FLOOD08407

• County: Jefferson
• County: Adams
• Community: Arvada, Westminster
• Stream Name: Little Dry Creek
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 4/1/1979
• Prepared For: Jefferson County / Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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IV
SUMMARY OF THE PHASE A ANALYSIS

The Little Dry Creek Study has been conducted in two phases. During Phase A, the
consulting team gathered information, formulated and evaluated various alternatives and
recommended a plan for the Little Dry Creek Drainage Basin. Upon completion of the
Phase A Study, the sponsors and representatives of the individual entities evaluated the
report and selected a revised version of the recommended plan. Following selection of
this plan by the sponsors, the study entered Phase B, which is a preliminary design of the
selected plan. The following is a synopsis of the work completed in the Phase A portion of
the study.

Flood Plains and Flood Damag'e
Analyses of the hydraulic characteristics of Little Dry Creek and its tributaries
were performed to provide estimates of the water surface elevations of each of the design
floods for the existi~ channel conditions. A total of approximately 15.4 miles of channel
was included in the analysis, however only 11.1 miles has been included in the Phase "A"
Study. All floodplains are delineated by routing the peak flow through the channel and
calculating the depth of flow in the channel and adjacent floodplain.
Water surface profiles and floodplains were computed using the most recent version
of the U.S. Army Corps of Engineers HEC-2 computer program, Reference F. Cross
sections were digitized when the topographic maps were compiled and have been
augmented by field surveys at culverts. Locations of the cross-sections and a more
detailed description of the 100-year floodplain is contained in the Flood Hazard Area
Delineation Report.
Channel roughness coefficients (Manning's "n" values) for these computations were
assigned on the basis of field inspection of the floodplain areas. Hydraulic losses through
culverts on Little Dry Creek have been determined with the use of nomographs published
by the Bureau of Public Roads and input into HEC-2 program. The normal bridge routing
of the HEC-2 program was used to calculate losses through non-standard culverts. The
structural integrity of existing culverts, bridges, detention dams and other channel
features was not evaluated in the analysis.
Computed water surface profiles and the floodplain delineation for the 100-year,
future basin conditions event are shown in the Flood Hazard Area Delineation Report for
the main stem and three tributaries at 1"=200' scale. The original mylar FHAD drawings
at a scale of 1"=100' are on file with the Urban Drainage and Flood Control District.
Floodplains for the 2-, 5-, and 10-year events have also been plotted on working drawings
and are on file with the office of the Urban Drainage and Flood Control District.

N- I

Flood damage to property was estimated by categorizing land use activities within
the floodplain. For each design flood, the floodplain was delineated and damage
calculated according to land use category and flow depth. Structural damage was
determined by estimating repacement costs for utilities, roads, culverts, and so forth. For
residential and commercial structures, flood damage was estimated by taking the assessed
property valuation obtained from County tax records, dividing by .30, and then applying a
damage factor according to the difference in the flood depth and the first floor elevation
of the structure. The damage factors were obtained from curves published by the Federal
Insurance Administration, Reference G, Content damage for residential structures was
estimated by applying similar depth related damage factors to 50% of the estimated
structural value. Content data for commercial and industrial properties was obtained by
interviewi~ individual management personnel on a case-by-case basis.
Listed in Table IV-l are the flood damages determined duri~ the Phase 'A' analysis
summarized by jurisdiction. These are the "baseline" conditions against which the
alternative plans were judged.

Alternatives
Because the expenditure of public funds by the Urban Drainage and Flood Control
District and participating local governments requires that the most feasible potential
flood control plans are identified, several flood control plans were considered for
implementation on Little Dry Creek. These alternative plans, considered on a reach by
reach basis, included the following:
A. Maintain the existi~ configuration.
B. A natural type waterway (where feasible) following the general historic
channel determined from old aerial photographs, old mapping, and from local
records.
C. The installation of major underground conduits along the general course of
the historic stream channel, using local right-of-way whenever possible.
D. The use of lined flood channels to reduce right-of-way requirements.
E. Selected or limited structural improvements or additions (culverts, bridges,
irrigation crossings, etc.) and channel improvements (re-alignment, erosion
control, low flow protection, maintenance access velocity control, and so
forth).
F. Detention and/or retention facilities.
G. Acquisition of flood prone properties and relocation of occupants.
H. Non-structural methods such as flash-flood warning, flood insurance,
evacuation plans, etc.