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

Drainage Design Manual for Maricopa County
Hydrology: Application
9.2.2 Procedures for Volume Calculations
Volume calculations should be done by applying the following equation:
V
P
= C
A
12
where:
(3.3)
V = calculated volume in, acre-feet.
C = runoff coefficient from Table 3.2.
P = rainfall depth, in inches.
A = drainage area, in acres.
In the case of volume calculations for stormwater storage facility design, P equals the 100-year,
2-hour depth, in inches, as discussed in Section 2.2, and is determined from Figure A.56 of
Appendix A.1.
9.2.3 Procedures for the Multiple Basin Approach
The Rational Method can be used to compute peak discharges at intermediate locations within a
drainage area less than 160 acres in size. A typical application of this approach is a local storm
drain system where multiple subbasins are necessary to compute a peak discharge at each proposed
inlet location. Consider the schematic example watershed shown in Figure 9.2. A peak
discharge is needed for all three individual subareas, subareas A and B combined at Concentration
Point 1 and subareas A, B and C combined at Concentration Point 2.
There are two accepted methods for computing peak discharges for multiple basins using the
Rational Method. The first method is the traditional approach that relies upon combining the subbasin
areas into a single watershed, computing a new T c , an arithmetically area-weighted value
of C for combined sub-basins, and then computing the peak discharge. This approach is
referred to as the “Combined Watershed Method.” The second method is the “Triangular Hydrograph
Method.” For this method, a triangular hydrograph is created for each sub-basin where the
time-to-peak is assumed equal to T c and the hydrograph time base is equal to 2.67T c , as shown
on Figure 3.2. Referring to Figure 9.2, the ordinates of hydrographs A and B at CP 1 are added
to obtain the total flow hydrograph. That hydrograph is then lagged downstream to CP 2 by the
estimated travel time in the roadway, pipe, or channel. The lagged hydrograph is then added to
the sub-basin C hydrograph to obtain the peak discharge at CP 2. The triangular hydrograph
method is incorporated in the DDMSW computer program, but the combined hydrograph method
is not. The combined hydrograph method is intended for use by engineers/hydrologists without
access to a computer and DDMSW. Either method may be used but the engineer/hydrologist
9-10 August 15, 2013