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

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
=
416.2 cfs
Notice that, for this example, all incremental rainfalls equal 0.0 from R 5 onward.
At the end of 30 minutes the available runoff is:
c
I 6 = ( A 3 R 4 + A 4 R 3 + A 5 R 2 ) × ----
Δt
=
60.5
[( 38) (.15) + ( 32) (.30) + ( 18) (.55)]
× ---------
5
=
304.9 cfs
At the end of 35 minutes the available runoff is:
c
I 7 = ( A 4 R 4 + A 5 R 3 ) × ----
Δt
=
60.5
[( 32) (.15) + ( 18) (.30)]
× ---------
5
=
123.4 cfs
At the end of 40 minutes the available runoff is:
c
I 8 = ( A 5 R 4 ) × ----
Δt
=
60.5
[( 18) (.15)]
× ---------
5
=
32.7 cfs
After 45 minutes (rainfall excess of 20 minutes plus travel time of 25 minutes) the available
runoff is:
I 9 = 0 cfs
The translation hydrograph (I i ) is shown in Figure 5.3(d). This theoretical hydrograph has
the correct volume of runoff from the watershed, however it does not reflect the effects of
routing through the watershed. The translation hydrograph is then routed and averaged
using Equation (5.2) through Equation (5.4) resulting in the final runoff hydrograph. For
example, assume that R = 15 minutes, and the runoff hydrograph is shown in Figure
5.3(d).
5-8 August 15, 2013