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

Drainage Design Manual for Maricopa County
Hydrology: Application
NOTE: There may be slight differences in results when DDMSW is used to perform these
calculations due to numerical rounding.
Using the above procedure, the T c and i for each subbasin are:
Subbasin T c , min i, in/hr
S1 13 5.68
S2 10 6.37
S3 12 5.90
S4 10 6.37
5. Compute the peak discharge for each subbasin using Equation (3.1):
Subbasin S1:
Subbasin S2:
Subbasin S3:
Subbasin S4:
Q = ( 0.66) ( 5.68) ( 65.99) = 247 cfs
Q = ( 0.50) ( 6.37) ( 12.60) = 40 cfs
Q = ( 0.69) ( 5.90) ( 21.18) = 86 cfs
Q = ( 0.65) ( 6.37) ( 27.80) = 115 cfs
6. Compute the peak discharge for concentration point C1 using the Combined Watershed
Method.
The combined area of subbasins S1 and S2 is 78.59 acres.
The area-weighted C coefficient is:
C1:
( 0.66) ( 65.99) + ( 0.50) ( 12.60)
C w
= ------------------------------------------------------------------------- =
( 78.59)
0.63
The area-weighted K b is:
C1:
=
------------------------------------------------------------------------------- ( 0.096) ( 65.99) + ( 0.065) ( 12.60)
=
( 78.59)
0.091
Use the length, L, and slope, S, from subbasin S1 since both subbasins S1 and S2 join at
C1 and subbasin S1 has the longer T c flow path.
C1: L = 0.729 miles
C1: S = 473.0 feet/mile
August 15, 2013 9-21