FLOOD07236

• County: Douglas
• County: Arapahoe
• Community: Greenwood Village, Aurora
• Stream Name: Cherry Creek
• Basin: South Platte
• Title: Probable Maximum Precipitation Study for Cherry Creek Reservoir - Related Technical Research Papers
• Date: 5/20/1990
• Prepared For: CWCB
• Prepared By: CWCB
• Floodplain - Doc Type: Project

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Increase
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Change in
Landcover
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----Iutotrpolat1on
Betveen %
Fun, Devl!loped
LoIndcover

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Landc:over Distribution

RG. 6. Percent Increase In PMF ~ tor Change In Spallal Land-
Cover Distribution from Homogeneous Undeveloped for lsohyetal
Axes Ratio 01 2.5 to 1.0 lor Base Point Comblnallon 01 Factors
(land Cover Distributions are Denoted as Follows: 1 = Homoge-
neous Fully Developed; 2 =, Downstream Hall Fully Developed; 3
= Upper Middle Hall Fully Developed; 4 = Upstream Hall Pully
Developed; and 5 =, Homogeneous Fully Developed) ,

in land-cover distribution, after using the adjustment factors
of Figs. 4 and 5,

Obtaining Design PMF Estimate

To obtain a design estimate of the PMF, the designer 1l!tlst
make sufficient HMR52/HEC-1 computer runs to obtain the
optimum combination of factors yielding the maximum PMF.

Dry Senec:. Creek
Subvatenhed
(20.21)

While the base point PMF may be a good point to start. the
. designer must perform a site-specific sensitivity analysis. The
amount of effort may be reduced by making use of the in-
forma~jon presented herein concerning: (1) The interaction
between the factors; and (2) the nature of the contribution
of each factor in estimating PME The research findings of
insignificant interactions among several of the factors imply
that the number of combination of values for the factors
required in the sensitivity analysis is significantly reduced.
The research findings on how each factor contributes to the
P1vIF t:~lilIlate, whether rnilinly in the volume of rainfall or
runoff, in the spatial distribution of rainfall or runoff, or in
the temporal distribution of rainfall or runoff, aid the designer
in selecting the initial and the final optimum values for each
factor. .

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VERIFICATION USING SENECA CREEK WATERSHED

As an initial verification of the results of the sensitivity
analysis a test of the sensitivity analysis was condli""Cted with
the. Seneca Creek drainage basin located in Seneca, Mary-
land. Although the hypothetical drainage basin assumed in
this research was only 60 sq mi (155,5 km'), the results are
verified with Seneca Creek, which is 129 sq mi (334.4 km'),
A successful verification of the results on a larger drainage
basin would imply the potential application or at least pre-
liminary support in the sensitivity charts just presented to
drainage areas that are 'different from the 60 sq mi (155,5
km') used in developing the sensitivity charts,
The 129 sq mi (334.4 km') drainage basin was subdivided
into 22 subbasins for'this study (see Fig, 7), The five major
subwatersheds are Little Seneca Creek, Great and Upper
Great" Seneca Creek, Lower Great -Seneca Creek, Seneca
Creek, and Dry Seneca Creek, Pertinent hydrologic data for
Seneca Creek was obtained from a study conducted by CH2M,
Hill (1983),

Results of Sensitivity Analysis for Seneca Creek

To verify the adjustment factors determined in the sensi-
tivityimalysis, a subjective comparison ofthe percent change

ereat and Uppe.r
Seneca Ct"l!ek
Subwate.rsbed
(I to 9)

Seneea Creek
Subvat'i!rl!'h'i!d
(19,22)

RG. 7. Seneca Creek Watershed Hydrologic Network
JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING / SEPTEMBER/OCTOBER 1995/335