Glossary of Coastal Terminology: N - R
NATIONAL TIDAL DATUM EPOCH (NTDE): A period of 19 years adopted by the National Ocean Service as the period over which observations of TIDES are to be taken and reduced to average values for TIDAL DATUMS.
NAUTICAL MILE: Also known as geographical mile, its length is 1852 meters (6076.115 feet), approximately 1.15 times as long as the statute mile of 5280 feet.
NEAP TIDE: TIDE of decreased range occurring semimonthly as the result of the moon being in quadrature. The NEAP RANGE of the TIDE is the average SEMIDIURNAL range occurring at the time of NEAP TIDES and is most conveniently computed from the harmonic constants. The NEAP RANGE is typically 10 to 30 percent smaller than the mean range where the type of TIDE is either SEMIDIURNAL or mixed and is of no practical significance where the type of TIDE is DIURNAL. The average height of the high waters of the neap tide is called neap high water or high water neaps (MHWN), and the average height of the corresponding LOW WATER is called neap low water or low water neaps (MLWN).
NEARSHORE: (1) In beach terminology an indefinite zone extending seaward from the SHORELINE well beyond the BREAKER ZONE. (2) The zone which extends from the SWASH ZONE to the position marking the start of the OFFSHORE zone, typically at WATER DEPTHS of the order of 20 m.
NEARSHORE CURRENT: The current system caused by WAVE action in and near the BREAKER ZONE, and which consists of four parts: the SHOREWARD MASS TRANSPORT of water; LONGSHORE CURRENTS; RIP CURRENTS; and the LONGSHORE movement of the expanding heads of RIP CURRENTS.
NECK: (1) The narrow strip of land which connects a PENINSULA with the mainland, or connects two ridges. (2) The narrow band (rip) of water flowing seaward through the SURF. See also RIP CURRENT. See Figure 7.
NETWORK: A set consisting of: (a) stations for which geometric relationships have been determined and which are so related that removal of one station from the set will affect the relationships (distances, directions, coordinates, etc.) between the other stations; and (b) lines connecting the stations to show this interdependence.
OCEAN: The great body of salt water which occupies two-thirds of the surface of the Earth, or one of its major subdivisions.
OCEAN CURRENT: A non-tidal current constituting a part of the general oceanic circulation.
OFFSHORE: (1) In beach terminology, the comparatively flat zone of variable width, extending from the SHOREFACE to the edge of the CONTINENTAL SHELF. It is continually submerged. (2) The direction seaward from the SHORE. (3) The zone beyond the NEARSHORE zone where SEDIMENT motion induced by WAVES alone effectively ceases and where the influence of the sea bed on WAVE action is small in comparison with the effect of wind. (4) The BREAKER ZONE directly seaward of the LOW TIDE line.
OFFSHORE WIND: A wind blowing seaward from the land in the coastal area.
ORDINARY HIGH WATER MARK (OHWM): (SMP) That mark that will be found by examining the bed and banks and ascertaining where the presence and action of waters are so common and usual, and so long continued in all ordinary years, as to mark upon the soil a character distinct from that of the abutting upland, in respect to vegetation as that condition exists on June 1, 1971, as it may naturally change thereafter, or as it may change thereafter in accordance with permits issued by a local government or the department.
ORDINARY TIDE: This expression is not used in a technical sense by the U.S. Coast and Geodetic Survey, but the word "ordinary" when applied to TIDES, may be taken as the equivalent of the word "mean". Thus "ordinary high water line" may be assumed to be the same as "mean high water line".
ORTHOPHOTOGRAPH: A photograph prepared from a perspective photograph by removing distortions and displacements of points caused by tilt, relief, and perspective.
OSCILLATION: A periodic motion backward and forward. To vibrate or vary above and below a mean value.
OVERWASH: (1) The part of the UPRUSH that runs over the crest of a BERM or structure and does not flow directly back to the OCEAN or lake. (2) The effect of WAVES OVERTOPPING a COASTAL DEFENSE, often carrying SEDIMENT landwards which is then lost to the BEACH system.
PARTICLE SIZE: in dealing with SEDIMENTS and sedimentary ROCKS it is necessary that precise dimensions should be applied to such terms as CLAY, SAND, PEBBLE, etc. Numerous scales have been suggested, but in this work, the Wentworth-Udden scale is used, as it is widely accepted as an international standard. In the table which follows, particle size ranges are shown, but within most groups further subdivision is possible; for example, SAND may be described as very fine, medium, coarse, very coarse, etc. Particle size is normally determined by hand measurement of PEBBLES, COBBLES, and BOULDERS; sieving of GRAVEL, SAND, and SILT; and ELUTRIATION of SILT and CLAY.
