Design and Construction of Outlet Works

Learn more about the differences between rehabilitation and replacement of outlet works. (Image Source: DamOutletWorks.org)

Outlet Works Sizing and Capacity

“The sizing of the outlet works should take into account the possibility of using it to reduce the size or frequency of spillway discharges. The necessity of emergency drawdown capability and low flow discharge capability should be considered during the outlet works planning phase. The selection of type and arrangement of outlet works structures should be based upon consideration of the costs of operation and maintenance likely to be incurred during the project life. Reliability under emergency flood conditions is a fundamental operational requirement of outlet works facilities”.^[1]

“Flows through river outlets and canal or pipeline outlets change throughout the year and may involve a wide range of discharge under varying heads. The accuracy and ease of control are major considerations, and a great amount of planning may be justified in determining the type of control devices that can be best utilized”.^[2]

“An outlet works may be used for diverting the river flow or portion thereof during a phase of the construction period, thus avoiding the necessity for supplementary installations for that purpose. The outlet structure size dictated by this use rather than the size indicated for ordinary outlet requirements may determine the final outlet works capacity”.^[2]

Elevation of Outlet Works Inlet(s) / Outlet(s)

“The establishment of the intake level is influenced by several considerations such as maintaining the required discharge at the minimum reservoir operating elevation, establishing a silt retention space, and allowing selective withdrawal to achieve suitable water temperature and/or quality. Dams which will impound water for irrigation, domestic use, or other conservation purposes must have the outlet works intake low enough to be able to draw the water down to the bottom of the allocated storage space. Further, if the outlets are to be used to evacuate the reservoir for inspection or repair of the dam, they should be placed as low as practicable. However, it is usual practice to make an allowance in a reservoir for inactive storage for silt deposition, fish and wildlife conservation, and recreation”.^[2]

“Reservoirs become thermally stratified, and taste and odor vary between elevation. Therefore, the outlet intake should be established at the best elevation to achieve satisfactory water quality for the purpose intended. Downstream fish and wildlife requirements may determine the temperature at which the outlet releases should be made. Municipal and industrial water use increases the emphasis on water quality and requires the water to be drawn from the reservoir at the elevation which produces the most satisfactory combination of odor, taste, and temperature. Water supply releases can be made through separate outlet works at different elevations if requirements for the individual water uses are not the same and the reservoir is stratified”.^[2]

Energy Dissipation

“The two types of energy dissipating devices most commonly used in conjunction with outlet works on concrete dams are hydraulic jump stilling basins and plunge pools. On some dams, it is possible to arrange the outlet works in conjunction with the spillway to utilize the spillway-stilling device for dissipating the energy of the water discharging from the river outlets. Energy-dissipating devices for free-flow conduit outlet works are essentially the same as those for spillways”.^[2]

Outlet Works Conduit Design Considerations

“The construction of conduits through embankments has long been recognized as creating an opportunity for seepage along the conduit surface. Possible consequences of uncontrolled seepage along the conduit are piping of soil material and subsequent embankment failure. This scenario has been identified as the primary cause of failure in many incidents. Therefore, it is critical in embankment design to provide adequate safeguards against this type of potential failure”.^[3]

“Proper pipeline installation involves much more than just covering up the pipe. A buried pipe is a structure that incorporates both the properties of the pipe and the properties of the soil surrounding the pipe. The structural design of a pipeline is based on certain soil conditions, and construction control is important to ensure these conditions are met”.^[4]

“The conduit-foundation contact must not be overlooked as a path for potential piping, particularly when the foundation is earth. Prevention of piping along the conduit consists of providing a smooth, firm contact surface free from loose or disintegrated materials and slush grouted to seal joints in rock foundations. If the foundation surface is subject to deterioration when exposed to the atmosphere, it may be necessary to protect the foundation surface with suitable earthfill, a concrete pad, or an acceptable sealing compound until conduit construction commences”.^[3]

“Filters placed around conduits to prevent piping should encircle conduits on earth foundations. Filters around conduits on firm formation materials should extend only to the foundation surface if the formation has to be excavated by blasting or ripping. The filters should meet the same criteria for dry unit weight and filtering as for other filters within the embankment”.^[3]

“Care should be exercised by the construction staff to ensure that backfill material quality, lift thickness, placement moisture content, and dry unit weight requirements are complied with and that hand-operated compactors are of the minimum mass. Special care should be exercised to ensure that within the impervious zone, no continuous layers or pervious material occur along the conduit. Controlling lift thickness and moisture content is important because the earthfill in the bottom of the lift must be forced laterally against the conduit during the compaction process. The entire construction process along the conduit must be carefully observed and inspected. An area of great concern is compaction of earthfill at the contact between earthfill compacted by special methods along the conduit and earthfill compacted by regular tamping rollers. The compaction of material by special methods along the conduit must extend a sufficient distance from the conduit to be overlapped by the compactive effort specified for rollers in the regular compaction zone”.^[3]

“There are two basic types of pipe, rigid and flexible. Rigid pipe must be supported on the bottom portion of the pipe. Flexible pipe must be supported on both the bottom and on the sides of the pipe. Proper soil support of the pipe is critical to the performance of both types of pipe, and proper inspection of pipe installation is essential in obtaining the required support. Inspection for proper soil support involves checking the: (1) adequacy of soil in trench walls and foundation; (2) type of soil used for bedding, embedment, and backfill; (3) distribution of soil around pipe; (4) density of soil around pipe; (5) deflection of flexible pipe”.^[4]

"The use of an externally shaped circular conduit through an embankment dam should be carefully evaluated due to concerns with the difficulty or inability to uniformly compact the earthfill around the conduit. Precast concrete pipe is the most often used externally shaped circular conduit. The earthfill beneath the haunches of the conduit cannot be adequately compacted with pneumatic tired equipment and requires compaction with hand held tampers. Efforts to obtain proper compaction using hand tampers could cause movement or displacement of smaller conduits. Improper compaction of the earthfill around the conduit and movement of the conduit can result in differential settlement and hydraulic fracture. The use of externally shaped circular conduits differs between the major dam building agencies. Reclamation does not use externally shaped circular conduits in their designs due to concerns about the inadequate compaction of earthfill against the conduit. NRCS allows use of externally shaped circular conduits, if they are constructed on cradles or bedding. The use of externally shaped circular conduits (e.g., precast concrete pipe) requires thorough inspection and strict adherence to proper construction techniques to achieve quality assurance of earthfill compaction."^[5]

“There are numerous reasons why a pipe may develop leaks, including improper foundation or embedment, overpressure and transients, poor jointing, inadequate restraint or blocking, and corrosion”.^[6]