U.S. Fish & Wildlife Service Low Hazard Dams - Association of State ...

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U.S. Fish & Wildlife Service Low Hazard Dams - Association of State ...

STANDING OPERATING PROCEDURES1.2.3 Service SpillwaysThe service spillway, also referred to as the principal or primary spillway, is the mainstructure over or through which flow is discharged from the reservoir and is the meansused to control the reservoir level. If the rate of flow is controlled by mechanical meanssuch as gates or stoplogs, it is considered a controlled spillway. If the elevation of thespillway crest is fixed, it is considered an uncontrolled spillway. Since service spillwaysare the primary means of controlling reservoir flows, they are generally designed to ahigh standard of performance so that they perform satisfactorily under all flow conditionswithout damage.Common types of service spillways on USFWS dams include stoplog-controlled concretedrop structures, concrete or corrugated metal riser structures, and concrete spillwayscontrolled with tainter gates. Some dams use steel sheet pile instead of concrete to createthe spillway structure. Many USFWS spillways are constructed using both steel sheetpile and reinforced concrete.1.2.4 Emergency SpillwaysEmergency spillways, also referred to as auxiliary or secondary spillways, are designed tooperate only during exceptionally large floods and their purpose is to pass major floodflows around the structure and prevent dam overtopping. Because emergency spillwaysare intended for infrequent use, many are designed to allow some damage to occur duringthe passage of the design flood provided the safety of the embankment is notcompromised.Most emergency spillways consist of an open trapezoidal channel excavated throughnatural materials having an approach channel, a level crest or control section, and anoutlet channel. They are normally covered with a layer of topsoil and vegetated withgrasses adapted to the local environment.Some emergency spillways are equipped with concrete control structures or are armoredwith articulating concrete blocks or other erosion resistant materials to prevent breachingor failure of the spillway where a grass cover cannot provide reliable erosion protection.1.2.5 Low-Level OutletsLow-level outlets provide a means for lowering or draining the reservoir and normallyconsist of a conduit through the embankment equipped with one or more valves. Thelow-level outlet must be able to evacuate the major portion of the reservoir storagevolume by gravity flow. Low-level outlets are sometimes incorporated as a featurewithin a concrete spillway structure such as a stoplog bay extending to the bottom of thestructure or a gated opening at the base of the structure. In some cases, at low hazarddams where the reservoir is very small, low-level outlets are omitted and the reservoir islowered or drained using portable pumps or by installing a temporary siphon. CurrentUSFWS policy requires all inventory dams to have a low-level outlet unless a waiver isobtained.Revision No. 01-4 October 2008

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