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

Drainage Design Manual for Maricopa County
Hydrology: Application
centration Point 2 using the arithmetically area-weighted value of C for all three
subareas and the total contributing drainage area at Concentration Point 2.
c. If the T c from subarea C is the longest, compute the total peak discharge using the
i for subarea C, the arithmetically area-weighted value of C for all three subareas
and the total contributing drainage area at Concentration Point 2.
The procedures for theTriangular Hydrograph Method are as follows:
1. Compute the peak discharge for each individual sub-basin using steps 1 through 5
from Section 9.2.1.
2. Plot triangular hydrographs for sub-basins A and B on a single sheet of graph paper
using the dimensionless triangular hydrograph shown in Figure 3.2 as the model. The
peak discharge occurs at time T c and the hydrograph time base is 2.67T c .
3. Add the hydrograph ordinates from sub-basins A and B to produce and plot a combined
hydrograph at CP 1.
4. Compute the travel time from CP 1 to CP 2 using the continuity equation or other
appropriate technique and hydraulic parameters for the conveyance path.
5. Plot the hydrograph for sub-basin C on a new piece of graph paper, starting at time =
0.0. Plot the hydrograph for CP 1 starting at time = travel time from CP 1 to CP 2.
6. Add the hydrograph ordinates from CP 1 and sub-basin C to produce and plot a combined
hydrograph at CP 2.
As an alternative to the above procedure, the DDMSW program may be used to calculate the
peak discharge at intermediate locations.
9.2.4 User Notes
1. The Rational Method is appropriate for watersheds less than 160 acres in size.
2. For drainage areas greater than 160 acres or for situations where hydrograph routing
is desired, the procedures described in Chapters 4 through 7 should be used.
3. The duration of T c should not be longer than 2 hours and normally it will be less than
1 hour.
4, The minimum duration of T c should not be less than 5 minutes, but is normally set to a
minimum of 10 minutes.
9-12 August 15, 2013