FLOOD02825

• Designation Number: 150
• County: Adams
• Community: Thornton
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Direct Flow Area 0054 - Including Major Drainageway Planning DFA 0054 Phase A Report 11/79
• Date: 10/1/1979
• Designation Date: 1/1/1980
• Floodplain - Doc Type: Floodplain Report/Masterplan

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In Reach 1, the implementation of the recommended plan would transpire in es-
sentially two stages. The initial stage calls for a 2-year grass lined channel with design
flows that are substantially reduced as a result of utilizing proposed upstream detention
improvements. The predominantly agricultura.11and use in Reach 1 does not substantiate a
larger channel due to the relatively low amount of probable flood damage. But the
absence of an established wasteway through this Reach necessitates the construction of a
formal channel to convey storm waters and allevia.te much of the damages that are
sustained during the higher frequency storm$. Regulation of the residual floodplain in
Reach 1 gives rise to the final phase of the plan. It is floodplain regulation which would
prevent the unwise development in the floodplain or would require adequate
channelization through any future development to safely convey flows through the reach
without increasing the probable flood damage. A floodway adequate to convey the 100-
year flood then becomes the final stage of tpe plan, with its construction a condition of
the development of adjacent lands. A flow separation structure is recommended where
the improved channel is to cross the Lower Clear Creek Canal, precluding the entry of
unwanted storm waters into the canal during the design flood. Right-of-way acquisition
and an access trail, which is to be constructed alongside the improved channel to provide
for economical and regular maintenance, are both necessary parts of the plan.

The recommendations for Reach 2 are similar to those of Reach 1. Specifically,
the construction of a 2-year grass channel Which is to be enlarged as a condition of the
future development of adjacent lands to convey the 100-year runoff, right-of-way
acquisition, and access trail construction. A 21 inch RCP was recently placed through the
entire length of Reach 2 as a temporary measure to alleviate various problems associated
with the constant base flow that has developed as a result of the subsurface drains and
lawn irrigation return flows that discharge into the drainage way. Field observations
indicate that this pipe is of sufficient size to accommodate outflow from the 18 inch RCP
beneath the Holly Street embankment situated immediately upstream. Since the total
improved detention volume upstream of Holly Street is slightly less than the 10-year
runoff volume, it is feasible at this juncture of the study to consider utilizing the 21 inch
RCP to convey runoff through Reach 2 from at least the 2-year storm. In either case, a .
formal right-of-way or maintenance easement is necessary to gain access to the improved
drainage channel and/or low flow pipe, The culvert beneath Riverdale Road will have to
be enlarged to pass the 10-year runoff, as required by Adams County, -to prevent any of
the initial design runoff from overtopping the road.

In Reach 3, proposed drainage way improvements are bounded by existing right-of-
way limits. The recommended plan in the upper half of the Reach calls for a 100-year
concrete lined channel that would extend from East 112th Avenue downstream for roughly
650 feet following the existing 30 foot wide right-of-way. In the lower half of the Reach,
two existing detention ponds are to be modified to better reduce flood peaks downstream
and to provide protection of pond embankments from larger floods. This would involve
enlarging the storage capacity of the Upper Medema Pond from the existing 4 acre feet to
approximately 16 acre feet, augmenting the existing storage capacity of 7 acre feet of the
Lower Medema Pond. In addition, the spillway of the Upper Pond is to be improved to
convey the 100-year flood and the flow rate of the outlet pipe is to be reduced to that of
the Lower Pond, assuring full utilization of its storage prior to filling the Lower Pond.
Finally, a spillway, designed to pass the 100-year flood, should be constructed and
protection provided on the Lower Pond embankment. This will include relocating the
Colorado Agricultural Canal to the East of Holly Street, thereby preventing the entry of
silt laden storm runoff into the Canal and rendering it inoperable. The narrow gorge
formed by the downstream face of the Lower Medema Pond and the upstream face of the
Holly Street embankment will then serve to collect spillway overflow from the Lower
Pond and funnel flow beneath Holly Street through new culverts, see Fig. 1-2.

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