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

Drainage Design Manual for Maricopa County
Hydrology: Indirect Methods
8.5 APPLICATIONS AND LIMITATIONS
The three indirect methods can be applied to any watershed gaged or ungaged in Maricopa
County. Limitations exist for the use of the Regional Regression Equations based on values of
the watershed characteristics as compared to the values of watershed characteristics that were
used to derive these regional regression equations. The interpretation and evaluation of the
results of these methods must be conducted with awareness of several factors.
1. It must be noted that these are empirical methods and the results are only applicable
to watersheds that are hydrologically similar to the database used to derive the particular
method.
2. The majority of the data in all three of these methods are for undeveloped, unregulated
watersheds. Urbanized watersheds can have significantly higher discharges
than the results that are predicted by any of these methods.
3. These methods (other than envelope curves) produce discharge values that are statistically
based averages for watersheds in the database. Conditions can exist in any
watershed that would produce flood discharges, either larger than or smaller than,
those indicated by these methods. Watershed characteristics that should be considered
when comparing the results of indirect methods to results by analytic methods
and/or flood-frequency analysis are:
a. The occurrence and extent of rock outcrop in the watershed.
b. Watershed slopes that are either exceptionally flat or steep.
c. Soil and vegetation conditions that are conducive to low rainfall losses, such
as clay soils, thin soil horizons underlain by rock or clay layers, denuded
watersheds (forest and range fires), and disturbed land.
d. Soil and vegetation conditions that are conducive to high rainfall losses, such
as sandy soil, tilled agricultural land, and irrigated turf.
e. Land-use, especially urbanization, but also mining, large scale construction
activity, and over-grazing.
f. Transmission losses that may occur in the watercourses.
g. The existence of distributary flow areas.
h. Upstream water regulation or diversion.
August 15, 2013 8-19