Lake Supply & Treatment Expansion

Lake Supply & Treatment Expansion

Location: Mountainburg, Ark.

Client: City of Fort Smith

Completion Date: Design: 2003; Lake Construction: 2006; Plant Construction: 2008

Burns & McDonnell, together with Shannon & Wilson as principal geotechnical consultants, 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 and Lake Shepherd Springs reservoirs at the site. When complete, the reservoir will impound approximately 84,000 acre-feet of conservation storage. The dam is constructed as a zoned earth and rockfill structure. The project includes construction of a new spillway system, a new intake tower and outlet works. The final design phase followed Burns & McDonnell’s completion of a comprehensive alternatives analysis and preliminary design for the project.

Burns & McDonnell has developed comprehensive hydrologic and hydraulic evaluations, analysis and design including development of the basin characterization, the water supply yield analyses, the flood hydrology and the spillway and outlet works hydraulic design. Further, U.S. Army Corps of Engineers, Federal Emergency Management Agency, National Resources Conservation Service and U.S. Bureau of Reclamation standards for dam design and dam safety engineering have been incorporated as general design methodology. The structure is designed as a single purpose reservoir impounded by a high hazard dam.

In conjunction with Alden Research Laboratory Inc., scale models of the reservoir, intake tower and principal spillway were constructed and hydraulically tested. Burns & McDonnell computer models, utilized in the hydrologic and hydraulic design development, include the USACE HEC-1, HEC-RAS and HMR-52 software to analyze the flood hydrology and spillway hydraulics. Also utilized was the Burns & McDonnell-developed software for yield and reservoir operations analysis — RESNET.

  • Supply reservoir enlargement
  • 30 million-gallon-per-day (MGD) water plant renovation
  • New 20-MGD dual train water plant addition
  • Pulsating clarifiers
  • Auto-backwash cluster filters
  • Corrosion control

Burns & McDonnell engineers developed the concept, preliminary and detailed hydraulic design of:

  • The principal spillway system with a peak design discharge of approximately 33,000 cfs.
  • The diversion works, as part of the intake/outlet works, will discharge to a baffled, impact-type stilling basin downstream which has an approximate peak capacity of 4,000 cfs.
  • An auxiliary spillway system is provided to discharge and pass flood event flows for greater than the 100-year event. This spillway is approximately 410 feet wide and is an uncontrolled, open channel cut into bedrock in the right abutment with a peak design discharge of approximately 136,000 cfs.

Lake Shepherd Springs, the upper lake of the pair, will be inundated by the new Lake Fort Smith after it has been raised to the design top of dam. General contactor for the reservoir construction is Granite Construction of Watsonville, Calif.

Influent flow to the Mountainburg Water Plant will be controlled by sleeve valves and meters in the flow control structure.

Key hydrologic and hydraulic design analysis featured:

  • Development of the water supply yield analysis for 2 percent probability yield and firm yield including detailed reservoir simulations. Extensive statistical analysis was developed extrapolating regional gage data to the ungaged watershed.
  • Basin characterization and hydrology for the more than 75-square-mile drainage basin was developed including infiltration and runoff analysis utilizing Horton-Holton methodology and development of Synder’s unit hydrograph with regional coefficient comparisons to accurately model the flood runoff.
  • Flood hydrology was developed for incremental floods from a five-year event up to and including the 100-year event. The probable maximum flood (PMF) was developed from HMR-52 software analysis for the probable maximum precipitation (PMP) and the preferred storm orientation.
  • Hydraulic design was based on use of the PMF event for hydrologic adequacy of the dam.
  • A study analysis of varying conservation pools and flood storage was developed to determine an optimum conservation pool level.
  • Downstream hydrology and flood inundation mapping was developed to incorporate the influence of the drainage from tributaries as part of compliance with the U.S. Army Corps of Engineers 404 permitting requirements.