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Lab 13 - Predicting Discharge and Soil Erosion

Lab 13 - Predicting Discharge and Soil Erosion. Estimating Runoff Depth using the Curve Number method Land use or cover type Hydrologic condition Soil type Estimating Peak Runoff A function of the Time of Concentration, Tc The faster the runoff time => the bigger the peak

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Lab 13 - Predicting Discharge and Soil Erosion

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  1. Lab 13 - Predicting Discharge and Soil Erosion • Estimating Runoff Depth using the Curve Number method • Land use or cover type • Hydrologic condition • Soil type • Estimating Peak Runoff • A function of the Time of Concentration, Tc • The faster the runoff time => the bigger the peak • Estimating Erosion Losses using the Universal Soil Loss Equation • Precipitation • Topography • Soils • Land use or land cover • Treatment or conservation practices

  2. Example calculation: • 100 ha Piedmont watershed • Residual soils (impermeable B/C horizons) • 4-6% slopes • 150 mm rainfall event • What is total runoff (mm) and peak runoff rate (L/sec)?

  3. Curve Number Method

  4. Hydrologic group: C • CN: 70 • Total (direct runoff) = 70 mm

  5. Time of concentration: Depends on watershed area, CN, and slope

  6. Time of concentration = 1.5 hr

  7. Unit peak discharge = 1.2 L/sec/mm/ha

  8. Peak Discharge • Qp = Up · A · Q • Qp is peak runoff (L/s) • Up is unit peak (L/s/ha/mm) • a function of Tc • Tc is the time of concentration, or the time for water to runoff of the watershed • the faster the runoff time, the bigger the peak • A is watershed area (ha) • Q is runoff depth (mm) • From the curve number method 1.2 L/s/mm/ha x 100 ha x 70 mm = 8400 L/s

  9. Universal Soil Loss Equation • A = R · K · LS · C · P • A is soil loss in tons per acre per year • R is rainfall erosivity factor • K is soil erodibility factor • LS is length-slope (topographic) factor • C is the land use or land cover factor • P is the treatment or conservation practices factor

  10. Example Calculation: • Cecil sandy loam, 3% humus • Clarke Co. GA • 5% slope grade, 200’ long • Initial land use: undisturbed forest, • 95% leaf litter, 95% canopy Estimate soil loss (t/a/yr)

  11. Rainfall Erosivity Factor

  12. R = 275 K = 0.215 LS = 0.758

  13. P Factor: Use only when CONTOUR TILLAGE is specified Tillage parallel to contour lines: P=0.5 All other cases: P=1 C = 0.0005 P = 1 A = 275 x 0.215 x 0.758 x 0.0005 x 1 = 0.022 t/a/y

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