FLOOD05467

• County: Adams
• Community: Thornton, Adams County
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 10/1/1979
• Prepared For: Thornton, Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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As this study was being conducted, homes were being constructed adjacent to the
channel in Reach 3. The developer had allotted a 30 foot right-of-way for channel
improvements in the upper portion of the reach, and approximately 4 acres of land area
for detention ponds in the lower portion of the Reach. However, these ponds as
constructed are capable only to effectively detain the 2-year runoff. Inadequate spillways
subject these ponds to potential erosion and subject the downstream reaches to additional
flood hazards.

In Reach 4, the basin is nearing total development. The storm sewer system in the
developed areas is generally adequate for floods up to the 2-year event. Except for the
area in the vicinity of the E. 112th Way cul-de-sac, flood problems are less frequent than
in the lower reaches. A detailed discussion of the existing flood hazards on DF A 0054 is
included in the Flood Hazard Area Delineation Report.

Flood Damag-e

The annual potential flood damage to public and private property within the flood-
plain is a function of the development within the floodplain. Unregulated, future develop-
ment would spread to the floodplain area and annual flood damage potential would
increase. With regulation, future flood damage potential would tend to decrease due to
the limitation of construction permitted in the floodplain. In the case of DFA 0054, a
Flood Hazard Area Delineation Report has been recently completed, so baseline conditions
have been based on the existing channel conditions subject to floods generated by the
estima ted future drainage basin conditions.

Flood damage to property is 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 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 G. 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 eleva tions
are lower than the first floor elevation of the structures. Content damage for residential
strllctures was estimated by applying similar depth related damage factors to 5Q% of the
estimated structural value.

Flood damage estimates for each category of land use and design flood are listed in
Table IV-I. These figures represent direct flood damages. 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 flood-
plain ecology, and parks and recreational opportunities. These intangibles have been
quantified in this analysis by increasing the direct flood damage estimates by 30%.

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