View
221
Download
0
Category
Preview:
Citation preview
February 2015 2WinTR-20 Sensitivity
Lesson Objectives
1. Identify the various WinTR-20 Input Parameters that affect the volume of runoff and peak discharge predictions.
2. Identify the relative sensitivity of WinTR-20 to its input parameters in predicting the peak and/or volume of runoff.
3. Identify the relative sensitivity of WinTR-20 to its input parameters in relation to channel routing.
February 2015 3WinTR-20 Sensitivity
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
February 2015 4WinTR-20 Sensitivity
WinTR-20 Watershed Input Variables
Drainage Area Runoff Curve Number (RCN) Time of Concentration (tc) Peak Rate Factor (PRF) of Dimensionless
Unit Hydrograph (DUH) Antecedent Runoff Condition (ARC)
February 2015 5WinTR-20 Sensitivity
WinTR-20 Rainfall Input Variables
Depth of Rainfall Rainfall Distribution (includes duration)
February 2015 6WinTR-20 Sensitivity
Effects of Variation in Drainage Area
Sensitivity to Drainage Area
0.70
0.80
0.90
1.00
1.10
1.20
1.30
0.6 0.8 1 1.2
% of Runoff Vol or Peak Compared to Unit
% o
f Dra
inag
e A
rea
Com
pare
d to
Uni
t
Volume Ratio
Peak Ratio
Linear Correlation
% 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).
February 2015 7WinTR-20 Sensitivity
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.
Sensitivity to RCN
0.00
0.50
1.00
1.50
2.00
2.50
60% 80% 100% 120% 140%% of RCN Compared to Unit (RCN 70)
% o
f R
un
off
Vo
l o
r P
ea
k C
om
pa
red
to U
nit Volume Ratio
Peak Ratio
LinearCorrelation
February 2015 8WinTR-20 Sensitivity
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.
Sensitivity to tc
0.500.600.700.800.901.001.101.201.30
60% 80% 100% 120% 140%
% of tc Compared to Unit (1.0 hr)
% o
f R
un
off
Vo
l or
Pe
ak
Co
mp
are
d t
o U
nit
Volume Ratio
Peak Ratio
Linear Correlation
February 2015 9WinTR-20 Sensitivity
Effects of Variation in Peak Rate Factor
% Change in PRF results in nearly similar change to predicted peak rate of runoff (no change in volume).
PRF is a watershed based response to excess rainfall assumed to be similar per inch of runoff.
Sensitivity to PRF
% of PRF Compared to 484
February 2015 10WinTR-20 Sensitivity
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).
February 2015 11WinTR-20 Sensitivity
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.
February 2015 12WinTR-20 Sensitivity
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.
Sensitivity to Rainfall Depth
0.50
0.75
1.00
1.25
1.50
70% 90% 110% 130%
% of Rainfall Compared to Unit (4in)
% o
f R
un
off
Vo
lum
e,
Pe
ak
Co
mp
are
d t
o U
nit
Volume Ratio
Peak Ratio
Linear Correlation
February 2015 13WinTR-20 Sensitivity
Effects of Variation in Rainfall Distribution
Design rainfall distributions normally set by location.
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.
February 2015 14WinTR-20 Sensitivity
Effects of Variation in Rainfall Distribution(Continued)
For this example: DA = 1.0 mi2, tc = 1 hr, RCN = 70, 4.0 inch 24 hr 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.
February 2015 15WinTR-20 Sensitivity
Effects of Variation in Rainfall Distribution(Continued)
For this example: DA = 1.0 mi2, tc = 1 hr, RCN = 70, 4.0 inch 24 hr Rainfall
Type NOAA A - Qp = 479 cfs Type NOAA B - Qp = 432 cfs Type NOAA C - Qp = 391 cfs Type NOAA D - Qp = 353 cfs
These rainfall distributions are used in 12 states and the District of Columbia covered in NOAA Atlas 14 Vol. 2.
February 2015 16WinTR-20 Sensitivity
Parameter Selection for Desired Change in WinTR-20 Runoff Volume
WinTR-20 Parameter to be Changed, Independent of Others
Desired Change in Runoff Volume (%)
-50% -25% -10% -5% +5% +10% +25% +50%
Required Change in Drainage Area -50% -25% -10% -5% +5% +10% +25% +50%
Required Change in Rainfall -26% -13% -5% -2.5% +2.5% +5% +12.5% +23%
Required Change in RCN -17% -8% -2% -1% +1% +2% +7% +13%
Required Change in Time of
Concentration N/C N/C N/C N/C N/C N/C N/C N/C
Required Change in PRF N/C N/C N/C N/C N/C N/C N/C N/C
N/C signifies, No Change possible to alter volume. This parameter does not effect volume prediction.
February 2015 17WinTR-20 Sensitivity
Parameter Selection for Desired Change in WinTR-20 Peak Runoff
WinTR-20 Parameter to be Changed, Independent of Others
Desired Change in Runoff Peak (%)
-50% -25% -10% -5% +5% +10% +25% +50%
Required Change in Drainage Area -50% -25% -10% -5% +5% +10% +25% +50%
Required Change in Rainfall -24% -12% -5% -2.50% +2.5% +5% +11% +21%
Required Change in RCN -13.5% -6% -2% -1% +1% +2% +5.5% +11%
Required Change in Time of
Concentration +150% +50% +15% +7% -6% -12% -26.5% -44%
Required Change in PRF -54% -29% -12% -6% +6% +13% +33% +72%
February 2015 18WinTR-20 Sensitivity
Combined Parameter Impacts
Assumed Normal Run DA = 1 mi2, RCN =70, tc = 1.0 hr, PRF = 484
Runoff Volume = 1.33”, Peak Rate = 437 cfs
Low Run DA = 1 mi2, RCN =63, tc = 1.25 hr, PRF = 300
Runoff Volume = 0.92”, Peak Rate = 148 cfs
High Run DA = 1 mi2, RCN =77, tc = 0.75 hr, PRF = 600
Runoff Volume = 1.81”, Peak Rate = 904 cfs
February 2015 19WinTR-20 Sensitivity
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.
February 2015 20WinTR-20 Sensitivity
WinTR-20 Channel and Reach Input Variables Selection of representative cross section Cross section rating table (slope and “n”) Channel length Floodplain length Shape of inflow hydrograph Base flow (if significant)
February 2015 21WinTR-20 Sensitivity
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
February 2015 22WinTR-20 Sensitivity
Two Inflow hydrographs
Red (higher) is the hydrograph for Tc = 0.5 hour. Green (lower) is the hydrograph for Tc = 1.0 hour.
February 2015 23WinTR-20 Sensitivity
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.
February 2015 24WinTR-20 Sensitivity
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.
February 2015 25WinTR-20 Sensitivity
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.
February 2015 26WinTR-20 Sensitivity
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.
Recommended