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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STUDY AREA DESCRIPTION

Direct Flow Area 0054 is situated predominantly in T 67 N, R 2 W, of the 6th Prime
Meridian, between Grange Hall Creek and Brantner Gulch. It is a west bank tributary to
the South Platte River. From its origin in the hills along the west bank of the Platte
River, the basin slopes to the east and consists of several short tributary channels which
either combine or flow directly into the South Platte River.

Drainage Basin Characteristics

The 0054 drainage basin, as shown on Sheet 9 of the drawings, is triangular in
shape, approximately 2.1 miles along the South Platte River and 2.8 miles from the South
Platte to the westernmost point of the drainage basin boundary. It drains approximately
4.3 square miles. Approximately 610 acres of the basin are within the City of Thornton
and 2,140 acres are in Adams County. The basin is also intersected by Riverdale Road,
the Lower Clear Creek Canal and Colorado Agricultural Canal, all of which, including
Holly Street, are directly impacted by the drainage patterns within the basin.

The development status of Direct Flow Area 0054 has been classified as "C" by the
Denver Regional Council of Governments (DRCOG) in their 1971 Project REUSE, meaning
the basin has undergone partial development and has been evaluated to have a moderate to
low probability of future development. As it stands now, however, the basin west of Holly
Street in Thornton is expected to experience nearly total development as single family
residential communities. East of Holly Street, the land use is still rural residential and
agricul tural and this status is not expected to change. For the purposes of developing
runoff hydrographs, the land use patterns shown on the "Future Land Use" map, Sheet 11
of the drawings, were assumed.

Study Reach Description

From the Platte River westward to Riverdale Road, there are three tributaries
which drain DFA 0054 and run roughly parallel to each other. The southern and longest
tributary basin, 0054-1, rises at an elevation of 5,270 feet above mean sea level and falls
about 240 feet in its 2.4 mile course. The northern most tributary, 0054-3, has two main

branches that merge below Riverdale Road to a common drainage swale that continues to
the South Platte River. The basin of the middle tributary, 0054-2, is bounded by the
basins of 0054-1 to the south and 0054-3 to the north, and is the smallest of the three. All
three tributaries have been included in the Flood Hazard Area Delineation analysis, but
only 0054-1 has been included in the Major Drainageway Planning study.

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 channel flows 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, a
historic channel is virtually undetectable. 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 locations other than 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 a fence line.
In December of 1978, a 21 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
Agricul tural Canal and the two detention ponds that have been constructed across the
channel by local developers. No emergency spill or flow diversion structure has been
constructed at the canal and when storm water runoff exceeds the capacity of the 18" 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, causing sediment
to be deposited in the canal and plugging the culverts.

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