4.74 CONVECTION OF LITTORAL MATERIALS
Sources and sinks of littoral materials are those processes that
resuit 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 sédiment transport
It is possible for straight exposed coastlines to hâve gross longshore
transport rates of more than 1 million cubic yards per year. On a coast
without structures, such a large
can occur, and yet not be apparent
because it causes no obvious beach changes. Other convecting processes
that may produce large rates of sédiment 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 resuit in érosion or accretion, it is
necessary that the sédiment budget quantitatively identify ail processes
convecting sédiment 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 rising. The level of inland seas may either rise or fall, generally depending on hydrologie 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 sédiment budget process, the net effect of these élévation changes is to move
the shoreline either landward (relative rise in sea level) or seaward
(relative fall in sea level). It thus can resuit in the appearance of a
gain or loss of sédiment volume.
The importance of relative change in sea level on Coastal engineering design dépends 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 diagrammatically in Figure 4-49 and listed in Table 4-14. The range of contributions
or losses from each of these éléments 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
Sources and sinks of littoral materials are those processes that
resuit 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 sédiment transport
It is possible for straight exposed coastlines to hâve gross longshore
transport rates of more than 1 million cubic yards per year. On a coast
without structures, such a large
can occur, and yet not be apparent
because it causes no obvious beach changes. Other convecting processes
that may produce large rates of sédiment 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 resuit in érosion or accretion, it is
necessary that the sédiment budget quantitatively identify ail processes
convecting sédiment 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 rising. The level of inland seas may either rise or fall, generally depending on hydrologie 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 sédiment budget process, the net effect of these élévation changes is to move
the shoreline either landward (relative rise in sea level) or seaward
(relative fall in sea level). It thus can resuit in the appearance of a
gain or loss of sédiment volume.
The importance of relative change in sea level on Coastal engineering design dépends 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 diagrammatically in Figure 4-49 and listed in Table 4-14. The range of contributions
or losses from each of these éléments 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