Glossary of Coastal Terminology: D - G
DAILY RETARDATION OF TIDES: The amount of time by which corresponding tidal phases grow later day by day (about 50 minutes).
DATUM PLANE: A horizontal plane used as a reference from which to determine heights or DEPTHS. The plane is called a TIDAL DATUM when defined by a certain phase of the TIDE. Datum planes are referenced to fixed points known as BENCH MARKS, so that they can be recovered when needed. See also REFERENCE PLANE.
DAVIDSON CURRENT: Deep-ocean boundary current off the west coast of the U.S. which brings warmer, saltier, low oxygen, high phosphate, equatorial-type water from low to high latitudes.
DECAY OF WAVES: The change which occurs in WAVES when they leave a GENERATING AREA and pass through a CALM (or region of lighter winds). In the process of decay the significant WAVE LENGTH increases and the SIGNIFICANT WAVE HEIGHT decreases.
DEFLATION: The removal of loose material from a beach or other land surface by wind action.
DEGRADATION: The geologic process by means of which various parts of the surface of the earth are worn away and their general level lowered, by the action of wind and water.
DELTA: (1) An ALLUVIAL DEPOSIT, usually triangular, at the mouth of a RIVER of other STREAM. It is normally built up only where there is no tidal or CURRENT action capable of removing the SEDIMENT as fast as it is deposited, and hence the DELTA builds forward from the COASTLINE. (2) A TIDAL DELTA is a similar deposit at the mouth of a tidal INLET, put there by TIDAL CURRENTS. (3) A WAVE DELTA is a deposit made by large WAVES which run over the top of a SPIT or BARRIER BEACH and down the landward side.
DENSITY STRATIFICATION: The lateral expansion of a SEDIMENT plume as it moves out of the distributary mouth, where salt and fresh water mix. This is most likely to occur where the speed of the RIVER flow is moderate to low and the distributary mouth is relatively deep.
DEPRESSION: A general term signifying any depressed or lower area in the ocean floor.
DESIGN STORM: Coastal protection structures will often be designed to withstand WAVE attack by the extreme design storm. The severity of the storm (i.e. RETURN PERIOD) is chosen in view of the acceptable level of risk of damage or failure. A design storm consists of a DESIGN WAVE condition, a design WATER LEVEL, and a DURATION.
DIAPOSITIVE: A positive photograph on a transparent medium.
DIFFERENTIAL EROSION / WEATHERING: These features develop in ROCKS which have varying resistance to the agencies of EROSION and/or weathering so that parts of the rock are removed at greater rates than others. A typical example is the removal of soft beds from between harder beds in a series of sedimentary ROCKS. The term may be applied to any size of feature, from small-scale 'etching' to the regional development of hills and VALLEYS controlled by hard and soft ROCKS.
DIFFRACTION: The phenomenon occurring when water WAVES are propagated into a sheltered region formed by a BREAKWATER or similar barrier that interrupts a portion of the otherwise regular train of WAVES, resulting in the multi-directional spreading of the WAVES.
DIRECTION OF WIND: Direction from which wind is blowing.
DISPERSE: (1) To spread or distribute from a fixed or constant source. (2) To cause to become widely separated.
DIURNAL CURRENT: The type of TIDAL CURRENT having only one flood and one EBB period in the TIDAL DAY. A ROTARY CURRENT is DIURNAL if it changes its direction through all points of the compass once each TIDAL DAY. See Figure 11.
DIURNAL INEQUALITY: The difference in height of the two high waters or of the two low waters of each day. Also, the difference in velocity between the two daily flood or EBB CURRENTS of each day. See Figure 11.
DOUBLE TIDE: A double-headed TIDE; that is, a HIGH WATER consisting of two maxima of nearly the same height separated by a relatively small DEPRESSION, or a LOW WATER consisting of two minima separated by a relatively small ELEVATION.
DREDGING: (SMP) Excavation or displacement of the bottom or SHORELINE of a water body. Dredging can be accomplished with mechanical or hydraulic machines. Most is done to maintain channel DEPTHS or berths for navigational purposes; other dredging is for shellfish harvesting or for cleanup of polluted sediments.
DRIFT SECTOR: (SMP) A particular reach of marine SHORE in which LITTORAL DRIFT may occur without significant interruption, and which contais any and all natural sources of such drift, and also any accretion shoreforms accreted by such drift.
DUNES: (1) Accumulations of windblown SAND on the BACKSHORE, usually in the form of small hills or ridges, stabilized by vegetation or control structures. (2) A type of bed form indicating significant SEDIMENT transport over a sandy seabed.
