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L-THIA
Impervious Input
Formula for calculating the
Curve Number (CN) value from percent impervious
cover:
From
simple area weighting,
If
n% is the percent impervious area
CNn%
is the CN value we are looking for
CN0%
is the CN value for the pervious area
98
(obtained from TR-55) is the CN value for the
impervious area
Then
CNn%
= CN0% + (n%/100) *
(98- CN0%)
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