Beach Processes

Beaches are dynamic as we saw in the film Portrait of a Coast. Waves breaking on the beach lose energy and create turbulence. Small waves carry sand up higher on the beach. Storm waves erode the beach and carry sand offshore. The water poured into the surf zone by breaking waves runs off in currents which carry sand along the bottom. Tides raise and lower sea level and modulate these processes. Wind carries sand inland and creates dunes.

Water flows along the beach, then turns offshore in a narrow current called a rip current. Swimmers caught sometimes panic as they are carried offshore. As they try to swim back to the beach they can tire and drown. The safest action is to swim parallel to the beach until out of the rip, then swim in.

Rip current at a beach. The current is only a few meters wide, extending mayne 100 yards offshore.
From NOAA web page on Rip Currents.

To understand some of the material in the movie, please read Pamela Gore's Shoreline and Coastal Processes web page, and Seafriends tutorial on Dunes and Beaches section of of the Oceanography Book published by Seafriends Marine Conservation and Education Centre.

Sand on beaches comes from the erosion of rocks on land. Rivers carry the sediments to the coast. The construction of dams has slowed the flow of sand to coastal regions.

Humans have simultaneously increased the sediment transport by global rivers through soil erosion (by 2.3 ± 0.6 billion metric tons per year), yet reduced the flux of sediment reaching the world's coasts (by 1.4 ± 0.3 billion metric tons per year) because of retention within reservoirs. Over 100 billion metric tons of sediment and 1 to 3 billion metric tons of carbon are now sequestered in reservoirs constructed largely within the past 50 years.
From Syvitski et al (2005).

Reference
Syvitski, J. P. M., C. J. Vorosmarty, et al. (2005). "Impact of Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean." Science 308 (5720): 376-380.

Revised on: 6 June, 2008