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):
a. Total amount of retreat = ______
b. Number of years of record = ___
c. 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)
a. The lowest average rate of retreat was __ feet/year
b. 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.
a. Number of years in interval from 1988 to 2010: _ years
b. Amount of retreat during the interval (number of years in interval multiplied by the average rate of retreat calculated in #1c.): ______feet c. Add the value you obtained to the total amount of retreat during the 1852-1988 interval: Total amount of retreat = ______________feet d. 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
e. 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?
- 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.
Actual shoreline retreat in nearby Surfside, TX:
A dome beach building in 1969 with a wide beach in front of it; and the same beach building in 2000 with no beach between it and the surf. (Watson, 2003)
COASTAL FLOODING DURING HURRICANES:
During a hurricane, the drop in air pressure that occurs, combined with air flow toward the center of the storm, causes the sea surface to rise. In intense storms, it can rise 20 feet, and in some cases, it has risen as much as 40 feet. Because much of the land adjacent to the Gulf coast is so flat, the rise in sea level associated with storm surges inundates much of the land along the coast, and this is one of the reasons storm surges are so hazardous. In addition, the storm waves that are generated on top of the storm surge also case damage – both because of the added height of the water level and because of the force of impact of the waves.
During a hurricane in 1969, a woman who lived in an apartment building across the street from the beach decided to throw a ‘hurricane party’. So she invited guests to come over and ride out the storm in the resumed safety of their concrete and brick apartment building. What this woman and her guests did not know about, or did not fully appreciate, was the rise in sea level associated with storm surges during hurricanes and the force with which the waves can strike shore. A cubic foot of water weighs 55.6 pounds and, during a storm, the waves can reach speeds of up to 55 mph.
- To illustrate what this means, calculate the weight of a wave striking the building based on the assumption that the wave struck the entire front of the building and that the building was 30 feet tall and 70 feet wide. In this problem we’ll make the simplifying assumption that the part of the wave that struck the building was a ‘wall’ of water 30 feet high, 70 feet wide, and 3 feet thick from top to bottom. (Obviously waves are thicker at the bottom than at the top, but we’re ignoring this to make the calculation easier.) To determine the weight of the wave, determine the volume of the wave (in cubic feet), and then multiply the volume by 55.6 pounds per cubic foot.
a. Volume = height x width x thickness = ___________________ft3
b. Weight = volume x 55.6 lbs/ft3 = ____________________________lbs.
This large number represents the weight of only one wave. Hundreds of waves would have hit the building during the course of the storm. Based on this calculation, it is easy to see why the apartment building was completely destroyed; nothing was left but the foundation. Tragically, the only person to survive the party was the lady who gave it – her 20+ guests were killed. - What was the moral of this story if you live along a beach and a hurricane is approaching?
