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Runoff Calculations Predicting Runoff Depth, Volume and Peak Flow

Runoff Calculations Predicting Runoff Depth, Volume and Peak Flow. Environmental Hydrology Lecture 11. Precipitation rate, p (mm/hr). Runoff Production A Refresher. Horton overland flow. Robert E. Horton Image source: American Geophysical Union. p > i  overland flow.

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Runoff Calculations Predicting Runoff Depth, Volume and Peak Flow

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  1. Runoff CalculationsPredicting Runoff Depth, Volume and Peak Flow Environmental Hydrology Lecture 11

  2. Precipitation rate, p (mm/hr) Runoff ProductionA Refresher • Horton overland flow Robert E. Horton Image source: American Geophysical Union p > i  overland flow Infiltration rate, i (mm/hr) time

  3. Infiltration The HydrographA Refresher Ward & Trimble, Fig. 5.11

  4. Predicting Runoff and Storm Flow • The Rational Equation (the SIMPLE model) • Peak runoff discharge • The Curve Number concept • Storm runoff depth  runoff volume • The Graphical Peak Discharge method • Peak runoff discharge • Empirical Regression models • e.g. USGS National Flood Frequency model

  5. The Rational Method • Where… • q is peak flow in ft3/sec • C in an empirical coefficient • I is the rainfall intensity (in/h) • A is the catchment area (acres) • The 1.008 factor is… • 1 in/h * 1ft/12in *1h/3600sec * 43,560ft2/acre

  6. Rational Coefficients

  7. Concept: Soil Hydrogroups • Group A: high infiltration (>0.3 in/h), low runoff • Group B: moderate infiltration when wet (0.15 to 0.30 in/h), moderate runoff • Group C: slow infiltration when wet (0.05 to 0.15 in/h), high runoff • Group D: very low infiltration when wet (<0.05 in/h), very high runoff

  8. Rational Method Assumptions and Limitations • Rainfall is uniform • Time of concentration (tc) is the time required for water to get from the most distant point in the watershed to the point of observation • Peak runoff ≈ Peak rainfall intensity lagged by tc • Frequency of runoff ≈ frequency of rainfall • Best for small watersheds (<1 mile2)

  9. CN = curve number Derivation of the CN equation Where… P = precipitation (in) Ia = initial abstraction (in) F = cumulative infiltration (in) Q = direct runoff (in) Mass Balance Proportionality Substitution S=f(P-Ia, Q) S = max. potential retention (in) λ = coefficient, usually 0.2 Proportionality Substitution TR-55 Manual, see also Mishra et al. 2006

  10. Concept: Curve Numbers TR-55 Manual

  11. Curve Numbers for Cover Types Ward and Trimble, after NRCS

  12. Concept: Antecedent Moisture Content(based on previous 5-day P) • AMC-1 (Dry) • Dormant season P <0.5 in • Growing Season P <1.4 in • AMC-2 (Average) • Dormant season 0.5<P <1.1 in • Growing Season 1.4<P <2.1 in • AMC-3 (Wet) • Dormant season P >1.1 in • Growing Season P >2.1 in

  13. Curve Number Adjustments for AMC Ward and Trimble, after NRCS

  14. Curve Number (TR-55)Assumptions and Limitations • Describes average and not extreme events • Does not explicitly account for storm duration or intensity • The term Ia is ill-defined and has important influences on the derivation of CN’s • Does not deal with snow or rain on frozen ground • Should not be used for storms <0.5 in • Does not explicitly deal with groundwater effects • Don’t use if the weighted CN is <40, inaccurate

  15. The Graphical Peak Discharge Method Where… • q is the peak runoff (ft3/sec) • qu is the unit peak discharge (ft3/sec per square mile per inch of rain or csm/in) • A is area (square miles) • Q is the total runoff depth (from CN, in inches) • F is a ponding adjustment factor Can be used to relate Q from CN approach to q. Essentially relates storm volume to storm peak q.

  16. The “graphs” of the Graphical Method

  17. The Graphical MethodAssumptions and Limitations • Homogenous land cover/use in catchment • Only one main stream, or an balance network • “ponding” is off-stream, not in-stream • Need some idea of Ia • Need tc = f(hydraulic length, maximum retention [S], land slope, return interval)

  18. Empirical Regression Models • Where • DA = drainage area • S = channel bed slope • P = reference precipitation (e.g. 2y/24h) • BDF = an index of development • IC = percent impervious cover • U = indices of other land uses • STR = percent lakes, ponds, wetlands • E = elevation effect • Qft = the reference discharge event (e.g. 2y/24h) • X, a, b, c,… are fitted coefficients This equation is used by FEMA for national flood forecasting (See NFF model)

  19. Later in Lab:Reconstructing Whole Hydrographs • Unit Hydrographs • Time-area method

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