INTRODUCTION A beach is a dynamic environment in which the sediment that compose
ID: 298058 • Letter: I
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INTRODUCTION
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. Use the data provided in this table to answer the following questions concerning beach erosion rates.
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
0
1930
-839
839
78
10.8
1933
-935
96
3
32.0
1943
-1164
1947
-1168
1952
-1310
1957
-1430
1963
-1450
1967
-1650
1972
-1710
1982
-1860
1988
-1869
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.
Year
Change in position relative to 1852
Amount of retreat (feet)
Number of years in interval
Average change ft/yr/interval
1852
0
1930
-839
839
78
10.8
1933
-935
96
3
32.0
1943
-1164
1947
-1168
1952
-1310
1957
-1430
1963
-1450
1967
-1650
1972
-1710
1982
-1860
1988
-1869
Explanation / Answer
Year
Change in position relative to 1852
Amount of retreat (feet)
Number of years in interval
Average change ft/yr/interval
1852
0
1930
-839
839
78
10.76
1933
-935
96
3
32.00
1943
-1164
229
10
22.90
1947
-1168
4
4
1.00
1952
-1310
142
5
28.40
1957
-1430
120
5
24.00
1963
-1450
20
6
3.33
1967
-1650
200
4
50.00
1972
-1710
60
5
12.00
1982
-1860
150
10
15.00
1988
-1869
9
6
1.50
We can use 1933, 1952 and 1967 retreat because these three year shown a large variation of retreat in small interval. These three are most accurate because the time interval is very small compared to other years.
Variability in estimates does not mean that the estmates are worthless, but the variability mean that the rate of erosion is varying with time. The rate of erosion depends on different oceanic and atmospheric parameters. These factors are changing with time. So the amount of erosion also changing.
Year
Change in position relative to 1852
Amount of retreat (feet)
Number of years in interval
Average change ft/yr/interval
1852
0
1930
-839
839
78
10.76
1933
-935
96
3
32.00
1943
-1164
229
10
22.90
1947
-1168
4
4
1.00
1952
-1310
142
5
28.40
1957
-1430
120
5
24.00
1963
-1450
20
6
3.33
1967
-1650
200
4
50.00
1972
-1710
60
5
12.00
1982
-1860
150
10
15.00
1988
-1869
9
6
1.50
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