Assessing Dunes

Sand dunes are fun to explore.  There are lots of ups and downs and little paths to follow and being just behind the beach, they are a good place to find shelter on a windy day.

However, sand dunes are a vulnerable environment that is easily disturbed.  Strong winds from the beach readily move the loose sand around, blowing it further inland and rearranging the dunes, covering whole plants in one place and exposing the roots in another.

As we go tramping around our feet kick the sand about.  The plant roots cling on to the sand as best they can, but it is easy to disturb everything just a little too much.

Let’s investigate:

  • Is there more evidence of people just behind the beach or further inland?
  • Does the impact decrease as you get further away from the car park?

To assess the impact on the dunes you need a chart:

This is a basic outline, but you can add more rows and more things to investigate.

You need a copy of the chart for each site that you are going to assess.  You’ll need to visit 3 or 4 sites, at different distances from the sea, or different distances from the car park. You could pace out the distance between them.

At each site look around you.  Then put one tick on each row of your chart.  So with the first row, if it is really noisy then put a tick under -2; if it is really quiet then tick +2.  If it is somewhere in between – well you have 3 options.  Do each row in the same way.

When you’ve finished work out the total score for each site.  The best site, with the least evidence of people, will have the most positive score overall.  The worst site will probably have a negative total score.

You can adapt your chart to other situations too, by changing the descriptions at the ends of the rows.  Just keep all the bad stuff on the left and all the good stuff on the right.

Which is the best beach?  The best footpath?  The best village?

Use Your Quadrat

A quadrat can be used in all sorts of ways and you might find it useful for biology fieldwork too.  Here are some ideas for the beach.

Last week we found out how to use the quadrat to estimate percentages, so now try that with beach material.

What are the percentages of seaweed and sand?

Or the percentages of sand and pebbles?

If you are measuring pebble sizes then you can use the quadrat to tell you which pebbles to measure.

You could measure the pebble underneath each point where the strings cross…

…or if they are bigger you could just take one from under each of the corner junctions.  It doesn’t matter how you do it, as long as you decide the rules before you throw the quadrat down.

Sometimes, behind the beach you find sand dunes.

If so, then take your quadrat into the dunes.  Measure the percentage vegetation cover, just like I showed you last week with my patio weeds.

Does the percentage change as you get further from the sea?

Now look at the plants in detail.  Use a book or the internet to identify the different types.  At each location see which species covers the largest area.

Does the main vegetation type change as you get further from the sea?

If you want an easier option than identifying plants, count the number of different species in your quadrat.  You don’t need to know what they are.

Does the number of plant species change as you get further from the sea?

We’ll look at dunes again next week – there are a few more things you can investigate, that don’t involve quadrats.

Hold on to that Beach

Seaside resorts would lose their popularity and their customers if they lost their beach.

Where the local council want to be sure to keep, and even build up their beach, groynes are often built – those wooden fences that point straight out to sea and can make a walk along the beach something of an obstacle course.

Groynes also provide a handy indicator of the predominant direction of the longshore drift.  The beach material piles higher, and the waves don’t come in so far, on the side that has the most longshore drift moving towards it.

You can see the effect on this satellite view of Bournemouth.

So if your beach has groynes, then you need to go armed with a tape measure.

Measure from the top of the groyne vertically down to the beach material.  Do two measurements at the same place, one on each side of the groyne.  You’ll get a smaller measurement where there is a bigger pile of beach material and that’s the side that the longshore drift is coming towards most of the time.

Here are some things to investigate:

  • Is the height difference bigger closer to the sea?
  • Is the height difference bigger at one end of the beach?
  • Is the longshore drift for a particular day (as shown by a float or pebbles) the same as the overall longshore drift direction (shown by the groynes)?

And don’t forget to check out the satellite view of your favourite resort.

Scilly’s Lost Land

Sea level around Britain is constantly changing, not just with the daily fluctuations in tide.  The current trend is for a steadily rising sea level.  This swamped Doggerland in the North Sea and has given the southern edge of Britain a submergent coastline.

The Scilly Isles have been affected in the same way.

Apart from St Agnes, which is separated from St Mary’s by a deep-water channel, the main islands of the Scilly group only have shallow water between them.  Indeed, at a very low spring tide it is possible to walk across the sand between Bryher and Tresco.

Take the sea level down by just 8 metres and you get a single island (Ennor) which was twice the current land area.

As the sea level rose, the low land between the islands was gradually swamped.  The population had to move, abandoning settlements and retreating to the higher ground.

Many historical sites were submerged, to be later rediscovered and explored by divers.

As recently as the 11th century, the islands were still joined at low water.  This is reflected in local place names, with the early Cornish language used on the outer edge of the islands, and places on the inward shores, bordering the shallow sea, having names with English roots, given since 1600.

The Atlantis on this part of the British coast is the land of Lyonnesse.  The low sea levels that allowed Doggerland to flourish, in the area that is now the North Sea, would have exposed land between the Scilly Isles and Cornwall.  The rising sea levels on the islands today are just the end of a very long process that has seen the Scilly’s change from high peaks in a vast landscape to low lying islands.  Legends abound.

Drifting Pebbles

Last week we learnt about longshore drift, and a simple method for finding out its direction on the day of your fieldtrip.

Longshore drift constantly shuffles beach material.

This is Chesil Beach in Dorset.

The beach material is clearly sorted, from small size at the west end…

…to large size at the east end.

(Zoom in on the photos to compare them – if I crop them any more you will lose the sense of scale.)

Longshore drift is usually from west to east on this part of our coast, as the most powerful waves tend to come from the south-west, causing all the beach material to move eastwards.  Less frequently the swash comes from the other direction, and longshore drift goes east to west, but waves from this direction are usually less powerful – only the smaller pebbles get shifted back to the west again.

Here’s another method for looking at longshore drift but this one needs a bit of preparation.  You will need to collect some pebbles, preferably a variety of different sizes, and paint them in a bright colour.  (Don’t use water soluble paint!)

At the beach, put your painted pebbles into the water, all at the same place.  Best to do this as the tide is coming in.  Otherwise they may not move very much before being left high and dry.  Leave them as long as you like, the longer the better.  Just make sure that you know where they started from.  Hopefully you will be able to find some of them at the end of your experiment.

  • Did they move along the beach?
  • Did they all move in the same direction along the beach?
  • Which moved further, bigger or smaller?