Definitions M – P

Marsh.   An area of soft, wet, or periodically submerged land, generally treeless and usually characterized by grasses and other low vegetation.

Mature.  Classification for streams which have established flat gradients not subject to further scour.

Maximum Historical Flood.  The maximum flood that has been recorded or experienced at any particular highway location.

Mean Annual Flood.  The flood discharge with a recurrence interval of 2.33 years.

Mean Depth.  For a stream at any stage, the wetted normal section divided by the surface width. Hydraulic mean depth.

Meander.  In connection with streams, a winding channel usually in an erodible, alluvial valley. A  reverse  or  S-shaped  curve  or  series of curves forme-d by erosion of the concave bank, especially at the downstream end, characterized by curved flow and alternating shoals and bank erosions.  Meandering is a stage in the migratory movement of the channel, as a whole, down the valley.

Meander Plug (Clay Plug).   Deposits of cohesive materials in old channel bend-ways.  These plugs  are  sufficiently resistant to erosion to serve as essentially semi-permanent geological controls to advancing channel migrations.

Meander  Scroll.    Evidence of historical meander patterns in the form of lines  visible  on  the inside of meander bends (particularly on aerial photographs) which  resemble  a  spiral  or convoluted form in ornamental design.  These lines are concentric and regular forms in high sinuosity channels and are largely absent in poorly developed braided channels.

Mesh.  Woven wire or other filaments used alone as revetment, or as retainer or container of masses of gravel or cobble.

Mud Flow.  A well-mixed mass of water and alluvium which, because of its high viscosity, and low  fluidity  as  compared  with  water,  moves  at  a  much  slower  rate,  usually  piling  up  and spreading out like a sheet of wet mortar or concrete.

Mulch.  A natural or artificial layer of plant residue or other material that covers the land surface and conserves moisture, holds soil in place, aids in establishing vegetation, and  reduces temperature fluctuations.

"n"  Value.    The roughness coefficient in the Manning formula for determination of the discharge coefficient in the Chezy formula, •  V = C(RS)1/2 , where C = (1/n )R1/6

National  Pollutant  Discharge  Elimination  System  (NPDES).    EPA.s  program to control the discharge  of  pollutants  to  waters  of  the  United States.  NPDES is a part of the federal CWA, which  requires point and non-point source dischargers to obtain permits.  These permits are referred to as NPDES permits.

Natural and Beneficial Floodplain Values.  Includes but are not limited to fish, wildlife, plants, open space, natural beauty, scientific study, outdoor recreation, agriculture,  aqua-culture, forestry, natural moderation of floods, water quality maintenance, and groundwater recharge.

Navigable Waters.  Those stream waters lawfully declared or actually used as such.  Navigable Waters of the United States are those determined by the Corps of Engineers or the U.S. Coast Guard to be so used  in  interstate  or  international  commerce.  Other  streams  have  been  held  as navigable by courts under the common law that navigability in fact is navigability in law.

Negative Projecting Conduits.   A  structure  installed  in  a  trench  with  the  top  below the  top  of trench, then covered with backfill and embankment.  See Positive Projecting Conduit.

Nonactive  Construction  Area.    Any area not considered to be an active construction area. Typically, active construction areas become nonactive construction areas whenever construction activities  are  expected  to  be  discontinued  for  a  period  of  20  or  more  days  during  the  winter season.

Non-Point Sources (NPS). Diffuse sources from which contaminants originate to accumulate in surface water or groundwater.  These sources can add to a cumulative problem with serious health or environmental consequences.

Nonuniform Flow.  A flow in which the velocities vary from point to point along the stream or conduit, due to variations in cross section, slope, etc.

Normal Depth.  The depth at which flow is steady and hydraulic characteristics are uniform.

Normal Water Surface (Natural Water Surface).   The free surface associated with flow in natural streams.

Notice of Intent (NOI).  A formal notice to the EPA or a state agency having delegated NPDES authority that a construction project seeking coverage under a General Permit is about to begin. The NOI provides information on the owner, location, and type of project, and certifies that the permittee will comply with conditions of the construction General Permit.  The NOI is  not  a permit application and no approval is required.  Some local permits may require submittal of a Notice of New Construction (NONC) in lieu of filing a NOI with the state or EPA.

Notice of Termination (NOT).  A formal notice to the EPA or delegated state agency for General Permit site terminating coverage under the permit.

