FLOOD09085

• Designation Number: 97
• County: Adams
• Community: Thornton
• Stream Name: Grange Hall Creek
• Basin: South Platte
• Title: Major Drainageway Planning - Grange Hall Creek, Phase B, Thornton, Adams County
• Date: 12/1/1977
• Designation Date: 1/19/1978
• Floodplain - Doc Type: Floodplain Report/Masterplan

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(3) The addition of two 53" x 83" HE-RCP culverts under the railroad to improve
the culvert capocity under the Union Pacific Railroad.
(4) The addition of two 48" x 76" HE-RCP culverts under York Street.
(5) The construction of one 3' concrete check drop below York Street and two 3'
concrete check drop structures below the private pond at Station 36+50.
(6) The construction of 0 35' wide concrete spillway with stilling basin for the
pond. Rip-rap is to be placed on the upstream face of the dam embankment.
The generol improvements are to meet the requirements of the State
Engineer's Office. Since the dam is privately owned, then the improvements
ore the responsibility of the owner.
19. Drawing TNE-2 (Sheet 24 of 30), Tributary NE, Station 30+00 to 0+00, Gronge
Hall Creek Confluence
The main chonnel in this reach is narrow and incised with 0 steep, longitudinal slope.
There is one private pond at the upper end of the reach without a formal spillway. The
flood plain is influenced by backwater and overtopping of 112th Avenue but generally
remains within the channel. The secondary flood plain which occurs along both sides of
112th Avenue (see olso discussion for Drawing NE- J) rejoins the main channel where the
main channel crosses 112th Avenue (opproximately Station 16+00). The proposed
improvements in this reach are as follows:
(J) A 3S' wide concrete spillwoy and stilling basin for the pond at Station 29+00.
Rip-rap will be placed on the upstream face of the dam as part of the
improvements to the dam ond spillway as required by the State Engineer.
(2) Selected structural improvements in the main channel will consist of the
construction of fifteen 3' concrete check drop structures throughout the reach
to the confluence with Gronge Hall Creek.

(3) The addition of two 60" RCP culverts ond 112th Avenue with an outlet stilling

basin.

The two dams at Stations 29+00 and 36+50 are shown with spillway and embankment

improvements. No overtopping of the dams occurs for the 100-year; however, there is

minimal freeboard of about one foot and embankment and spillway improvements would

probably be required by the State Engineer in the future. The cost of the improvements to

both ponds ore not jusitifed from a flood control standpoint; therefore, the cost and

responsibility of the improvements is to be born by the owner.

20. Drawing TSE-I (Sheet 25 of 30), Tributary SE, Station 41 +00 to 20+00

The channel in this reach is a broad V-type channel with a steep, longitudinal slope.

The flood plain in the reach is determined by the inadvertent storage obtained upstream of

the Colorado Blvd. embankment. The remainder of the reach would have shallow flow.

There ore no proposed channel improvements except flood plain zoning and flood

insurance and improvements to the Union Pacific Roilroad and Colorado Blvd. crossings.

There is an existing 27" CMP culvert under the Union Pacific Railroad crossing and there

is an additional 42" RCP culvert proposed for the Union Pocific Railroad crossing. At

Colorado Blvd. there is an existing 18" x 30" CMP arch culvert and an additional 48" RCP

is proposed.

21. Drawing TSE-2 (Sheet 26 of 30), Tributary NE, Station 20+00 to 0+00, Grange

HolI Creek Confluence

The channel in this reach is essentially non-existent except for a portion between

the Colorado Agricultural Canal and the dam embankment. However, this portion of the

channel does not receive any significant amount of storm runoff since the low areo in the

dam embankment (j.e. unimproved spillway) discharges towards 104th Avenue and away

from the main chonnel area. With the existing channel conditions and the runoff storage

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