4.74 CONVECTION OF LITTORAL MATERIALS 



Sources and sinks of littoral materials are those processes that 

 result in net additions or net subtractions of material to the selected 

 control volume. However, some processes may subtract at the same rate 

 that they add material, resulting in no net change in the volume of lit- 

 toral material of the control volume. 



The most important convecting process is longshore sediment transport. 

 It is possible for straight exposed coastlines to have gross longshore 

 transport rates of more than 1 million cubic yards per year. On a coast 

 without structures, such a large Q^ can occur, and yet not be apparent 

 because it causes no obvious beach changes. Other convecting processes 

 that may produce large rates of sediment transport with little noticeable 

 change include tidal flows, especially around inlets, wind transport in 

 the longshore direction, and wave-induced currents in the offshore zone. 



Since any structure that interrupts the equilibrium convection of 

 littoral materials will normally result in erosion or accretion, it is 

 necessary that the sediment budget quantitatively identify all processes 

 convecting sediment through the study area. This is most important on 

 shores with high waves. 



4.75 RELATIVE CHANGE IN SEA LEVEL 



Relative changes in sea level may be caused by changes in sea level 

 and changes in land level. Sea levels of the world are now generally ris- 

 ing. The level of inland seas may either rise or fall, generally depend- 

 ing on hydrologic influences. Land level may rise or fall due to tectonic 

 forces, and land level may fall due to subsidence. It is often difficult 

 to distinguish whether apparent changes in sea level are due to change in 

 sea level, change in land level, or both. For this reason, the general 

 process is referred to as relative change in sea level. 



While relative changes in sea level do not directly enter the sedi- 

 ment budget process, the net effect of these elevation changes is to move 

 the shoreline either landward (relative rise in sea level) or seaward 

 (relative fall in sea level). It thus can result in the appearance of a 

 gain or loss of sediment volume. 



The importance of relative change in sea level on coastal engineer- 

 ing design depends on the time scale and the locality involved. Its 

 effect should be determined on a case-by-case basis. 



4.76 SUMMARY OF SEDIMENT BUDGET 



Sources, sinks, and convective processes are summarized diagrammati- 

 cally in Figure 4-49 and listed in Table 4-14. The range of contributions 

 or losses from each of these elements is described in Table 4-14 measured 

 as a fraction of the gross longshore transport rate, or as a rate given 

 in cubic yards per year per foot of beach front. The relative importance 



4-131 



