FLOOD10481

• County: Douglas
• Basin: South Platte
• Title: Chatfield Reallocation Study - Roadway Analysis Study
• Date: 5/11/2004
• Prepared For: CWCB
• Prepared By: Sear Brown
• Floodplain - Doc Type: Floodplain Report/Masterplan

The conceptual plan proposes to move the roadway to the north out of the areas
to be inundated along the north (left) side of the reservoir. The proposed plan also
contemplates raising the roadway embankment around the upstream end of the
reservoir or building a new roadway further west and upstream of the west end of the
reservoir at its new normal water elevation. A new bridge will be built across Deer Creek
at nearly the toe of the Colorado Highway 121 embankment. The existing bridge across
the South Platte River will be replaced if raising the western stretch of the existing
roadway embankment is the option selected, A new bridge across the South Platte
River will be built if the selected option is to build the new stretch of roadway around the
west end of the reservoir above the elevation of the new reservoir water level.
The stretches of the existing roadway to be rebuilt. with the exception of the
eastern-most approximately 0.4 miles or so, are located on the fiood plain created by
the confluence of Deer Creek and the South Platte River and the flood plain of the
South Platte River. Several ravines and draws also end at the flood plain so soils and
rocks washed down these ravines and draws have contributed to the building of the
flood plain. The exposures of the ground that can be seen and our experience suggest
the soils in the flood plain are a variable thickness of clays overlying alluvial sands and
gravels mixed with some cobbles and probably boulders, The clays appear to be
reasonably stiff through parts of the flood plain and softer through others, depending
upon where runoff water ponds or runs off the ground surface. The underlying sands
and gravels are likely dense, The ground water level appears to be several feet below
the ground surface except where there are depressions of the ground surface and the
ground water will likely be shallower.
The eastern-most approximately 0.4 miles or so of the proposed new roadway
alignment crosses higher ground and is routed down the gently sloping side of the Deer
Creek valley, The ground under this stretch of the rerouted alignment appears to be a
thin layer of clays overlying claystone bedrock that has probably weathered to a clay in
its upper several feet. The clays and the claystone are expansive and will swell when
they get wet.
The proposed location for the new bridge across Deer Creek is over-grown by
brush and trees, The ground visible in the bottom of Deer Creek is sands, gravels, and
cobbles, The east abutment appears to be clays and claystones like exposed on the
valley slope just to the east of the creek and the west abutment appears to be clays
over the alluvial sands, gravels and cobbles.
The location of the existing bridge across the South Platte River and the
proposed new bridge location across the river are also over-grown with brush and trees.
The ground visible in the sides and on the bottom of the South Platte River channel is
mostly alluvial sands, gravels, and cobbles, At places there appears to be a thin layer of
very sandy clays or silts over the alluvial materials,
Roadway. The segments of the realigned roadway will cross expansive clays
overlying claystone (east approach to Deer Creek Bridge) or clays overlying the
SEAR.BROWN 2
CHATFIELD RESERVOIR REALLOCATION STUDY
"ROADWAY ANALYSIS STUDY'
ClL I T JOB NO. FC.2998

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