FLOOD05467

• County: Adams
• Community: Thornton, Adams County
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 10/1/1979
• Prepared For: Thornton, Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Typical of smaller, direct South Platte River tributaries, the channels in Direct
Flow Area 0054 are not well defined and can contain only minor flows. Development in
the floodplains and agricultural activities have encroached into the channel area partly
because there has never been well defined flow areas and the channels have been cut off
by roads, stock watering or irrigation ponds, and irrigation canals. Until the development
west of Holly Street, there was no regular low flow from the area.

From the confluence with the South Platte River westward to Riverdale Road, the
channel is very small, having been flanked through the years by cropland. The floodplain
widens in this reach, as Figure II-4 depicts during the storm of May 2, 1979. At about 200
feet east of Riverdale Road the channel is intersected by the Lower Clear Creek Canal.
Since no wasteways or diversion channels have been constructed, storm waters are usually
carried by the canal until they overflow and spill back into the fields at points downstream
from the original point of interception.

Between Riverdale Road and Holly Street in Reach 2, the channel historically has
been a grassy swale since there was no regular low flow. In recent years, however, the
runoff from the developing basin upstream has been eroding a channel along the fence
line, see Figure II-5. In December of 1978, a 2l inch diameter concrete culvert was
placed under the swale to conduct the trickle flows which have become a nuisance to the
property owners in recent years. However, the pipe can contain only a small part of the
total storm runoff from a two-year event.

Upstream from Holly Street in Reach 3, the channel is intersected by the Colorado
Agricultural Canal and two detention ponds have been constructed across the channel by
local developers, see Figure 1I-6. No spill or flow diversion structure has been constructed
at the canal and when storm water runoff exceeds the capacity of the lS" RCP placed
beneath Holly Street, the canal intercepts some storm flow and conducts it beneath Holly
Street through the 36" R.C.P. culverts which are part of the canal. This causes sediment
to be deposited in the canal and plugging the culverts, see Figure II-7.

The total detention capacity of the existing ponds in Reach 3 is l2 acre feet. When
this capacity is exceeded and the flow rates surpass about 12 to l4 cfs, runoff will overtop
Holly Street. This has occurred twice in the last 3 years causing extensive erosion damage
to the Holly Street embankment, see Figure II-S.

A 36 inch concrete pipe conducts flow beneath 112th Avenue from Reach 4. The
vacant area at the upstream side of 112th Avenue presently acts as a small detention area
ponding water behind the road embankment, see Figure 1I-9. Most of the basin upstream
from 112th Avenue has been developed and minor storm runoff is handled by underground
sewer systems in the subdivisions. Most of the systems including the 3,400 linear feet of
storm sewer from Cherry Drive to the upstream Study Limits and included in the project
area, are capable of handling runoff from a two-year event.

II-2

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