Lake Fort Smith dam & reservoir enlargement

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Courtesy of Burns & McDonnell

Client: City of Lake Fort Smith, Ark.
 
Completion Date: 2006
 
Services Provided
  • Intake tower with microtunnels
  • Road relocation
  • Raw water lines
  • Outlet works
  • Pressure reducing station
  • Dam embankment
  • Principal spillway
  • Auxiliary spillway

Project Summary

Burns & McDonnell, together with Shannon & Wilson as principal geotechnical consultant, completed the final design phases for the development of the enlarged Lake Fort Smith Water Supply Reservoir as part of continuing services to the City of Fort Smith, Ark. The project entails the design of a single, enlarged dam and impoundment to replace the existing Lake Fort Smith (8,900 acre-feet) and Lake Shepherd Springs (17,600 acre-feet) reservoirs at the site.

When completed, the enlarged reservoir impounds approximately 84,000 acre-feet of conservation storage. The enlarged dam is constructed as a zoned earth and rockfill structure, which had to be constructed while maintaining the water supply capability of the existing Lake Fort Smith Reservoir. The final design phase followed Burns & McDonnell’s completion of a comprehensive alternatives analysis and preliminary design for the project.

The general contractor for the project is Granite Construction Co. of Watsonville, Calif.

Project Features

Major project components include the following:

  • Intake tower with microtunnels: This new four-cell reinforced concrete structure is 46 feet square at its base and 225 feet tall. Two of the cells, each rated at 100 percent capacity, are for raw water supply and the other two cells are for flow diversion. The intake tower is constructed in the left abutment since the water supply capability of the existing Lake Fort Smith Reservoir had to be maintained during its construction. Existing rock in the left abutment was used as a cofferdam to allow construction of the intake tower in relatively dry conditions. The lower portion of the intake tower was poured directly against a shaft wall that was formed by blasting and excavating the existing rock materials. Each of the two raw water supply cells in the intake tower contains six sluice gates located at various elevations to allow draw off of raw water from the reservoir. Each sluice gate is connected to a 60-inch diameter opening in the intake tower wall. The lowest opening in each raw water supply cell was approximately 52 feet below the water level in the existing reservoir. Thus, the two conduits between the intake tower shaft and the reservoir were installed by the microtunneling method to prevent the reservoir from filling the intake tower shaft during installation of the conduits. Each microtunnel liner pipe had an inside diameter of 60 inches and approximately 150 feet of fine-grained sandstone, with thin shale laminations, separated the intake tower shaft from the point of break through into the existing reservoir.
  • God’s Ranch Road: Relocation of this existing county road was required to allow room for construction of the new intake tower and its associated access pad with standby generator building.
  • Raw water lines: Two new 48-inch diameter PCCP raw water supply lines, each rated at 100 percent capacity, are installed from the new intake tower to the existing Mountainburg Water Treatment Plant. Each line contains pigging facilities to facilitate their cleaning.
  • Outlet works: This facility is comprised of a new 1,300-foot-long tunnel through the left abutment of the dam in addition to a new reinforced concrete stilling basin at the tunnel portal and a new riprap lined outlet channel, which connects to existing Frog Bayou. The bottom portion of the tunnel contains the raw water supply lines and the top portion contains a new reinforced concrete diversion conduit, which connects to the reinforced concrete stilling basin.
  • Pressure reducing station: Initially this structure is a pump station that can be used to deliver raw water to the Mountainburg Water Treatment Plant at any time adequate head is not available after lowering the existing Lake Fort Smith Reservoir level to allow closure of the existing spillway for construction of the new embankment. As the new reservoir level increases the structure will be converted to control and meter raw water flow to the water treatment plant.
  • Dam embankment: The new 190-foot-high by 3,300-foot-long embankment incorporates the existing 89-foot-high Lake Fort Smith Dam, which will allow the reservoir’s normal pool elevation to be raised by 86 feet. The new embankment comprises a clay core, fine and coarse filters, and upstream and downstream rock shells (shale and sandstone) with a total volume of over five million cubic yards.
  • Principal spillway inlet weir/trough: The principal spillway will pass flows (33,000 cfs) up to the 100-year flood event. Flows will pass over a new 250-foot-long reinforced concrete ogee weir and into a new 445-foot-long reinforced concrete trough section.
  • Principal spillway box culvert: This new section of the spillway connects the trough section to the chute section and is designed to carry flow through the new dam embankment. This reinforced concrete structure contains four box sections and is 260 feet long, 68 feet wide and more than 21 feet high.
  • Principal spillway chute: This new 1,400-foot-long section of the spillway connects the box culvert section to the new reinforced concrete stilling basin. The reinforced concrete chute section is 55 feet and 152 feet wide at the upstream and downstream ends, respectively, and contains vertical walls up to 21 feet in height.
  • Auxiliary spillway: More than 5.5 million cubic yards of excavation (shale and sandstone) was required to construct this spillway, which will pass flows (136,000 cfs) in excess of the 100-year flood event. The excavated portion of the spillway is more than 3,300 feet long and 365 feet wide at its control sill. The majority of the excavated shale and sandstone is used to construct the rock shells of the dam embankment. Portions of the spillway are rock bolted to provide stability and exposed shale faces are shotcreted to prevent erosion.

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