FLOOD04309

• County: Eagle
• Community: Eagle County
• Stream Name: Stone Creek
• Basin: Colorado Mainstem
• Title: Flood Hazard Identification Report - Stone Creek, Eagle County, Colorado
• Date: 9/1/1986
• Prepared For: Eagle County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Hydraulic Analysis

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An analysis of the hydraulic characteristics of Stone Creek was
carried out to provide estimates of the water surface elevations due to
the 100-year flood. No accurate topographical information was available
for the studied area as all of it had been significantly altered when the
Eagle-Vail Subdivision was constructed. No as-built drawings were ever
filed with the County and none were required at that time. A map of the
area was created by CWCB for this study by superimposing various maps and
aerial photos available from various sources. In addition, a field
survey was conducted by CWCB and Eagle County personnel (Ref. 3) in order
to measure culvert sizes and survey elevations and typical
cross-sections. The gathering of all of this data allowed for a detailed
flood study to be undertaken for Stone Creek.

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Two significant findings were noted in the field. The first of
these was that cross-section shapes remained quite uniform over long
distances; thus there was not a need for many cross-sections to be
surveyed. The second finding was that water would be diverted out of the
creek itself in many locations because of intercepting drainageways and
swales. The most significant drainageway was one platted for exactly
such purposes. It is located at station 51+80 (see Plate 2).
Calculations indicate that approximately 40 cfs of the 100-year flow
would flow into this drainageway, leaving approximately 210 cfs in the
channel downstream from this point. Other locations where the 100-year
flow would be diverted are at the Stone Creek Drive crossing (station
27+90) and at the Deer Boulevard crossing (station 10+00). Flow diverted
at these locations was ignored; the effect on the floodplain was thought
to be minor below these points.

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The water surface elevations were computed through use of
of Engineers' HEC-2 step-backwater computer program (Reference
following assumptions were made in the computer analysis:

the Corps
8). The

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1. Blockage of culverts by debris and sediments was assumed to
be the same as that when the final field survey was
performed (August 20, 1986).

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2. All of the small rock "dams" creating the small ponds along
Stone Creek were assumed to fail. The only dam assumed to
hold was an earthen dam that diverts water to the platted
drainageway at station 51+80.

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Roughness coefficients (Manning's n values) varied from 0.020 to
0.040 in both the channel and in the overbank areas. The starting water
surface used in the computer program was critical depth at sta 2+95,
since a very steep drop begins at this location.

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