FLOOD07075

• County: Denver
• County: Jefferson
• Community: Denver, Edgewater, Lakewood, Wheatridge
• Stream Name: Sloans Lake Basin
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 10/1/1977
• Prepared For: Denver / Lakewood
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Hydrologic Engineering Center in Davis, California, and has been accep-
ted as one of the most thorough and comprehensive methodologies for cal-
culating water surface profiles in natural and urban channels. This
program has been used in other drainage studies in the Denver Metropoli-
tan Area for the Urban Drainage and Flood Control District.

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Criteria

The flows used for the water surface computations were obtained from the

unit hydrographs and discharge probabi lity curves developed in the hydro-

logic investigation. The flood routing program was applied to Sloans

Lake from which water surface elevations for the various design flood

frequencies were developed. The computed water surface elevations were

plotted on the 1"=100' scale mapping from which flood plains for the

5-, 10-, 25- and 100-year design floods were drawn. Only the 100-year

flood plain is shown in this report.

In the evaluation of Sloans Lake Basin, 190 drainageway sections were
taken from the 1"-100' scale mapping completed for the project. Input
to the HEC-2 water surface profile program included the drainageway
flows, cross section data along the drainageways, control data of drain-
age structures and obstructions such as buildings, vegetation, and man-
made embankments, and channel roughness coefficients. The Mannings
roughness factors used In the computat ion of water surface profi les
varied from 0.013 to 0.10 In the drainageways and 0.015 to 0.10 in the
overbank areas. Transition coefficients used to denote changes In the
channel cross sectton causing energy losses varied from 0.3 to 0.9
for expansion and O. I to 0.5 for contraction. All coefficients were
based on field observatIon of each dralnageway within the Basin.

Analysis

Flood profiles were run on each of the drainageways within the Basin.
the 5-, 10-, 25- and 100-year flood profi les were calculated for the
totally developed basin utilizing peak flows developed in the hydro-
logic investigation. The I imits of the computed flood plain for the
100-year event is presented on the drawings contained In this re-
port. From the 1"=100' plotted flood plain information, an analysis
of potential flood plain damages was made.

Structures along the channel were analyzed concurrently with the drain-
agewayevaluatlon. Street crossing structure capacities were computed
along with rating curves for significant crossings using the HEC-2
Water Surface Profiles Computer Program. Particular examination was
made of culverts under Sheridan Boulevard leading to Sloans Lake where a
change in the lake's water surface elevation had a distinct effect on
the culvert capacity, as well as the backwater flood elevation.

Both the Basin hydrologic investigation and the flood plain delinea-
tion analysis identified numerous potential problem areas for damage
to existing development. The location by drainage reach, defined as
the length of drainageway between design points, of some of the major
potential problem areas are I isted as follows:

Ora i nageway
Reach

Potential Flood Damage Problem Areas

A-B

Flood water diversion from lower Co[fax Avenue between
Federal Boulevard and the South Platte River (Sub-basin
A) into the adjacent direct flow sub-basin (A4) on
streets located under the Colfax viaduct.

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