Environmental Engineering Reference
In-Depth Information
Fig. 2.8
Depleted beach and backshore erosion on the shore of Cleeland Bight, Phillip Island,
following the stabilisation of dunes that previously supplied sand to this beach. © Geostudies
2.5 Submergence and Increased Wave Attack
Deepening of nearshore waters allows larger waves to reach the shore and erode
the beach, withdrawing sand or gravel to the sea floor. Such deepening occurs
briefly during storms, when strong onshore winds raise sea level along the coast
and larger-than-usual waves break on the shore, eroding beaches. Longer-term
deepening occurs as the result of coastal submergence, produced either by land
subsidence, an actual rise of sea level, or some combination of land and sea move-
ment that results in the sea standing higher relative to the land. Larger waves
then reach the shore, causing erosion and the re-shaping of the nearshore profile:
erosion of the upper beach and transference of sand or gravel from the beach to
the adjacent sea floor causes the transverse shore profile to migrate upward and
landward.
Beach erosion has become widespread on coasts where the sea has been rising
because land subsidence is in progress, as on the Gulf and Atlantic coasts of the
United States. Coastal land subsidence resulting from extraction of groundwater
has resulted in beach erosion on the northern Adriatic coast of Italy, as at Ravenna,
and beaches were cut back suddenly on sectors of the Alaskan coastline that sub-
sided during the 1964 earthquake.
There is evidence from tide gauge records of a sea level rise of 1-2 mm/year
during the past few decades, offset on some coasts by equal or greater land uplift,
and varying also in relation to the geophysical factors that complicate the surface
