FLOOD04145

• County: Adams
• Community: Thornton, Adams County
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 10/1/1980
• Prepared For: Thornton, Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The selected plan for Reach 2 is similar to that of Reach 1. Specifically, the
construction of a 2-year grass channel without increased detention which is to be
enlarged, as a condition of the future development of adjacent lands to convey the 100-
year runoff with increased detention. A constant base flow has developed in recent years
as a result of the subsurface drains and lawn irrigation return flow that discharge into the
drainageway. A 21-inch reinforced concrete pipe, placed through the entire length of
Reach 2 as a temporary measure to alleviate various problems stemming from the base
flow, is to remain under the selected plan. A formal right-of-way or maintenance
easement is necessary to gain access to the improved drainage channel and low flow pipe.
During the initial phase, the culvert beneath Riverdale Road is to be enlarged to pass the
10-year runoff, as required by the Urban Drainage and Flood Control Districts' road
overtopping criteria, to prevent any of the initial design runoff from overtopping the road.
The last phase calls for a final enlargement of the culvert to pass the 100-year flow.

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 calIs for an
underground culvert and overflow channel that would extend from East 112th Avenue
downstream for roughly 650 feet following the existing 30 foot wide right-of-way and
would accommodate the 100-year flow. 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 embankments from larger floods. This would involve enlarging the
storage capacity of the Upper Medema Pond from the existing 4 acre feet to
approximately 12.3 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
and the outlet pipe size reduced to assure full utilization of its storage prior to filling the
Lower Pond. Finally, an outlet, designed to pass the 100-year flood beneath Holly Street
is to be constructed and erosion protection provided on the face of the Holly Street
embankment. This will involve relocating and enclosing the Colorado Agricultural Canal
within the Holly Street embankment, thereby preventing the entry of sil t-iaden storm
runoff into the canal and rendering it inoperable.

The focal point of the recommended plan in Reach 4 is the construction of a 24.8
acre feet capacity detention pond just north of East 112th Avenue. The pond provides a
collection basin for the various storm sewers which drain upland areas, and effectively
detains the flows before discharging them to the proposed double 6 foot by 6 foot rein-

forced concrete box culverts beneath 112th Avenue. This is the change to the directive
memorandum referred to in the introductory paragraph, as the selected plan had called for
the existing 42 inch pipe beneath 112th Avenue to be the sole means of conveying flows
beneath 112th. The former plan had called for runoff beyond the capacity of the existing
pipe to overtop 112th and be directed to the downstream channel by regrading 112th
Avenue. Under the modified plan, a lower level constricted inlet restricts flows to fill the
pond, whereupon an upper level drop type inlet conveys the 100-year flow into the
proposed box culvert.

Further upstream in Reach 4, it is recommended that the storm sewer at the end of
the East 112th Way cul-de-sac be enlarged to accommodate the 100-year storm. The
enlargement would entail the expansion of existing inlets and the installation of necessary
underground culvert which has been tentatively located between the houses at the end of
the cul-de-sac. Further north, on Cherry Drive, the construction of catch basins and
accompanying underground storm sewer is proposed. Both this storm sewer pipe and the
one from East 112th Way are to continue to the pond north of East 112th Avenue. Flows
in excess of the capacity of these storm sewers flow down 112th, where curb cuts and
catch basins direct them into the pond or the channel below.

Improvements to the Middle and Northern Branches of Direct Flow Area 0054 are
not economically warranted at the present time due to la.ck of potential flood damage.
Some measures, however, have been proposed to better accommodate flow which will
increase with the onset of upper basin development. These include: culverts beneath
Riverdale Road and 112th Avenue, flow separation structures at the Colorado Agricultural
and Lower Clear Creek Canals, and erosion control measures. A storm sewer system is
recommended in the upper area of the Middle Branch to retain runoff in the basin of origin
and minimize flood hazards. Formal channels must be cut at the lower end of each of
these tributary branches to link them with the main stem. Finally, a 100-year detention
pond at 112th Avenue of sufficient size to maintain a historic 100-year peak outflow is
proposed for the Middle Branch.

Inherent to any flood control plan is the regulation of existing floodplain and the
careful monitoring of all drainageway improvements. Unregulated, future development
would spread to the floodplain area and annual flood damage potential would increase.

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