FLOOD05894

• Designation Number: 342
• County: Jefferson
• Community: Arvada
• Stream Name: Moon Gulch Watershed
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Moon Gulch Watershed
• Date: 8/1/1992
• Designation Date: 9/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

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In several locations the predicted flood-waters would diverge into two or more separate flow

paths. In general, one of the paths would convey only a minor amount of water, and is

described as "indeterminate shallow flooding" on the FHAD sheets. The course and extent

of such flooding is predicted based solely upon the contour mapping and engineering

judgement. Hydraulic computations are made only for the main channel flow.

Split Flow Analysis

Sections were refined so that flow was allowed to pass only through areas of "effective

conveyance." Areas which had negligible "effective conveyance" (space and slope to allow

passage of water) were eliminated from the applicable cross sections. In general, it was

assumed that the flood water, after passing through a constriction (for example, between two

buildings) cannot expand at a rate greater than four to one (4:1). Cross sections

downstream from constrictions were modeled in such a way that for each four feet of

distance along the channel axis, the width of effective flow would expand only one foot on

each side of the axis. If areas of inundation existed outside of the lines of "effective

conveyance" drawn as described, they were shown on the FHAD Sheets as flooded, but were

modeled to be ineffective in carrying the flow.

As previously stated, the lower reach of Moon Gulch is currently developed with single

family residential houses, After the initial HEC-2 analysis and delineation of the lower

reach, it was found that the flow was not entirely confined to a north-south path down

Beech Street south of 72nd Avenue. It was determined that a portion of the total flow was

also being conveyed down Braun Court, Cole Court, and Coors Court. To accurately define

the floodplain at the lower reach of Moon Gulch (below 72nd Avenue), a HEC-2 split flow

analysis was performed to determine the proportion of flows down the four residential

streets in the lower reach of Moon Gulch. This analysis is described in the following

section.

Computations were made and are presented in tabular form along the channel. A detailed,

technical appendix of hydraulic calculations is available at the office of the Urban Drainage
and Flood Control Distikt.-

The hydraulic cross sections in the lower reach were extended (using 1"=100' aerial

topographic maps) to include Beech Street, Beech Court, Braun Court, Cole Court, and

Coors Court. After the cross sections were extended, a HEC-2 model was constructed to

model the flow down Beech Street and Braun Court. Another model was built for Cole and

Coors Courts. These additional HEC-2 models utilized the extended cross sections for the

original model, and the conveyance down each street was determined in the extended cross

section using the effective area option in HEC-2. Additional cross sections were added to

each split flow model to detail areas of flow between the extended cross sectionS. -All cross

sections used for the split flow analysis are shown on Sheet 3 of the FHAD maps. Once the

HEC:Z models of each flow split were constructed, a range of discharges was input into each

model to calculate a rating curve at cross section #8 (station 20+65). Ooss section #8

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