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

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
At the end of the first 5 minutes of rainfall excess, the available runoff at the outlet of the
watershed is:
c
I 1 = ( A 1 R 1 ) × ----
Δt
where:
c = 60.5 cfs/acre-inch/minute
Δt = 5 minutes
I 1
= (8 acres)(0.10 inch)(60.5 cfs/acre-inch/minute)/(5 minutes)
= 9.7 cfs
At the end of 10 minutes the available runoff is:
I 2 = ( A 1 R 2 + A 2 R 1 )----
c Δt
=
60.5
[( 8) (.55) + ( 24) (.10)]
× ---------
5
=
82.3 cfs
At the end of 15 minutes the available runoff is:
c
I 3 = ( A 1 R 3 + A 2 R 2 + A 3 R 1 ) × ----
Δt
=
60.5
[( 8) (.30) + ( 24) (.55) + ( 38) (.10)]
× ---------
5
=
234.7 cfs
At the end of 20 minutes the available runoff is:
c
I 4 = ( A 1 R 4 + A 2 R 3 + A 3 R 2 + A 4 R 1 ) × ----
Δt
=
60.5
[( 8) (.15) + ( 24) (.30) + ( 38) (.55) + ( 32) (.10)]
× ---------
5
=
393.3 cfs
At the end of 25 minutes the available runoff is:
c
I 5 = ( A 1 R 5 + A 2 R 4 + A 3 R 3 + A 4 R 2 + A 5 R 1 ) × ----
Δt
60.5
= [( 8) ( 0) + ( 24) (.15) + ( 38) (.30) + ( 32) (.55) + ( 18) (.10)]
× ---------
5
August 15, 2013 5-7