Sediment Supply and Human Activity 



Sediment supply at a given coastal segment can be a critical factor in the 

 morphology of unconsolidated coastal features such as beaches, deltas, and 

 capes. The volumetric accounting of the material lost or gained constitutes a 

 sediment budget. Longshore transport out of or into an area is the chief cause 

 of loss or gain along mainland coasts and most barrier beaches (Bowen and 

 Inman 1966). Other important reasons for gains include river transport, sea 

 cliff erosion, onshore transport, biogenic deposition, hydrogenic deposition, 

 wind transport into an area, and beach nourishment. Significant losses are 

 caused by wind transport out of an area, offshore transport, solution, 

 abrasion, dredging, and mining. Human modifications along the coast, 

 including engineering structures, dredging, and beach nourishment, can 

 profoundly influence the patterns and amounts of losses and gains. Human 

 modifications of river basins can also change the amounts and patterns of the 

 supply of incoming materials. 



An analysis of sediment supply may involve the investigation of input at a 

 relatively great distance from a given project site. The relationships of supply 

 and transport are complex, in that littoral sediments supplied by one factor, 

 such as river sources, may be transported or removed by another, such as 

 longshore processes. A multitude of techniques including field measurements, 

 charts, maps, photographs and documents, numerical analyses, and computer 

 simulations can be employed. Sometimes, analyses can be facilitated by 

 breaking up the coast into a series of compartments, or cells, to assist in 

 identifying sources and sinks (Carter 1988). The time scale over which data 

 are collected is important, as sediment budgets also reflect the geomorphic 

 variability of coasts, and may be influenced by cyclic and noncyclic changes 

 and long-term trends. 



Engineering structures often create obstruction to alongshore sediment 

 transport. Structures most commonly affecting alongshore sediment move- 

 ment are groins and jetties (Figure 11). Groins are constructed for the pur- 

 pose of obstructing or retarding alongshore sediment transport to mitigate the 

 effects of erosion. However, they usually cause accelerated erosion of 

 downdrift beaches by cutting off or reducing the amount of material that 

 reaches them (Figure 12). If spaced incorrectly, groins can also cause 

 localized erosion. Offshore breakwaters are being used in some places to 

 reduce the effects of wave erosion, while at the same time allowing along- 

 shore transport processes to continue. 



Other engineering structures may create an obstruction to onshore-offshore 

 sediment transport. Seawalls and bulkheads are often built to protect cliffs 

 and dunes from being eroded by direct storm wave activity or by slumping 

 (Figure 13). Where cliffs are composed of unconsolidated material they are 

 an important source of sediment both to the adjacent shore and to the 



Chapter 2 Relevant Processes and Factors 



33 