EBB CURRENT: The movement of a TIDAL CURRENT away from SHORE or down a tidal stream. In the SEMIDIURNAL type of reversing current, the terms greater ebb and lesser ebb are applied respectively to the EBB CURRENTS of greater and lesser velocity of each day. The terms of maximum ebb and minimum ebb are applied to the maximum and minimum velocities of a continuously running EBB CURRENT, the velocity alternately increasing and decreasing without coming to a slack or reversing. The expression maximum ebb is also applicable to any EBB CURRENT at the time of greatest velocity.
EBB TIDAL DELTA: The bulge of SAND formed at the seaward mouth of tidal inlets as a result of interaction between TIDAL CURRENTS and WAVES. Also called inlet-associated bars and estuary entrance shoals.
ECOSYSTEM: The living organisms and the nonliving environment interacting in a given area.
EDDY: A current of air, water, or any fluid, forming on the side of the main current, especially one moving in a circle; in extreme cases a whirlpool.
EDGE WAVE: An ocean WAVE parallel to the COAST, with crests normal to the SHORELINE. An edge wave may be standing or PROGRESSIVE. Its height diminishes rapidly seaward and is negligible at a distance of one WAVE LENGTH OFFSHORE.
EELGRASS: A submerged marine plant with very long narrow leaves.
ELEVATION: The distance of a point above a specified surface of constant potential; the distance is measured along the direction of gravity between the point and the surface.
ELUTRIATION: The process by which a granular material can be sorted into its constituent PARTICLE SIZES by means of a moving STREAM of fluid (usually air or water). Elutriators are extensively used in studies of SEDIMENTS for determining PARTICLE SIZE distribution. Under certain circumstances wind, RIVERS and STREAMS may act as natural elutriating agents.
ENDEMIC: Native to a specific geographic area.
EQUATORIAL CURRENTS: (1) OCEAN CURRENTS flowing westerly near the equator. There are two such currents in both the Atlantic and Pacific Oceans. The one to the north of the equator is called the North Equatorial Current and the one to the south is called the South Equatorial Current. Between these two currents there is an easterly flowing STREAM known as the Equatorial Countercurrent. (2) TIDAL CURRENTS occurring semimonthly as a result of the moon being over the equator. At these times the tendency of the moon to produce DIURNAL INEQUALITY in the current is at a minimum.
EROSION: Wearing away of the land by natural forces. On a BEACH, the carrying away of BEACH material by WAVE action, TIDAL CURRENTS or by DEFLATION. (2) (SMP) The wearing away of land by the action of natural forces.
ESTUARY: (1) A semi-enclosed coastal body of water which has a free connection with the OPEN SEA. The seawater is usually measurably diluted with freshwater. (2) The part of the RIVER that is affected by TIDES. (3) (SMP) The zone or area of water in which freshwater and saltwater mingle and water is usually brackish due to daily mixing and layering of fresh and salt water.
FACIES: The sum total of features such as sedimentary rock type, MINERAL content, sedimentary structures, bedding characteristics, fossil content, etc. which characterise a SEDIMENT as having been deposited in a given environment.
FAIRWAY: The parts of a waterway kept open, and unobstructed, for navigation.
FAULT: A fracture in rock along which there has been an observable amount of displacement. Faults are rarely single planar units; normally they occur as parallel to sub-parallel sets of planes along which movement has taken place to a greater or lesser extent. Such sets are called fault or fracture-zones.
FAUNA: The entire group of animals found in an area.
FETCH LENGTH: (1) The horizontal distance (in the direction of the wind) over which a wind generates SEAS or creates WIND SETUP. (2) The horizontal distance along open water over which the wind blows and generates WAVES.
FLOATING BOG: A grass or moss growth floating on a pool of water with high content of decayed vegetation, grass and moss roots.
FLOCCULATION: The change which takes place when the DISPERSED phase of a COLLOID forms a series of discrete particles which are capable of settling out from the DISPERSION medium. In geological processes, flocculation is almost inevitably a result of a COLLOIDAL solution mixing with a solution containing electrolytes, e.g., sea water.
FLOOD CURRENT: The movement of a TIDAL CURRENT toward the SHORE or up a tidal STREAM. In the SEMIDIURNAL type of reversing current, the terms greater flood and lesser flood are applied respectively to the flood currents of greater and lesser velocity each day. The terms maximum flood and minimum flood are applied to the maximum and minimum velocities of a flood current the velocity of which alternately increases and decreases without coming to slack or reversing. The expression maximum flood is also applicable to any flood current at the time of greatest velocity.
FLOOD INTERVAL: The interval between the transit of the moon over the meridian of a place and the time of the following flood.
FLOOD MARK: Proof of any kind on the SHORELINE used to determine the highest level attained by the water surface during the flood (note: the height of the flood mark usually includes the WAVE RUN-UP).
