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:

"Review Report of the Great Western Reservoir Pre-Feasibility Study." Pre-
Decisional Draft. Interagency No. DE-AI04-89 AL3290S. U,S. Army Corps of
Engineers. October 1989.

Flows in excess of 260 cfs in the Upper Church Ditch and McKay Ditch west
of the South Boulder Diversion Canal are assumed to spill into the canal and
not to flow onto the plant site.

4.

This detailed report was prepared for the U.S. Department of Energy to assist in assessing
hydrologic and hydraulic engineering work by Rocky Mountain Consultants (RMC) in regard
to the diversion of flood waters from Rocky Flats around Great Western Reservoir.

5.

The A- and B-series ponds are assumed to be 10 percent full at the start of
the precipitation event.

The U.S. Army Corps of Engineers (USACE) performed a hydrologic analysis of the Walnut
Creek basin using the SWMM and HEC-1 computer models. A hydraulic analysis of RMC's
three proposed diversion/detention facilities is provided in the document. Flood discharges
and volumes in Walnut Creek at Indiana Street were calculated for the lOG-year, 3-hour,
and probable maximum precipitation (PMP), 16-hour storms.

6. Flows in the Walnut Creek Diversion Canal in excess of 1,3OG cfs would spill
from the canal onto the plant site.

Data Included

Limitations

1. The peak 100-year flood discharge and runoff volumes for Walnut Creek at
Indiana Street are 1850 cfs and 244 acre-feet. Probable maximum flood
(PMF) flow is 32,710 cfs (see Table 4 of the document).

1.

2.

The peak lOG-year flood discharge and runoff volumes for a sub-basin of
Walnut Creek not tributary to the main stem of Walnut Creek at Indiana
Street (USACE sub-basins 40-42) are 420 cfs and 33 acre-feet. PMF flow is
3,300 cfs (see Table 4 of the document).

The Woman Creek basin is not addressed.

2.

Flood discharges at Indiana Street are provided, but flood discharges at

intermediate points are not.

Assumntions

Drainage sub-basin delineation is shown on Plate 1 of the document.

3.

1.

The PMP, 16-hour precipitation of 31.0 inches yields 23.4 inches of runoff (see
Table 3 of the document). The 100-year, 3-hour rainfall of 3.35 inches yields
2 inches of runoff.

A uniform infiltration rate of 0.5 inches per hour was used.

4.

2.

The 100-year, 3-hour storm point precipitation value was taken from the
NOAA Atlas. This was distributed in 10-minute intervals with the maximum
precipitation occurring in the middle of the storm (see Table 1 of the
document).

5.

The report contains extensive additional information regarding the site
hydraulics, geology, geotechnical and environmental characteristics.

3. The point precipitation values for the PMP were taken from the
Hydrometeorological Report No. 55A. The 16-hour storm was distributed in
half-hour increments with the maximum precipitation occurring in the middle
of the storm (see Table 3 of the document)."