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WinTR-20 Sensitivity to Input Parameters

Understand the impact of WinTR-20 input parameters on runoff predictions and peak discharge. Learn how variations in watershed and rainfall variables affect modeling outcomes. Explore sensitivity to drainage area, curve number, time of concentration, and more.

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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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