PARTICLE SIZE RANGES:
PERIGEAN RANGE: The average SEMIDIURNAL range occurring at the time of the PERIGEAN TIDES and most conveniently computed from the harmonic constants. It is larger than the mean range where the type of TIDE is either SEMIDIURNAL or mixed and is of no practical significance where the type of TIDE is DIURNAL.
PERIODIC CURRENT: A current caused by the tide-producing forces of the Moon and the Sun, a part of the same general movement of the SEA that is manifested in the vertical rise and fall of the TIDES. See FLOOD CURRENT and EBB CURRENT.
PERMANENT CURRENT: A current that runs continuously independent of the TIDES and temporary cause. Permanent currents include the fresh water discharge of a RIVER and the currents that form the general circulatory systems of the OCEAN.
PHOTIC ZONE: The zone extending downward from the OCEAN surface within which the light is sufficient to sustain photosynthesis. The DEPTH of this layer varies with water clarity, time of year and cloud cover, but is about 100 m in the open OCEAN. It may be considered the DEPTH to which all light is filtered out except for about one percent and may be calculated as about two and one-half times the DEPTH of a SECCHI DISK reading.
PHOTOGRAMMETRY: The science of deducing the physical dimensions of objects from measurements on images (usually photographs) of the objects.
PHOTOMOSAIC: An assemblage of photographs, each of which shows part of a region, put together in such a way that each point in the region appears once and only once in the assemblage, and scale variation is minimized.
PILE: A long substantial pole of wood, concrete or metal, driven into the earth or sea bed to serve as a support or protection.
PILING, SHEET: Interlocking member of wood, steel, concrete, etc., subject to lateral pressure, driven individually to form an obstruction to percolation, to prevent movement of material for SEAWALLS, stabilization of foundations, etc.
PIXEL: An element of surface resulting from subdividing an image into the smallest identically shaped figures that give information about the location, intensity and perhaps color of the source, but such that no smaller subdivision will provide more information.
PLACER DEPOSITS: MINERAL deposits consisting of dense, resistant and often economically valuable MINERALS which have been weathered from TERRIGENOUS ROCKS, transported to the SEA and concentrated in marine SEDIMENTS by WAVE or current action.
PLANFORM: The outline or shape of a body of water as determined by the stillwater line.
PLATEAU: (1) (Geographical) an elevated plain, tableland or flat-topped region of considerable extent. (2) (Oceanographical) an ELEVATION from the bottom of the OCEAN with a more or less flat top and steep sides.
POINT: (1) The extreme end of a CAPE, or the outer end of any land area protruding into the water, usually less prominent than a CAPE. (2) (SMP) A low profile SHORELINE PROMONTORY of more or less triangular shape, the top of which extends SEAWARD.
PORT: A place where vessels may discharge or receive cargo.
PRESERVATION: Static protection of an area or element, attempting to perpetuate the existence of a given 'state'.
QUATERNARY: (1) The youngest geologic period; includes the present time. (2) The latest period of time in the stratigraphic column, 0 - 2 million years, represented by local accumulations of glacial (Pleistocene) and post-glacial (Holocene) deposits which continue, without change of fauna, from the top of the Pliocene (Tertiary). The quaternary appears to be an artificial division of time to separate pre-human from post-human sedimentation. As thus defined, the quaternary is increasing in duration as man's ancestry becomes longer.
RADAR: An instrument for determining the distance and direction to an object by measuring the time needed for radio signals to travel from the instrument to the object and back, and by measuring the angle through which the instrument's antenna has traveled.
RADIOACTIVE DATING (RADIOMETRIC DATING): the most reliable method of obtaining a 'date' for a rock depends upon the observation that the rate of decay of a radioactive element is a constant. The earliest methods, using uranium and thorium MINERALS as the starting material, yielded evidence that the extent of geological time was at least 2 million years. The development of knowledge concerning radioactive processes since 1939 has made available a number of refined techniques for radioactive dating which are nowadays routine processes.
RANGE OF TIDE: The difference in height between consecutive high and low waters. The MEAN RANGE is the difference between MEAN HIGH WATER and MEAN LOW WATER. The GREAT DIURNAL RANGE or DIURNAL RANGE is the difference in height between MEAN HIGHER HIGH WATER (MHHW) and MEAN LOWER LOW WATER (MLLW). Where the type of TIDE is DIURNAL, the mean range is the same as the DIURNAL range. See Figure 11.
