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

Drainage Design Manual for Maricopa County
Hydrology: Indirect Methods
8
8 INDIRECT METHODS
TABLE OF CONTENTS
8 INDIRECT METHODS
8.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1
8.2 INDIRECT METHOD NO. 1 - UNIT PEAK DISCHARGE CURVES . . . . . . . . . . . . . 8-2
8.3 INDIRECT METHOD NO. 2 - USGS DATA FOR ARIZONA. . . . . . . . . . . . . . . . . . . 8-4
8.4 INDIRECT METHOD NO. 3 - REGIONAL REGRESSION EQUATIONS . . . . . . . . 8-12
8.5 APPLICATIONS AND LIMITATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-19
8.6 PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-20
8.1 GENERAL
The estimation of peak discharges by analytic methods (the Rational Method or by rainfall-runoff
modeling using the HEC-1 program) is based on various assumptions, and in the case of HEC-1
modeling, requires the correct input of numerous parameters. Therefore, the resulting peak discharges
that are computed by analytic methods should always be verified, to the extent possible,
to guard against erroneous design discharges that can result from questionable assumptions
and/or faulty model input.
Since the majority of discharge estimates are made for ungaged watersheds, usually only indirect
methods can be used to check the discharge estimates obtained from either the Rational
Method or rainfall-runoff modeling. When the watershed is gaged, or is near a gaging station, a
flood-frequency analysis can be performed and the results of that analysis can be used for
design or used to check the results from analytic methods. The results of flood-frequency analyses,
because of variability of flooding in both the time and space regime, and because of uncertainties
in the data and the analytic procedures, should also be checked by indirect methods.
True verification of design discharges cannot be made by any of the methods (analytic methods,
flood-frequency analyses, or indirect methods) because for none of these methods is there
“absolute assurance” that the discharges obtained are the “true” representations of the flood discharge
for a given frequency of flooding. However, the results of the various methods, when
compared against each other and when qualitatively evaluated, can provide a basis for either
acceptance or rejection of specific estimates of design discharges for watersheds in Maricopa
County.
In this chapter, three indirect methods are presented for “verifying” flood discharges that are
obtained by analytic methods.
August 15, 2013 8-1