FLOOD04129

• County: Gilpin
• Community: Black Hawk
• Basin: South Platte
• Title: Black Hawk Floodplain Information Study Technical Addendum
• Date: 3/1/2011
• Prepared For: CWCB
• Prepared By: Owen Ayres & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION 1

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HYDRAULICS

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Standard Step Computations

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Water surface profiles were computed using the llEC-2 computer program, a
sample of which follows (pgs 1-2 - 1-10). As each of the waterways studied
were generally in a condition of supercritical gradually varied flow, com-
putations began upstream and proceeded downstream. Computed profiles exist
for the 10, 50, 100 and 500 year floods and floodway for each stream studied
and the computation printouts can be obtained from the Colorado Water Con-
servation Board.

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Stream Roughness Factor

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In the computer computations use is made of Bernoulli1s Theorem for
the total energy at each cross section and Manning's formula for the fric-
tion head loss between cross sections. In Manning's formula a factor known
as Manning's "nil must be chosen to reflect the stream's hydraulic roughness.
This is done using proven techniques (the "SCS" method) in combination with
pictorial techniques (U.S.G.S. Water Supply Paper 1849) with picture and on
site examination of the specific stream characteristics. A summary of "011
values so determined, stream bed descriptions and stream bed pictures follow
(pgs 1-11 - 1-47 and 1-53 - 1-84). The coding of "n" values, expansion and
contraction coefficients, channel and overbank distances was done a'single
time (as is standard practice for this type of work) reflecting projected
100 year values and these values used for the Gomputation of all four floods.

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Cross Sections

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Cross sections were field surveyed at regular intervals and locations
necessary for the hydraulic computations. firidges were measured individually
to obtain the appropriate necessary detail. This information can be found
on pages 1-11 - 1-27 and pages 1-85 - 1-97. Complete field survey notes
Gan be obtained fro~ the Colorado Water Conservation Board. Individual hand
plotted cross sections are available from the Colorado Water Conservation
Board. Pages 1-48 - 1-52 show a tabulation of stream channel distances measured
along the talweg. Note that the cross section and reference point numbering
systems differ and pages 1-98 - 1-103 cross reference the two systems.

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Flow Diversion

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Occasionally floodwater will encounter obstructions or high points in
the stream or floodplain. When this occurs and the obstruction is not sub-
merged, flow diverts around the item and continues downstream until it

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