FLOOD04017

• County: Jefferson
• County: Denver
• Community: Columbine Valley, Denver
• Stream Name: Dutch Creek, Lilley Gulch, Coon Creek,
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 5/1/1978
• Prepared For: Jefferson County / Arapahoe County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Description of Flood Hazards

Factors Which Reduce Flood Hazard

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Natural obstructions to flood flow include brush, trees, and other low-

land vegetation which naturally grow along drainage channels and low-

lands adjoining drainageways. During periods of high flooding, these

obstructions tend to slow the flood velocities and as a result, cause

greater depths and backwater conditions that increase flood elevations.

Often the natural vegetation found within the flow area becomes dis-

lodged from its normal location and accumulates as debris within the

flood waters to be collected at bridges and culverts and other manmade

obstructions. In addition to the vegetation obstructions, flood waters

tend to erode channel banks and carry the suspended soil material as

well as rocks and boulders further downstream, adding to the debris

and damage at the manmade obstructions. As these masses of debris

accumulate, the artificial barrier soon breaks loose and a wall of water

and debris surges further downstream to continue the hazard and destruction.

Flood control improvements, including channel improvements, reservoirs

and bridge improvements, as well as good flood plain management prac-

tices such as flood proofing and avoiding building in flood plains,

will reduce the potential hazards associated with flood prone areas.

For the essentially developed flood plains delineated in this report,

the enforcement of sound flood plain regulations will only help serve

the needs of the study area.

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The National Oceanic and Atmospheric Administration (NOAA) maintains

year-round surveillance of weather and flood conditions. Although

intense cloudburst activity in the general region can be forecast, such

storms and resultant flooding in specific small drainage areas cannot

be predicted accurately and, in any event, forecasts would provide very

little advance warning.

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In general, both natural and manmade obstructions exist which tend to

restrict flood flows and cause overbank or non-channel flooding. These

obstructions will often result in unpredictable flood flow patterns.

This is particularly true of irrigation canals crossing natural drainage-

ways. The obstructions also tend to become sources of debris and further

obstruction when structural failure results. Also, manmade flood plain

obstructions tend to concentrate flood flows which in turn result in

increased velocities and increased hazards in adjacent flooded areas.

Flood fighting and emergency evacuation plans are also factors which

can reduce the flood hazard. No formal plans for flood fighting or

emergency evacuation of people and personal property from flood plain

areas are known to have been prepared for the Basin. However, during

times of flooding, agencies such as the local police, the sheriff's

office, the State Highway Patrol, and road maintenance crews assist

in flood fighting and evacuating people from flooded areas.

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It is impossible to predict the degree of accumulation of debris within

flood waters and the resulting effect of such accumulation. This re-

port assumed that existing structures within the flood flow area would

see variable clogging and reduced efficiency during a major flood event.

The capacity for bridge structures and box culverts was reduced by

25 per cent to account for debris accumulation at the inlet. Culvert

structures greater than 36 inches in diameter were analyzed for blockage

of 75 per cent. Culvert structures smaller than 36 inches in diameter

were considered to be totally blocked and ineffective.

The most effective means to reduce flood hazards, is to pe aware of

such hazards and avoid them. Implementing flood plain regulation

practices will keep damageable land uses and improved property out of

the flood plain and out.of danger. Flood proofing will tend to reduce

damage to the existing buildings and structures within the flood plain.

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