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 2
Ms. Davidson
August 19, 1994

Inlet

The inlet will be hydraulically llimiIar to a curb inlet with a two-foot vertical opening. The inlet will be
250 feet long commencing on Mill Site 28. The Chunnel inlet wiD have a bed slope of 8% while the low
flow channel is sloped at 1%. By the end of the inlet zone, the low flow channel wiD be above the
Chunnel. A echematic drawing is attached.

Gregory Gulch Flume, which drains 12% of the total basin, wiD be routed direcUy into the Chunnel.
Only flood flows from North Clear Creek will use the inlet on Mill Site 28. About 1,500 cfs will enter
the Chunnelinlet, 970 cfs will enter from the Gregory Flume and 200 ds will remain in the low flow
channel if Q..dl" - (Owen A.Yers-type b,ydrology). Additional Bhorter inlets will be placed llIong the low
flow channel to take local runoff into the ChunneL

Outlet

The outlet design has not been selected at this time. However, there are several energy dissipator
options available such as impact-type, U .8.B.R. dentated,9pe, a water filii, and others. The important
feature is to property revet the transition from the Chunnel back to the Datura! channel.

Barrel

Barrel hydraulics will be llimiIar to storm sewer analyses. The ~draulic grade line will be kept below
the additional inlets and below the elevations of:Main Street ln1ets which will be tapped into the
Chunnel.

Local drainage structures will be tapped into the Chwmel. These have not been inventoried yet. A
few ere S.H. 119 culverts and Main Street pavement drainage Btructures.

Sediment Transport

..

Sediment transport is an important design consideration. Stream power inside the Chunnel will be
maintained to keep all sediments which enter moving all the way through. The preliminary Cbunnel
design will keep sediments up to 12.inch diameter moving during the 1llO-year event. Lower level
. events will be analyzed also be analyzed.

Sediments will be contributed by North Clear Creek, Gregmy Gulch and locaIlIOUI'CeS. The North
Clear Creek aediment contnoution wiD be controlled by the relativel,y flatter alope upstream of
BuDwhackers and the S.H. 279 culvert. Since North Clear Creek flood water will be entering from the
low flow channel, sediment clogging of the low flow channel is critical. The width of the low flow
channel will be narrowed along the inlet Btructure to maintain stream power. Below the end of the
inlet, the low flow channel will be widened to provide additional ~draulic capacity around sediments
which drop out.

The Gregory Gulch connection wiD carry more sediments (in the near term or) until the Gregory Gulch
channel and upper basin (mine tailings) lire stabwed. Existing bed load in the Gregmy Flume will be
analy~ed to predict lediment load size (diameter) B11d amount.

Your assistance with FEMA in establishing design discharge wiD provide ~ect laving&. We are
essembling base mapping and are working on plan/profile and CI'OIIS eection Bheets. The 404 permit