The idéal resuit of post-storm beach recovery is a wide backshore
that will protect the shore from the next storm. Beach recovery may be
prevented when the period between successive storms is too short. Maintenance of Coastal protection requires knowledge of the necessary width
and élévation of the backshore appropriate to local conditions, and
adéquate surveillance to détermine when this natural sand réservoir is
diminished to a point where it may not protect the backshore during the
next storm.
4.525 Bar-Berm Prédiction. High, steep waves scour the beach,eroding the
foreshore into a simple concave-upwards profile. The material eroded from
the beach is deposited offshore as a longshore bar. Waves of low steepness
tend to push sand onto the beach, usually as migrating longshore-bar Systems which eventually become part of the beach. In contrast to the concave
up eroded profile discussed previously, the accreted profile is concavedownward. Idealized eroded and accreted profiles (measured in a prototypescale wave tank) across the beach and nearshore zone are shown in Figure
4-28.
To design a beach that contains a réservoir of sand in the backshore
sufficient to survive a design storm, a minimum requirement is the ability
to distinguish wave conditions that cause eroded profiles from those that
cause accretion. Usually, it is assumed that a berm characterizes an accreted profile and that a bar characterizes an eroded profile. (See Figure
4-28.) This picture is somewhat idealized. A sharp berm crest between
backshore and foreshore is often lacking, and on some beaches the berm is
absent, so that the top of the foreshore reaches the dune or cliff line.
Berms are illustrated in Figures 4-1 and 4-27.
Similarly, the idealized longshore bar seaward of an eroded beach,
(middle profile of Figure 4-18) is often absent, and in its place there
may be several subdued bars or a platform extending to the breaker line
at nearly constant depth.
a* Longshore Bars. The term bar has been applied to a number of
quite different Coastal features, including barrier islands (the "offshore bar of Johnson, 1919), ridge-and-runnel Systems, and linear shoals.
(Duane, et al., 1972.) Longshore bars are unrelated to any of these features. Ihey appear to most nearly resemble a ridge-and-runnel System,
but differ in that longshore bars are located at the breaker position and,
at least in part, are eroded out of the bottom by the falling breaker,
whereas the ridge-and-runnel System is an accretionary feature migrating
landward across the surf zone.
The typical longshore bar, as described from the observations of
Shepard (1950) and the experiments of Keulegan (1948), is a ridge of sand
parallel to the shore and formed at the breaking position of large plunging breakers. Longshore bars seem most directly related to the height of
larger breakers (not necessarily of maximum height). The depth to the seaward bar increases with the height of the larger breakers along the Pacific
coast. (Shepard, 1950.) Bars forai readily in tidal seas, but seem better
4-78
that will protect the shore from the next storm. Beach recovery may be
prevented when the period between successive storms is too short. Maintenance of Coastal protection requires knowledge of the necessary width
and élévation of the backshore appropriate to local conditions, and
adéquate surveillance to détermine when this natural sand réservoir is
diminished to a point where it may not protect the backshore during the
next storm.
4.525 Bar-Berm Prédiction. High, steep waves scour the beach,eroding the
foreshore into a simple concave-upwards profile. The material eroded from
the beach is deposited offshore as a longshore bar. Waves of low steepness
tend to push sand onto the beach, usually as migrating longshore-bar Systems which eventually become part of the beach. In contrast to the concave
up eroded profile discussed previously, the accreted profile is concavedownward. Idealized eroded and accreted profiles (measured in a prototypescale wave tank) across the beach and nearshore zone are shown in Figure
4-28.
To design a beach that contains a réservoir of sand in the backshore
sufficient to survive a design storm, a minimum requirement is the ability
to distinguish wave conditions that cause eroded profiles from those that
cause accretion. Usually, it is assumed that a berm characterizes an accreted profile and that a bar characterizes an eroded profile. (See Figure
4-28.) This picture is somewhat idealized. A sharp berm crest between
backshore and foreshore is often lacking, and on some beaches the berm is
absent, so that the top of the foreshore reaches the dune or cliff line.
Berms are illustrated in Figures 4-1 and 4-27.
Similarly, the idealized longshore bar seaward of an eroded beach,
(middle profile of Figure 4-18) is often absent, and in its place there
may be several subdued bars or a platform extending to the breaker line
at nearly constant depth.
a* Longshore Bars. The term bar has been applied to a number of
quite different Coastal features, including barrier islands (the "offshore bar of Johnson, 1919), ridge-and-runnel Systems, and linear shoals.
(Duane, et al., 1972.) Longshore bars are unrelated to any of these features. Ihey appear to most nearly resemble a ridge-and-runnel System,
but differ in that longshore bars are located at the breaker position and,
at least in part, are eroded out of the bottom by the falling breaker,
whereas the ridge-and-runnel System is an accretionary feature migrating
landward across the surf zone.
The typical longshore bar, as described from the observations of
Shepard (1950) and the experiments of Keulegan (1948), is a ridge of sand
parallel to the shore and formed at the breaking position of large plunging breakers. Longshore bars seem most directly related to the height of
larger breakers (not necessarily of maximum height). The depth to the seaward bar increases with the height of the larger breakers along the Pacific
coast. (Shepard, 1950.) Bars forai readily in tidal seas, but seem better
4-78