NPDP Dam Dictionary

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Term AKA Definition
Losing stream A stream or reach that contributes water to a zone of saturation.
Lotic Flowing water, such as rivers and streams.
Low density material A material having a low weight per unit volume either as it occurs in its natural state or after compacting it, such as loess with a density of 80 pounds per cubic foot or less.
Low hazard A downstream hazard classification for dams in which no lives are in jeopardy and minimal economic loss (undeveloped agriculture, occasional uninhabited structures, or minimal outstanding natural resources) would occur as a result of failure of the dam. This classification also applies to structures other than dams.
Macroclimate The climate representative of relatively large area.
Macrohabitat An extensive habitat presenting considerable variation of the environment, containing a variety of ecological niches and supporting a large number and variety of complex flora and fauna.
Macrophytes A plant large enough to be seen by the naked eye.
Magnitude A rating of a given earthquake, independent of the place of observation. It is calculated from measurements on seismographs and it is expressed in ordinary numbers and decimals based on a logarithmic scale. A measure of the strength of an earthquake, or the strain energy released by it, as determined by seismographic observations. See Richter scale.
Main channel The deepest or central part of the bed of a stream, containing the main current.
Main channel pool Reach of a stream with a low bed elevation, relative to rapids or riffles.
Mainstream The main course of a stream where the current is the strongest.
Maintenance All routine and extraordinary work necessary to keep the facilities in good repair and reliable working order to fulfill the intended designed project purposes.
Masonry dam Any dam constructed mainly of stone, brick or concrete blocks jointed with mortar.
Mass Concrete Any large volume of concrete cast-in-place, generally as a monolithic structure. Dimensions of the structure are of such magnitude that measures must be taken to cope with the generation of heat and the resulting volume changes and cracking.
Massive head buttress dam A buttress dam in which the buttress is greatly enlarged on the upstream side to span the gap between buttresses. See solid head buttress dam.
Maximum controllable water surface The highest reservoir water surface elevation at which gravity flows from the reservoir can be completely shut off.
Maximum credible earthquake MCE The largest earthquake that a fault or other seismic source could produce under the current tectonic setting. It is a believable event which can be supported by all known geologic and seismologic data. The MCE can be associated with specific surface geologic structures and can also be associated with random or floating earthquakes (movements that occur at depths that do not cause surface displacements). The seismic evaluation criteria determines which faults or seismic sources are assigned an MCE.
Maximum design earthquake MDE The earthquake selected for design or evaluation of the structure. This earthquake would generate the most critical ground motions for evaluation of the seismic performance of the structure among those loadings to which the structure might be exposed. For example, if a site were assigned MCEs from two separate sources, the MCE which would be expected to generate the most severe ground motions would be the maximum design earthquake. The response of the structure to specific ground motion parameters (frequency, duration, etc.) needs to be considered in specifying this event. In certain cases, more than one maximum design earthquake may be specified to reflect the differing response of various components of the structure to earthquake loading. Maximum controllable water surface. The highest
Maximum water surface The highest acceptable water surface elevation with all factors affecting the safety of the structure considered. It is the highest water surface elevation resulting from a computed routing of the inflow design flood through the reservoir under established operating criteria. This surface elevation is also the top of the surcharge capacity.
Mean number The average number
Median Middle value in a distribution, above and below which lie an equal number of values.
Medium-thick arch dam An arch dam with a base thickness to structural height ratio between 0.2 and 0.3 (previously defined as between 0.3 and 0.5).
Megawatt MW One million watts.
Megawatt-hour MWh One million watt- hours.
Membrane diaphragm A membrane or sheet or thin zone or facing, made of a flexible impervious material such as asphaltic concrete, plastic concrete, steel, wood, copper, plastic, etc. A cutoff wall or core wall, if thin and flexible, is sometimes referred to as a diaphragm wall." "
Metalimnion Middle layer of a lake or reservoir with a rapid temperature decrease with depth. See stratification and thermocline.
Metamorphic Rock compressed or changed by pressure, heat, or water.
Microclimate The climate of a small area, particularly that of the living space of a certain species, group or community.
Microhabitat A small, specialized, and effectively isolated location. See macrohabitat.
Microsystem irrigation Method of precisely applying irrigation water to the immediate root zone of the target plant at very low rates.