(2) Sédiment transport in the nearshore zone. Seaward of the
breakers, sand is set in motion by waves moving over ripples, either
roi lin g the sand as bed load, or carrying it up in vortices as suspended
load. The sand, once in motion, is transported by mean tidal and wmdinduced currents and by the mass transport velocity due to waves. The
magnitude and direction of the resulting sédiment transport are uncertain under normal circumstances, although mass transport due to waves
is more than adéquate to retum sand lost from the beach during storms.
It appears that bottom mass transport acts to keep the sand close to the
shore, but that some material, probably finer sand, escapes offshore as
the resuit of the combined wind- and wave-induced bottom currents.
(3) The shape and expectable changes in shape of nearshore and
beach profiles. Storms erode beaches to produce a simple concave-up
beach profile with déposition of the eroded material offshore. Rates of
érosion due to individual storms vary from a few cubic yards per foot to
10’s of cubic yards per foot of beach front. The destructiveness of the
storm in producing érosion dépends on its intensity, duration, and orienta
tion, especially as these factors affect the élévation of storm surge and
the wave height and direction. Immediately after a storm, waves begin to
retum sédiment to the eroded beach, either through the motion of bar-andtrough (ridge-and-runnel) Systems, or by berm building. The parameter,
Fo = Ho/(VyT), given by Equation 4-20 détermines whether the beach erodes
or accretes under given conditions. If Fo is above critical value
between 1 and 2 the beach erodes. (See Figures 4-29, and 4-30.)
(4) The slope of the foreshore. There is a tendency for the foreshore to become steeper as grain size increases, and to become flatter
as mean wave height increases. Data for this relation exhibit much scatter and quantitative relationships are difficult to predict.
4.833 Sédiment Budget. Section 4.76 summarizes material on the sédiment
budget. Table 4-14 tabulâtes the éléments of the sédiment budget and indicates the importance of each element. Table 4-13 classifies the éléments of the sédiment budget.
A sédiment budget carefully defines the littoral control volume,
identifies ail éléments transferring sédiment to or from the littoral
control volume, ranks the éléments by their magnitude, and provides an
estimate of unknown rates by the balancing of additions against losses
(Equation 4-46).
If prepared with sufficient data and expérience, the budget permits
an estimate of how proposed improvements will affect neighboring segments
of the littoral zone.
4-154
«j
breakers, sand is set in motion by waves moving over ripples, either
roi lin g the sand as bed load, or carrying it up in vortices as suspended
load. The sand, once in motion, is transported by mean tidal and wmdinduced currents and by the mass transport velocity due to waves. The
magnitude and direction of the resulting sédiment transport are uncertain under normal circumstances, although mass transport due to waves
is more than adéquate to retum sand lost from the beach during storms.
It appears that bottom mass transport acts to keep the sand close to the
shore, but that some material, probably finer sand, escapes offshore as
the resuit of the combined wind- and wave-induced bottom currents.
(3) The shape and expectable changes in shape of nearshore and
beach profiles. Storms erode beaches to produce a simple concave-up
beach profile with déposition of the eroded material offshore. Rates of
érosion due to individual storms vary from a few cubic yards per foot to
10’s of cubic yards per foot of beach front. The destructiveness of the
storm in producing érosion dépends on its intensity, duration, and orienta
tion, especially as these factors affect the élévation of storm surge and
the wave height and direction. Immediately after a storm, waves begin to
retum sédiment to the eroded beach, either through the motion of bar-andtrough (ridge-and-runnel) Systems, or by berm building. The parameter,
Fo = Ho/(VyT), given by Equation 4-20 détermines whether the beach erodes
or accretes under given conditions. If Fo is above critical value
between 1 and 2 the beach erodes. (See Figures 4-29, and 4-30.)
(4) The slope of the foreshore. There is a tendency for the foreshore to become steeper as grain size increases, and to become flatter
as mean wave height increases. Data for this relation exhibit much scatter and quantitative relationships are difficult to predict.
4.833 Sédiment Budget. Section 4.76 summarizes material on the sédiment
budget. Table 4-14 tabulâtes the éléments of the sédiment budget and indicates the importance of each element. Table 4-13 classifies the éléments of the sédiment budget.
A sédiment budget carefully defines the littoral control volume,
identifies ail éléments transferring sédiment to or from the littoral
control volume, ranks the éléments by their magnitude, and provides an
estimate of unknown rates by the balancing of additions against losses
(Equation 4-46).
If prepared with sufficient data and expérience, the budget permits
an estimate of how proposed improvements will affect neighboring segments
of the littoral zone.
4-154
«j