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

Subiect:

"Storm-Runoff Quantity For Various Design Events." ASI Task 6. January 8,
1991,

This hydrologic analysis of Woman Creek and Walnut Creek was prepared for EG&G
Facilities Engineering, Plant Civil/Structural Engineering in response to the Agreement in
Principle between the Colorado Department of Health and the U.S. Department of Energy.

This report presents the peak-flood flows and volumes for Woman Creek and Walnut Creek
at Indiana Street, downstream of Ponds A-4, B-5, and C-2, plus three selected upstream
locations. TR-20 analyses were done for 25-, 100-, 5OD-year, and PMP recurrence intervals
for durations of 2-, 24-, and 72-hours. The analyses were done with the terminal ponds (A-
4, B-5, and C-2) empty, !,-full, and full.

Assumntions

1. When there is more than 600 cfs flowing in Coal Creek, water spills into
ditches that contribute flows to Woman Creek and Walnut Creek. Coal Creek
spills up to 94 cfs into Upper Church Ditch and 170 cfs into McKay Ditch
which both contribute to North Walnut Creek. Coal Creek spills up to 1600
cfs into Kinnear Ditch which contributes to Woman Creek.

2. PMF flows from west of the South Boulder Diversion Canal will breach the
berm of the canal and contribute 3,200 cfs to Woman Creek and 1,100 cfs to
North Walnut Creek. This assumption is based on a Woodward-Clyde study
in 1984.

Limitations

1. Subsequent analyses by private investigators and the USACE have shown that
assumptions on Coal Creek transfer of flood water to Rocky Flats are
incorrect.

Data Included

1. Estimated peak flows and runoff volumes for four locations on Woman Creek
are shown in Table 5 of the document. IOO-year, 2-hour peak flows at Indiana
Street ranged from 3,400 to 3,800 cfs, depending on initial Pond C-2 volume.

2.

Estimated peak flows and runoff volumes for six locations on Walnut Creek
are shown in Table 6 of the document. loo-year, 2-hour peak flow at Indiana
Street ranged from 2,300 to 3,600 cfs depending on the initial Pond A-4 and
B-5 volumes.

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Design precipitation values for recurrence intervals of 2-, 5-, 10-, 25-, 50-, 100-,
and 500-years, and PMP, and durations of 5-, 10-, 15-, 30-minutes, and 1-,2-,
6-, 24-, and 72-hours are listed in Table 1 of the document. Sources of rainfall
data are documented.

4.

Design precipitation distributions for the recurrence intervals and durations
listed in item 3 are listed in Appendix B of the document.

5.

TR-20 input parameters of drainage area, time of concentration (Tc), and
curve number (CN) are listed in Table 3 of the document. CN was 85 for
undeveloped areas and ranged between 85 and 90 for developed areas. Basin
sizes ranged from 0.08 to 1.63 square miles. The corresponding Tcs ranged
from 0.22 to 1.65 hours.

6.

The report includes an extensive description of the Woman Creek and Walnut
Creek watersheds.

7.

The report provides analyses and data for the Woman Creek and Walnut
Creek basins which are useful for correlation with master planning studies and
preliminary design engineering for specific drainageways. II