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Parameter Sensitivity in Hydrologic Modeling - page 122 / 163

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TxDOT IDF (Intensity-Duration-Frequency) curves and an alternating block cumulative precipitation hyetograph for Travis county and a three-hour design storm. Equation 5.4 and Table 5.12 are used to calculate the cumulative rainfall for a three-hour storm in six-minute increments for each sub-basin. A six-minute increment is chosen because this is the value TxDOT used.

I=

a (T d +b)c

Equation 5.4

a

56

69

77

87

91

103

b

8.1

8.6

8.6

8.6

8.6

8.1

c

0.796

0.780

0.775

0.766

0.751

0.752

Table 5.12: TxDOT IDF (Intensity-Duration-Frequency) Curves

100 Years

50 Years

25 Years

10 Years

5 Years

Return Period

2 Years

5.2.2.2 Lag Time Variation

The next step was to determine the discharge elasticity from changes in

lag time.

This was done by proportionally changing the lag time values for each

of the sub-basins in the HEC-1 model.

Table 5.13 gives the original SCS lag

value for each sub-basin, followed by 5%, 10%, and 20% increases in lag time,

then 5%, 10%, and 20% decreases in lag time.

112

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