Environment and development
in coastal regions and in small islands

Coastal region and small island papers 19

5

  Beach composition

The powder white sand
on this beach at Vlingilli,
Madives, 2003, originates
from the surrounding
coral reefs.

  



Different sizes of material on a beach in
Rarotonga, Cook Islands, 2003.

Background

A beach consists of loose material, of varying sizes. The actual material itself can tell a lot about the stability of the beach.
 

Activity 5.1

 

Finding out where beach material comes from

  
Observe and record

 


Observe, describe and record the type of beach material. A beach may be composed of just one type of material, e.g. sand, or there may be a mixture of materials, e.g. sand, gravel and boulders. Beach material can be classified into different sizes (see the table below). Sand is just one size range.

Note and record the colour, size and texture of the material on the beach. A simple ruler or tape measure can be used to distinguish between the larger sizes, although obviously not for clay and silt. Use plastic bags to collect samples of material from different parts of the beach and label the location, e.g. near high water mark, beneath cliff face and so on.

SEDIMENT SIZES
Clay Less than 0.004 mm Less than 0.00015 inches
Silt 0.004–0.08 mm 0.00015–0.003 inches
Sand 0.08–4.6 mm 0.003–0.18 inches
Gravel 4.6–77 mm 0.18–3 inches
Cobbles 77–256 mm 3–10 inches
Boulders Greater than 256 mm Greater than 10 inches

WHAT IS SAND?

Sand consists of small pieces of stone or shell and can be
classified into three main types:


Common components of mineral sand include the following:

  • Quartz grains are clear, quartz is one of the most common
    minerals found in sand and is extremely weather resistant
  • Feldspar grains are pink, light brown to yellow
  • Magnetite grains are black and strongly magnetic
  • Hornblende grains are black and prism-shaped

Common components of biogenic sand include the following:

  • Coral may be identified by its many rounded holes
  • Shell fragments may come from scallops, mussels, clams and
    be a variety of colours
  • Sea urchin spines appear as small rods or tubes and may be
    a variety of colours

Sand samples may also include some organic material.
 

This black sand at Londonderry,
Dominica, 1994, is volcanic and
is transported to the coast
by the rivers.

 

This yellow-brown silica sand at Walkers Pond, Barbados, 1983, originates from the erosion of
inland rocks.

 

Discuss where the beach material originates

Back in the classroom, make a sketch map showing the different features (e.g. river mouth, rocky outcrop, cliff) on the beach and the different types of material. Discuss where the different types of material might originate.

Sand is composed of small pieces of stone or shell and its colour depends on its origin. Sand may come from inland rocks and be carried to the coast by rivers and streams. It may originate from nearby cliffs, or even far distant cliffs and be carried to a particular beach by longshore currents (see Chapter 10). Or the sand may have its source in the offshore coral reefs and seagrass beds.

The pure white sands of many tropical beaches are derived from coral reefs or coral reef limestone rocks. Yellow to brown silica sand found along some coasts comes from the erosion of inland rocks, while the black sand beaches of many volcanic islands consist of grains of olivine and magnetite, derived from the erosion of volcanic rocks.

Ask students to write a story about the life of a grain of sand, starting perhaps in an inland mountain and travelling to the beach by a stream, or originating on a coral reef and being moved by waves and currents to a beach. Ask them to imagine their life on a beach and what happens when a storm strikes or a sand miner moves them. A ‘letter from a grain of sand’ in the accompanying box provides some further ideas.

    
Ernesto Ardisana Santa (fourth
from right) presenting ‘Letter from
a grain of sand’, Cuba, February
2004.
 

LETTER FROM A GRAIN OF SAND

Hello friends!

I am a tiny grain of sand, bathed by the sea spray, created by the waves of the Caribbean Sea. I live in a marvellous place where, every morning at sunrise, I listen to the tremulous murmur of flying fish shooting out of the transparent sea water. Many birds inhabit this place, particularly the small, delicate and dark sea swallows which fl y constantly in search of food.

The sea is sweet and beautiful, but it can also be cruel and can become angry all of a sudden. Perhaps you may be surprised at my referring to the sea in Spanish as if it were feminine. This is the way we, those that love her, refer to the sea. I consider her as belonging to the feminine gender and as someone who concedes or denies big favours, and if she does perverse deeds, it is because she cannot help it.

My Mom and Dad are also sand grains, already hundreds of thousands of years old, since in this beach toxic substances that could have degraded us have never been used. Those persons who visit us are sorry to tread on us, which explains their walking warily and their not leaving food leftovers behind. We are always tended by children and the young of the local beach community, who remove the plant litter that comes out of the sea.

Through this letter I wish to express my solidarity with all the suffering grains and tiny grains of sand in this world, and especially so those of the coasts of Galicia in Spain who are bearing the effects of an oil spill.

I wish to invite you all to my unpolluted world. You can find me at the following e-mail address: letstakecare@everybody.world. I will receive you with pleasure. I now say goodbye with a great marine salutation, since it is the time to go to listen to the classes given by the snail on how to recycle the trash left daily on the coasts by humans, in order that this, my small paradise, may remain clean and pure and that I may be proud to live in my blue planet, helping to make it liveable for others too.

