FLOOD00479

• Designation Number: 575
• County: Douglas
• Community: Douglas County
• Stream Name: Plum Creek and East Plum Creek
• Title: FHAD - Plum Creek and East Plum Creek
• Date: 9/3/2004
• Prepared For: Doulgas County
• Floodplain - Doc Type: Floodplain Report/Masterplan

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IV. FLOOD HAZARD AREA DELINEATION

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A,

GEl"ERAL

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A Flood Hazard Area Delineation (FHAD) has been completed for Plum Creek and East Plum Creek.
FlIAD maps have been generated and an: contained in the appendix of this report. The purpose of the
FI fAD mapping is to identify areas. structures, and property which have the potential of being inundated by
the I DO-year naod event. In addition to the FHAD mapping, Jloodways have been defined along Plum
Creek and East Plum Creek to establish the portion of the channel that must remain free of obstmction to
allow for conveyance of the IOO-year flood event without significant increases in water surface devation.
Two floodway limits have been developed for Plum Creek and East Plum Creek, reflecting 0.5 foot
increa..<:;cs and 1.0 foot increases in the energy grade of the channel. The Federal Emergency !\1anagement
Agency (FEMA) uses the 1.0 foot tlood\".ay as part of the Flood Insurance Study (FIS). The 0.5 foot
tloodv,:ay has been adopted by many local entities, including the Urban Drainagt: and Flood Control District.
for regulatory purposes.

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B. FLOODI'LAIl" AND FLOOD\VAY DATA

The lOO-year floodplain delineation and corresponding nood profiles are presented on FHAD dra\\;ngs I
through 17. Additionally, profiles for the 10-year event, typical channel cross-sections, and cross-sections
at hydraulic structures are also presented on the FHAD maps. The pertinent floodplain and floodw.ay
information are shown in Tables I and 2 for Plum Creek and East Plum Creek respectively. These tables
identify the channel cross-section locations: thalweg elevations; 10-,50-, 100-. and SOD-year discharges and
water surface elevations; 100-year velocities. channel topwidths, and cross-sectional area: and left. right.
and total floodway widths for both the 0.5 foot and 1.0 foot floodways.

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111e 10-, 50-, 100-, and 500-year water surface elevations were detemlined using the U,S. Aml)' Corps of
Engineer's step backwater program HEC-RAS (Reference 7). The 0.5 foot and 1.0 foot floodwavs were
established predominately using encroachment method -l of the IIEC-RAS computer program, :-..icthod '"
encroachment provides an equal loss of conveyance in the crosswsection o\"erbanks to achieve a target
change in water surface elevation and resulting energy grade line. In areas \\"-here the method -l
encroachment "\"'as resulting in flood way surcharges outside of the allov..'ablc change in energy grade. or
negative floodway surcharges occurred, a method I encroachment option was used. For a method 1
encroachment, the exact location of the tloodway stationing was defined manually. Typically, when the
method I encroachment was used. the floodway width was set equivalent to the IOO-year floodplain
topwidth (i.e. no encroachment). The tloodway left and right widths are measured from the cha.nnel
stationing line which correlates to Ihe centerline of the channel.

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c.

CO~lI'ARISON WITH THE EFFECTIVE FLOOD Il"SURAl"CE STUDY

:\ comparison was made between the information presented in the FHAD and the effective FlS for Douglas
County and Unincorporated Areas (Reference I). Typically. the Plum Creek I DO-year water surface pr;file
indicates some significant increases as compared to the effective FlS information. For example, the
computed water surface elevation on Plum Creek, just upstream of the Rio Grande A ".enue Bridge is
57-H.S. 111e FlS reports the water surface elevation to be around 57-l2.6 (adjusted to NA YD 1988). The
computed water surface on East Plum Creek, just upstream of State Highway 67 and the Burlington
Northern and Santa Fe Railroad is 5791.6 and 590-l,Q respectively. The FlS reports these water surface
elevations to be 5788.3 and 5900.6. In general. the channel thalweg elevations have decreased, as compared
to the etTective information. Therefore, these increases in water surface elevations observed with the FHAD
data are predominately attributed to differences in frequency of cross-sections used in the hydraulic
analysis, general changes in topography that have occurred since the effective FIS was prepared. and
differences in computational methodology. The effective FIS water surface profiles were based on
computations using an older \VSP-2 computer program (Refen:nce 10). When comparing the 100-year
floodplain delineation from the FHAD \vith the Flood Insurance Rate Map (FIRM) from the FlS, it is
evident that some topography changes may exist between the two studies, as the FIRM floodplain is often
defined well outside of the current [)lul11 Creek and East Plum Creek channel banks, even though the FIS
water surface elevations remain lower than the FIIAD elevations, It should be noted that in most-places the
Plum Creek and East Plum Creek channel banks are very well ddined and often extend tcn or more feet
above the elevation of the tloodplain.

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