f^^l 



Figure 13. Beach backed by seawall near Galveston, TX. Note riprap added for further 

 protection 



alongshore transport system. Therefore, protecting the cliffs can cause net erosion of the 

 adjacent and downdrift beaches. 



Structures and activities in river basins also affect the availability of sediment 

 supply to the coast. Sediment supply can be greatly reduced by the building of dams and 

 reservoirs in watersheds because sands, silts, and clays are impounded behind the dams. 

 Therefore, when the rivers ultimately reach the coast, they transport lower sediment loads 

 than they did before the dams were built. As an example, recent erosion of the Nile Delta is 

 attributed to the construction of the Aswan High Dam (Carter 1988). Other structures, such 

 as revetments which reduce bank caving, and levees, which prevent rivers from overtopping 

 their banks, also reduce sediment supply to coastal and wetland areas. Stream diversion, 

 whether a natural event or by design, can cut off important sources of sediment to the coastal 

 areas formerly receiving sediment from those sources. In some circumstances, diversion of 

 streams may supply sediment to an area that formerly had been bypassed. Land-use changes 

 in watersheds, including deforestation, agriculture, and urbanization, may affect the fluxes 

 and timing of sediment supply to coasts. 



Chapter 2 Relevant Processes and Factors 



35 



