Geoscience Reference
In-Depth Information
Factors recognized as controlling beach erosion (based on Bird, 1988).
Table 8.2
Exclusively Human-induced
•
Reduction in longshore sand supply because of construction of breakwalls.
•
Increased longshore drift because of wave reflection off breakwalls behind, or updrift of, the beach.
•
Quarrying of beach sediment.
•
Offshore mining.
•
Intense recreational use.
•
Revegetation of dunes.
Exclusively Natural
•
Sea level rise.
•
Increased storminess.
•
Reduction in sand moving shoreward from the shelf because it has been exhausted or because the beach profile is
too steep.
•
Increased loss of sand shoreward from the beach by increased wind drifting.
•
Shift in the angle of wave incidence due to shifts in the location of average pressure cells.
•
Reduction in sediment volume because of sediment attrition, weathering, or solution.
•
Longshore migration of large beach lobes or forelands.
•
Climatic warming in cold climates, leading to melting of permanently frozen ground and ice lens at the coastline.
•
Reduction in sea-ice season leading to increased exposure to wave attack.
Either Human-induced or Natural
• Decrease in sand supply from rivers because of reduced runoff from decreased rainfall or dam construction.
• Reduction in sand supply from eroding cliffs, either naturally, or because they are protected by seawalls.
• Reduction in sand supply from dunes because of migration inland or stabilization.
• Increase in the beach watertable because of increased precipitation, use of septic tanks or lawn watering.
Source: Based on Bird, 1988
structures. Classic examples of breakwall obstruction
or longshore sediment drift, leading to erosion down-
drift, include the Santa Barbara harbor breakwall in
California, and the Tweed River retaining walls south
of the Queensland Gold Coast in eastern Australia.
Offshore mining may affect refraction patterns at shore
or provide a sediment sink to which inshore sediment
drifts over time. Erosion of Point Pelée, Ontario, in
Lake Erie, can be traced to gravel extraction some
distance offshore. Many cases also exist, along the
south coast of England, of initiation of beach erosion
after offshore mining. Dam construction is very
prevalent on rivers supplying sediment to the coastline.
Californian beaches have eroded in many locations
because dams, built to maintain constant water
supplies, have also trapped sediment that maintains a
stable, coastal sand budget. The best example is
the case of the Aswan Dam in Egypt. Since its con-
struction, the Nile Delta shoreline has retreated by
1 km or more. Even the simple act of revegetating
dunes can cause shoreline retreat if the sand budget of
a beach is fixed. Sand blown into vegetated dunes
cannot return to the beach on offshore winds, so the
beach slowly narrows over time.
Two little-known factors may be more important
than any of the above. Each time someone walks across
a beach, that person becomes a mini-excavator. A
person who does not disrobe and endeavor to shake all
clothing free of sand after swimming can remove 5-10 g
of sand on each visit to the beach. On popular bathing
beaches, this can amount to several tonnes per kilome-
ter of beach per year. Secondly, since the Second World
War rapid urban expansion near the coastline has led to
modification of urban drainage channels and water-
tables. Water pumped from coastal aquifers to supply
urban water may not only cause local subsidence - as is
the case for most cities along the United States east
coast - but may also lead to accelerated beach erosion
because that water may raise local watertables if it is
passed through septic systems or used to water lawns.
