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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In Colorado, the 100-year flood plain is an area of state interest as

defined in H,B, 1041, Lesser floods with higher probabilities of

occurrence have, therefore. not been considered here. Flood magnitudes

greater than the 100-year flood magnitude can and will occur and plans for

land improvement adjacent to the 100-year flood plain should consider the

probability of flood damage, Flood hydrographs at key locations are shown

in Figures 3 through 6 in the Appendix,

The 100-year flood plain limits and water surface profiles for the reach of

the Spring Gulch flood plain are shown on the Flood Hazard Area Delineation

drawings, Sheets 2 through 6 (see Appendix), and the computed 100-year

flood elevations for the various cross section locations are tabulated in

Table 3 as discussed earlier, A brief description of the significant

flooding aspects for this reach follows:

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The peak flow profiles given in Figure 7, in the Appendix, shows the

discharges developed in the hydrologic portion of this study for the

100-year flood event and are the discharges used in the hydraulic analysis

portion of the flood hazard area delineation, The discharge values shown

are those which could occur under fully developed conditions without any

mitigating measures being considered to control or retard the increased

rates of runoff. A previous report entitled "Master Plan Of Drainage

Detention for Spring Gulch" has addressed reduced discharge rates

which are more representative of final design conditions and

describes the mitigating measures which are to be taken to achieve a

discharge rate which should not exceed pre-development conditions at

the downstream edge of Highlands Ranch development, (i,e, D,S, 85),

The downstream terminus of the Spring Gulch Flood Plain (STA 1+00) is

located just upstream of the Army Corps of Engineers Detention Embankment,

As previously mentioned in this report and the Master Plan of Drainage for

Spring Gulch, the backwater elevation at this location was based on a flood

routing of the developed storm through the Corp's outlet structure for this

embankment (see Alternative 2 - Master Plan of Drainage Detention). This

ponding condition is contained within the Corps property and backs water to

STA 8+00. The Corps of Engineers (CaE) facility is capable of holding

approximately 16 inches of runoff from the Spring Gulch watershed, The

limits of P,M,F, ponding are shown on sheets 1 and 2 of 7, The spillway is

located on the north side of the CaE property,

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The 100-year flood discharges shown in the figures mentioned above are

conservative in that the highest discharge rate calculated for a subreach

is used for the entire subreach length, not just for the point for which

it was calculated, In general, discharge values were selected such that

they increased in step fashion as the flood progressed downstream,

An overtopping of an existing private road 'Old Ranch Road' occurs at STA

42+00 during the 100-year storm, Flood depth over the top of the road is

approximately 5 feet, It is anticipated that this roadway will cease to

exist when development begins in the Spring Gulch basin,

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B, FLOOD PLAIN AND FLOODWAY DATA

The only future road crossing scheduled for Spring Gulch at this time is

Westridge Parkway - located at STA 58+00, Backwater conditions have been

evaluated for this crossing and the following was concluded: The maximum

allowable backwater elevation will be 5654,5 - this backwater extends

approximately 1600' upstream to STA 74+00.

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The Spring Gulch flood plain has been characterized as one main reach and

is stationed to readily identify the various points of interest of the

waterway, The Basin Boundary Map (sheet 1 of 7) in the Appendix of this

report provides a more exact location of this reach.

In addition to the backwater conditions mentioned above, minor backwater

conditions exist at 6 check dams located at various points along the Gulch,

With the exception of these check dams, it was found the lOa-year flood

event would be confined within the existing natural channel for a major

part of the Gulch. For this reason no cross~sections were found where

meaningful floodway data could be tabulated (see Table 3 - Appendix),

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