NPDP Dam Dictionary

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Term AKA Definition
Obligate riparian species A species that depends completely upon habitat along a body of water.
Off-peak energy Electric energy supplied during periods of relatively low system demand.
Ogee crest The shape of the concrete spillway crest that represents the lower profile of the undernappe of a jet of water flowing over a sharp-crested weir at a design depth.
Ohm The unit of resistance. The value of resistance through which a potential difference of one volt will maintain a current of one ampere.
Omnivore Animal that eats both vegetable and animal substances.
On-peak energy Electric energy supplied during periods of relatively high system demand.
Onfarm Activities (especially growing crops and applying irrigation water) that occur within the legal boundaries of private property.
Operating basis earthquake OBE The earthquake that the structure must safely withstand with no damage. All systems and components necessary to the uninterrupted functioning of the project are designed to remain operable during the ground motions associated with the OBE. This includes the dam, appurtenant structures, electrical and mechanical equipment, relays, spillway gates, and valves. For most usage in Reclamation, the OBE is specified to have a 90% probability of nonoccurrence in a 25-year-exposure period. This is equivalent to a recurrence interval of 237 years. Economic considerations for specific projects may lead to consideration of other values.
Operating log See logbook.
Operational losses Losses of water resulting from evaporation and seepage.
Optimum moisture content Optimum water content The one water content (percent of dry weight of the total material) of a given soil and a given compactive effort that will result in a maximum dry density of the soil.
Option value Option value Value associated with people who know they can visit an area in the future if they so desire. Also a reversible decision or an option to develop at some time in the future have option value.
Original ground (surface) The surface of the earth as it exists in an unaltered state (i.e. prior to any earthwork). See existing ground.
Outage Period during which a generating unit, transmission line, or other facility is out of service.
Outflow The amount of water passing a given point downstream of a structure, expressed in acre-feet per day or cubic feet per second. Water flowing out of a body of water
Outlet An opening through which water can be freely discharged from a reservoir to the river for a particular purpose.
Outlet capacity The amount of water that can be safely released through the outlet works.
Outlet channel Exit channel Channel downstream from terminal structure that conveys releases back to the natural" stream or river. Channel can be excavated in rock or soil with or without riprap soil cement or other types of erosion protection."
Outlet works A combination of structures and equipment required for the safe operation and control of water released from a reservoir to serve various purposes, i.e., regulate stream flow and quality; release floodwater; and provide irrigation, municipal, and/or industrial water. Included in the outlet works are the intake structure, conduit, control house-gates, regulating gate or valve, gate chamber, and stilling basin. A series of components located in a dam through which normal releases from the reservoir are made.
Outlet works tower A tower within a reservoir that contains the mechanisms to open the entrance to the outlet works
Overamping Exceeding the rated capacity of a system.
Overexcavation Excavation beyond specified or directed excavation. Removing unsuitable foundation material.
Overflow spillway Ogee spillway A spillway that has a control weir that is ogee-shaped (S-shaped) in profile.
Overhaul Movement of (earth) material far enough so that payment, in addition to excavation pay, is made for haulage. The distance in excess of that given as the stated haul distance to haul excavated material.
Owner Economy Loss Description Describe the damages/losses that occurred to facilities owned by the dam owner, including damage to the dams, power house, etc.
Owner Name The name of the owner of the dam.
Owner Type The type of owner could be one the following: 1.Federal 2.State 3.Local Government 4.Public Utility 5.Private
Oxbow channel A natural U- shaped channel in a river as viewed from above.