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Chapter 2 Estimating Runoff Volume and Peak Discharge

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Chapter 2 Estimating Runoff Volume and Peak Discharge Title 210 – National Engineering Handbook Part 650 Engineering Field Handbook National Engineering Handbook Chapter 2 Estimating Runoff Volume and Peak Discharge (210-650-H, XX Edition, D R A F T, ____date____) Title 210 – National Engineering Handbook Issued __________ In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights regulations and policies, the USDA, its Agencies, offices, and employees, and institutions participating in or administering USDA programs are prohibited from discriminating based on race, color, national origin, religion, sex, gender identity (including gender expression), sexual orientation, disability, age, marital status, family/parental status, income derived from a public assistance program, political beliefs, or reprisal or retaliation for prior civil rights activity, in any program or activity conducted or funded by USDA (not all bases apply to all programs). Remedies and complaint filing deadlines vary by program or incident. Persons with disabilities who require alternative means of communication for program information (e.g., Braille, large print, audiotape, American Sign Language, etc.) should contact the responsible Agency or USDA's TARGET Center at (202) 720-2600 (voice and TTY) or contact USDA through the Federal Relay Service at (800) 877-8339. Additionally, program information may be made available in languages other than English. To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, AD-3027, found online at How to File a Program Discrimination Complaint and at any USDA office or write a letter addressed to USDA and provide in the letter all of the information requested in the form. To request a copy of the complaint form call (866) 632-9992. Submit your completed form or letter to USDA by: (1) mail: U.S. Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) email: [email protected]. USDA is an equal opportunity provider, employer, and lender. (210-650-H, XX Edition, D R A F T, ____date____) 650-2.i Title 210 – National Engineering Handbook Chapter 2 - Estimating Runoff Volume and Peak Discharge Table of Contents 650.0200 Introduction ................................................................................................................... 1 650.0201 Factors Affecting Surface Runoff .................................................................................. 1 A. General ................................................................................................................................. 1 B. Rainfall Amount ................................................................................................................... 2 C. Rainfall Distribution ............................................................................................................. 2 D. Hydrologic Soil Groups ........................................................................................................ 5 E. Cover Type ........................................................................................................................... 6 F. Land Use ............................................................................................................................... 6 G. Land Treatment .................................................................................................................... 6 H. Hydrologic Conditions.......................................................................................................... 7 I. Topography ............................................................................................................................ 7 650.0202. Hydrographs ................................................................................................................. 7 A. General ................................................................................................................................. 7 B. Types of Hydrographs........................................................................................................... 7 C. Dimensionless unit hydrograph peak rate factors .................................................................. 8 650.0203 Runoff Volume .............................................................................................................. 9 A. General ................................................................................................................................. 9 B. NRCS Runoff Equation ........................................................................................................ 9 C. Runoff Curve Numbers ....................................................................................................... 10 D. Estimating Runoff Volume using the NRCS CN Method ................................................... 10 650.0204 Time of Concentration ................................................................................................. 16 A. General ............................................................................................................................... 16 B. The NRCS Lag Equation .................................................................................................... 16 (2) Flow Length ................................................................................................................... 17 C. Estimating Tc Using the NRCS Lag Equation ..................................................................... 18 650.0205 Peak Discharge ............................................................................................................ 19 A. General ............................................................................................................................... 19 B. Estimating Peak Discharge Manually ................................................................................. 20 (1) Unit peak discharge curves ............................................................................................ 20 (2) Ia/P ratio ......................................................................................................................... 20 (3) Estimating Peak Discharge ............................................................................................ 20 C. Estimating Peak Discharge Using the EFH-2 Computer Program ...................................... 26 (210-650-H, XX Edition, D R A F T, ____date____) 650-2.ii Title 210 – National Engineering Handbook 650.0206 Limitations ................................................................................................................... 26 650.0207 Examples ..................................................................................................................... 27 A. Example - Estimating weighted CN .................................................................................... 27 B. Example - Estimating Time of Concentration ..................................................................... 27 C. Example – Manual Estimation of Peak Discharge .............................................................. 27 650.0208 References ................................................................................................................... 30 650.0209 Acknowledgements ..................................................................................................... 30 650.0210 Exhibits ........................................................................................................................ 30 A. Runoff Curve Number Worksheet ...................................................................................... 30 B. Time of Concentration and Peak Discharge Worksheet ...................................................... 30 Table of Figures Figure 2-1. Approximate Geographic Boundaries for NRCS Standard Rainfall Distributions ...... 3 Figure 2-2. Temporal Plots of Type I, IA, II, and III rainfall distributions .................................... 4 Figure 2-3. NOAA Atlas 14, Volume 2 2 – Geographic Boundaries of NRCS-developed Rainfall Distributions ................................................................................................................................... 4 Figure 2-4. Temporal plots of the NRCS-derived NOAA Atlas 14 Rainfall Distributions for the Ohio Valley and Neighboring States .............................................................................................. 5 Figure 2-5. Hydrograph plot illustrating a natural (or measured) hydrograph and a synthetic (or computed) hydrograph for the same watershed .............................................................................. 8 Figure 2-6. Dimensionless curvilinear unit hydrograph (PRF=484) and equivalent dimensionless triangular unit hydrograph .............................................................................................................. 9 1 Figure 2- 7a. Runoff Curve Numbers - (a) Cultivated Agricultural Lands ............................... 11 Figure 2-8. Solution to the NRCS Runoff Equation .................................................................... 15 1 Figure 2-9. Runoff Depth for Selected CNs And Rainfall Amounts .......................................... 16 Figure 2-10. Natural watershed ................................................................................................... 18 Figure 2-11. Watershed with Terraces ........................................................................................
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