Win TR20 Sensitivity to Input Parameters Win TR20

Win. TR-20 Sensitivity to Input Parameters Win. TR-20 Sensitivity March 2009 1

Lesson Objectives 1. Identify the various Win. TR-20 Input Parameters that affect the volume of runoff and peak discharge predictions. 2. Identify the relative sensitivity of Win. TR-20 to its input parameters in predicting the peak and/or volume of runoff. 3. Identify the relative sensitivity of Win. TR -20 to its input parameters in relation to channel routing. Win. TR-20 Sensitivity March 2009 2

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

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

Win. TR-20 Rainfall Input Variables n Depth of Rainfall n Rainfall Distribution (includes duration) Win. TR-20 Sensitivity March 2009 5

Effects of Variation in Drainage Area n % Change in DA results in comparable change to predicted volume and peak of runoff. n Be sure DA is being properly identified (be aware of non-contributing areas). Win. TR-20 Sensitivity March 2009 6

Effects of Variation in RCN n % Change in RCN results in exaggerated change to predicted volume and peak of runoff. n RCN can be influenced by stage of vegetal growth and/or antecedent rainfall at time of storm event. Win. TR-20 Sensitivity March 2009 7

Effects of Variation in tc n % Change in tc results in decreased change to predicted peak rate of runoff (no change in volume). n A decrease in tc results in an increase in predicted peak discharge. Win. TR-20 Sensitivity March 2009 8

Effects of Variation in Unit Peak Factor n % Change in UPF results in nearly similar change to predicted peak rate of runoff (no change in volume). n UPF is a watershed based response to excess rainfall assumed to be similar per inch of runoff. Win. TR-20 Sensitivity March 2009 9

Effects of Variation in Antecedent Runoff Condition (ARC) n ARC values of 1 or 3 alter the RCN selected for assumed ARC 2 conditions. n ARC 2 is normally assumed for design. n ARC 1 can be used to help calibrate for a known “drought” condition prior to the target storm event (not necessarily accurate). n ARC 3 can be used to help calibrate for a known “saturated soil” condition prior to the target storm event (not necessarily accurate). Win. TR-20 Sensitivity March 2009 10

ARC Adjustments (Continued) n For this example: DA = 1. 0 mi 2, tc = 1 hr, RCN = 70, 4. 0 inch 24 hr Type II Rainfall n n n 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 n Win. TR-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. Win. TR-20 Sensitivity March 2009 11

Effects of Variation in Rainfall Depth n % Change in Rainfall Depth results in exaggerated change to predicted volume and peak of runoff. n Be sure that the actual Rainfall that has occurred and is being calibrated to is properly identified for the entire watershed. Win. TR-20 Sensitivity March 2009 12

Effects of Variation in Rainfall Distribution n Design rainfall distributions normally set by criteria (e. g. Type I, IA, II, or III). n Can attempt to calibrate to a historical rainfall event of known varying intensity (recording rain gage). n Rainfall distribution alone (not depth) only effects the rate of runoff, not the volume. Win. TR-20 Sensitivity March 2009 13

Effects of Variation in Rainfall Distribution (Continued) n For this example: DA = 1. 0 mi 2, tc = 1 hr, RCN = 70, 4. 0 inch 24 hr Type II Rainfall n n Type II - Qp = 437 cfs Type I - Qp = 221 cfs Type IA - Qp = 106 cfs Type III - Qp = 383 cfs n Win. TR-20 peaks are very sensitive to selection of rainfall distribution. Calibrate with the best known rainfall distribution. Win. TR-20 Sensitivity March 2009 14

Parameter Selection for Desired Change in Win. TR-20 Runoff Volume Desired Change in Runoff Volume (%) Win. TR-20 Parameter to be Changed, Independent of Others -50% -25% -10% -5% +10% +25% +50% Required Change in Drainage Area -50% -25% -10% -5% +10% +25% +50% Required Change in Rainfall -26% -13% -5% -2. 5% +5% +12. 5% +23% Required Change in RCN -17% -8% -2% -1% +2% +7% +13% Required Change in Time of Concentration N/C N/C Required Change in Unit Peak Factor N/C N/C N/C signifies, No Change possible to alter volume. This parameter does not effect volume prediction. Win. TR-20 Sensitivity March 2009 15

Parameter Selection for Desired Change in Win. TR-20 Peak Runoff Desired Change in Runoff Peak (%) Win. TR-20 Parameter to be Changed, Independent of Others -50% -25% -10% -5% +10% +25% +50% Required Change in Drainage Area -50% -25% -10% -5% +10% +25% +50% Required Change in Rainfall -24% -12% -5% -2. 50% +2. 5% +11% +21% Required Change in RCN -13. 5% -6% -2% -1% +2% +5. 5% +11% Required Change in Time of Concentration +150% +15% +7% -6% -12% -26. 5% -44% -54% -29% -12% -6% +13% +33% +72% Required Change in Unit Peak Factor Win. TR-20 Sensitivity March 2009 16

Combined Parameter Impacts n Assumed Normal Run n DA = 1 mi 2, RCN =70, tc = 1. 0 hr, UPF = 484 n Runoff Volume = 1. 33”, Peak Rate = 437 cfs n Low Run n DA = 1 mi 2, RCN =63, tc = 1. 25 hr, UPF = 300 n Runoff Volume = 0. 92”, Peak Rate = 148 cfs n High Run n DA = 1 mi 2, RCN =77, tc = 0. 75 hr, UPF = 600 n Win. TR-20 Sensitivity Runoff Volume = 1. 81”, Peak Rate = 904 cfs March 2009 17

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

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

Win. TR-20 Channel Routing Sensitivity Test n Trapezoidal cross section, BW = 15, SS = 2: 1 n Slope = 0. 001 and 0. 004 n Manning n = 0. 03, 0. 04, 0. 05 n Channel length, 0. 8 to 1. 2 mile n Inflow hydrograph, DA = 1, CN = 80, Tc = 0. 5 and 1. 0, RF = 3. 2 inches, Type II storm n Base flow = 0. 0 n 60 Win. TR-20 runs Win. TR-20 Sensitivity March 2009 20

Two Inflow hydrographs n Red (higher) is the hydrograph for Tc = 0. 5 hour. n Green (lower) is the hydrograph for Tc = 1. 0 hour. Win. TR-20 Sensitivity March 2009 21

Effects of Variation in Length and “n” n % Change in length results in less change to predicted peak outflow. n % Change in Manning “n” results in less change to predicted peak outflow. Win. TR-20 Sensitivity March 2009 22

Effects of Variation in Length and “n” n % Change in length and “n” results in less change to predicted peak outflow. n Length and “n” less sensitive for Tc = 1. 0 hydrograph. Win. TR-20 Sensitivity March 2009 23

Effects of Variation in Length and “n” n % Change in length and “n” results in less change to predicted peak outflow. n Results for steep slope are less sensitive. Win. TR-20 Sensitivity March 2009 24

Effects of Variation in Length and “n” n % Change in length and “n” results in less change to predicted peak outflow. n Results for Tc = 1. 0 hydrograph are even less. Win. TR-20 Sensitivity March 2009 25

Porcupine Mountains State Park, Michigan Win. TR-20 Sensitivity March 2009 26

Questions? ? ? Win. TR-20 Sensitivity March 2009 27

The End Win. TR-20 Sensitivity March 2009 28