Because tides are influenced by many factors and show great temporal and 

 spatial variation, they play an important role in the variations of both 

 processes and landforms in the coastal zone. The length of the drying period 

 of the intertidal zone, changes in the water table of beaches, and the intensity 

 and duration of tidal currents are all influenced by the type and magnitude of 

 tides. Tides also influence the timing of morphologic changes, since cliff 

 erosion and beach building may occur only intermittently at high tide. Long- 

 term tidal variations, including solar semi-annual and 18.6-year components, 

 may also be related to cycles of progradation and stabilization by mangroves 

 on fluid mudflat coasts (Wells and Coleman 1981b). 



Several landforms showing geomorphic variability are strongly influenced 

 by tides. Tidal flats are depositional surfaces, alternately flooded and 

 exposed. The formation, dimensions, and spacing of inlets are related to the 

 total volume of water or tidal prism. Flood and ebb deltas, as well as asso- 

 ciated features at inlets, are controlled by tidal current hydraulics. Tidal 

 channels of various types, including intertidal channels, tidal creeks, and tidal 

 rivers, are influenced to differing degrees by tides. 



Currents 



Near the coast, currents are effective agents of erosion and transport of 

 sediments and, consequently, have an impact on the geomorphic variability of 

 coasts. Mechanisms responsible for the generation of coastal currents include 

 winds, waves, and tides. 



Wind-driven currents, in combination with density currents, comprise 

 much of the large-scale oceanic circulation, as well as much of the circulation 

 in marginal seas. These currents, generated by the drag of wind on the ocean 

 surface, are deflected up to 45 deg as a result of the Coriolis effect, although 

 appreciably less in shallower waters. Ocean circulation following the parallels 

 is predominantly wind-driven, whereas circulation following the meridians is 

 largely density-driven (Mosetti 1982). Wind-driven currents show greater 

 influence on geomorphic variability in marginal seas than oceans, because 

 sediment transport and beach modification are facilitated by shallower depths. 



Wave-generated currents can influence any part of the shore, shoreface, 

 and Continental Shelf; however, the most important currents for coastal 

 development are longshore currents. Longshore currents are generated 

 between the breaker zone and shore by waves approaching the shore at an 

 angle. The wave height, period, direction of approach, and bottom slope are 

 important in determining the direction and characteristics of longshore cur- 

 rents. Flow velocities of longshore currents are typically around 30 cm/sec, 

 but can be substantially higher. While such velocities are often unable to 

 entrain materials, suspension by concurrent waves may greatly increase the 

 effectiveness of sediment transport. 



Chapter 2 Relevant Processes and Factors 



17 



