Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 11—Appropriate Emergency Actions11.4.2 PumpingPumping of the reservoir water can be used to drain relatively small reservoirs. Asingle or a series of high capacity (e.g., 3,000-gal/min) portable pumps can bedelivered to the damsite to assist in draining of the reservoir. The pumps can bepositioned in the spillway entrance or on the embankment dam crest (figure 152) andallowed to discharge into the spillway or outlet works, or the pump can be placedover and down the downstream face of the embankment dam to the downstreamtoe. At the discharge end of the pumps, the area should be properly protected toensure that the discharging does not cause erosional damage.Pumps are usually self-contained and trailer or skid mounted. They can usually berented in nearby major metropolitan communities, or they can be delivered and setup by the supplier. State or local flood control agencies may be another readilyavailable source for high capacity pumps. Pumps are typically gasoline, diesel, orelectrically powered. If electrically powered, a reliable electrical source should beavailable at the damsite. Purchasing pumps requires continuing maintenance on thepump and has a greater one-time expense compared to renting. Purchase may bejustified for remote locations or to fulfill other needs for a high capacity pump by thedam owner, especially if one pump can service multiple dams. See the BalmanReservoir Dam case history in appendix B for an example of using a pump toevacuate a reservoir.Another consideration with pumps is the limited net positive suction head available,(NPSHA) which essentially is the atmospheric pressure less any suction line frictionlosses and the height of the lift. To avoid cavitation, all pumps are rated with a netpositive suction head required (NPSHR). If the NPSHA does not exceed theNPSHR, the pump will not operate. Also, as the NPSHA approaches the NPSHR,the pump capacity decreases. Placing the pump as close to the reservoir watersurface as possible and using large diameter suction lines to minimize friction lossmaximizes pump capacity. For reservoirs deeper than about 15 to 20 feet, pumpslocated on the dam crest or spillway crest may not be able to totally drain thereservoir because the height of the lift from the water surface to the pump by itselfexceeds the NPSHA when reservoir levels are down, unless the pump can be movedalong with the receding reservoir water surface. Because of the potential forcomplications caused by terrain and reservoir sediments, the frequent movement oftrailer-mounted pumps may not be practical. However, floating pumps are availablefor these situations but are usually more expensive and probably not as available forrent as trailer-mounted units.Totally draining the reservoir is usually not necessary to successfully evacuate thereservoir to levels that mitigate embankment dam failure. Usually, only a portion ofthe total reservoir requires evacuation to stop or control the erosion processesoccurring within the embankment dam.281