Flood, levee, and erosion control glossary - M

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Macroalgae

Non-rooted aquatic plants commonly referred to as seaweed.

MAIA

The MAIA program is an integrated environmental assessment program being conducted by USEPA, Region 3, and USEPA's Office of Research and Development, in partnership with other federal and state agencies. See http://www.epa.gov/maia.

Major Disaster

Any natural catastrophe or, regardless of cause, any fire, flood, or explosion in any part of the United States which in the determination of the President, causes damage of sufficient severity and magnitude to warrant major disaster assistance under the Robert T. Stafford Disaster Relief and Emergency Assistance Act.

Major Flood

A general term indicating high water that causes extensive inundation and property damage, usually characterized by evacuation of people and animals and closure of highways.

Major Flooding

Flood conditions resulting in extensive inundation and property damage. Typically characterized by the evacuation of people and livestock and the closure of both primary and secondary roads. Also see Minor Flooding and Moderate Flooding.

Major Storm

A precipitation event that is larger than the typically largest rainfall for a year.

Manning's Equation

An equation used to predict the velocity of water flow in an open channel or pipelines. V= 1.486R2/3S1/2 / n where: V is the mean velocity of flow in feet per second, R is the hydraulic radius in feet, S is the slope of the energy gradient or, for assumed uniform flow, the slope of the channel in feet per foot; and n is Manning's roughness coefficient of the channel lining.

Manure

The fecal and urinary defecation of livestock and poultry.

Map, Topographic

A representation of the physical features of a portion of the earth's surface as a plane surface, on which terrain relief is shown by a system of lines, each representing a constant elevation above a datum or reference plane. See Topographic Map.

Marien Chronic Criteria

Water quality criteria established by the U.S. EPA which concentrations of constituents in water which, if not exceeded, are protective of aquatic ecosystems.

Marsh

An area of soft, wet, or periodically submerged land, generally treeless and usually characterized by grasses, woody plants, such as trees and shrubs and other low vegetation.

Photo of Marsh Vegitation.

Marsh, Diked

A former salt marsh which has been protected by a dike.

Marsh, Salt

A marsh periodically flooded by salt water.

Mass Wasting

The movement of large volumes of earth material downslope.

Maximum Contaminant Level (MCL)

The maximum level of certain contaminants (set by EPA under the federal Safe Drinking Water Act) permitted in drinking water supplied by a public water system. MCLs are enforceable standards and are established on the basis of health effects, treatment capability, monitoring availability, and costs. These standards apply to finished (treated) drinking water.

Maximum Probable Flood

The largest flood for which there is any reasonable expectancy.

Mean Annual Flood

The average of all the annual flood stages or discharges of record. It may be estimated by regionalization, correlation, or any other process that can furnish a better estimate of the long-term average than can the observed data. Some investigators arbitrarily define the mean annual flood as the stage or discharge having an exceedence interval of 2.33 years.

Mean Depth

Average depth; cross-sectional area of a stream or channel divided by its surface or top width.

Mean High Water (MHW)

The average height of the maximum elevation reached by each rising tide over a specific 19-year period. MHW is the reference base for structure heights, bridge clearances, etc.

Mean Low Water (MLW)

The average height of the minimum elevation reached by each falling tide over a specific 19-year period. All depth measurements in coastal waters and all depths shown on navigation charts are referenced to MLW.

Mean Rise

The height of mean high water above the elevation of the reference or datum of chart.

Mean Sea Level (MSL)/ Mean Tide Level (MTL)

MSL is the arithmetic mean of hourly heights observed over a specific 19-year period. MSL is very nearly the same as Mean Tide Level MTL (also known as half-tide level) is the arithmetic mean of mean high water and mean low water.

Mean Velocity

The average velocity of a stream flowing in a channel or conduit at a given cross-section or in a given reach. It is equal to the discharge divided by the cross-sectional area of the reach.

Mean Water Level

The mean surface level as determined by averaging the heights of the water at equal intervals of time, usually at hourly intervals.

Meander

A broad, looping bend in a stream channel.

Meandering Stream

An alluvial stream characterized in planform by a series of pronounced alternating bends. The shape and existence of the bends in a meandering stream are a result of alluvial processes and not determined by the nature of the terrain (geology) through which the stream flows.

Measure

This refers to an individual flood proofing method.

Measured Sediment Discharge

The quantity of sediment passing a stream cross section in a unit of time as computed with data measured by sampling. (i.e.Sampling with suspended sediment samplers provides the measured sediment discharge of suspended sediment. There will be an unmeasured sediment discharge which must be added to that value to obtain the total sediment discharge for the cross section.)

Mechanically Stabilized Earth

Use of steel bars or other material to stabilize the earthen mass behind a retaining wall.

Mesohaline Waters

Waters having salinity between 5 and 18 ppt.

Mesotrophic

Waters containing a moderate nutrient content, and therefore, more biological productivity.

Metalimnion

The middle layer of a thermally stratified lake or reservoir. In this layer there is a rapid decrease in temperature with depth. Also called the thermocline.

Metals

Elements, such as mercury, lead, nickel, zinc and cadmium, that are of environmental concern because they do not degrade over time. Although many are necessary nutrients, they are sometimes magnified in the food chain, and they can be toxic to life in high enough concentrations. They are also referred to as heavy metals.

Methemoglobinemia

Blood related condition found in babies due to nitrate poisoning. This poisoning limits the blood's ability to carry oxygen thereby causing a baby to look blue-hued, also known as "blue baby syndrome."

Method Detection Limit (MDL)

The minimum concentration of a substance being analyzed that has a 99 percent probability of being identified.

Microphytes

Microorganisms found in soil.

Microscopic Soil Particles

Clay and silt; particles that cannot be observed by the unaided human eye.

Mid-Atlantic Estuaries

For MAIA, the Mid-Atlantic Estuaries are defined as Delaware Bay and its tributaries, Chesapeake Bay and its tributaries, and the coastal bays of the Delmarva Peninsula.

Mid-Atlantic Region

For MAIA, the Mid-Atlantic Region is defined as the watershed of the Chesapeake Bay, Delaware Estuary, and the Delmarva coastal bays. It includes all or portions of Virginia, West Virginia, Maryland, Delaware, Pennsylvania, New Jersey, and New York. Specifically, the proper boundaries for the MAIA region includes the Upper New Cataloging Unit (05050001), portions of the Allehgeny Cataloging Unit (05010001), Conewango Cataloging Unit (05010002), and French Cataloging Unit (05010003) that extends into New York.

Migration

The movement of chemical contaminants through soils or groundwater.

Million-Gallons Per Day (MGD)

A measure of water flow.

Mine Tailings

Tailings are waste products from the mining industry that are finely ground material left over in containment areas or discharged to receiving waters after valuable metals are extracted.

Mineral Soil

A soil consisting predominantly of, and having its properties determined by, mineral matter usually containing less than 20% organic matter, but sometimes containing an organic surface layer up to 30 cm thick.

Minimal Flood Hazard Areas

Areas between the 100-year and the 500-year flood boundaries are termed Moderate Flood Hazard Areas. The remaining areas are above the 500-year flood level and are termed Minimal Flood Hazard Areas.

Minor Flooding

Flooding resulting in minimal or no property damage but some public inconvenience. Also referred to as Nuisance Flooding. Also see Major Flooding and Moderate Flooding.

Mississippi River Levee System

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