Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Application
Subbasin ID
IA inches
S6 0.27
S7 0.24
S8 0.21
S9 0.27
S10 0.30
S11 0.17
4. Compute the arithmetically area-weighted percent impervious (RTIMP) for each subbasin for
natural conditions using the data in Table 9.7:
Subbasin S1:
A T
=
3,
480.4 acres
( 5.6) ( 0) + ( 873.3 + 167.2) ( 15) + ( 1723.5) ( 35)+ ( 50.6 + 2.4 + 137.1) ( 30)
+ ( 3.0 + 67.3 + 340.3 + 72.5) ( 60) + ( 36.0 + 1.6) ( 50)
RTIMP N
= -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
3,
480.4
RTIMP N =
112 -------------------- , 499
=
3,
480.4
32%
Using the above procedure, the area-weighted RTIMP for each subbasin is shown in the
table in Step 5.
5. Compute the arithmetically area-weighted RTIMP for each subbasin for developed conditions:
Subbasin S3:
A D
=
2,
113.3 acres
( 106.9) ( 80) + ( 742.5) ( 15) + ( 81.3) ( 30) + ( 1,
103.1)
( 5) + ( 79.6) ( 0)
RTIMP D = ----------------------------------------------------------------------------------------------------------------------------------------------------------------------
2,
113.3
RTIMP
27,
644
D
= ------------------ = 13 %
2,
113.3
August 15, 2013 9-31