RECTIFICATION: The process of producing, from a tilted or oblique photograph, a photograph from which displacement caused by tilt has been removed.
REFERENCE STATION: A TIDE or current station for which tidal or TIDAL CURRENT constants have previously been determined and which is used as a standard for the comparison of simultaneous observations at a second station; also a station for which independent daily predictions are given in the TIDE or current tables from which corresponding predictions are obtained for other stations by means of differences or factors.
REFERENCE ZONE: In regard to BEACH measuring procedure, the part of the FORESHORE subject to WAVE action (between the LIMIT OF UPRUSH and the LIMIT OF BACKWASH) at mid-tide stage. In areas of great TIDAL RANGE a more complex definition is needed.
REFRACTION: The process by which the direction of a WAVE moving in SHALLOW WATER at an angle to the bottom contours is changed. The part of the WAVE moving shoreward in shallower water travels more slowly than that portion in deeper water, causing the WAVE to turn or bend to become parallel to the contours. See Figure 13.
RESOLUTION: (1) In general, a measure of the finest detail distinguishable in an object or phenomenon. (2) In particular, a measure of the finest detail distinguishable in an image.
RETARDATION: The amount of time by which corresponding tidal phases grow later day by day (about 50 minutes).
REVERSING TIDAL CURRENT: A TIDAL CURRENT that flows alternately in approximately opposite directions with a SLACK WATER at each reversal of direction. Currents of this type usually occur in RIVERS and STRAITS where the direction of flow is more or less restricted to certain CHANNELS. When the movement is towards the SHORE, the current is said to be flooding, and when in the opposite direction it is said to be ebbing.
REVETMENT: (1) A facing of STONE, concrete, etc., to protect an EMBANKMENT, or shore structure, against EROSION by WAVE action or currents. (2) A retaining wall. (3) (SMP) Facing of STONE, concrete, etc., built to protect a SCARP, EMBANKMENT or shore structure against EROSION by WAVES of CURRENTS.
RIP CURRENT: A strong surface current of short duration flowing seaward from the SHORE. It usually appears as a visible band of agitated water and is the return movement of water piled up on the SHORE by incoming WAVES and wind. A rip current consists of three parts: the FEEDER CURRENT flowing parallel to the shore inside the BREAKERS; the NECK, where the FEEDER CURRENTS converge and flow through the breakers in a narrow band or "rip"; and the HEAD, where the current widens and slackens outside the breaker line. See Figure 7.
RIPPLE: (1) The light fretting or ruffling on the surface of the water caused by a breeze. (2) The smallest class of WAVES and one in which the force of restoration is, to a significant degree, both surface tension and gravity.
RIPPLE MARKS: Undulations produced by fluid movement over SEDIMENTS. Oscillatory currents produce symmetric RIPPLES whereas a well-defined current direction produces asymmetrical RIPPLES. The crest line of RIPPLES may be straight or sinuous. The characteristic features of ripples depend upon current velocity, PARTICLE SIZE, persistence of current direction and whether the fluid is air or water. SAND DUNES may be regarded as a special kind of 'super'-ripple.
RIPRAP: (1) Broken STONES used for REVETMENT, TOE protection for BLUFFS, or structures exposed to WAVE action, foundations, etc. (2) Foundation of wall or STONES placed together irregularly. (3) (SMP) A layer, facing or protective mound of STONES placed to prevent EROSION, scour or sloughing of a structure or EMBANKMENT; also the STONE so used.
RIPS: Agitation of water caused by the meeting of currents or by rapid current setting over an irregular bottom.
RISK ANALYSIS: Assessment of the total risk due to all possible environmental inputs and all possible mechanisms.
ROCKS: An aggregate of one or more MINERALS rather large in area. The three classes of rocks are the following: (1) Igneous rock - crystalline rocks formed from molten material. Examples are granite and basalt. (2) Sedimentary rock - A rock resulting from the consolidatrion of loose SEDIMENT that has accumulated in layers. Examples are sandstone, shale and limestone. (3) Metamorphic rock - Rock that has formed from preexisting rock as a result of heat or pressure.
ROTARY CURRENT, TIDAL: A TIDAL CURRENT that flows continually with the direction of flow changing through all points of the compass during the TIDAL PERIOD. Rotary currents are usually found OFFSHORE where the direction of flow is not restricted by any barriers. The tendency for the rotation in direction has its origin in the deflecting force of the earth's rotation and, unless modified by local conditions, the change is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The velocity of the current usually varies throughout the tidal cycle, passing through two maxima in approximately opposite directions and two minima with the direction of the current at approximately ninety degrees from the direction at the time of maximum velocity.