Coastal Processes Lab INTRODUCTION A beach is a dynamic environment in which the
ID: 298057 • Letter: C
Question
Coastal Processes Lab
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.
Determine the amount of retreat per interval (complete column C). Then add these values together and divide by the number of years of observation (as listed below):
Total amount of retreat = __________________
Number of years of record = _______________
Average amount of retreat per year = ______________ (round to the nearest tenth)
Calculate the average rate of retreat per year per interval and record those values in column E. To do this, first determine the number of years per interval (column D). Then divide the amount of retreat per interval by the number of years in the interval. (Example: 1852-1930 interval: 839 feet of retreat ÷ 78 years = 10.8 ft/yr)
The lowest average rate of retreat was ______________ feet/year
The highest average rate of retreat was ______________ feet/year
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
Calculate how far the beach will retreat (relative to the 1852 shoreline) by the year 2010 using the average rate of retreat you just calculated. Follow the steps given below.
Number of years in interval from 1988 to 2010: __________ years
Amount of retreat during the interval (number of years in interval multiplied by the average rate of retreat calculated in #1c.): _________feet
Add the value you obtained to the total amount of retreat during the 1852-1988 interval: Total amount of retreat = ______________feet
Repeat steps B and C using the lowest and highest average rates of retreat obtained in question 2:
i.Total amount of retreat based on lowest rate: ______________feet
ii.Total amount of retreat based on highest average rate: _________________ feet
What is the maximum difference in shoreline retreat estimates for the year 2010? _______________feet
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?
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
Totalb amount of retreat = 839+96+229+4+142+120+20+200+60+150+9=1869f
Number of years of record = including 1852 = 12 years
Average amount of retreat per year = 1869/12 = 155.75 nearest to tenth = 156 f/y
lowest average rate of retreat = 1 feet/ year
highest average amount rate of retreat = 50feet/ year
No. of years in interval from 1988- 2010 = 22 years
Amount of retreat during the interval = no.of years * average amount of retreat per year
= 22* 156
= 3432feet
Total amount of retreat with 2010 = 1869 + 3432 = 5301feet
Total amount of retreat upto 1988 based on lowest rate = 4feet
total amount of retreat based on highest rate = 200feet
Average rate of retreat in 2010 = 5310/22
= 240feet / year
maximum difference in shoreline retreat estimates for the year 2010 relative to 1952 = 5301 -1869
[ amount of retreat in 2010 - total amount of retreat ]
= 3432feet
every year theshore line is decreasing and somewhat increasing to balance the sediments at shoreline.
But relative to 1852 the retreat is somewhat decreasing .
no. Year Change in position relative to 1852 Amount of retreat (feet) Number of years in interval Average change feet/year/interval 1 1852 0 2 1930 -839 839 78 10.8 3 1933 -935 96 3 32.0 4 1943 -1164 229 10 22.9 5 1947 -1168 4 4 1 6 1952 -1310 142 5 28.4 7 1957 -1430 120 5 24 8 1963 -1450 20 6 3.33 9 1967 -1650 200 4 50 10 1972 -1710 60 5 12 11 1982 -1860 150 10 15 12 1988 -1869 9 6 1.5Related Questions
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