Burns & McDonnell

Water system master plan


Courtesy of Courtesy of Burns & McDonnell

Client: City of Fort Smith, Ark.

Completion Date: 2002

Services Provided

  • Water supply/delivery
  • Hydraulic modeling and design
  • Water quality analyses
  • Water supply hydrology
  • Flood hydrology
  • 3-D project rendering
  • Physical hydraulic models
  • Spillway hydraulics
  • Design plans and specifications
  • Construction services

Project Summary 

Burns & McDonnell has completed a multiphase water system master plan for the City of Fort Smith, Ark. Potential water sources were identified to provide projected water supply needs. In all, 14 potential surface water sources, as well as groundwater, were studied. Each alternative was evaluated in regard to potential yield, water quality, water treatability, environmental/permitting issues, and construction and operation cost. The selected alternative includes a major enlargement of the Lake Fort Smith Dam and Reservoir, the most downstream of two dams in series on Frog Bayou that serves as the City’s primary source of supply.

Burns & McDonnell completed the water supply and flood hydrology of the spillway hydraulics design, the diversion design; general concept design memorandum; the technical design memorandum, the contract’s bid documents – plans and specifications for the design of the enlarged Lake Fort Smith Dam and Water Supply Reservoir. The Burns & McDonnell team developed Contract 2 bid documents for the dam construction. When completed in 2006, the construction included a raising of the zoned dam embankment; a new spillway system; a new intake tower and outlet works.

Project Features

As part of the preliminary design development on this project, staff conducted and developed a 2 percent risk yield and firm yield water supply analysis for the basin; developed extensive economic profiles and cost analysis for the selection of design alternatives; and conducted extensive site investigations for the analysis of environmental, geotechnical, geologic and cultural impact of the project.

The work included comprehensive hydrologic and hydraulic evaluation for spillway and diversion design. The hydraulic design of the dam and outlet works was being developed using USACE HEC-1, HEC-RAS and HMR-52 software. General design methodology utilized for the project includes USACE and USBR standards.

Analysis and design:

  • Water supply yield analysis and capacity projections including detailed reservoir simulations and yield risk analysis.
  • Basin characterization and hydrologic analysis of the 75 square mile drainage basin including development of the Probable Maximum Flood.
  • Hydrologic and hydraulic design of the new spillway and outlet works system, including detailed hydraulic design of: a principal spillway system encompassing a side channel ogee weir discharging to a concrete chute; two baffled jump basins for the stilling basins of the principal spillway and outlet works tunnel conduit; a 410-foot-wide rock cut auxiliary spillway; outlet works with diversion conduit and high head flood gates.
  • Raising of the zoned, earthen/rock fill dam embankment by an additional 100 feet, incorporating approximately 4 million cubic yards of additional fill and resulting in the impoundment of a single water supply reservoir covering nearly 1,500 surface acres replacing the two current reservoirs at the site (the existing Lake Fort Smith and Lake Shepherd Springs).

The peak design rate of discharge through the principal spillway is approximately 28,000 cfs; the peak design rate of discharge through the auxiliary spillway is approximately 136,000 cfs. The outlet works will consist of an 8-foot diameter horseshoe arch tunneled through the left abutment; the outlet works will serve for diversion of flows during construction and is designed for a peak rate of discharge of 4,000 cfs. An unusual requirement of the design was the need to develop a construction diversion and overall sequencing scheme capable of maintaining an adequate supply of water in the existing reservoirs while properly providing for the safety of the work in progress.

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