FLOOD10152

• County: Boulder
• County: Adams
• County: Jefferson
• Community: Broomfield, Westminster
• Stream Name: Big Dry Creek Tributaries
• Basin: South Platte
• Title: FHAD - Broomfield Area
• Date: 6/1/1979
• Prepared For: Broomfield, Westminster
• Prepared By: UDFCD
• Floodplain - Doc Type: Floodplain Report/Masterplan

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of Carbon Roan is able to cross Lowell Boulevard and enter West Lake. This

flow has been determined to be approximately 150 cfs for the 100-year event.

.,as defined as the I imit of encroachment into the existing channel floodplain

where the flood profiles are not increased by more than 0.5 feet.

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Sheets 29 and 30 show the floodplains for the two tributaries and their

confluence upstream of West Lake. The west tributary flows east in a mild

swale and overtops Lowell Boulevard before entering West Lake. The total

100-year flow entering the lake is 470 cfs.

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The overtopping of the West lake embankment is shown on Sheet 31. The

flow which spills over the northeast corner (80 cfs) is directed through

the West Lake Community open space and rejoins the main flow at West lake

Drive (Sheet 32). The major portion of the flow (390 cfs) proceeds south

and through the West Lake Community along King Circle to West Lake Drive.

Most of the flow is then directed west to the low point along West lake

Drive while some flow overtops the curb. At the low point the flow com-

bines with the 80 cfs from the lake and overtops the curb to flow south in a

wide shallow floodplain adjacent to the mobile home community. Approximately

100 cfs is lost from the basin downstream from West lake Drive due to the

flat topography and lack of a channel with sufficient capacity. The remain-

ing West lake Basin flow combines with flow from the Gay Reservoir Basin

south of West 128th Avenue.

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FlOOD\,AY DATA

The floodplain and floodway reference data are tabulated in Tables IV-I

through IV~4, The tables list the station, the thalweg elevation, the 100'

year floodplain elevation, the floodway top width and location relative to

floodway centerline, and the floodway surcharge elevation at each cross

section studieu,

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The floodway is that portion of the floodplain which is required to pass

the 100-year flood event without raising the flood profile more than an

acceptable amount and also represents that portion of the floodplain that

is most hazardous to personal safety and welfare. In this study, the floodway

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