A beach is a dynamic environment in which the sediment that composes a beach is

ID: 298096 • Letter: A

Question

A beach is a dynamic environment in which the sediment that composes a beach is in constant motion. The movement is caused by waves and currents acting along the beach, and the amount of movement varies depending upon the number and size of waves that strike the beach and the speed and direction of the currents. Under stable conditions, the amount of sediment removed from a segment of the beach is balanced by the amount that is brought into that area, so that no net loss or gain occurs. If more sediment is brought in than is lost, the beach increases in area, i.e., accretion occurs. On the other hand, if more sediment is removed than is deposited, beach erosion occurs. Sediment losses increase during storms when wind and wave action are stronger than normal. However, the erosion that occurs during a storm is often balanced by deposition that occurs after the storm; the result being that the beach returns to its former state. Nonetheless, if you happen to have a home on a beach that erodes during a storm, then you lose your home! Erosion can also occur when vegetation is removed from a beach during development. Removal of vegetation allows both wind and water to transport more sediment than normal, so that net losses may occur from the devegetated area. In addition, erosion can occur when the supply of sediment to a coast is diminished, as for example, when dams are built across rivers leading to the sea. When this happens, sediment that was headed to the coast becomes trapped behind dams and sediment loss for the beaches exceeds sediment gain. Because development is occurring on most beaches, and because dams have been built across most rivers, the rate of sediment removal is increasing while the rate of sediment supply is decreasing. Consequently, most of the beaches in the United States and around the world are eroding.

BEACH EROSION RATES

The data table below lists shoreline changes that occurred along Sergeant Beach, TX between 1852 and 1988. There has been substantial coastal retreat since 1852.

Rate of Retreat at Sergeant Beach, TX

A B C D E

Year

Change in position relative to 1852

Amount of retreat (feet)

Number of years in interval

Average change ft/yr/interval

1852

1930

-839

839

10.8

1933

-935

32.0

1943

-1164

1947

-1168

1952

-1310

1957

-1430

1963

-1450

1967

-1650

1972

-1710

1982

-1860

1988

-1869

The above estimates show a substantial variability of rates of shoreline retreat. What does this suggest about the reliability of estimates of future shoreline positions?

If it were your job to inform the people living in the Sergeant Beach area that their beach was eroding, which of the three estimates of retreat would you use and why? (Which do you think is the most accurate?)

Does the variability in estimates mean that the estimates are worthless and can be ignored – especially if you don’t like the implications of the estimates? Explain.