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

UDFCD has previously studied the Big Dry Creek basin extensively and much of the basin
has been subject to comprehensive urban drainage and flood control master plans.
However, the master planning has not previously extended upstream of either Standley Lake
or Great Western Reservoir.

6.

Prepare a master plan for drainage and flood control management with an
array of options for implementation to improve the drainage system.

7.

Utilize a report format familiar and acceptable to UDFCD and members of
local government and which meets the relevant goals and objectives of the
State Legislature in forming UDFCD in 1969.

MASTER PLAN PRIMARY AND SECONDARY OBJECTIVES

The primary objective of this Master Plan is to provide EG&G and DOE with reliable and
usable hydrology and drainage data for planning purposes, and innovative options for
dealing with drainage issues.

PLANNING AREA

The Master Plan hydrologic study area encompasses approximately 15.8 square miles (10,100
acres) upstream and west of Indiana Street. To assure compatibility with the already-
approved UDFCD master plans, flood hydrology was analyzed to include the drainage basins
of Great Western Reservoir and Standley Lake, covering a total of approximately
24.8 square miles (15,900 acres). The outline of the hydrological study area is shown on
Figure III-I in Section III along with the additional hydrologic study area east of the Rocky
Flats Plant.

This subject Master Plan deals mainly with the upper reaches of Big Dry Creek above
Standley Lake and Great Western Reservoir. Additionally, specific hydrologic and hydraulic
aspects of Coal Creek were studied to analyze the effect of Coal Creek on the Upper Big
Dry Creek Basin. Also, the Rock Creek basin upstream of the Plant boundary was
analyzed. Due to the integrated and complimentary nature of the Rocky Flats drainage and
flood control master planning area with those local government master plans downstream,
several secondary objectives were adopted. These are to:

ANALYTICAL PROCEDURES

1.

Determine suitable rainfall and storm runoff relationships for a full range of
frequencies of occurrence utilizing the published UDFCD storm drainage and
flood control guidelines and design criteria for both hydrologic and hydraulic
engineering (Wright McLaughlin, 1969).

For this Master Plan, analytical procedures were followed consistent with accepted national
practice of the American Society of Civil Engineers and the standards of UDFCD and the
Colorado Water Conservation Board. Review letters were received from the latter two
entities and the Denver Water Department which are included as Appendix II-A.

2.

Determine channel flood hydraulics across Rocky Flats and delineate lOO-year
floodplains.

The following procedures were followed:

3.

Conform to already-adopted and approved downstream planning, hydrology
and regulatory floodplains of UDFCD and the Federal Emergency
Management Agency (FEMA).

1.

Utilization of reliable topographic mapping. Mapping at 1 inch to 100 feet
with two-foot contour intervals was available for the Rocky Flats Plant site.

2.

Selection and definition of drainage basins suitable to achieve stated
objectives.

4.

Define Core Area drainage characteristics and determine what deficiencies
exist.

3.

Definition of precipitation intensities and durations for a full range of
frequencies of occurrence.

5.

Consult with personnel of UDFCD to assure reasonableness and full
appropriateness of methodology, computations and results.