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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development conditions with Englewood Dam only, as shown on Table 5.

The difference of 4,100 cfs would flow around the conduit, flooding the

area. The water surface elevation under these conditions is estimated to

be 5,313 feet at the upstream end of the conduit. Flood waters spillover

the banks of Little Dry Creek at the conduit entrance at about elevation

5,3ll.

The maximum discharge capacity of the conduit is limited by its constricted

entrance. Only about 3,000 cfs can pass into the existing conduit under

the foregoing 100-year flood conditions. The entrance reach of the con-

duit has a capacity of about 3,400 cfs and the middle and exit reaches of

the conduit have capacities of about 4,750 cfs. The higher capacities can

not be realized presently because of the existing entrance constriction.

Improvements

By improving the conduit entrance configuration the capacity of the present

conduit could be increased to about 3,400 cfs. This would require cons-

truction of a relatively simple entrance transition structure having rounded

walls and top. Conduit capacity could be increased to 4,700 cfs by cons-

tructing a 550-foot long, l2 by 4-foot conduit from the conduit entrance to

the beginning of the middle reach of the present conduit. In order to in-

crease the conduit capacity by 2500 cfs to carry the full 7,100 cfs Engle-

wood Dam only flood peak, it would be necessary to add a full length 12-

foot high conduit,17-feet wide at the upper end diminishing to l2 feet wide

about 500 feet below the entrance. An additional full length conduit pro-

bably could not be built economically along side the present conduit because

of recent building construction. Some earth dike construction would be

needed to prevent 100-year flood flows from overtopping the Little Dry Creek

banks just above the entrance to the conduit.

When the five recommended additional flood detention dams are built on the

Little Dry Creek watershed, the 100-year flood peak discharge at the con-

duit entrance would be reduced to 4,700 cfs and the only improvements needed

to carry the lOa-year flood through the conduit would be to the entrance

reach, including the 530-foot long adjacent l2 by 4-foot conduit.

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