Environment and development in coastal regions and in small islands

Coastal region and small island papers 19

4

	The exposed tree roots and leaning palm tree are indicators of erosion at this beach in the Rock Islands, Palau, July 2002.

Background

Beaches change their shape and size from day to day, month to month and year to year, mainly as a response to waves, currents and tides. Sometimes human activities also play a role in this process, such as when sand is extracted from the beach for construction, or when jetties or other structures are built on the beach.

For more information on erosion and accretion as well as waves, tides and currents, see Cambers, 1998, and other texts dealing with coastal processes.
 

Activity 4.1

Measuring erosion and accretion over time

  
What to measure

One very simple way to see how the beach changes over time, and whether it has eroded or accreted, is to measure the distance from a fixed object behind the beach, such as a tree or a building, to the high water mark.

The high water mark is the highest point to which the waves reached on that particular day. It is usually easy to identify on a beach, by a line of debris such as seaweed, shells or pieces of wood, or by differences in the colour of the sand between the part of the beach that has recently been wetted by the water and the part that remains dry.

Figure 5 Determining the high water mark, Savannah Bay, Anguilla, 1996. (The arrow shows the position of the high water mark on that date.)

Figure 5 shows a photograph of a beach in Anguilla; the arrow shows the high water mark which, in this case, is the land-most edge of the band of seaweed.

Alternatively, in countries where tide tables are published in the local newspapers, the visit to the beach can be timed to coincide with high tide, in which case the measurement is made to the water’s edge. One note of caution here, in the Caribbean the tidal range is very small, approximately 1 ft (0.3 m), so the state of the tide – whether high, mid or low tide – does not matter very much. But in the Pacific for example, the tidal range is greater, 3 ft+ (1 m+), so in this case it will be necessary to always repeat these measurements at the same tidal state, e.g. if the first measurement is done at high tide, then subsequent measurements should also be done at high tide.

Sometimes there may appear to be more than one line of debris on a beach. In such cases, take the line closest to the sea; the other debris line may well be the result of a previous storm some weeks or months ago.

Most beaches show variation in erosion and accretion, for instance, sand may move from one end to the other end. So if monitoring the physical changes in the beach, it is recommended to carry out these measurements at a minimum of three sites on the beach, one near each end and one in the middle (see Figure 6).

Figure 6 Plan view of a sample beach showing suggested points for measuring beach width.

How to measure

  
When to measure

What the measurements will show

The data may show seasonal changes in the measurements, e.g. the beach may be wider in summer than in winter. Figure 8 shows this type of seasonal pattern in a bar graph.

 
What to measure

If the structure is a jetty or a groyne, select a measurement point on each side of the structure, and measure the distance from a fixed object behind the beach to the high water mark, as in the previous activity (4.1).

Measuring the beach width in front of this wall, as well as in front of the grassy area to the left, could yield interesting results, Grand Mal, Grenada, 1998.

Alternatively if there is a seawall at the back of the beach, you may wish to set up a measurement point in front of the seawall as well as one on an adjacent part of the beach where there is no seawall.

 

 

 
How to measure

 
What the measurements will show

Beaches in front of seawalls may also react differently to beaches where there are no seawalls. Often the beaches in front of seawalls may change very dramatically, e.g. a beach in front of a seawall may completely disappear one week, only to re-appear the following week.
 

Activity 4.3

 
What to measure

Figure 10 shows how a beach profile eroded as a result of a tropical storm.

Figure 10 Changes in a beach profile before and after Tropical Storm Lilli, Port Elizabeth, Bequia, St Vincent and the Grenadines, 2002.

	Group of students measuring a beach profile at Hamilton, Bequia,  St Vincent and the Grenadines, 2000.
	Group learning how to measure slope with an Abney level at Beau Vallon, Mahe, Seychelles, 2003.

The measurements show how the beach profile changes over time. For instance, Figure 10 shows how the beach profile became steeper and the beach width narrower after a tropical storm. The computer program allows successive profiles to be plotted on the same graph to see the changes.

Regular measurements of profiles can show not only how a beach responds to a storm or hurricane, but also how/if it recovers afterwards and the extent of that recovery. Removing sand for construction or building a seawall also impacts a beach, and only by carefully measuring beach profiles before and after the activity is it possible to say accurately how the beach has changed. Government authorities, as well as beachfront house and hotel owners may also be interested in the information collected from beach profiles. Designing a successful tree planting project requires knowledge of how the beach changes over time. The applications are numerous. Many people think they can tell how a beach has changed simply by looking at it, but it is much more complex than that, and often people’s memories are not as accurate as they like to think. Accurate data, such as beach profiles, are the basis for sound development planning.

New threats to beaches

Today, there is a new threat facing beaches – that of sea level rise. While sea levels may rise naturally in some parts of the world, this is a very slow and gradual process. However, global warming caused by excess production of greenhouse gases, notably carbon dioxide, by human activities, can greatly accelerate this process. This warming of the atmosphere is believed to cause glaciers to melt and ocean water to expand thermally. Both effects will increase the volume of the ocean, raising its surface level. This means many of our beaches may erode and disappear faster than before.

Scientists also believe that global warming may cause changes in the frequency and intensity of tropical storms, hurricanes, cyclones or typhoons. These weather systems bring extremely strong winds, torrential rain and huge waves which impact beaches, coasts and in some cases entire islands.

Related research and discussion topics might include:

• Climate change and climate variation – how do they differ?

• Research the number of hurricanes/cyclones coming within 100 miles (160 km) of your
  country or island in the 1970s and each following decade. Discuss the results, is there a
  trend?

• How many really severe hurricanes/cyclones (category 3 or higher) have come within
  100 miles (160 km) of your country or island in past decades?

• Have there been changes in the climate in your country or island? Are the summers
  getting hotter? Or the dry season getting longer?

• What happens to beaches and dunes when hurricanes/cyclones strike?

• Has the sea level surrounding your country or region changed over the last 50 years?

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