Hydrology - NCTCOG

1 ISWMTM Technical Manual Hydrology Hydrology : Hydrological Analysis Downstream Assessment Streambank Protection Water Balance Rainfall Tables Hydrologic Soils DataiSWMTM Technical Manual Hydrology Hydrology HO-i April 2010, Revised 9/2014 Table of Contents Hydrological Analysis .. HO-1 Estimating HO-1 Introduction to Hydrologic Methods .. HO-1 Symbols and Definitions .. HO-4 Rainfall Estimation .. HO-5 Rational Method .. HO-7 Introduction .. HO-7 Application .. HO-7 Equations .. HO-7 Time of Concentration .. HO-8 Rainfall Intensity (I) .. HO-10 Runoff Coefficient (C) .. HO-10 Example Problem .. HO-10 SCS Hydrological Method.

2 HO-15 Introduction .. HO-15 Application .. HO-15 Equations and Concepts .. HO-15 Runoff Factor (CN) .. HO-17 Urban Modifications of the SCS Method .. HO-18 Travel Time Estimation .. HO-19 Simplified SCS Peak Runoff Rate Estimation .. HO-23 Example Problem 1 .. HO-25 Hydrograph Generation .. HO-28 Example Problem 2 .. HO-32 Hydrologic Stream Routing .. HO-33 Snyder s Unit Hydrograph Method .. HO-33 Introduction .. HO-33 Application .. HO-33 Urbanization Curves .. HO-34 Determination of Percent Urbanization and Percent Sand .. HO-35 Modified Rational Method .. HO-38 Introduction .. HO-38 Design Equations .. HO-39 Example Problem.

3 HO-40 iSWMTM Technical Manual Hydrology Hydrology HO-ii April 2010, Revised 9/2014 USGS and TxDOT Regression Methods .. HO-41 Introduction .. HO-41 USGS Equations for Urban Basins .. HO-41 Application of USGS Equations .. HO-42 Peak Discharge Limitations for Urban Basins .. HO-42 TxDOT Equations for Rural (or Undeveloped) Basins .. HO-43 Rural (or Undeveloped) Basin Application .. HO-43 Example Problem .. HO-46 References .. HO-47 Downstream Assessment .. HO-48 Introduction .. HO-48 Downstream Hydrologic Assessment .. HO-48 Reasons for Downstream Problems .. HO-49 Methods for Downstream Evaluation .. HO-50 Example HO-51 Streambank Protection Volume Estimation.

4 HO-53 Streambank Protection Volume Calculation .. HO-53 Example HO-55 Water Balance .. HO-57 Introduction .. HO-57 Basic Equations .. HO-57 Example HO-61 References .. HO-63 Rainfall Tables .. HO-64 Methodology .. HO-64 References .. HO-64 Hydrologic Soils Data .. HO-82 Electronic Soil Maps .. HO-82 Online Web Soil Survey .. HO-82 Downloading Soil Surveys .. HO-82 iSWMTM Technical Manual Hydrology Hydrology HO-iii April 2010, Revised 9/2014 List of Tables Table Applications of the Recommended Hydrologic Methods .. HO-2 Table Constraints on Using Recommended Hydrologic Methods .. HO-3 Table Symbols and Definitions.

5 HO-4 Table Frequency Factors for Rational Formula .. HO-8 Table Times of Concentration .. HO-9 Table Recommended Runoff Coefficient Values .. HO-11 Table Weighted Runoff Coefficient .. HO-12 Table Composite Curve Number Calculation Example .. HO-18 Table Runoff Curve Numbers1 .. HO-20 Table Roughness Coefficients (Manning's n) for Sheet HO-22 Table Ia Values for Runoff Curve Numbers .. HO-26 Table Dimensionless Unit Hydrograph With Peaking Factor of 484 .. HO-31 Table Example of Dimensionless Unit Hydrograph With Peaking Factor of 484 .. HO-32 Table Typical Values of Cp .. HO-34 Table Percent Urbanization and Imperviousness Summary with Associated Land Use Categories.

