FLOOD01081

• Designation Number: 286
• County: Douglas
• Community: Unincorporated Douglas County
• Title: Flood Hazard Area Delineation - Spring Gulch
• Date: 12/1/1986
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Therefore, no floodway data was shown. For all cross-sections the floodway

is assumed to be identical to the lOO-year flood plain. Floodway data

within those locations where future backwater conditions have been

evaluated would not be meaningful since the proposed backwater has

already been set at its maximum allowable elevation and dictates

required right-of,way drainage and flood control purposes.

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C. FLOOD VELOCITIES AND EROSION POTENTIAL

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Average flow velocities are presented on the Flood Hazard Area Delineation

drawings 1,6 for specific cross,sections which have been detailed. The

range of average flow velocities for the lOO-year flood event is tabulated

below for the reach. The mean velocity is representative for all the

cross-sections analyzed within the reach.

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Velocity
Range
1 to 13 fps

Mean
Velocitv
6.76 fps

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Reach
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Water traveling at a rate greater than six to eight feet per second (fps)

can cause erosion of stream banks and is capable of transporting silt,

cobbles and large rocks. Water travelling at 4 fps or greater will

transport colloidal silts and can erode embankment and/or bridge abutment

fills. Water flowing at less than 2 fps will deposit debris and silt and

will tend to produce clear water at least near the water surface. It is

concluded that relatively high velocities can be expected in the Spring

Gulch drainage basin resulting in an enlarged natural channel capable of

conveying the lOO'year flood event in most cases.

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This does not mean that the existing channel is now stable; erosion can and

will continue to occur and special precautions should be taken to assure

that structures, utilities, and other development facilities close to the

flood plain are protected from the erosive effects of high velocity flow.

As part of development planning, grade checks should be considered as a

means of controlling bank sloughing and stream bed degradation.

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VI. REFERENCES

A.

"Urban Storm Drainage Criteria Manual", Urban Drainage and Flood

Control District, March, 1969, Revised May 1975, Revised May, 1984.

B.

Colorado Urban Hvdrop.:raoh Procedure Comnuter Proe:ram, User Manual

CUHPE/PC, Urban Drainage and Flood Control District, January, 1985.

C.

PreciDitation - Freauencv Atlas of the Western United States, Vol.

III, Colorado, National Oceanic and Atmospheric Administration, 1973.

D.

Storm Water Management Model User's Manual. Version II,

U.S.

Environmental Protection Agency, EPA-670/2,75,017, March 1975. See

also Revisions to Runoff Block of Storm Water Management Model",

Missouri River Division, U.S. Army Corps of Engineers. Revised for

UDFCD by Boyle Engineering, ' March 1982.

E.

"HEC,2 Water Surface Profiles Users Manual", September 1982, by

Hydrologic Engineering Center, U.S. Army Corps of Engineers, Davis

California.

F.

"STORM DRAINAGE DESIGN AND TECHNICAL CRITERIA MANUAL", Douglas County,
January, 1986.

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