Glossary of Coastal Terminology: U - Z
UNCONFORMITY: A surface that represents a break in the geologic record, with the rock unit immediately above it being considerably younger than the rock beneath. There are three major aspects to consider: (1) Time. An unconformity develops during a period of time in which no SEDIMENT is deposited. This concept equates deposition and time, and an unconformity represents unrecorded time. (2) Deposition. Any interruption of deposition, whether large or small in extent, is an unconformity. This aspect of unconformity pre-supposes a standard 'scale' of deposition which is complete. Major breaks in sedimentation can usually be demonstrated easily, but minor breaks may go unrecorded until highly detailed investigations are made. (3) Structure. Structurally, unconformity may be regarded as planar structures separating older ROCKS below from younger ROCKS above, representing the 'break' as defined in (1) and (2) above. A plane of unconformity may be a surface of weathering, EROSION or denudation, or a surface of non-deposition, or possibly some combination of these factors. It may be parallel to the upper strata, make an angle with the upper strata, or be irregular. Subsequent Earth movements may have folded or faulted it.
UNDERTOW: (1) A current below water surface flowing seaward; the receding water below the surface from WAVES breaking on a shelving beach. (2) Actually undertow is largely mythical. As the BACKWASH of each WAVE flows down the BEACH, a current is formed which flows seaward. However, it is a periodic phenomenon. The most common phenomena expressed as undertow are actually RIP CURRENTS.
UPWELLING: The process by which water rises from a deeper to a shallower DEPTH, usually as a result of OFFSHORE surface water flow. It is most prominent where persistent wind blows parallel to a COASTLINE so that the resultant EKMAN TRANSPORT moves surface water away from the COAST. See Figure 5.
VARIABILITY OF WAVES: (1) The variation of heights and periods between individual WAVES within a WAVE TRAIN. (2) The variation in direction of propagation of WAVES leaving the GENERATING AREA. (3) The variation in height along the crest, usually called "variation along the WAVE".
VISCOSITY: Resistance to flow.
WATER LEVEL: The ELEVATION of a particular point or small patch on the surface of a body of water above a specific point or surface, averaged over a period of time sufficiently long to remove the effects of short period disturbances.
WATER LINE: (1) The juncture of land and SEA. This line migrates, changing with the TIDE or other variation of the WATER LEVEL. Where WAVES are present on the BEACH, this line is also known as the LIMIT OF BACKRUSH. (2) The common boundary between the water surface and any immersed structure.
WATER, NAVIGABLE: The waters which are or can be used as water highways for commerce.
WATER TABLE: The upper surface of a zone of saturation, where the body of groundwater is not confined by an overlying impermeable formation. Where an overlying confining formation exists, the AQUIFER in question has no water table.
WAVE: (1) An oscillatory movement in a body of water manifested by an alternate rise and fall of the surface. (2) A disturbance of the surface of a liquid body, as the OCEAN, in the form of a ridge, SWELL or hump. (3) The term wave by itself usually refers to the term SURFACE GRAVITY WAVE (PROGRESSIVE). See also CAPILLARY WAVE, GRAVITY WAVE, PROGRESSIVE WAVE, STANDING WAVE, TIDE WAVE, TSUNAMI. See Figure 10.
WAVE DRIFT: The small net forward displacement of water in the direction of the WAVE travel, particularly in WAVES of large AMPLITUDE, so that the orbits are not quite closed, and the water, while in the crests, moves slightly further forward than it moves backward while in the TROUGHS. See also MASS TRANSPORT, SHOREWARD.
WAVE STAFF: An instrument consisting of a graduated vertical pole for measuring WAVE HEIGHTS, and, by introducing a timing device, WAVE PERIODS. The staff may support a strip or series of electrical contacts for activating a recorder.
WAVE VARIABILITY: (1) The variation of heights and periods between individual WAVES within a WAVE TRAIN. WAVE TRAINS are not composed of WAVES of equal heights and periods, but rather of heights and periods which vary in a statistical manner. (2) The variability in direction of WAVE travel when leaving the GENERATING AREA. (3) The variation in height along the crest.
WELL MIXED ESTUARY: In this circulation type, tidal fluctuations dominate, and the water column is mixed vertically.
WIND CURRENT: A current created by the action of the wind. From theoretical considerations, currents produced by winds in the OPEN SEA will set to the right of the direction towards which the wind is blowing if in the Northern Hemisphere and to the left of this direction if in the Southern Hemisphere.
WIND ROSE: Diagram showing the long-term distribution of wind speed and direction.
WIND SETUP: (1) The vertical rise in the STILLWATER LEVEL on the LEEWARD side of a body of water caused by WIND STRESSES on the surface of the water. (2) The difference in STILLWATER LEVES on the WINDWARD and the LEEWARD sides of a body of water caused by WIND STRESSES on the surface of the water. (3) synonymous with WIND TIDE and STORM SURGE. STORM SURGE is usually reserved for use on the OCEAN and large bodies of water. Wind setup is usually reserved for use on reservoirs and smaller bodies of water.
WINDWARD: The direction from which the wind is blowing.
WITNESS MARK: A material mark placed at a known distance and direction from a property corner, an instrument station or a survey station, as an aid in its recovery and identification.