FLOOD03149

• County: Gilpin
• Community: Central City
• Stream Name: Eureka and Nevada Gulches
• Basin: South Platte
• Title: FEMA LOMR Application Hydrologic and Hydraulic Study
• Date: 10/1/1991
• Prepared For: Central City
• Prepared By: RMC
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Most inlets are of the trench type, that is they have a wide grated
cover over a channel which crosses the road, all owing interce~t i on
across the full road width. Minimum interception capacities were
determined by use of the Federal Highway Administration's H~C-12
guidel ines. (See Reference No. 12 in the Bibl iography, and excerpts
of that document in Appendix "E".) Despite the size and configuration
of the inlets, and other measures taken to minimize sediment cloS9in9,
inlets were overdesigned to account for clogging.

Analyses of flume capacities were ultimately performed by Mar.1ings
equation. However, lower limits of "n" values were determined tj the
Darcy equation and Moody diagrams, which can account not onlj' for
surface roughness, but turbul ence and vi scosit i es other than for ~'ater.
Peak flows in the flumes are without question turbulent, and must be
analyzed accordingly. Also, stormwater laden with sediment will have
a viscosity significantly higher than water. Ultimate Manning values
used for the Opera House flume are:

1. RCP, n = 0.030
2. CMP, n = 0.035
3. Rock, n . 0.035

Ultimate Manning values used for the Nevada flume are:

1. RCP, n = 0.025
2. Concrete with soil bottom, n = 0.035

Procedures used to select the above "n" values are explained in Section
C, and lower limit derivations are provided in Appendix "F".

Channel hydraulics are generally calculated by HEC-2. (See Reference
No. 14 in the Bibliography.) Minor street and unnatural sections may
be analyzed using Mannings equations where slopes are known to result
in super critical flows so that backwater methods are not required.
HEC-2 results are provided in Appendix "G".

Due to sediment burden, shallow street flows were assumed to have an
"n" of 0.04. Natural channel "n" values without vegetation were
assumed to have an "n" value of 0.06, and vegetated sections having an
"n" value of 0.08. These are based on "n" values presented in the
Federal Highway Administration's HDS No.3, modified to account for
substantial sediment which effectively increases wetted perimeter, flow
quantity, and volume. (See Reference No. 15 in the Bibliography.)
Hand hydraulic calculations are provided in Appendix "H".

C. Procedures

With diversions from surface runoff as was done in this study, inlet,
cul vert, and fl ume capacit i es had to be known pri or to performing
hydrological analyses. All diversions through culverts and inlets into
flumes are based on flow/ interception tables, resulting in real time
diverted hydrographs which are combined as appropriate and routed
through conveyance systems. Pipes and flumes can then be analyzed for
realistic pea.ks, not assumed singular or additive peaks.

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