I am looking forward to your messages. I will give you my address later, because it is difficult, very difficult to understand, since unfortunately you must find your way through the paths of dreams.

With best wishes
The happy tiny grain of sand

Source: Instituto Pre Universitario Vocacional De Ciencias Exactas,
Comandante Ernesto Che Guevara, 2004
 

Activity 5.2   Exploring what happens when sand and stones are removed for construction

 
Observe and record

   

Visit a beach that has been heavily mined for construction material as well as a beach that has not been mined. Observe and record the differences between the two beaches and relate them to the mining activity. Features to look for and discuss might include the following:
Mined beach at
Brighton,
St Vincent and
the Grenadines,
1995.
  • How is the material being extracted – with heavy equipment or by people using spades?
  • Are there vehicle tracks all over the beach?
  • Are there deep holes where material has been extracted?
  • Does the water reach further inland?
  • Are there trees that have been undermined or vegetation that has been trampled?
  • Might the deep holes affect baby turtles if they nest on this beach?
  • Does the beach look like a nice place to visit?
  • Are there other sources of construction material besides the beach?

 
Discuss how the
beach material
is used in
construction

   

Ask the students to think about the construction materials used for houses and buildings in their country. Topics to discuss might include:
  • What materials were used to build houses in the past?
  • Compare and contrast the differences between concrete houses and wooden houses.
  • What materials are needed to make concrete?

Activity 5.3

  Measuring beach sand - size, shape and sorting

 
What to measure

  

Sand samples can be collected from different parts of the beach and the size, sorting and shape of the sand grains can be measured. These characteristics are likely to vary from one part of the beach to another.
 
 
How to measure
 

During a visit to the beach, sand samples can be collected from different areas, e.g. from a river mouth, from the inter-tidal zone where the sea is wetting the sand, from the dry sand at the back of the beach, from a dune behind the beach, or from beneath an eroding rock face or cliff.

Place the sand samples in clean plastic bags, label each bag and keep notes on exactly where the sample was collected.

THREE ‘S’S’ OF SAND: SIZE, SHAPE AND SORTING

Sand size depends on the origin of the sand and the wave energy. Strong wave action, such as found on exposed coasts, washes out the finer sand particles leaving only coarse sand and a steep beach profile. Often stones and boulders may be present on such beaches. However, on more sheltered coasts, finer sand is deposited and a gently sloping beach results. Near mangroves and river mouths, silt and organic material also collects.

Sorting relates to the mixture of sizes, e.g. if all the sand grains are the same size, then the sample is well sorted. If there are a lot of different size grains in the sample, then it is poorly sorted. As sand is moved about by the waves, it tends to get better sorted, in other words all the sand grains are about the same size.

The shape of the sand grains relates to whether the individual grains are angular and pointed or whether they are smooth and rounded. As the sand grains are moved about by the waves, they tend to become rounded with very few sharp points.
 

   
    

Measuring the shape of sand
grains with a magnifying glass,
St Lucia, 2001.

 

On return to the classroom, the samples should be spread out on a flat surface to dry (if they are wet). Then sprinkle some dry grains on to a plastic sheet. Place the plastic sheet with the sand grains on top of the size charts in Figure 11. If the sand grains are light coloured use the left hand chart, while if the grains are dark coloured use the right hand chart. With a magnifying glass, determine the size category matching most of the grains and record the results. Then compare the sand grains on the plastic sheet with the sorting chart, and with the magnifying glass determine the best-fit sorting category. Finally, compare the sand grains in the sample with the angularity charts to determine the shape.

If the beach is made up of stones only, these can also be measured. Collect at least 20 stones, picking them randomly, measure the length along the longest axis and then calculate the average. The chart in Figure 11 can be used for determining the shape of the stones.

Figure 11
Sediment analysis charts for size, sorting and shape
(adapted from Kandiko and Schwartz, 1987; and Powers, 1953).

 

 
When to measure
 
 
You may wish to collect sand samples from different parts of the beach one time only, and compare the different samples.

Alternatively you may decide to collect and measure sand samples from the inter-tidal zone, at different times of the year and after different wave events, e.g. after the summer when the waves have been relatively calm and then again after a high wave event. Some beaches show marked differences in composition, having sand in the summer and stones in the winter. Size comparisons can be made and related to the wave energy (see Chapter 9).
 

 
What the measurements show
 
 


Variations in size, sorting and angularity will provide information about the different zones on the beach and the processes that shape these zones. For instance, dunes are formed by the wind lifting dry sand grains and carrying them to the back of the beach. So, dune sand might be expected to be smaller in size than sand in the inter-tidal zone. Similarly, sand near a river mouth might be expected to have more organic material in it than the sand in the inter-tidal zone.

 
In the summer months (April to
October), Bunkum Bay in
Montserrat is a sandy beach;
     while in the winter months
(December to March) the sand is
replaced by stones.

Comparisons of sand size over time might be shown in a bar graph, such as is shown in Figure 12. In this example the beach consisted of black and grey stones in January 2002, while at other times of the year, the beach was made up of black sand (see also photographs of Bunkum Bay in Montserrat where similar changes take place).

Figure 12
Bar graph showing changes in sediment size.

 

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