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

Drainage Design Manual for Maricopa County
Hydrology: Application
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. Verification with 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. Verification with Regional Regression Equations:
1. Determine the flood region (Figure 8.6).
2. If the basin(s) fall within Region 12 on Figure 8.6, then calculate the mean basin elevation
(ELEV). 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
outcome, but clearly note if the variable values are not within the “cloud of common
values.”
4. Calculate the peak discharge estimates using the applicable regression equations for
the flood region within which the project site is located.
5. Plot the 100-year peak discharge estimate on a copy of the appropriate Q 100 data
points and 100-year peak discharge relation graph (Figure 8.8 or Figure 8.9).
D. For all three Indirect Methods:
1. Quantitatively and qualitatively analyze the results of the primary and the secondary
peak discharge estimates. Address watershed characteristics that may explain differences
between the primary and secondary estimates.
2. Prepare a summary of results by all methods and a qualitative evaluation of the
results.
9-66 August 15, 2013