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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The sequence of procedures used was to determine inlet, culvert, and
flume capacities, and model stormwater runoff with diversions from the
surface. HEC-2 models take the balance of flow not diverted to the
fl umes.

1. Appendix "0", Culvert Capacities

Culverts have substantial slope, and unlike flumes, do not have
additional inflow or intermediate openings to the surface. Inlet
control was assumed to govern.

Flume capacities were analyzed for more than just inlet control.
Hydraulic gradients are calculated to determine overall maximum
capacity, which is the lessor of pipe or inlet capacity.

2. Appendix "E", Inlet Capacities

Inlet capacities were discussed in the Criteria Section. However,
it may be beneficial to note here that the HEC-12 procedures were
based on grate types, flow velocity, percent of flow over the
grate, splash over velocity, and length of grate. The method is
very suitable to existing conditions.

9

3. Appendix "F", Flume Capacities

Kinematic: viscosity of water at 62'F is 1.185 x 10.5 ft2/sec. This
would likely be a minimum viscosity for stormwater. As a maximum
amount, 38.6 x 10.5/ft2 sec. is assumed, which is the value for
linseed oil. Sediment laden stormwater is assumed to be between
these two (2) extremes. It is apparent from the photographs of
stormwater flow in the street in Blackhawk, which has similar
slopes and watershed conditions, that the flow has low viscosity
and would be within the above range. (See photographs in Appendix
"F", pages F-4 and F-5.)

Roughness is determined by using roughness depth divided by pipe
size. Using the above values and the Darcy equation, and limiting
total headloss in the flumes to the average flume slope to
approximate a non-surcharged condition, it can be seen from the
Moody diagram that the range between water and linseed oil for the
given conditions lies on a flat portion of the friction curve
entirely in the complete turbulence range for CMP and rock flumes
and nearly all in the complete turbulence range for concrete.
Accordingly, use of either viscosity produces the same friction
value in the Darcy equation for a given headloss. The Darcy
equation can then be equated to Manning's equation, setting
headlosses as equal, and an equivalent turbulence and viscosity
adjusted "n" value in Manning's equation can be determined. This
procedure resulted in a minimum CMP and rock wall Un" value of
0.027, which is less than the 0.035 value used in analysis. The
turbul ence and vi scosity adjusted "n" val ue for concrete was
determined to be 0.013, which is less than the 0.025 value used in
the Nevada flume and 0.030 value used in the Eureka flume. The
conservative "n" values are selected to account for additional flow
quantity due to sediment, minor blockage, other unknowns in