4. 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 littoral 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 750,000 cubic meters (1 million cubic yards) per year. On 

 a coast without structures, such a large Q- can occur and yet not be appar- 

 ent 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. 



5. Relative Change in Sea Level . 



Relative changes in sea level may be caused by changes in sea level and/or 

 changes in land level. Sea levels of the world are now generally rising. The 

 level of inland seas may either rise or fall, generally depending on 

 hydrologlc Influences. Land level may rise or fall due to tectonic forces, 

 and land level may fall due to subsidence. It is often difficult to dis- 

 tinguish 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 sediment 

 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). Relative sea level change thus can result in the 

 appearance of a gain or loss of sediment volume. 



The importance of relative change in sea level on coastal engineering 

 design depends on the time scale and the locality involved. Its effect should 

 be determined on a case-by-case basis. 



6. Summary of Sediment Budget . 



Sources, sinks, and convective processes are summarized diagrammatlcally 

 in Figure 4-49 and listed in Table 4-16. The range of contributions or losses 

 from each of these elements is described in Table 4-16 measured as a fraction 

 of the gross longshore transport rate, or as a rate given in cubic meters per 

 year per meter (cubic yards per year per foot) of beach front. The relative 

 Importance of elements in the sand budget varies with locality and with the 

 boundaries of the particular littoral control volume. 



In most localities, the gross longshore transport rate significantly 



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