What are the causes and effects of beach erosion?
Beaches are temporary features. There is always sand being removed and sand=
being added to them. Often, they change drastically during the year,
depending upon the frequency of storms.
Ultimately, a beach erodes because the supply of sand to the beach can not keep up with the loss of sand to the sea. Most sand is transported from inland via rivers and streams. The damming of most waterways in the US has thus prevented a major supply of sand from getting to our beaches. Sand can also be transported from beach to beach along a shoreline but this is mostly just a redistribution of sand that is already on the coast. The problem of beach loss can be exaggerated if sea level rises relative to the land (either due to true sea level change or geological sinking of the coast line). On an uninhabited shoreline, new beach can be created further inland. But, when the encroaching sea comes against people's property, the tendency is for people to try and stop the encroaching sea. They armor the shoreline with seawalls, revetments, jetties, etc. These have a negative effect on beaches because once sea water reaches them, it "bounces" off them with more energy than a wave washing back off a normal sand beach. More sand is carried off shore, promoting beach loss. Additionally, jetties placed perpendicular to the beach, disrupting along-beach currents and causing sand loss downstream of the jetty.
Dr. Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822