FLOOD10485

• County: Arapahoe
• County: Douglas
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Big Dry Creek ‎(Arapco)‎ & Tributaries - Major Drainageway Planning - Phase B Preliminary Design Report
• Date: 4/1/1998
• Prepared For: Arapahoe County, Douglas County, Englewood, Greenwood Village, Littleton, UDFCD
• Prepared By: WRC Engineering, Inc.
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

As part of the first phase (Phase A) of this study, a public awareness program was initiated to inform
the general public of the project and obtain comments from interested persons. Two public meetings were
held for this purpose. The first publ ic meeting held on July 24, 1995, introduced the purpose and scope of
the project to the public and solicited comments regarding drainage issues and problems within the Big Dry
Creek watershed. The second public meeting, held on September 16, 1995, was used to present some of the
initial findings of the project and to discuss a number of alternatives that mayor may not be possible along
various reaches of Big Dry Creek.
During the second phase (Phase B) of this study, the public awareness program was continued with
a third public meeting. At this meeting, held on May 28, 1997, the results of the alternatives evaluation
phase were presented as well as the project sponsors selected improvements. In addition, preliminary design
concepts of the selected improvements were presented for review and comment by the general public.
The Big Dry Creek watershed encompasses about 20 square miles of land area in Arapahoe and
Douglas Counties (See Figure I). The lower end of Big Dry Creek flows through portions of the Cities of
Littleton, Englewood, and Greenwood Village before discharging to the South Platte River near Union
Avenue. Several changes have occurred in the watershed since it was first studied in 1975. These changes
have included increased urbanization (primarily in Douglas County), dedication oflarge areas of open space
(previously planned for development), and construction of various drainageway improvements. Most
notably, several regional detention ponds have been constructed as part of the Highlands Ranch development
in Douglas County.
The combined result of these changes since 1975 has been a 29 to 68 percent reduction in the
computed I OO-year peak flows downstream of High lands Ranch. These changes are demonstrated in the
following peak flows comparison and are graphically presented on Figure 2. The peak flow diagrams for
Tributaries A and B are included on page 111-25 and 111-26 of this report. This reduction has a significant
benefit in allowing for generally smaller and less-costly drainageway facilities to be constructed for
conveyance of the 100-year and more frequent flood events. Some of these benefits have been offset by
natural changes in the drainageway's ability to convey flood flows, most notably an increase in stream bank
vegetation (i.e. trees and shrubs) which has occurred subsequent to the 1975 master plan. As a result, the
100-year floodplain levels remain close to those identified in 1975 in the lower reaches of Big Dry Creek
(downstream of Arapahoe Road).
ES-1

BIG DRY CREEK (ARAPCO) AND TRIBUTARIES
MAJOR DRAINAGE WAY PLANNING
PHASE - B
PRELIMINARY DESIGN REPORT
EXECUTIVE SUMMARY
This report presents the development, analysis, and results of Major Drainageway Planning of Big
Dry Creek (ARAPCO) and Tributaries. This study is ajointly sponsored effort of the City of Littleton, City
of Englewood, City of Greenwood Village, Arapahoe County, Douglas County and the Urban Drainage and
Flood Control District (UD&FCD). It is being coordinated by the UD&FCD. The project consultant is
WRC Engineering, Inc. who was responsible for the preparation of this report.
The purpose of this study is to prepare an updated drainageway plan for Big Dry Creek (ARAPCO)
and it's two major tributaries. The three main goals of this study are as follows:
1. Prepare an estimate of the peak flows and volumes of runoff for various locations
along Big Dry Creek and tributaries.
2. Prepare a new delineation ofthe I OO-year floodplain based on current peak flow and volume
estimates and existing channel conditions.
3. Use this information to develop a plan to maintain the stability, integrity, and flood carrying
capacity of Big Dry Creek and tributaries and reduce safety hazards and damage associated
with probable flooding along this drainageway.
A project advisory committee consisting of the project sponsors and representatives ofthe Colorado
Department of Transportation, the Denver Water Department, and the South Suburban Parks and Recreation
District has 'met periodically during the progress of this study. These meetings have been used primarily to
exchange information and to discuss ideas and findings of the study.

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