FLOODPLAIN: (1) A flat tract of land bordering a RIVER, mainly in its lower reaches, and consisting of alluvium deposited by the RIVER. It is formed by the sweeping of the meander belts downstream, thus widening the VALLEY, the sides of which may become some kilometers apart. In time of flood, when the RIVER overflows its banks, SEDIMENT is deposited along the valley banks and plains. (2) (SMP) Synonymous with 100-year floodplain. The land area susceptible to being inundated by stream derived waters with a 1 percent chance of being equaled or exceeded in any given year. The limits of this area are based on flood regulation ordinance maps or reasonable method that meets the objectives of the SMP (WAC 173-22-030(2)).
FLOOD ROUTING: The attenuating effect of storage on a river-flood passing through a VALLEY by reason of a feature acting as control (e.g. a reservoir with a spillway capacity less than the flood inflow, or the widening or narrowing of a VALLEY).
FLOOD TIDE: A nontechnical term used for rising TIDE or FLOOD CURRENT. In technical language flood refers to current. The portion of the tidal cycle between LOW WATER and the following HIGH WATER. See Figure12.
FLORA: The entire group of plants found in an area.
FOAM: The light-colored substance which is made up of an aggregation of bubbles, formed on the surface of liquids by violent agitation.
FOG: Vapor condensed to fine particles of water and obscuring vision near the ground.
FORESHORE: (1) The part of the SHORE, lying between the BERM CREST and the ordinary LOW WATER mark, which is ordinarily traversed by the UPRUSH and BACKWASH of the WAVES as the TIDES rise and fall. (2) The same as the BEACH FACE where UNCONSOLIDATED material is present. (3) (SMP) In general terms, the BEACH between MEAN HIGHER HIGH WATER and MEAN LOWER LOW WATER.
FULLY-DEVELOPED SEA: The WAVES that form when wind blows for a sufficient period of time across the open OCEAN. The WAVES of a fully developed sea have the maximum height possible for a given wind speed, FETCH and DURATION of wind.
GABION: (1) Steel wire-mesh basket to hold STONES or crushed rock to protect a BANK or bottom from EROSION. (2) (SMP) Structures composed of masses of ROCKS, rubble or masonry held tightly together usually by wire mesh so as to form blocks or walls. Sometimes used on heavy EROSION areas to retard wave action or as a foundation for BREAKWATERS or JETTIES.
GENERATION OF WAVES: (1) The creation of WAVES by natural or mechanical means. (2) The creation of and growth of WAVES by a wind blowing over a water surface for a certain length of time. The area involved is called the GENERATING AREA or FETCH.
GEODESY (OR GEODETICS): The science of dealing with the investigation of scientific questions connected with the shape and dimensions of the Earth.
GEOGRAPHIC INFORMATION SYSTEM (GIS): A system of spatially referenced information, including computer programs that acquire, store, manipulate, analyze, and display spatial data.
GEOMORPHOLOGY: (1) That branch of physical geography which deals with the form of the Earth, the general configuration of its surface, the distribution of the land, water, etc. (2) The investigation of the history of geologic changes through the interpretation of topographic forms.
GEOPHYSICS: The study of the physical characteristics and properties of the Earth.
GLOBAL POSITIONING SYSTEM (GPS): A navigational and positioning system developed by the U.S. Department of Defense, by which the location of a position on or above the Earth can be determined by a special receiver at that point interpreting signals received simultaneously from several of a constellation of special satellites.
GRADIENT: (1) A measure of SLOPE (SOIL- or water-surface) in meters of rise or fall per meter of horizontal distance. (2) More general, a change of a value per unit of distance, e.g. the gradient in LONGSHORE TRANSPORT causes EROSION or ACCRETION. (3) With reference to winds or currents, the rate of increase or decrease in speed, usually in the vertical; or the curve that represents this rate.
GRAVITY WAVE: A WAVE whose velocity of propagation is controlled primarily by gravity. Water WAVES more than 5 cm long are considered gravity waves. WAVES longer than 2.5 cm and shorter than 5 cm are in an indeterminate zone between CAPILLARY and GRAVITY WAVES. See RIPPLE. See Figure 10.
GROIN: (1) A shore-protection structure (built usually to trap LITTORAL DRIFT or retard EROSION of the SHORE). It is narrow in width (measured parallel to the SHORE) and its length may vary from tens to hundreds of meters (extending from a point landward of the SHORELINE out into the water). GROINS may be classified as permeable (with openings through them) or impermeable (a solid or nearly solid structure). (2) (SMP) A barrier-type structure extending from the BACKSHORE or stream bank into a water body for the purpose of the protection of a SHORELINE and adjacent upland by influencing the movement of water and/or deposition of materials.
GROUND PENETRATING RADAR (GPR): A geophysical technology that uses radio waves to detect buried objects and the internal structure of landforms.