FLOOD08658

• County: Adams
• Community: Thornton, Adams County
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 11/1/1986
• Prepared For: Thornton, Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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and crosses the floodplain at a severe angle of skew. The
elevated road surface would cause the water surface to rise
to a wide floodplain immediately upstream of the crossing.
An industrial greenhouse complex encroaches into the
resulting floodplain.

include the 160th Avenue crossing, which would be submerged
to a depth of 8.6 feet. Several farmsteads would be
inundated along the fringes of this pond, as well as one
located in the overbank area upstream of the 160th Avenue
crossing.

3. A major flow obstruction results from the Union Pacific
Railroad crossing, which spans the floodplain atop an
embankment upstream of Colorado Boulevard. The culvert
through this embankment is sized for a storm smaller than
the 10-year event. Di scharges exceedi ng thi s frequency
wou 1 d overtop the embankment near its intersect i on with
168th Avenue. At this site, the configuration of the
highway and the railroad embankment would split the flood
flows, causing half to flow north over 168th Avenue, and
half to flow east over the railroad embankment.

Floodplain and Floodway Data

The Flood Hazard Area Delineation Floodplain Maps show the outline of the
100-year flood event for present channel and floodplain conditions. The
drawings are supplemented by Table VII, "Floodplain and Floodway Reference
Data. "

The northern flow is not addressed, since it would exit the
study area as it crosses 168th Avenue. However, the
eastern flow over the rail road embankment would return to
the Big Dry Creek channel along the reach between the
railroad embankment and 168th Avenue. Topographic features
in this area would distribute the return flow along much of
the reach length.

Table VII presents a listing of descriptive information, 100-year flood
discharges, stream stationing, thalweg elevations, 100-year flood
elevations, 100-year floodplain widths, and average channel velocities at
cross sections used in this study.

The floodplain is illustrated for this distributed flow
situation, as well as for the situation which would result
if the culvert at the rail road crossing were improved to
convey the 100-year flow. The floodway along this reach is
calculated for the latter situation only.

The floodway data in this table define the area required to convey the
100-year flood without causing significant increases in stage or velocity
due to future development outs i de the estab 1 i shed fl oodway. The fl oodway
was determined based on the criterion specified by the Urban Drainage and
Flood Control District which allows a maximum rise of 0.5 feet in the
energy grade line, assuming approximately equal reductions in conveyance on
both sides of the floodplain. The floodway is a high hazard area
characterized by relatively deep water and high flow velocities and should
remain free of any development which would obstruct flow.

4. The Union Pacific Railroad embankment rises 32 feet above
the channel thalweg at the crossing structure. The
upstream inundation resulting from a 100-year flood would
form a pond of approximately 2800 acre-feet in volume
upstream of the embankment. The extent of this pond would

The floodway data consist of floodway widths to the right and left of the
channel station line, the total floodway width, the mean velocity in the
floodway, and the 100-year flood elevations resulting from flows being
confined to the floodway.

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