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WinTR-20 Sensitivity to Input Parameters

WinTR-20 Sensitivity to Input Parameters. Lesson Objectives. Identify the various WinTR-20 Input Parameters that affect the volume of runoff and peak discharge predictions.

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WinTR-20 Sensitivity to Input Parameters

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  1. WinTR-20 Sensitivity to Input Parameters March 2009

  2. Lesson Objectives • Identify the various WinTR-20 Input Parameters that affect the volume of runoff and peak discharge predictions. • Identify the relative sensitivity of WinTR-20 to its input parameters in predicting the peak and/or volume of runoff. • Identify the relative sensitivity of WinTR-20 to its input parameters in relation to channel routing. March 2009

  3. WinTR-20 Hydrology Model • Predicts Volume of Runoff • Predicts Peak Rate of Runoff • Predicts Entire Hydrograph of Runoff • Based on Watershed and Rainfall Characteristics Modeled as Input Parameters • Changes to Input Parameters Will Change the Volume and Rate of Runoff Predicted March 2009

  4. WinTR-20 Watershed Input Variables • Drainage Area • Runoff Curve Number (RCN) • Time of Concentration (tc) • Unit Peak Factor (UPF) of Dimensionless Unit Hydrograph (DUH) • Antecedent Runoff Condition (ARC) March 2009

  5. WinTR-20 Rainfall Input Variables • Depth of Rainfall • Rainfall Distribution (includes duration) March 2009

  6. Effects of Variation in Drainage Area • % Change in DA results in comparable change to predicted volume and peak of runoff. • Be sure DA is being properly identified (be aware of non-contributing areas). March 2009

  7. Effects of Variation in RCN • % Change in RCN results in exaggerated change to predicted volume and peak of runoff. • RCN can be influenced by stage of vegetal growth and/or antecedent rainfall at time of storm event. March 2009

  8. Effects of Variation in tc • % Change in tc results in decreased change to predicted peak rate of runoff (no change in volume). • A decrease in tc results in an increase in predicted peak discharge. March 2009

  9. Effects of Variation in Unit Peak Factor • % Change in UPF results in nearly similar change to predicted peak rate of runoff (no change in volume). • UPF is a watershed based response to excess rainfall assumed to be similar per inch of runoff. March 2009

  10. Effects of Variation inAntecedent Runoff Condition (ARC) • ARC values of 1 or 3 alter the RCN selected for assumed ARC 2 conditions. • ARC 2 is normally assumed for design. • ARC 1 can be used to help calibrate for a known “drought” condition prior to the target storm event (not necessarily accurate). • ARC 3 can be used to help calibrate for a known “saturated soil” condition prior to the target storm event (not necessarily accurate). March 2009

  11. ARC Adjustments (Continued) • For this example: DA = 1.0 mi2, tc = 1 hr, RCN = 70, 4.0 inch 24 hr Type II Rainfall • ARC 2 – (RCN 70), Qv = 1.33”, Qp = 437 cfs • ARC 1 – (RCN 51), Qv = 0.37”, Qp = 65 cfs • ARC 3 – (RCN 85), Qv = 2.46”, Qp = 874 cfs • WinTR-20 results are very sensitive to changes in ARC. Be sure that assumed change is appropriate or alter RCN within ARC 2 conditions for finer adjustment. March 2009

  12. Effects of Variation in Rainfall Depth • % Change in Rainfall Depth results in exaggerated change to predicted volume and peak of runoff. • Be sure that the actual Rainfall that has occurred and is being calibrated to is properly identified for the entire watershed. March 2009

  13. Effects of Variation in Rainfall Distribution • Design rainfall distributions normally set by criteria (e.g. Type I, IA, II, or III). • Can attempt to calibrate to a historical rainfall event of known varying intensity (recording rain gage). • Rainfall distribution alone (not depth) only effects the rate of runoff, not the volume. March 2009

  14. Effects of Variation in Rainfall Distribution(Continued) • For this example: DA = 1.0 mi2, tc = 1 hr, RCN = 70, 4.0 inch 24 hr Type II Rainfall • Type II - Qp = 437 cfs • Type I - Qp = 221 cfs • Type IA - Qp = 106 cfs • Type III - Qp = 383 cfs • WinTR-20 peaks are very sensitive to selection of rainfall distribution. Calibrate with the best known rainfall distribution. March 2009

  15. Parameter Selection for Desired Change in WinTR-20 Runoff Volume N/C signifies, No Change possible to alter volume. This parameter does not effect volume prediction. March 2009

  16. Parameter Selection for Desired Change in WinTR-20 Peak Runoff March 2009

  17. Combined Parameter Impacts • Assumed Normal Run • DA = 1 mi2, RCN =70, tc = 1.0 hr, UPF = 484 • Runoff Volume = 1.33”, Peak Rate = 437 cfs • Low Run • DA = 1 mi2, RCN =63, tc = 1.25 hr, UPF = 300 • Runoff Volume = 0.92”, Peak Rate = 148 cfs • High Run • DA = 1 mi2, RCN =77, tc = 0.75 hr, UPF = 600 • Runoff Volume = 1.81”, Peak Rate = 904 cfs March 2009

  18. WinTR-20 Channel Routing Model • Predicts hydrograph (including peak) at downstream end of reach. • Based on cross section and reach characteristics modeled as input parameters. • Changes to input parameters will change the peak discharge and hydrograph shape predicted at the end of the reach. March 2009

  19. WinTR-20 Channel and Reach Input Variables • Selection of representative cross section • Cross section rating table (slope and “n”) • Channel length • Flood plain length • Shape of inflow hydrograph • Base flow (if significant) March 2009

  20. WinTR-20 Channel Routing Sensitivity Test • Trapezoidal cross section, BW = 15, SS = 2:1 • Slope = 0.001 and 0.004 • Manning n = 0.03, 0.04, 0.05 • Channel length, 0.8 to 1.2 mile • Inflow hydrograph, DA = 1, CN = 80, Tc = 0.5 and 1.0, RF = 3.2 inches, Type II storm • Base flow = 0.0 • 60 WinTR-20 runs March 2009

  21. Two Inflow hydrographs • Red (higher) is the hydrograph for Tc = 0.5 hour. • Green (lower) is the hydrograph for Tc = 1.0 hour. March 2009

  22. Effects of Variation in Length and “n” • % Change in length results in less change to predicted peak outflow. • % Change in Manning “n” results in less change to predicted peak outflow. March 2009

  23. Effects of Variation in Length and “n” • % Change in length and “n” results in less change to predicted peak outflow. • Length and “n” less sensitive for Tc = 1.0 hydrograph. March 2009

  24. Effects of Variation in Length and “n” • % Change in length and “n” results in less change to predicted peak outflow. • Results for steep slope are less sensitive. March 2009

  25. Effects of Variation in Length and “n” • % Change in length and “n” results in less change to predicted peak outflow. • Results for Tc = 1.0 hydrograph are even less. March 2009

  26. Porcupine Mountains State Park, Michigan March 2009

  27. Questions??? March 2009

  28. The End March 2009

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