IV. Retreating Shorelines
163
Storms with a recurrence interval of a year can erode as much as 4200 to
21.000 m3/km (10,000 to 50,000 cu.yd/mi), a large amount, but modest compared
to longshore transport that can surpass, for the same area 765,000 mVyr
(1,000,000 cu.yd/y).
A storm can thus remove in record time a large segment of a beach because at the
mean water-line the increased ground-water flow goes seaward, its up- flow
tendency provides a quicksand effect. An off-shore bar can even be built as the
sediment-loaded return flow, of above normal density, travels outwards near the
bed, a process encouraged by the action of onshore winds creating a strong
surface shoreward drift.
Material accumulation on the bar persists till it has reached a height sufficient to
break most incoming waves. The volume of the material required depends
upon the off-shore depth to be filled. Bar formation may increase beach erosion :
water keeps circulating from beach to bar to beach with a surface inward flow
and a storm-profile apron bed outward flow. Material is kept suspended but the
bar is consolidated at the beach's expense. A continued pattern of storms nets a
beach erosion.
Though the foreshore and the beach face are considered synonymous by some
authors, U.S. Army Corps of Engineers texts point out that during storms the
beach face (sensu stricto) is moved shoreward by the cutting action of the waves
on the profile and cuts a scarp into the berm.
As a result of a heavy storm, or several mild ones, the backshore may be
completely eroded, allowing waves to erode dunes, cliffs, or even the land behind
the beach.
2.3
Standing Waves
Standing waves, as well as storm waves, promote erosion. The beach slope
determines the degree of wave reflection, and the coarser the material transported
the steeper the beach face ; the greater the reflection, the stronger the wave
capacity to suspend and roll sediment. Reflection at walls (man-made) and cliffs
(nature-built) create angled wave-trains, and survival of beaches at their front is
very difficult.
163
Storms with a recurrence interval of a year can erode as much as 4200 to
21.000 m3/km (10,000 to 50,000 cu.yd/mi), a large amount, but modest compared
to longshore transport that can surpass, for the same area 765,000 mVyr
(1,000,000 cu.yd/y).
A storm can thus remove in record time a large segment of a beach because at the
mean water-line the increased ground-water flow goes seaward, its up- flow
tendency provides a quicksand effect. An off-shore bar can even be built as the
sediment-loaded return flow, of above normal density, travels outwards near the
bed, a process encouraged by the action of onshore winds creating a strong
surface shoreward drift.
Material accumulation on the bar persists till it has reached a height sufficient to
break most incoming waves. The volume of the material required depends
upon the off-shore depth to be filled. Bar formation may increase beach erosion :
water keeps circulating from beach to bar to beach with a surface inward flow
and a storm-profile apron bed outward flow. Material is kept suspended but the
bar is consolidated at the beach's expense. A continued pattern of storms nets a
beach erosion.
Though the foreshore and the beach face are considered synonymous by some
authors, U.S. Army Corps of Engineers texts point out that during storms the
beach face (sensu stricto) is moved shoreward by the cutting action of the waves
on the profile and cuts a scarp into the berm.
As a result of a heavy storm, or several mild ones, the backshore may be
completely eroded, allowing waves to erode dunes, cliffs, or even the land behind
the beach.
2.3
Standing Waves
Standing waves, as well as storm waves, promote erosion. The beach slope
determines the degree of wave reflection, and the coarser the material transported
the steeper the beach face ; the greater the reflection, the stronger the wave
capacity to suspend and roll sediment. Reflection at walls (man-made) and cliffs
(nature-built) create angled wave-trains, and survival of beaches at their front is
very difficult.