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

Drainage Design Manual for Maricopa County
Hydrology: Indirect Methods
8.6 PROCEDURES
The following instructions should be followed as confidence checks on the validity of peak discharges
that are derived by analytic methods, (Rational Method or rainfall-runoff modeling).
These procedures are typically applied for floodplain delineation studies, dam safety designs and
studies, and where the hydrologic model results are to be used for defining high hazard areas or
for design of facilities used to provide protection in high flood risk areas. Watersheds with an
area of less than one square mile are exempt. The agency may require application of these procedures
for larger watersheds depending on the intended application.
A. Confidence Check using Unit Peak Discharge Curves:
1. For a given watershed of drainage area (A), in square miles, divide the 100-year primary
peak discharge estimate by A.
2. Plot the unit peak discharge on a copy of Figure 8.1. Note the location of the plotted
point in relation to the various curves in that figure.
B. Confidence Check using USGS Data for Arizona:
1. Calculate the 100-year peak discharge estimate by Equation (8.1)
2. Select Figure 8.3 or Figure 8.4 according to watershed drainage area size, and plot
the 100-year peak discharge estimate on a copy of that figure.
3. Using watershed drainage area as a guide, identify gaged watersheds of the same
approximate size from Table 8.1. Tabulate the peak discharge statistics and watershed
characteristics for those gaged watersheds by using the USGS report (Garrett
and Gellenbeck, 1991). Compare these to the computed peak discharge estimates
and watershed characteristics for the watershed of interest.
C. Confidence Check using Regional Regression Equations:
1. Determine the flood region (Figure 8.6).
2. Calculate the regression equation variables, such as mean basin elevation (ELEV) for
Region 12. This can be done by placing a transparent grid over the largest scale topographic
map available. The grid spacing should be selected such that at least 20
elevation points are sampled. The elevation at each grid point is determined and the
elevations are then averaged.
3. Check the drainage area using the appropriate scatter diagram to determine if the values
are in the “cloud of common values.” Proceed with the analysis regardless of the
8-20 August 15, 2013