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<br />Maximum flood control level. The highest elevation of the flood control <br />storage. <br /> <br />Minimum operating level. The lowest level to which the reservoir is drawn <br />down under normal operating conditions. <br /> <br />Observation well. A hole used to observe the groundwater surface at <br />atmospheric pressure within soil or rock. <br /> <br />Outlet. An opening through which water can be discharged. <br /> <br />Outlet Works. A device to provide controlled releases from a reservoir. <br /> <br />Parapet wall. A solid wall built along the top of a dam (upstream or <br />downstream edge) used for ornamentation, for safety of vehicles and <br />pedestrians, or to prevent overtopping caused by wave runup. <br /> <br />Penstock. A pressurized pipeline or shaft between the reservoir and hydraulic <br />machinery. <br /> <br />Phreatic surface. The free surface of water seeping at atmospheric pressure <br />through soil or rock. <br /> <br />Piezometer. An instrument used for measuring fluid pressure (air or water) <br />within soil, rock, or concrete. <br /> <br />Piping. The progressive development of internal erosion by seepage. <br /> <br />Plunge pool. A natural or artificially created pool that dissipates the <br />energy of free falling water. <br /> <br />Probability. The likelihood of an event occurring. <br /> <br />Probable Maximum Precipitation (PMP). Theoretically, the greatest depth of <br />precipitation for a given duration that is physically possible over a given <br />size storm area at a particular geographical location. <br /> <br />Reservoir. A body of water impounded by a dam and in which water can be <br />stored. <br /> <br />Reservoir regulation (or operating) procedure. Operating procedures that <br />govern reservoir storage and releases. <br /> <br />Reservoir surface area. The area covered by a reservoir when filled to a <br />specified level. <br /> <br />Riprap. A layer of large uncoursed stone, precast blocks, bags of cement or <br />other suitable material, generally placed on the upstream slopes of an embank- <br />ment or along a watercourse as protection against wave action, erosion or <br />scour. Riprap is usually placed by dumping or other mechanical methods and in <br />some cases is hand placed. It consists of pieces of relatively large size as <br />distinguished from a gravel blanket. <br /> <br />-10- <br />