February 2015 1WinTR-20 Sensitivity

WinTR-20 Sensitivity to Input Parameters

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

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are

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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.

February 2015 27WinTR-20 Sensitivity

Porcupine Mountains State Park, Michigan

February 2015 28WinTR-20 Sensitivity

Questions???

February 2015 29WinTR-20 Sensitivity

The End