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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Basin B 1 discharge has been proposed to be routed around and through the proposed 300
Main Street Parking Structure by others (Map 3A). The stonn sewer is proposed to
discharge' directly to North Clear Creek. A site plan level drainage report for this
structure has recently been submitted to the City (Reference 4) and is currently under
review. Infonnation in the site plan level drainage report, including lower peak
discharges, flow routing, and basin areas differ from those shown on Map 3A.

Basin B2

Basin B2 historically sheet flows across an approximately 910 linear feet length of basin
boundary defined by Bobtail Street, through existing development, and onto Backus
Street. Stonn peak discharges are approximately 21 and 53 cfs for the 5- and 100-year
event, respectively. Proposed downstream developments located on lands which
historically received stonnwater from Basin B2 include 300 Main Street Parking
Structure (Reference 4) on the western side and Lot 2, Block 1, Miners' Mesa Filing No.
I Dakota Works Casino Parking Lot (Reference 5) on the eastern side. Since the
longitudinal slope of Bobtail Street is approximately 8 percent along the subbasin
boundary, the drainage design capacity of Bobtail Street, where it intercepts Subbasin
B2A, is approximately 2.6 and 19 cfs for the 5- and 100-year event, respectively.

For the discussion of proposed improvements, Basin B2 is divided into two sub-basins,
Subbasins B2A and B2B (Map 2). Final street design directs Subbasin B2B flows to the
western section of Subbasin B2A.

Subbasin B2A storm peak discharges equal approximately 12 and 31 cfs for the 5- and
100-year events, respectively. Subbasin B2A is proposed to drain into a culvert below
Bobtail Street and then into a stonn sewer system (see the discussion of Basin B I)
located on the western side of the proposed 300 Main Street Parking Structure being
designed by others. Please refer to Map 3A. The eastern section of Subbasin B2A,
which comprises approximately 20 percent of Subbasin B2A, will be captured by a small
open channel or storm sewer located on top of and along the highwall. Storm peak
discharge estimates for the eastern section of Subbasin B2A, based on the proportional
area method, equals approximately 2 and 6 cfs for the 5- and 100-year event,
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, this discharge can likely be accommodated in
a I-foot deep, 3: I side slope triangular channel lined with rough rock. Otherwise, an 18-
inch diameter RCP or HDPE pipe with inlets located every 50-feet along the pipe should
be used to capture and convey flows.

Subbasin B2B storm peak discharges equal approximately 9 and 22 cfs, or 0.020 and
0.048 cfsllinear foot along the approximately 460 linear feet of basin boundary defined
by Bobtail Street, for the 5- and 100-year event, respectively. Approximately 75 percent
of the drainage from the basin, or approximately 7 and 17 cfs for the 5- and 100-year
events, respectively, can be intercepted by a stonn sewer located along the new pump

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