FLOOD10256

• County: Douglas
• Community: Douglas County
• Stream Name: Spring Creek
• Basin: South Platte
• Title: FHAD - Spring Gulch
• Date: 12/1/1986
• Prepared For: Douglas County
• Prepared By: UDFCD
• Floodplain - Doc Type: Floodplain Report/Masterplan

I

I

ABSTRACT

This roadway crossing is proposed to be used to detain flows and reduce

peak discharges, The predicted maximum flooding limits for conditions

before and after the construction of this future road crossing is shown on

the Flood Hazard Area Delineation drawings, in the Appendix of this

report, The maximum water surface elevations shown on the drawings are

based upon anticipated roadway elevations and not upon a storage or

discharge requirement. This maximum backwater limit will be used as a

control elevation for determining avc.ilable storage at the roadway

crossings and/or a high water elevation to guide in the location of

development areas upstream, The design of outlet control facilities in the

roadway embankment shall be such that the maximum backwater elevation never

be exceeded during the lOa-year storm event.

I

I

This report was prepared for the Urban Drainage and Flood Control District,

Douglas County, and Mission Viejo Company, and describes that portion of

the Spring Gulch drainage basin within Highlands Ranch that would be

inundated by a flood with a return frequenc~ of 100 years, Fully developed

site conditions were assumed, but without any mitigating features which

would maintain flood discharges at pre-development levels, The flooding

limits described in this report, therefore, represent the worst possible

lOa-year flooding conditions after the site is developed,

I

I

I

I

For the most part, Spring Gulch has a well defined natural channel and,

with a few exceptions, the lOa-year flood is contained within its existing

banks, Presently no major structures cross Spring Gulch between U,S, 85

(downstream limit of this report) and the upper limits of the basin, An

unimproved, privately owned dirt road (for purposes of this report is

called 'Old Ranch Road') presently crosses Spring Gulch and will be subject

to flooding during the lOa-year storm, The Army Corps of Engineers has

developed a detention embankment at the downstream end of the Gulch, near

U,S, Highway 85 (see Figure 2 - Master Plan of Drainage Detention), which

was designed in conjunction with Chatfield Dam and is capable of containing

the probable maximum flood (in excess of 1300 Acre Foot), A 36" diameter

concrete lined steel pipe carries runoff from the upstream side of the

detention facility and releases it near U,S, 85 downstream, The backwater

created at the Corps detention facility depicted in this report correlates

to the routed lOa-year storm hydrograph addressed in the Master Plan of

Drainage for Spring Gulch,

I, INTRODUCTION

I

I

A, SCOPE OF WORK

I

I

This study was prepared to determine the drainage infrastructure facilities

necessary in the Spring Gulch basin of Highlands Ranch to support

development,

I

I

Maximum lOa-year flood backwater conditions were also evaluated at the site

of the Westridge Parkway crossing of Spring Gulch,

This report studies that portion of the Spring Gulch Drainage Basin located

within Highlands Ranch that could be inundated by a storm event of a return

frequency of 100 years after buildout development, in accordance with the

approved Development Plan for Highlands Ranch. The 100-year flood flows

determined in this report are to be used as maximum values and do not

reflect expected peak flow reductions that would result from the

construction of retarding basins and other mitigating measures which will

be employed during and after development of the area,

I

I

I

1

I

I