Fine, cohesive sediments respond differently to hydrodynamic forces than 

 do non-cohesive sediments. Fine sediments are made up largely of clay min- 

 erals with an interlayered crystal structure, and normally have a negative 

 surface charge. Cohesion results from inter-particle surface attractions 

 between clay minerals, which are promoted in sea water, and then reinforced 

 by organic secretions. Thus, for noncohesive sediments, the main stabilizing 

 force is the particle weight, whereas cohesive sediments are stabilized by 

 interparticle adhesion and organic binding. 



The relative importance of running water, wind, ice, or groundwater in 

 transporting materials from inland areas to the coast varies gready with local 

 conditions. Running water is most important for transporting solid and 

 dissolved materials where large rivers meet the coast. Locally, smaller rivers 

 may be important, and reworking of ancestral river deposits is also often 

 significant. As with running water, groundwater may be important for trans- 

 portation of solutes or dissolved materials. Wind is important in arid areas, 

 although not necessarily more important than running water. Transportation 

 by ice is much slower than by water and wind, so that ice is probably only 

 important over long-term periods at high latitudes. 



Between the offshore and coastal zones, mass transport may take place by 

 a variety of mechanisms. Erosion and transport of offshore sediments are 

 chiefly accomplished by waves and wave-generated littoral currents in the 

 shore and upper shoreface areas. Considerable difficulties arise when attempt- 

 ing to apply sediment movement thresholds in oscillatory flow, as the water 

 particles under waves reverse their direction of flow and accelerate and 

 decelerate under each pulse. The threshold condition for movement appears 

 to be better related to the ratio between grain diameter and orbital diameter of 

 the water particles (Komar and Miller 1973). 



Storm-generated waves and currents can effectively erode and transport 

 material in deeper waters of the continental shelf, and it is likely that they 

 play a role in modifying and moving large sediment features, such as shelf 

 shoals. Mass transport has both a longshore component, parallel to the shore, 

 and a cross-shore component, which may be onshore or offshore. Lower- 

 energy events mostly transport sediment onshore, while larger events transport 

 sediment offshore and sometimes onshore, if overwashing and overtopping 



Sediment Character 



Unconsolidated coastal sediments may be composed of a variety of mate- 

 rials, which range in size and shape, mineralogy, density, and other proper- 

 ties. Clastic sediments are comprised of rock detrital grains, whereas biogenic 

 sediments are comprised primarily of calcium carbonate grains from shells, 

 skeletons, and invertebrates. The characteristics of these materials account for 



Chapter 2 Relevant Processes and Factors 



29 



