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

Drainage Design Manual for Maricopa County
Hydrology: Application
LL
---------- ca
S 0.5
=
(
------------------------------
4.59) ( 2.30)
= 0.66
255 0.5
b. Select a value for K n
From Appendix D, Section 2 for mountain and foothill watersheds, the Santa Anita
Creek and Medicine Bow River watersheds were found to have similar physical characteristics
to Subbasin S1. The K n values for those watersheds are 0.053 and
0.0534, respectively with lagtimes of 1.10 and 0.89 hours, respectively. Comparison
of the K n values for these two watersheds to the general values of K n for the Phoenix
Mountain S-graph, provided in Table 5.6, indicate that a value for K n of 0.053 is appropriate.
K n = 0.053
c. Compute the lag time using Equation (5.11)
The source of the K n values for the two similar watersheds is unknown, therefore use
the Corps of Engineers version of the lag equation.
C = 24K n
= ( 24) ( 0.053) = 1.272
m = 0.38
Lag
C LL ca m
= ----------
= 1.272( 0.66) 0.38 = 1.09 hours
S 0.5
The lag of 1.09 hours compares favorably to the lag times of the similar watersheds
used for the selection of K n .
d. Compute Q ult using Equation (5.10)
Q ult = -------------------
645.33A
Δt
Δt = 0.15 lag = ( 0.15) ( 1.09) = 0.164 hours, therefore
use
Δt =
10 minutes
August 15, 2013 9-49