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

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IV, HYDRAULIC ANALYSIS

Computer runs were made in the upstream direction for flow in the

sub-critical to critical depth mode, The flow was found to be sub-critical

at the lower end of the study area where backwater is created by the

various culverts. Upstream, the flow tends to oscillate between

sub-critical and critical,

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The flow rates developed in the hydrologic analysis were then used in the

hydraulic analysis to determine the peak water surface profile and flood

plain limits resulting from a 100-year return frequency storm within the

Spring Gulch drainage basin, As mentioned earlier, the existing small

check darns within Spring Gulch have been included in the hydraulic analysis

since their presence, although probably temporary in nature, could produce

higher water surface profiles and wider flood plain limits until such time

as the check darns are washed out by flood overtopping,

The HEC-2 computer program utilizes the Manning formula for analyzing the

flood plain hydraulics. A Manning's "n" value of 0.035 was used both for

the main channel and the overbank areas.

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Additional studies were performed to determine the maximum water surface

profile and flood plain limits for backwater conditions which could result

from the future construction of roadway crossings at various locations

along the water course. Floodway limits and water surface elevations were

also evaluated for those areas where future encroachments within the

overbank portions of the flood plain could cause a rise in the peak water

surface elevation of up to, but not more than, 0.5 feet above the 100-year

storm water surface elevation without encroachments.

On completion of the HEC-2 computer runs, the water surface profile and

100-year flood plain limits were plott~d on the Flood Hazard Delineation

drawings provided in the Appendix of this report, Backwater conditions

resulting from tentative culvert-type road crossings have also been

plotted, The flood plain data used for these plots as well as computed

floodway data are tabulated on the Flood Plain and Floodway Reference Data

sheets, Table 3 which is also included in the Appendix, The culvert type

road crossing backwater condition referenced above tabulated at the end of

Table 3, Page 3,

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Flood plain cross-sections were taken from I inch ~ 40 feet scale
topographic maps, These cross-sections were digitized and incorporated
into the HEC-2 Water Surface Profiles computer program developed by the
Corps of Engineers Hydrologic Engineering Center, Davis, California
(Reference E),

V, FLOOD HAZARD AREA DELINEATION

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A, FLOOD FREOUENCY AND DISCHARGE

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Tentative culvert-type road crossings were evaluated separately assuming

that the individual culverts would be sized such that the resulting

backwater would peak at an elevation three feet below the minimum roadway

elevation at the crossing, Spillways might also be provided where

necessary to assure that the maximum assumed backwater elevation is not

exceeded. The actual sizing of these culverts and spillways is not needed

for this analysis and will, therefore, be address~d in a later phase of

development planning,

Floods will continue to occur in the future along Spring Gulch with

magnitudes equal to and greater than those that have occurred in the past,

As development progresses within the drainage basin, the flooding potential

will increase and actual flooding will occur more frequently,

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The Spring Gulch drainage basin has been analyzed to determine the

discharge at various key locations along the Spring Culch channel for the

100-year flood event, The 100-year flood event is defined as that which

has a one percent chance of being equalled or exceeded in anyone year,

The 100-year flood is considered by the Urban Drainage and Flood Control

District, the Colorado Water Conservation Board, and the Federal Insurance

Administration, as the flood magnitude for which flood plains should be

designated for regulatory and improvement purposes.

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