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CE 374 K – Hydrology

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  1. CE 374 K – Hydrology Runoff Processes Daene C. McKinney

  2. Watershed • Watershed • Area draining to a stream • Streamflow generated by water entering surface channels • Affected by • Physical, vegetative, and climatic features • Geologic considerations • Stream Patterns • Dry periods • Flow sustained from groundwater (baseflow) http://www.epa.gov/owow/watershed/whatis.html

  3. Atmospheric Moisture Snow Rain Energy Evaporation Interception Throughfall and Stem Flow Snowpack Snowmelt Watershed Boundary Surface Pervious Impervious Infiltration Evapotranspiration Soil Moisture Percolation Overland Flow Groundwater Groundwater Flow Evaporation Streams and Lakes Channel Flow Runoff Streamflow • Atmospheric Water • Evapotranspiration • Precipitation • Subsurface Water • Infiltration • Groundwater • Surface Water

  4. Basin Lag Centroid of Precipitation Peak Time of Rise Recession Limb Rising Limb Discharge, Q Inflection Point Baseflow Recession Baseflow Recession Time Beginning of Direct Runoff End of Direct Runoff Streamflow Hydrograph

  5. Discharge, Q Baseflow Recession Time Baseflow Separation • No inflow added to groundwater - depletion (recession) curve • Continuity equation

  6. Discharge, Q Direct Runoff B A Baseflow Time Baseflow Separation Techniques • Straight – line method • Draw a horizontal line segment (A-B) from beginning of runoff to intersection with recession curve

  7. Discharge, Q D B A C Time Baseflow Separation Techniques • Fixed Base Method • Draw line segment (A – C) from baseflow recession to a point directly below the hydrograph peak • Draw line segment (C-D) connecting a point N time periods after the peak Direct Runoff Baseflow

  8. Discharge, Q E B A C Time Baseflow Separation Techniques • Variable Slope Method • Draw line segment (A-C) forward from baseflow recession to a point directly below the hydrograph peak • Draw line segment (B-E) backward from baseflow recession to a point directly below the inflection point • Draw line segment (C-E) Direct Runoff Baseflow

  9. Abstraction (Losses) EstimationPhi – Index Method • Excess (effective) rainfall • Rainfall that is not retained or infiltrated • Becomes direct runoff • Excess rainfall hyetograph (excess rainfall vs time) • Abstraction (losses) • Difference between total and excess rainfall hyetographs • Phi – Index • Constant rate of abstraction yielding excess rainfall hyetograph with depth equal to depth of direct runoff

  10. Example Have precipitation and streamflow data, need to estimate losses Time Observed Rain Flow in cfs 8:30 203 9:00 0.15 246 9:30 0.26 283 10:00 1.33 828 10:30 2.2 2323 11:00 0.2 5697 11:30 0.09 9531 12:00 11025 12:30 8234 1:00 4321 1:30 2246 2:00 1802 2:30 1230 3:00 713 3:30 394 4:00 354 4:30 303 No direct runoff until after 9:30 And little precip after 11:00 Basin area A = 7.03 mi2

  11. Example (Cont.) • Estimate baseflow (straight line method) • Constant = 400 cfs baseflow

  12. Example (Cont.) • Calculate Direct Runoff Hydrograph • Subtract 400 cfs Total = 43,550 cfs

  13. Example (Cont.) • Compute volume of direct runoff • Compute depth of direct runoff

  14. Example (Cont.) • Neglect all precipitation intervals that occur before the onset of direct runoff (before 9:30) • Select Rm as the precipitation values in the 1.5 hour period from 10:00 – 11:30

  15. Example (Cont.) fDt=0.27

  16. SCS Curve Number Method • Soil Conservation Service(SCS) Curve Number (CN) model estimates precipitation excess as a function of cumulative precipitation, soil cover, land use, and antecedent moisture • SCS developed the method for small basins (< 400 sq. mi.) to "before" and "after" hydrologic response from events. • Classify soils (60 or 70 types) into four hydrologic soil groups • Method is simple enough to be used by people that have little experience with hydrology. • Converts basin storage into something simpler and more manageable (a “curve number” CN)

  17. Precipitation Time Abstractions – SCS Method • In general • After runoff begins • Potential runoff • SCS Assumption • Solve for Rainfall Excess

  18. SCS Method (Cont.) • Surface • Impervious: CN = 100 • Natural: CN < 100 • Experiments showed • So

  19. SCS Method (Cont.) • S and CN depend on antecedent rainfall conditions • Normal conditions, AMC(II) • Dry conditions, AMC(I) • Wet conditions, AMC(III)

  20. SCS Method (Cont.) • SCS Curve Numbers depend on soil conditions

  21. Example - SCS Method - 1 • Rainfall: 5 in. • Area: 1000-ac • Soils: • Class B: 50% • Class C: 50% • Antecedent moisture: AMC(II) • Land use • Residential • 40% with 30% impervious cover • 12% with 65% impervious cover • Paved roads: 18% with curbs and storm sewers • Open land: 16% • 50% fair grass cover • 50% good grass cover • Parking lots, etc.: 14%

  22. Example (SCS Method – 1, Cont.)

  23. Example (SCS Method – 1 Cont.) • Average AMC • Wet AMC

  24. Example (SCS Method – 2) • Given P, CN = 80, AMC(II) • Find: Cumulative abstractions and excess rainfall hyetograph

  25. Example (SCS Method – 2) • Calculate storage • Calculate initial abstraction • Initial abstraction removes • 0.2 in. in 1st period (all the precip) • 0.3 in. in the 2nd period (only part of the precip) • Calculate continuing abstraction

  26. Example (SCS method – 2) • Cumulative abstractions can now be calculated

  27. Example (SCS method – 2) • Cumulative excess rainfall can now be calculated • Excess Rainfall Hyetograph can be calculated

  28. Time of Concentration • Different areas of a watershed contribute to runoff at different times after precipitation begins • Time of concentration • Time at which all parts of the watershed begin contributing to the runoff from the basin • Time of flow from the farthest point in the watershed