FLOOD04145

• County: Adams
• Community: Thornton, Adams County
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 10/1/1980
• Prepared For: Thornton, Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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IV.

SUMMARY OF PHASE A ANALYSIS

As was discussed at the outset of this report, the Major Drainageway Planning
Study for Direct Flow Area 0054 transpired in two phases. During the initial phase,
Phase A, the consulting team gathered information, formulated and evaluated various
alternatives, and recommended a plan to alleviate flood hazards in the Direct Flow Area
Basin. Upon completion of the study, the sponsors and representatives from the Urban
Drainage and Flood Control District, Adams County, and the City of Thornton evaluated
the recommendations, and directed the consultants in the preliminary design of their
selected plan, which culminates in this, the Phase B Report. The following is a synopsis of
the work completed in the Phase A portion of the study.

Flood Plains and Flood Damage

Urban Drainage and Flood Control projects are presently evaluated by comparing
amortized improvement costs with the average annual flood damage costs. The magnitude
of flood damages and costs for flood control improvements are directly dependent upon
estimates of flood flows and limits of the respective flOOdplains. Given the flood
hydrographs for floods in a range of recurrence intervals, dollar amounts for flood damage
and costs for implementing preventative measures are estimated based on an analysis of
the existing hydraulic conditions.

Analyses of the hydraulic characteristics of DFA 0054 and its tributaries were
performed to provide estimates of the water surface elevations of each of the design
floods for the existing channel conditions. A total of approximately 6.3 miles of channel
was included in the analysis; however, only 2.4 miles was included in the Phase A study.
All floodplains are delineated by routing the peak flood flow through the channel and
calculating the depth of flow in the channel and adjacent floodplain. The depth of a flood,
or water surface elevation, is directly related to the conveyance characteristics of the
channel and floodplains: channel geometry, roughness, longitudinal slope, and presence of
obstructions such as culverts, houses and large trees.

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 E. Cross-
sections were digitized when the topographic maps were compiled and have been aug-
mented by field surveys at culverts. Locations of the cross-sections appear on the
Drawings, but 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
cul verts on Direct Flow Area 0054 were determined with the use of nomographs publiShed
by the Bureau of Public Roads, Reference M, and input into HEC-2 program. The
structural integrity of existing culverts, street embankments, detention dams and other
channel features has not been evaluated in this 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, 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.

Structural damage was determined by estimating replacement costs for utilities,
roads, culverts, and so forth. For residential structures, flood damage was estimated by
taking the property valuation obtained from local developers 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 F. As a result of the flood which occurred on May 2,
1979, these curves were modified to reflect less damage than is indicated by the curves
when the flood elevations are lower than the first floor elevation of the structures.
Content damage for residential structures was estimated by applying similar depth related
damage factors to 50% of the estimated structural value.

Other "damages" occur during a flood which are not so easily quantified. Probably
the most significant hazards associated with the occurance of a flood are the threats to
the health, both physical and emotional, and safety of people. Additionally, there are
flood "damages" which result from inconvenience: interruption of traffic flows,
obstruction of emergency vehicle movement, and interruption of domestic services.
Further, there is destruction of floodplain ecology, and parks and recreational
opportunities. These intangibles have been quantified in this analysis by increasing the
direct flood damage estimates by 30%.

For each design flood, the dollar amounts of flood damage were plotted against the
exceedence probability (inverse of the return period) by reach. These plots illustrated
where the bulk of potential flood damages occur. In the lower reaches of DFA 0054,
heavy flood damages result from the more frequent floods. This indicates minor channel
capacity. In Reaches 3 and 4, flood damages rise only in the less frequent floods.

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