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

Drainage Design Manual for Maricopa County
Hydrology: Indirect Methods
8.3 INDIRECT METHOD NO. 2 - USGS DATA FOR ARIZONA
The U.S. Geological Survey (USGS) provides streamflow and statistical data for 138 continuousrecord
streamflow-gaging stations and 176 partial-record gaging stations in Arizona (Garrett and
Gellenbeck, 1991). The streamflow data were analyzed by the USGS by Log-Pearson Type 3
(LP3) analyses and flood magnitude-frequency statistics are provided in that report along with
the maximum recorded discharge for each station. Figure 8.2 is a plot of the 100-year peak discharge
(from LP3 analyses) versus drainage area (for stations with drainage areas smaller than
2,000 square miles). Lines were fit to the data by least-squares of the log-transformed data. The
equation for the 100-year peak discharge (Q 100 ) line is:
Q 100
= 850A 0.54
(8.1)
where:
A is the drainage area in square miles.
Figure 8.2 also shows 75 percent tolerance limit lines about the 100-year discharge line from
Equation (8.1). The tolerance limits are a statistical measure of the spread of the data about that
line.
As an aid to using Figure 8.2, that figure is reproduced with larger drainage area scales in Figure
8.3 and Figure 8.4. Those larger scale plots of the data also show 75 percent tolerance limit lines
about the 100-year discharge line from Equation (8.1).
A listing of the data that was used to produce Figure 8.2 through Figure 8.4 is shown in Table 8.1.
This table includes USGS streamflow-gaging station numbers, the associated drainage areas
and the 100-year flood peak discharge estimates by LP3. Watershed characteristics for each of
these gaging stations are provided in the USGS report (Garrett and Gellenbeck, 1991). A map of
Arizona showing the locations of the gaging stations for this data compilation are shown in Figure
8.5.
8-4 August 15, 2013