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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Results

Map 2 indicates basins whose runoff historically sheetflowed over Bobtail Street will
now be conveyed within the street. This street conveyance will modifY historic flow
paths, decrease surface runoff in certain areas, and increase flood peaks at other locations.
Because of the currently proposed street design, there is no way to maintain historical
sheet flow discharges. Additional drainage facilities or conveyance capacities are
required to convey stonnwater within the street. Ifno additional stonnwater conveyance
capability is provided besides the street, nuisance flows are expected to create localized
areas of ponding on the street, and flows are expected to enter the driving lanes, creating
a potential hazard to drivers.

PROPOSED DRAINAGE IMPROVEMENTS

Each of the subbasins draining to Bobtail Street is unique, and a single set of design
criteria cannot be arbitrarily applied to each basin. A description of drainage control for
individual drainage basins is presented in this section. Discharge points are defined for
drainage basins which, under historic land use conditions, sheetflowed over the proposed
location of Bobtail Street. Proposed or future development activities are incorporated
into the proposed design. Drainage control recommendations have been placed on Maps
3A and 3B.

Desi!ln Objectives

The design objectives for drainage control along Bobtail Street include:

. IdentifY probable discharge points and flow paths based on natural topography and
street profile;
. Maintain historic point discharge areas to the maximum extent possible;
. Provide conveyance for the 100-year stonn peak discharge;
. Direct historic sheet flow areas which are proposed to be concentrated by
construction of Bobtail Street safely through developed areas;
. Discharge flows directly to North Clear Creek, when possible;
. Utilize existing drainage infrastructure, when possible;
· Incorporate City and UDFCD drainage design criteria into the design of Bobtail
Street; and,
. Remove sediment and debris from stonnwater flow to the maximum extent possible
before it reaches Bobtail Street.

Basin B I

Basin B I (unnamed gulch) historically flows in a channel located just west of Miners
Mesa Street below the Bobtail Street Bridge and discharges onto Backus Street. Storm
peak discharge estimates for Basin Blare approximately 80 and 190 cfs for the 5- and
100-year event, respectively.

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