge Curve .. Figure Riser Flow Diagrams .. Figure Weir and Orifice Flow .. Figure Reverse Slope Pipe Outlet .. Figure Hooded Outlet .. Figure Half-Round CMP Orifice Hood .. Figure Internal Control for Orifice Protection .. Figure Example of Various Trash Racks Used on a Riser Outlet Structure .. Figure Minimum Rack Size vs. Outlet Diameter .. Figure Emergency Spillway .. Commented [RC4]: Figure list to be updated at 100% Volume 2 (Technical Handbook) Georgia STORMWATER Management Manual METHODS FOR ESTIMATING STORMWATER RUNOFF Introduction to Hydrologic Methods HYDROLOGY deals with estimating peak flows, volumes, and time distributions of STORMWATER runoff.