Changes

Jump to navigation Jump to search
→‎Where you can go for help: added Seasonal Dams sections
Line 69: Line 69:  
*Alternatives to Push-Up Dams (video), U.S. Bureau of Reclamation, Oregon Department of Environmental Quality, et al.
 
*Alternatives to Push-Up Dams (video), U.S. Bureau of Reclamation, Oregon Department of Environmental Quality, et al.
 
*“Fish Screening Criteria for Anadromous Salmonids,” National Marine Fisheries Service, http://swr.ucsd.edu/hcd/fishscrn.htm.
 
*“Fish Screening Criteria for Anadromous Salmonids,” National Marine Fisheries Service, http://swr.ucsd.edu/hcd/fishscrn.htm.
 +
==Seasonal Dams==
 +
Seasonal dams are temporary structures that can be erected to store water for immediate or later diversion, or removed to allow flows and (in most cases) fish to pass. Inflatable dams and flashboard dams (also known as stop log dams) are the most common types of seasonal dams. When in operation, both types of dams raise the river level allowing water to be diverted through a channel or pipe.
    +
===Inflatable dams===
 +
Inflatable dams are made of thick, laminated rubber and nylon tubes that are anchored to a concrete foundation across the streambed. The tube can be filled automatically or manually with air or water to create a barrier, and subsequently deflated to lie flat on the foundation (see photo). The inflatable tubes usually last between 25 and 50 years.
 +
 +
===Flashboard dams===
 +
Flashboard dams usually involve a concrete foundation and frame into which boards are inserted to block the stream flow and raise the water level to allow for diversion.
 +
 +
====Advantages====
 +
Seasonal dams provide the flexibility to store and divert water or allow water, sediment and fish to pass when the dam is not in use. In certain cases, pools created by temporary dams can provide cool water habitat for species to over-summer in warm streams.<ref> Marty Gingras, California Department of Fish and Game, personal communication, 31 October 2001.</ref> Seasonal dams are usually designed to deliver water by gravity, thus avoiding costs associated with pumping.
 +
 +
====Disadvantages====
 +
Despite the flexibility of seasonal dams, they can cause significant problems for fish populations. For example, a dam operator might need to block the flow when fish are migrating to or from the ocean, thus delaying or entirely stopping their up or downstream movement. In addition, seasonal dams can block juvenile or adult fish from moving to cold-water refuges that help them survive high summer temperatures.<ref> NOAA Fisheries and California Department of Fish & Game has increasingly denied requests for permits to operate seasonal dams, in part because they can prevent juveniles from accessing cold-water areas.</ref> In some cases, the concrete structure that anchors flashboards or inflatable tubes can create barriers to fish passage even when the dam is not in operation, if scouring below the structures lowers the streambed elevation significantly, or if the water flowing over the foundation or tube is too shallow or too fast. These foundations inhibit the dynamic nature of the river, interfering with natural stream migration. This can modify sediment transport processes and cause problems with excessive scour or undesirable deposition. In addition, the pipe or channel diverting water from the temporary pool can entrain fish if not properly screened. Seasonal dams can affect streams negatively in other ways as well, including increasing water temperatures, harboring predator species, eliminating water flows and associated aquatic habitat downstream and inducing erosion of the bed and banks of streams and introducing major fluctuations in water levels upstream of the dam impacting biota, aquatic vegetation and riparian homeowners. In recent years, operators have experimented with strategies to change the shape of the tubes used in inflatable dams to improve downstream passage while the tube is inflated. The most common strategy is to create a notch or to place a strap over the tube so that it cannot fully inflate at that location. These notches increase flow depth over the tube, which is safer and more appealing to out-migrating juveniles. These notches can sometimes also be used for adults migrating upstream if the jump is not too high.
 +
 +
====Costs====
 +
The cost of inflatable and flashboard dams depends on many factors, including the size of the stream to be impounded, channel shape and material and the complexity of the required design. In 1989, the Alameda County Water District in California constructed a 300-foot long 13-foot tall air filled inflatable dam on Alameda Creek. The concrete foundation cost $1.6 million and the bladder cost $1.6 million.<ref> Steve Peterson, Alameda County Water District, personal communication, 13 December 2002.</ref>
 +
 +
===Case Study, Seasonal Dams===
 +
The Susquehanna River in Pennsylvania is home to the Adam T. Bower Dam (popularly referred to as Sunbury Dam), which is the world’s largest inflatable dam.  Shikellamy State Park maintains the dam, inflating it with air each spring and deflating it each fall in order to create a seasonal three-thousand-acre compound called Lake Augusta. The lake, which is approximately eight feet deep at the dam, provides 13 miles for recreation such as boating and water skiing. The rubber bags measure twelve millimeters thick and sit flat upon cement casings when not in use.
 +
This dam exemplifies in many ways, however, how inflatable dams can be misused. For example, during the 2003 season this dam was inflated in April to accommodate recreational and commercial interests and remained inflated until early fall, effectively blocking the Susquehanna during the entire migratory season (April – July) for American shad. Because of the pressures to inflate the dam early in the year, the state has agreed to let the dam operator meet migratory fish passage obligations through the construction of a fish ladder. The dam is currently providing no fish passage and has not provided any since its installation even though an inflatable dam was chosen over a more permanent structure entirely for the purpose of providing for fish passage. To learn more about the Adam T. Bower Dam, visit http://www.visitcentralpa.org/OUTDOORS/Fabridam.htm.
 +
 +
====Where you can go for help====
 +
*For more information, contact your state natural resources agency, such as the Department of Natural Resources or Department of Environmental Protection.
 +
*“Rubber Dam Hydraulics: Hydraulic Design of Inflatable Flexible Membrane Dams.” University of Queensland, Australia, http://www.uq.edu.au/~e2hchans/rubber.html
    
==References==
 
==References==
 
<references/>
 
<references/>
 
[[Category:Water]]
 
[[Category:Water]]

Navigation menu