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INFLOW RUNOFF - Runoff Curve Number Direct Precipitation

INFLOW RUNOFF - Runoff Curve Number Direct Precipitation. Runoff Equation. RO= [P - Ia] 2 [P - Ia] + S Ia=0.2S, S= 1000/CN-10, therefore = [P - ((200/CN) - 2)] 2 [P + (800/CN) - 8] where, P>0.2S. 50% Chance Event. How does a 50% chance of 7 days

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INFLOW RUNOFF - Runoff Curve Number Direct Precipitation

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  1. INFLOW RUNOFF - Runoff Curve NumberDirect Precipitation

  2. Runoff Equation • RO= [P - Ia]2 • [P - Ia] + S • Ia=0.2S, S= 1000/CN-10, therefore • = [P - ((200/CN) - 2)]2 • [P + (800/CN) - 8] • where, P>0.2S

  3. 50% Chance Event • How does a 50% chance of 7 days • or 14 days runoff differ from runoff • generated from a 24 hour 2 year • frequency storm????

  4. Runoff Curve Number (RCN) • EFM (EFH) Ch. 2 or EFM2 Computer Program NRCS Engineering Hydrology or • NRCS Engineering Hydrology Training Series Module 104 (Runoff Curve Number Computations)

  5. CN Information Needed • Hydrologic Soil group • Cover Description • Condition of Cover • Acres or Percent of Total Area Training Series Module 104 (Runoff Curve Number Computations) part of pre-course assignment.

  6. Muti-year Crop Rotation • - For assistance with computation of the RCN for seasonal adjusted cropland, refer to Appendix A.

  7. Adjustment from Type II ARC (During Growing Season) • ARC Type 5-Day Antecedent Rainfall (AMC) • I < 1.4 in. • II between 1.4 in. • and 2.1 in. • III > 2.1 in. • Table 6.1

  8. Dormant Season Adjustment • ARC Type 5-Day Antecedent Rainfall(AMC) • I < 0.5 in. • II between 0.5 in. • and 1.1 in. • III > 1.1 in. • Table 6.1

  9. CN Adjustment from Type II ARC • CN for CN for CN for • Type II Type I Type III • 65 45 82 • 70 51 85 • 75 57 88 • 80 63 91 • 85 70 94 • Table 6. 2 (From NEH Sec. 4 Table 10.1)

  10. Steps for Determining Runoff • 1. Compute RCN for drainage area • 2. Determine the growing season • 3. Determine the appropriate inundation or saturation period • 4. Obtain daily precipitation data • (30 years minimum)

  11. RO Steps continued • 4a. - Compute each runoff event • - determine maximum continuous runoff for given number of day for each year • - obtain 50% chance value by ranking

  12. Direct Precipitation • The Runoff (RO)= [P - (200/CN) - 2]2 • [P + (800/CN) - 8] • for example: RO = 0.17 in. • CN for Type II ARC = 85 • Trial & Error Equation is: • 0.17= [P - (200/85) - 2]2 • [P + (800/85) - 8]

  13. Average Precipitation Value • PARC I = 0.0 + P70 • PARC II = 1.4 + P85 • PARC III = 2.1 + P94 • P = (PARC I + PARC II + PARC III ) / 3

  14. Break

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