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

.'
I
I
I
II
I
I
I
I,
I
I
I
I
I)
I
I
I
I
I

7. Drowing GH-7 (Sheet 9 of 30), Gronge Hall Creek Sto. 130+00 to 100+00
The main channel in this reach is narrow and incised with 0 relatively steep,
longitudinal slope. The majority of the storm flows ore carried in the main channel with
high flow velocities. Overbank flooding also occurs which increases the flood plain width.
The channel is crossed by the Colorodo Agricultural Conal (pipe). The potential flood
damoges in the reach consist of erosion and c1eon-up, vegetol growth wash-out, ond
potential loss of livestock.
The improvements consists of the construction of four concrete check drop
structures along the main chonnel to control erosion.
8. Drawing GH-8 (Sheet 10 of 30), Grange HolI Creek Sta. 100+00 to 70+00
The main channel in this reach is narrow and incised with 0 steep longitudinal slope
from Stotion 100+00 to Riverdale Rood. Below Riverdale Rood the channel has been
altered by numerous years of forming with the resulting Grange HolI Creek flood plain
being very brood and shallow. A low flow or pilot channel below Riverdale Rood is in
evidence through this reach, although it oppears to be used os on irrigation droinage ditch.
The potential flood damages consist of erosion, loss of crops and livestock, damage to
form equipment, and limited residential inundation.
The improvements in this reach consist of the following:
(I) Two concrete check drop structures above Riverdale Rood.
(2) Construction of 0 triple wide 4' x 8' RCBC culvert (60') and the 40' extension of
the existing bridge ot the Riverdale Rood crossing (20' each side of Riverdale
Rood).
(3) Construction of 0 30' extension on the north end of the existing Lower Clear
Creek Conal siphon at approximotely Station 79+30.
(4) A channelizotion of the area of the Grange Hall Creek from Riverdale Rood to
Station 70+00. The channel is designed to keep the 100-yeor flood within 0
2('0' right-of-way below Riverdale Rood.

The channelization improvements below Riverdale Rood ore to be the responsibility

of the private property owners in the area, if the property owners determine that the

improvements are economically justified when considering potentiol flood damage costs.

The confluence of the Undefined Tributary with Grange Hall Creek occurs just above

Riverdale Rood. The investigation of the flood hazards and drainage improvements for

the tributary were not port of this study.

9. Drawings GH-9 (Sheet II of 30), GH-I 0 (Sheet 12 of 30), and GH-II (Sheet Q

of 30), Grange HolI Creek Station 70+00 to 0+00

The channel in this reach has been altered by numerous years of forming with the

resulting Grange Hall Creek flood plain being very brood and shallow. A low flow or pilot

channel is in evidence through this reach, although it appears to be used os on irrigation or

drainage ditch. The potential flood damages consist of erosion, loss of crops, loss of

livestock, damage to form equipment, limited residential inundation, and interrupted

operation of the sand and gravel plant along the bonks of the South Platte River. A

portion of this reach is also subject to flooding from the South Platte River and lies within

the currently defined South Platte River flood plain.

The improvements in the area included on the GH-9, GH-IO, and GH-II drawings

consist of channelization for the area of Grange Hall Creek from Station 70+00 to 4+00.

The channel is designed to keep the 100-year flood within 0 200' right-of-way in this area.

The channelization improvements below Riverdale Rood ore to be the responsibility of the

private property owners in this area if the property owners determine that the

improvements ore economically justified when considering potential flood damage costs.

No public monies ore to be allocated to the improvements below Riverdale Rood.

16