FLOOD02559

• County: Gilpin
• Community: Black Hawk
• Basin: South Platte
• Title: Black Hawk Chunnel Technical Information
• Date: 2/3/1995
• Prepared For: Black Hawk
• Prepared By: Futura Engineering Inc
• Floodplain - Doc Type: Project

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Page 4
Chunnel Committee
December 2. 1994

of data from 1952 to 1955. The average high flow for six consecutive months was 18
cfs with a range from 4 to 37.

2. West Fork of Gear Creek above Empire, drainage area of 40.5 square miles, four
years of data from 1943 to 1946. The average high flow for six consecutive months
was 111 cis.

3. Fall River near Idaho Springs, drainage area 23.5 square miles, eight years of data
from 1931 to 1938. The average high flow for six consecutive months was 35 cis.

The preliminary recommendation for North Gear Creek construction water control through
Black Hawk is 55 cis. A 48-inch pipe can convey water around the construction zone. The
contractor may select a smaller or larger pipe depending on the risk he is willing to assume.
A specification has been drafted modeled after COOT detour sizing specifications recognizing
level of risk and duration of construction (i.e., does the contractor leave a smaller pipe in as
spring approaches). Detour (of water around the work area) risk will be discussed with Black
Hawk during final design.

Chunnel HydraulicslPreliminary Design

Chunnel sizing was based on the hydrology study noted above. The Chunnel is expected to
carry 2,400 cis with the low flow channel to carry 200 cis. Cross sections have been
examined to determine the potential for the hydraulic grade line exceeding the elevation of
Main Street, inlets on Main Streets or inlets along the Chunnellength necessary to convey
local drainage into the ChunneI. Main Street near Jazz Alley is the most critical area and
will receive special attention for connection of local drainage.

Transition Structure HydraulicslPreliminary Design

The Gregory Gulch flume, which drains 12% of the total basin area, is routed directly into
the ChunneI. Only flood flows from North Gear Creek will use the transition structure on
Mill Site 28. The transition structure takes 1,430 cis, 970 cis will enter from the Gregory
flume and 200 cfs will remain in the low flow channel. Additional shorter inlets will be
placed along the low flow channel to take local runoff into the ChunneI.

The transition structure has been designed to be hydraulically similar to a curb inlet with a
two-foot vertical opening. The preliminary transition structure design length is 250 feet
commencing at the Gregory flume on Mill Site 28. The transition has a bed slope of 3%
while the low flow channel is sloped at 1 %. By the end of the 250 feet, the low flow
channel will be above the ChunneI. A plan/profile drawing for the inlet is not complete.

The capacity of the existing box culvert under Black Hawk Street is an important component