FLOOD04421

• County: Arapahoe
• County: Douglas
• Community: Arapahoe, Douglas
• Stream Name: Piney Creek, Cottonwood Creek, Lone Tree,
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 10/1/1975
• Prepared For: UDFCD
• Prepared By: Arapahoe, Douglas
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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9

- FUTURE FLOODS -

A detailed description of the hydrologic analysis is available

at the offices of the Urban Drainage & Flood Control District.

Flood Frequency and Discharge

Floods of the same and greater magnitude as those that have oc-

curred in the past will occur in the future. With the planned

development in the basins, flooding will occur more frequently.

The discharges reported on Figures 7 and 8 for the 10-, 50- and

100-year flood frequencies represent the relative extent and

impact of each flood event. The discharge information is usable

not only for flood plain regulation but also for planning, engi-

neering and flood plain improvement.

Hydraulic Analysis

Hydraulic analyses of each stream were completed to determine

the water surface elevations for the 100-year storm event.

The elevations were compiled using the Corps of Engineers' HEC-2

Water Surface Profiles computer program. Valley cross sections

were measured on the 1" ; 100' scale, 2-foot contour topographic

mapping provided by the District. Estimates of channel and over

bank roughness factors were made after field investigation of the

study area. Typical roughness, or Manning's "n" values used in

this study are:

channel - .030 to .040
over bank - .035 to .050

The 100-year flood event, which can be expected to occur at any

time in a given area, based upon recorded historical precipitation

and other valid data, has a one percent chance of being equalled

or exceeded during anyone year. The 100-year flood event pro-

vides a lower risk of failure than the 10-year or SO-year flood

event, when considering structural flood plain improvements.

The computed flood profiles are plotted on the Flood Hazard

Area Delineation Map Drawings. Also, the computed flood elevations for

Piney Creek, Cottonwood Creek, Lone Tree Creek and Murphy Creek

are tabulated at each reference point in Tables 6, 7, 8 and 9,

respectively.

The 100-year flood event is considered by the Urban Drainage &

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. In Colorado, the 100-year flood plain

represents an area of state interest as defined in H.B. 1041.

Detailed information on the hydraulic calculations is available

from the Urban Drainage and Flood Control District.

The 10- and SO-year recurrence flood magnitudes have also been

calculated at various design points along the four creeks. These