FLOOD08905

• County: Gilpin
• Community: Black Hawk
• Basin: South Platte
• Title: 1998 Black Hawk Flood
• Date: 9/8/1998
• Prepared For: Clear Creek County
• Prepared By: Consultants
• Floodplain - Doc Type: Flood Documentation Report

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station access road (Map 3A). This capture of Subbasin B2B flows by the access road
stonn sewer reduces street flows in Bobtail Street to approximately 2 cfs and 5 cfs for the
5- and 100-year event, respectively. Since the street capacity of Bobtail Street is 2.6 and
19 cfs for the 5- and 100-year events, respectively, no additional alternative conveyance
facilities are required in the roadway to meet street drainage conveyance standards.

Subbasin B2B discharges onto the western section of Subbasin B2A. It is proposed to
direct the access road stonn sewer discharges from Subbasin B2B to an open channel or
stonn sewer located on the top of the highwall above the road in Subbasin B2A. Storm
peak discharge estimates from the western section of Subbasin B2A equal approximately
10 and 25 cfs for the 5- and 100-year event, respectively, calculated by the proportional
area method. When Subbasin B2A flows are combined with Subbasin B2B access road
flows, the discharges in Subbasin B2A equals approximately 19 and 42 cfs for the 5- and
100-year events, respectively. Although it has been suggested to use an open channel to
capture flows, it is currently unknown if the highwall provides positive drainage to the
proposed culvert. However, if an open channel is to be used, assuming its slope does not
exceed 10 percent, the 100-year peak discharge of 42 cfs can be accommodated in a 2.4-
foot deep, 3: I side slope triangular channel lined with rough rock. Otherwise, a 24-inch
diameter RCP or HDPE pipe, with a minimum slope of 4 percent and inlets located every
50-feet along the pipe, should be used to convey flows. Energy dissipation will be
required at the outlet of the open channel or the pipe.

Basin B2 flows will be directed to a culvert below Bobtail Street (Map 3A). The culvert
will convey discharges to a proposed storm sewer by others discussed in the section on
Basin B 1. The design capacity of the culvert equals 53 and 21 cfs, respectively, for the 5-
and 100- year events. The invert of the culvert compared to the road elevation and the
slope of the culvert are unknown. A 36-inch RCP would require a headwater depth of
approximately 4.4 feet above the invert of the pipe to pass 53 cfs. In comparison, a 42-
inch RCP would require approximately a headwater approximately 3.85 feet above the
invert of the pipe to pass 53 cfs.

It is additionally recommended to place approximately 300-feet of 12-inch diameter
slotted drain in the gutter to remove nuisance flows, as shown on Map 3A.

Basin B3

Under current conditions, Basin B3 point discharges from the location of Bobtail Street
over an existing highwall into the parking lot behind Canyon Casino. Stonn peak
discharges equal approximately 37 and 94 cfs for the 5- and 100-year event, respectively.

Drainage control for the parking lot (once known as Harrah's Parking Lot) was designed
in 1995 (Reference 6). The report (Reference 6) indicated storm peak discharges for the
100-year event were approximately 86 cfs, similar to the 94 cfs peak discharge estimated
for Basin B3. The 1995 report recommended installing a 36-inch RCP to pass this event
through the parking lot.

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