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.
 

Erosion and accretion

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

 
  Erosion takes place when
sand or other sediment is lost
from the beach and the beach
gets smaller, and the opposite
process –
accretion – takes
place when sand or other
material is added to the beach,
which as a result gets bigger.


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

At the first point, select the building or tree that you are going to use. Write down a description of the tree or building (and if possible photograph it). This will help you to return to the same point to re-measure. With two people, one standing at the building and one at the high water mark, lay the tape measure on the ground and pull the tape measure tight. Note the distance either in feet and inches, or metres and centimetres, whichever system the students are familiar with, record the measurement together with the date and the time of measurement. Then proceed to the next point and repeat the measurement. Label your three points either with physical names or a notation system (A, B, C or 1, 2, 3).
Taking a photo
of your reference tree
or building is always
advisable, Magazin
Beach, Grenada, 1996.
Measuring the beach
width, Sandy Beach,
Puerto Rico, 1997.

If your beach or beach section is about 1 mile (1.6 km) long then a minimum of three points is recommended. However, you can always add additional points.

The measurements can be supplemented with photographs of the beach taken from the same position and angle on different dates.

  
When to measure

  
 
Ideally these measurements could be repeated monthly, but even if only repeated every two or three months, they will still yield some interesting information.
 

What the measurements will show

The data will show how the beach has changed over the monitoring period, whether it has gained or lost sand, possibly one part of the beach has increased in size while another section has decreased in size. Figure 7 shows line graphs from three points on a sample beach, the beach at Site A accreted (it gained sand), at Site B there was very little change and at Site C the beach eroded (it became smaller).
 
Figure 7
Line graph showing
erosion and accretion changes over time.
 
   

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.

 
Figure 8
Bar graph showing beach width changes over time.
 
    If the students are also measuring waves (see Chapter 9), then these measurements may be related to the changes in beach width. Figure 9 shows beach width and wave height recorded on the same graph. In this case the beach width was greatest in August and September when the wave height was lowest.
 

Figure 9
Mixed graph showing changes in beach width and wave height.
 
Activity 4.2   Determining the effects of man-made structures on erosion and accretion

 
What to measure

   

Look for any man-made structures on the beach (also called sea defences) such as jetties, groynes, seawalls on or behind the beach. Note their numbers and where they are positioned.

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

   

Use the same techniques as described above in the activity dealing with erosion and accretion (Activity 4.1)

 
What the measurements will show

   

Again the measurements will show how the beach changes over time. In the case of the measurements on either side of the jetty, the data may well show that the beach on one side of the structure gets bigger, while the beach on the other side gets smaller. These changes can also be related to measurements in waves and longshore currents (see Chapters 9 and 10).

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

  Measuring beach profiles

 
What to measure

  

This activity is better suited to older students in secondary school. A beach profile or cross-section is an accurate measurement of the slope and width of the beach, which when repeated over time, shows how the beach is eroding or accreting. It builds on ‘Activity 4.1 Measuring erosion and accretion’ and includes measurement of the slope of the beach.

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.
 
 
How to measure
 

There are many different ways of measuring beach profiles, the method described in Annex 2 is one of the simpler methods, and is currently used in many small islands to determine beach changes over time. The annex describes how to measure beach profiles and also provides information on the use of a simple computer program available to analyse the data. The program is available free on request from UNESCO-CSI (csi@unesco.org).
 
When to measure
 
 
Beach profiles should be repeated at three month intervals or more frequently if time permits.
 
What the measurements show
 
 
   
 

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?

Start     Chapter 5

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