Nourishment.    The  process of replenishing a beach. It may be brought about naturally, by accretion due to the longshore transport, or artificially, by the deposition of dredged materials.

Off-Site Drainage.  Flow of water that originates outside the property.

On-Site Drainage.  Flow of water that originates inside the property.

Open Channel.  Any conveyance in which water flows with a free surface.

Ordinary High Water Mark.  The line on the shore established by the fluctuation of water and physically indicated on the bank (1.5 + years return period)

Outfall.  Discharge or point of discharge of a culvert or other closed conduit.

Outwash.    Debris transported from a restricted channel to an unrestricted area where it is deposited to form an alluvial or debris cone or fan.

Overflow.  Discharge of a stream outside its banks; the parallel channels carrying such discharge.

Overtopping  Flood.    The flood described by the probability of exceedance and  water  surface elevation at which flow occurs over a hydraulic structure, highway, watershed divide, or through structure(s) provided for emergency relief.

Peak Flow.  Maximum momentary stage or discharge of a stream in flood.  Design Discharge.

Pebble.  Stone 10 to 75 mm in diameter, including coarse gravel and small cobble.

Perched Water.  Ground water located above the level of the water table and separated from it by a zone of impermeable material.

Percolating  Waters.   Waters  which  have infiltrated the surface of the land and move slowly downward and outward through devious channels (aquifers)  unrelated  to  stream  waters,  until they reach an underground lake or regain and spring from the land surface at a lower point.

Permeability.    The  property  of  soils  which  permits  the  passage  of  any  fluid.    Permeability depends on grain size, void ratio, shape and arrangement of pores.

Permeable.    Open  to  the  passage of fluids, as for (1) pervious soils and (2) bank-protection structures.

Permit. An authorization, license, or equivalent control document issued by EPA or an approved state agency to implement the requirements of an environmental regulation.

Physiographic Region.  A geographic area whose pattern of landforms differ significantly from that of adjacent regions.

Pier.  Vertical support of a structure standing in a stream or other body of water. Used in  a general sense to include bents and abutments.

Pile.  A long, heavy timber or section of concrete or metal that is driven or jetted into the earth or bottom of a water body to serve as a structural support or protection.

Piping.  The action of water passing through or under an embankment and carrying some of the finer material with it to the surface at the downstream face.

Plunge.  Flow with a strong downward component, as in outfall drops, overbank falls, and surf attack on a beach.

Point  of  Concentration.    That point at which the water flowing from  a  given  drainage  area concentrates.

Point  Sources. A source  of  pollutants  from  a  single  point of conveyance such as a pipe.  For example, the discharge pipe from a sewage treatment plant or factory is a point source.

Poised Stream.  A term used by river engineers applying to a stream that over a period of time is neither degrading or aggrading its channel, and is nearly in equilibrium as to sediment transport and supply.

Positive Projecting Conduit.  A structure installed in shallow trench with the top of the conduit projecting above the top of the trench and then covered with embankment.  See  Negative Projecting Conduit.

Potamology.  The hydrology of streams.

Practicable.  Capable of being done within reasonable natural, social, and economic constraints.

Precipitation.  Discharge of atmospheric moisture as rain, snow or hail, measured in depth of fall or in terms of intensity of fall in unit time.

Prescriptive Rights.   The operation of the law whereby rights may  be  established  by  long exercise of their corresponding powers or extinguished by prolonged failure to  exercise  such powers.

Preserve.   To avoid modification to the functions of the natural floodplain environment or to maintain it, as closely as practicable, in its natural state.

Probability.  The chance of occurrence or recurrence of a specified event within a unit of time, commonly expressed in 3 ways. Thus a 10-year flood has a chance of 0.1 per year and is also called a 10%-chance flood.

Probability of Exceedance.   The  statistical  probability,  expressed  as  a  percentage,  of  a hydrologic event occurring or being exceeded in any given year.  The probability (p) of a storm or flood is the reciprocal of the average recurrence interval (N).

Probable  Maximum  Flood.    The flood discharge that may be expected from the most  severe combination of critical meteorological and hydrological conditions that are  reasonably  possible in the region.

Pumping  Plant.    A complete pumping installation including a storage box, pump or  pumps, standby pumps, connecting pipes, electrical equipment, pumphouse and outlet chamber.