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

i

I

The proposed cross section of Bobtail Street (Figure 2) indicates the uphill (eastbound)
side of Bobtail Street is typically a near-vertical highwall connected to a 6-inch curb and
I-foot wide gutter, and the downhill (westbound) lane is bounded by a 32-inch high
Modified Type 4 Vehicle Barrier. Offsite basins discharge stonnwater only along the
eastbound lane of Bobtail Street.

I

I

t

Bobtail Street is classified as a collector, and USDCM requires collectors maintain one
open lane,during minor stonn events. Since offsite drainage enters only the eastbound
lane, the eastbound lane is assumed to be completely inundated during the minor storm
for calculation purposes, while the westbound lane remains open. During the major
stonn, USDCM recommends the depth of flow should not be greater than 18-inches
higher than the gutter invert. However, since the street flows are confined by barriers on
both sides of Bobtail Street, flow depths are limited to 6-inches over the gutter flow line
during the 100-year event to reduce the possibility of flood-induced accidents along the
Street.

I
I
I

I

Street conveyance capacities for Bobtail Street were calculated using the typical street
section shown on Figure 2. Rating tables showing allowable street capacities for street
slopes between I and 10 percent are presented on Table 5. The reduction factor for
street flows in USDCM, which estimates the impact on surface irregularities and other
disturbances on the street surface on theoretical street flows, is used to calculate design
street capacities.

I

I
I

When inlets are required for storm sewers, all stonn sewer inlets are assumed to be or
have the equivalent capacity of Type "R" inlets with a 6-inch throat height and a 2-inch
sump. Following UDFCD recommendations, an 80 percent reduction coefficient is
applied to inlet capacity. Supporting information on inlet design can be found in the
Appendix of this report.

I

Slotted drains are employed in the drainage design primarily to eliminate nuisance flows.
However, in some cases, the slotted drain does provide additional stormwater conveyance
in the street. Slotted drains are assumed to have a continuous opening. If the slotted
drain is used as part of the street stonnwater conveyance system, the capacity of the
slotted drain is calculated assuming the energy grade line within the pipe does not exceed
the top ofthe pipe (See Table 6).

I

I

I
I

I
I

10