FLOOD10336 ‎(2)‎

• County: Adams
• Community: Brighton, Commerce City and Thornton
• Stream Name: South Platte River
• Title: Flood Hazard Area Delineation, South Platte River, Adams County
• Date: 4/1/2005
• Prepared For: UDFCD; Adams County; Brighton, Commerce City, Thornton, Metro Wastewater
• Prepared By: CDM
• Floodplain - Doc Type: Floodplain Report/Masterplan

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4/8/05

oscillates with the 1977 FHAO floodplain. The east portion has decreased in size compared to the 1977 FHAO
through the Worthing Pit Lake 4 reach. However, near Tower Lake, the existing floodplain extends approximately
300 feet beyond the 1977 FHAO.

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Upstream of l04th A venue, between stations 625+00 to 603+00, the west bank FHAD floodplain is approximately
200 feet narrower than the previous floodplain. As with several of the differences already described, this is a
relatively flat area, and small changes in water surface elevations can result in substantial changes in floodplain
extent. Although the Master Plan model used a split flow analysis in the vicinity of 104th A venue, this study
modeled the area as divided flows because it was determined that there were multiple hydraulic connections
between the main channel and the east overbank.

The floodplain extending from Franklin Street to 1-270 is significantly wider than the previous FHAD. Here, the
existing floodplain flows into an area that previously was not floodplain. In April 1998, when the aerial
photography used for this study was updated, construction was in progress in this area, resulting in this area being
added to the floodplain. The west overbank in this area was modeled as a split flow reach using Denver FHAD
split flows (UDFCO 1985b). Upstream of 1-270, between stations 969+00 to 962+00, the FHAD floodplain extends
roughly 350 feet further west than that in the previous FHAD. The change in water surface elevations between the
two models is less than one foot in this area. The revised floodplain delineation is within the National Map
Accuracy Standards (:I: 1 foot). The revised water surface elevation lies on the other side of the elevation contour
line compared to the Master Plan water surface elevation; therefore, the resultant floodplain is slightly different.
The difference in extent (350 feet) is attributable to the lack of topographic relief in this area.

Section 4
Hydraulic Analyses

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Downstream of Henderson Road, the construction of the E-470 Bridge has significantly altered the shape of the
floodplain. The bridge was designed to fully pass the 100-year flood flows and thus the floodplain has been
narrowed at this spot. Due to the construction of the bridge and associated earthwork, the topography in this area
has been altered. The floodplain was mapped based on the LOMR data, not based on the mapping used as the
basis for this FHAO.

Upstream of 1-76, between stations 905+00 to 900+00, the FHAD floodplain extends roughly 1,400 feet east of the
main channel, whereas the previous floodplain does not. There is a narrow swale, approximately 30 feet wide in
this area, which allows water to flow into the topographically depressed area east of the main channel. The
algorithm used to generate the previous FHAO floodplain did not account for this hydraulic connection, and
excluded this area from the floodplain.

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Upstream of the Brighton split flow reach (station 290+00 to 250+00) the west FHAO floodplain is approximately
300 feet narrower than the Master Plan floodplain. The east bank FHAD floodplain (station 275+00 to 260+00)
extends approximately 150 feet further than the Master Plan floodplain. Oownstream of the Brighton Ditch
diversion, the 100-year floodplain shows similar results as the 1977 FHAD. The existing floodplain matches the
1977 floodplain with minor exceptions. These exceptions occur along both sides, with the current floodplain
boundary lying within the 1977 boundary.

Immediately upstream and downstream of the Highway 224 Bridge (station 885+00 to 887+50) the FHAO
floodplain is approximately 400 ft narrower than the floodplain shown on the previous FHAD. This appears to be a
result of topographical changes to the area, notably the filling of a former gravel pit.

Downstream of Engineers Lake to 88th A venue, the 100-year floodplain is significantly wider than that shown on
the 1977 FHAD, but similar to that shown on the 1995 Flood Insurance Rate Map (FIRM) for Adams County and
Incorporated Areas. Along the west side of this reach, West Gravel Lake No.3 is now included in the floodplain.
Differences in the immediate vicinity of 88th Avenue can be attributed to the different model algorithms used for
predicting water surface elevations at bridges. In addition, this area was modeled as a split flow in the previous
FHAD, but was modeled as a single reach in this study

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The existing 100-year floodplain downstream of 88th Avenue to Fulton Ditch is similar to the previous FHAD with
differences occurring primarily along the west side, where development and degradation on the channel bed has
reduced the extent of the floodplain. Further floodplain reductions under existing conditions were observed
southeast of the intersection of McKay Road and 96th Avenue. The floodplain has expanded slightly near McKay
Road, where the eastern embankments north and south of McKay Road are overtopped under existing conditions
while the 1977 FHAD shows these areas to be dry.

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From Fulton Ditch to Brantner Ditch, the 100-year floodplain matches the 1977 FHAO floodplain with one
exception. The west side of the reach starting at approximately the Fulton Ditch headgate has increased flooding.
This occurs over 104th A venue and into a gravel pond and extends north to the Grange Hall Creek outfalL The rest
of the reach closely matches the 1977 FHAD floodplain boundary.

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North of Brantner Ditch to Henderson Road has two distinct differences occurring, one along the west side and the
other along the east. The west side has experienced an increase in floodplain extent while the east side has been
reduced. Inside the floodplain, additional islands have formed, mainly due to the formation of new gravel ponds
in the area.

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Significant changes in the 100-year floodplain have occurred around Henderson Road. North of Henderson Road
on the west side of the reach, the Riverdale Dunes Golf Course has been constructed. This has created a reduced,
discontinuous floodplain resembling a braided channel. Downstream of the golf course, the floodplain boundary

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