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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obtained above Colorado Blvd. little runoff will be discharged from the pond and,

therefore, the flood plain is limited to the flow in the ditch along 104th Avenue and some

overland flow.

The following are the proposed improvements in the reach:

(I) Raise the dam embankment at the southeast end to an elevation of 5,172 feet

to prevent overtopping in this area.

(2) A 25 foot wide concrete spillway is proposed for Brewer Reservoir with a rip-

rap stilling basin.

(3) Rip-rap is to be placed on the upstream face of the dam as part of the

reservoir and spillway improvements.

(4) An 18" CMP pipe is proposed for installation to serve as the outlet work for

the reservoi r.

(5) A 48" RCP is proposed for construction under the Colorado Agricultural Canal.

With the proposed improvements the flood plain will be along the main channel as is

shown on the drawing.

22. Drawing S-I (Sheet 27 of 30), Typical Channel Cross Sections

This drawing presents existing cross sections for the Grange Hall Creek basin. The

proposed improved channel cross sections, where applicable, are also shown over the

existing cross sections.

23. Drawing S-2 (Sheet 28 of 30), Typical Channel Cross Sections

This drawing presents the existing channel cross sections and improved sections for

the tributary drainage basins.

24. urawing S-3 (Sheet 29 of 30), Typical Cross Sections

This drawing presents the existing channel cross sections and improved sections for

the tributary drainage basins.

25. Drawing D-I (Sheet 30 of 30), Details

This drawing presents selected details for the preliminary design of the proposed

drainage facilities. Also shown is the recommended check drop structure design with a

tabulation of structure dimensions for each check drop structure indicated on the plan and

profi Ie drawings (Sheets 3 through 26).

E. James L. Carpenter Dam

A water supply/recreation dam and reservoir for the cities of Thornton and

Northglenn has been previously designed (1973) to be constructed in Grange Hall Creek

just upstream of Colorado Blvd. The reservoir was designed to contain 2, I 00 acre-feet of

storage and have a surface area of about 135 acres at a normal high water elevation of

5,185 feet. The dam was designed as a zoned earth embankment with a maximum height

of about 65 feet with a crest length of about 4,000 feet. The spillway for the structure is

a 135 foot wide, broad-crested weir and is located in the center of the dam.

The reservoir was to serve as 0 water supply source and recreational facility for the

cities of Thornton and Northglenn. Due to the relatively low average annual damage

potential for the storm flows in Grange Hall Creek the dam and spillway was not designed

specifically for flood control and, therefore, was not considered a part of the alternative

matrix to be evaluated in the Grange Hall study. In the Phase A portion of the Grange

Hall Major Drainageway Planning Study an evaluation of the flood control benefits of the

proposed Carpenter Dam was made as a necessary part of the analysis.

The hydraulic evaluation of the dam and reservoir in Phase A was based upon the

data presented in the specifications and drawings for the dam (Ref. 15). The 5-year and

I DO-year storm flows for the future developed basin were routed through the reservoir and

channel routed through to the South Platte River. The results of the reservoir routings

indicate a significant reduction in the flood peaks below the dam. Since the presence of

Carpenter Dam would have a significant effect on the peak flows below the dam and

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