6 HO-36 Table Permeability Rating for the Determination of Percent Sand .. HO-37 Table Hydrologic Loss Rates .. HO-38 Table Rainfall Factors a and b for the Modified Rational Method (1-year through 100-year return periods) .. HO-40 Table USGS Peak Flow Regression Equations for Dallas-Fort Worth Urban Areas .. HO-43 Table Regression Equations for Estimation of Peak-Streamflow Frequency for Hydrologic Regions of Texas1 .. HO-44 Table Monthly Precipitation Values .. HO-58 Table Saturated Hydraulic Conductivity .. HO-59 Table Evaporation Monthly Distribution Grapevine, Texas as a % of Annual Total .. HO-60 Table Summary Information for the North Dallas Site.

7 HO-61 Table Collin County Rainfall Data .. HO-66 Table Dallas County Rainfall Data .. HO-67 Table Denton County Rainfall Data .. HO-68 Table Ellis County Rainfall Data .. HO-69 Table Erath County Rainfall Data .. HO-70 Table Hood County Rainfall Data .. HO-71 Table Hunt County Rainfall Data .. HO-72 Table Johnson County Rainfall Data .. HO-73 iSWMTM Technical Manual Hydrology Hydrology HO-iv April 2010, Revised 9/2014 Table Kaufman County Rainfall Data .. HO-74 Table Navarro County Rainfall Data .. HO-75 Table Palo Pinto County Rainfall Data .. HO-76 Table Parker County Rainfall Data .. HO-77 Table Rockwall County Rainfall Data.

8 HO-78 Table Somervell County Rainfall Data .. HO-79 Table Tarrant County Rainfall Data .. HO-80 Table Wise County Rainfall Data .. HO-81 List of Figures Figure The 16 Counties Participating in NCTCOG .. HO-6 Figure Example IDF Curve (Dallas County, Texas) .. HO-6 Figure Rational Formula - Overland Time of Flow Nomograph .. HO-13 Figure Manning s Equation Nomograph .. HO-14 Figure Approximate Geographic Boundaries for SCS Rainfall Distributions .. HO-16 Figure SCS Solution of the Runoff Equation .. HO-17 Figure Composite CN with Connected Impervious Areas .. HO-21 Figure Composite CN with Unconnected Impervious Areas.

9 HO-21 Figure Average Velocities - Shallow Concentrated Flow .. HO-24 Figure SCS Type II Unit Peak Discharge Graph .. HO-27 Figure Dimensionless Unit Hydrographs for Peaking Factors of 484 and 300 .. HO-30 Figure Modified Rational Definitions .. HO-38 Figure Hydrologic Regions for Statewide Rural Regression Equations .. HO-44 Figure Detention Timing Example .. HO-49 Figure Effect of Increased Post-Development Runoff Volume with Detention on a Downstream Hydrograph .. HO-50 Figure Example of the Ten-Percent Rule .. HO-52 Figure Detention Time vs. Discharge Ratios .. HO-54 Figure Approximate Detention Basin Routing for Rainfall Types I, IA, II, and III.

10 HO-55 Figure Average Annual Free Water Surface Evaporation (in inches) .. HO-60 Figure Dallas/Fort Worth Precipitation Information .. HO-62 Figure County Rainfall Data Location Map .. HO-65 Figure Example SSURGO Soil Map West Tarrant County .. HO-85 iSWMTM Technical Manual Hydrology Hydrology HO-1 April 2010, Revised 9/2014 Hydrological Analysis Estimating Runoff Introduction to Hydrologic Methods Hydrology deals with estimating flow peaks, volumes, and time distributions of stormwater runoff. The analysis of these parameters is fundamental to the design of stormwater management facilities, such as storm drainage systems and structural stormwater controls.