FLOOD10207

• Designation Number: 503
• County: Jefferson
• Community: Unincorporated Jefferson County
• Stream Name: Woman Creek, Walnut, Upper Big Dry and Rock Creek
• Basin: South Platte
• Title: Drainage & Flood Control Master Plan - Rocky Flats Plant
• Date: 4/1/1992
• Designation Date: 9/1/1999
• Floodplain - Doc Type: Floodplain Report/Masterplan

A sample hydrograph showing the 100-year, 50-year, 25-year, 10-year, 5-year and 2-year
present development conditions is included below. The vertical axis shows discharge in cfs,
while the horizontal axis shows the time in hours.

Due to the small period of historical stream flow data and small precipitation events
available for Walnut Creek and Woman Creek (USGS, 1976), it was not possible to
accurately calibrate the model to published historical Rocky Flats stream flow data.

FIGURE C-.4
INFLOW TO POND A-4
(SWMM ELEMENT 169)
6-HOUA STORM, PRESENT DEVELOPMENT CONDITION

During this study, the output flood flows and volumes were verified two ways. First, the
data were checked internally to ensure consistency of flows between sub-basins taking into
account their different basin characteristics. Second, the data were checked with those
presented in previous reports by the U.S. Army Corps of Engineers (USACE), ASI, and
UDFCD for the same hydrological design points. These data are presented in Tables IV-9
and IV-lO.

~.
100 YEAR
~.
'0 ,yEAR
;;;
c
8
~.
.,
".

"

Two checks were performed to ensure internal consistency of the calculated CUHP /SWMM
peak flows. First, the peak flow per unit area in units of cfs per square mile for each of the
Buffer Zone sub-basins were plotted on a semi-log scale. The sub-basins were separated
into four categories with similar soil infiltration characteristics. The results are presented
in Figures IV-3 to IV-6. As expected, there is progressively less runoff from basins with
higher soil infiltration rates. Also, as expected, the smaller basins had higher runoff per unit
area than the larger basins.

.
.

TIME (HRS)

SAMPLE HYDROGRAPH

A second check of the internal consistency of the CUHP /SWMM calculated flow data was
done to compare flow calculated by CUHP for the sub-basins and routed by SWMM against
flow calculated using CUHP by combining sub-basins into one large aggregate basin. The
results are presented in lines I, J, K, P, Q, and R of Table IV-9. The flows calculated using
the two methods were similar in the Standley Lake Basin (P, Q, R) and the tributary to
Great Western Reservoir (J). Because of the relatively homogeneous nature of these basins,
this result can be expected. The calculated flows for the Great Western Basin (I, K) were
higher using the SWMM analysis. Part of the basin consists of the highly developed Core
Area with correspondingly low permeable area; the remainder is generally undeveloped,
with high permeability. CUHP combines these areas into one area with aggregate
characteristics and hence does not as closely model the physical situation as SWMM. The
high Core Area runoff is routed quickly downstream. The corresponding peak flows have
similar time to peak and, being additive, result in higher flows. The calculated SWMM flow
can therefore be expected to be higher than the CUHP flow.

The total100-year volume of runoff leaving the plant site through Woman Creek, Walnut
Creek, and unnamed tributaries to Standley Lake and Great Western Reservoir (SWMM
elements 129, 127, 126, 124, 123, 108, 180) is 692 acre-feet for present conditions and 795
acre-feet for future basin development.

Calibration and Verification of COOP ISWMM

Calibration and verification of any CUHP /SWMM model is desirable to better ensure the
reliability of the results obtained. CUHP was developed by the UDFCD using data from
the Denver area, and is suitable for Rocky Flats application.