FLOOD00850

• Designation Number: 31
• County: Arapahoe
• Community: Unincorporated Arapahoe County
• Title: Major Drainageway Planning - Little Dry Creek, Report, Volume I
• Date: 2/1/1974
• Designation Date: 6/1/1974
• Floodplain - Doc Type: Floodplain Report/Masterplan

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F L 0 0 D

S TAG E

D E T E R M I N A T ION

Flood stages were determined along Little Dry Creek and its major tribu-

taries for several different conditions. Flood stage determinations were

made for both present and estimated ultimate future condition of urbani-

zation in the basin in order to make the information useful for planning

future drainage works and enforcing land use regulations. By comparing

Tables 5 and 6 it will be seen that flood peaks and volumes under esti-

mated future conditions are only slightly greater than those to be ex-

pected under present conditions.

Flood stages were computed for 10, 25, and lOa-year frequency floods in

order to develop flood damage estimates. Flood stages were also computed

for three degrees of flood detention dam development: (1) with no dams,

(2) with Englewood Dam only, and (3) with Englewood Dam plus the five pro-

posed additional detention dams. The six detention dam development results

in the greatest reduction in flood peaks in the Little Dry Creek Basin.

Flood water levels were computed by conventional open channel hydraulic

analysis methods. Channel roughness characteristics were determined by

a field inspection of the waterways in May 1972. These roughness factors

were increased slightly to reflect the greater growth of vegetation that

could be expected in the summer months. It is estimated that natural chan-

nel roughness will not increase significantly in the future as a result of

vegetal growth, although the construction of fences and buildings in the

flood plains would impede flow and cause flood levels to rise.

Water levels were calculated at waterway cross sections that were normally

300 to 500 feet apart. In a few cases sections were 600 to 700 feet apart.

Where abrupt channel constrictions occurred, cross section spacing as close

as 10 feet was used. A major part of the water level calculations were done

using an IBM 370 computer. The computer program used was a Reclamation

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