DFHADprototype071409.. - Urban Drainage and Flood Control District

SECTION 2 – STUDY AREA
Flows from the Rainbow Split which are not diverted into the Railroad Split pass over the railroad tracks
and enter a prestressed concrete plant. Within the plant property, these flows split around the primary
building. The flows that split to the south become the Industrial Split, which is discussed below. Flows
which split around the north side of the building continue along the Rainbow Split over Steele Street.
Below Steele Street, the Rainbow Split flows through the corner of a mobile home park and into sparsely
developed horse pastures. The Rainbow Split flows into a series of gravel ponds and discharges into the
South Platte River.
Industrial and Industrial South Splits
The Industrial Split begins as flows splits around the south side of the m ain bui lding of the prestressed
concrete plant. The Industrial Split continues south, paralleling the Railroad through an undeveloped parcel.
Runoff in the Industrial Split which is not div erted into th e Industrial So uth Split, d iscussed below, flows
around the north side of the commercial propert y, overtopping a detention pond and Steele Street, and
rejoins the Rainbow Split east of Steele Street.
South of the aforementioned undeveloped parcel is a newly developed commercial property (Parrot H.C.C.
Subdivision). When the commercial property was constructed, several feet of fill material was added. As a
result of the fill m aterial being pl aced in this location, flows from the Industrial Split are divided again,
creating the Industrial South Split. A ditch along the ra ilroad tracks carries flow in the Industrial South
Split. W ater in this split follows the railroad around the west side and the south side of the comm ercial
property to Steele Street. Flows then overtop Steele Street, flow through sparsely developed land and rejoin
the Rainbow Split at the gravel lakes.
With the existing topog raphy, there is no way for flows fr om the Rainbow Split to rejo in th e Hoffm an
Drainageway. The Rainbow Split overtops the UPRR and splits and rejoins several times before continuing
easterly toward the South Platte River.
2.4 PREVIOUS STUDIES AND FLOOD HISTORY
HOFFMAN MAJOR DRAINAGEWAY PLANNING
FLOOD HAZARD AREA DELINEATION
According to the Flood Insurance Rate Map (FIRM) , there is a Zone A (approxim ate) floodplain along
Hoffman Way. In August 1974 a detailed study of the drainageway between 88 th Avenue and Welby Road
was completed and a Zone AE floodpl ain was recognized. Downstream of Welby Road, the FIRM panel
notes, “500-year flood contained in [the] channel”.
According to the current FIRM, a num
ber of hous es along Hoffma n W ay and in the Hope Court
neighborhood are within a 100-year floodplain. Figure 2-3 illustrates the approximate location of the FIRM
effective floodplain and how it compares to the FHAD 100-year floodplain.
The residents in the watershed have reported their experiences with flooding. At the public meeting, several
of residents in the Hope Court area relayed their stories of flooding. At the same meeting, two members of
the mobile home community southeast of the LCCC and 86 th Avenue imparted their stories of being flooded
multiple times from backwater up the LCCC.
2.5 WETLAND AND RIPARIAN ZONES
No riparian or wetland vegetation ha s been observed within the study area with the ex ception of the South
Platte River banks at the downstream study limits. Ho wever, a thorough investigation should be perform ed
prior to construction.
2.6 FLORA, FAUNA AND THREATENED OR ENDANGERED SPECIES
There are no known threatened flora or fauna with in the study area; however a thorough investigation
should be performed prior to any construction.
The mainstem of Hoffman runs along the south side of 86 th Avenue in an open channel. At the LCCC, the
at-grade crossing with the irrigation ditch is to o sm all to r elease th e larger sto rm events and during the
100-year storm event, water breaks out of the irrigation stru cture overtopping 86 th Avenue. This results in
shallow flooding north of 86 th Avenue.
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