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.

Make a Quadrat

There are plenty more beach fieldwork options to come, but first you need to make yourself another piece of equipment – a quadrat.

To make one of these you need something to make the outside frame and then string to divide the central area into smaller squares.  You also need a tape measure to help you get everything in the right place.

For the frame I used garden canes, but lengths of wood would work too and would probably be more robust.  You need to cut 4 lengths of about 60 cm.  The inside edge of the frame will be 50 cm x 50 cm, but you need the extra length to fix them together.

Use string to lash the frame together at the 4 corners.  (Lengths of wood could be screwed together.)

Make sure the internal area of the square is exactly 50 cm by 50 cm.

You are going to divide the frame every 10 cm.  Mark the edge with permanent marker so that you know where to fix the strings, and so that you can see where they should be, if they slip out of position.  There should be marks at 10, 20, 30 and 40 cm on each side.

Now tie strings across.  Go first one way…

…and then the other way, twisting around each of the first set as you go.

Check the squares are still square (10 cm by 10 cm) and adjust if necessary.

And there you have it…

…a very useful piece of kit.

Here’s one way to use it.

As you can see my patio needs weeding!  We can use the quadrat to help us estimate the percentage of my patio that is covered in weeds.

Throw the quadrat down.  Yes, don’t place it, since that would be you choosing the results.

The quadrat has 25 small squares so each of them represents 4%.  Give each square a score:

  • Square is all weeds = 4
  • Square is ¾ weeds = 3
  • Square is ½ weeds = 2
  • Square is ¼ weeds = 1
  • Square has no weeds = 0

Add them up and you can see that 45% of my patio is covered in weeds.

I’m off to do some gardening, but I’ll be back next week to tell you how to use your quadrat at the beach.

 

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.

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?

Longshore Drift

When a wave comes in, breaks and flows up the beach towards you, the movement is known as “swash”.  The water then drains back down the beach, which is called “backwash”.  The backwash always drains down the slope of the beach, but the swash can come from any direction, depending on the direction of the wind that formed the waves.

If the swash comes in at an angle and the backwash goes straight back, beach material gets gradually shifted along the beach.

To see the direction of longshore drift you need something that you can put into the water and find again easily.

One way to do this is to use an orange.  It will float so you will be able to keep it in sight.  Follow the backwash out and put the orange down as the next wave comes in.  Keep an eye on its movement for 10 minutes.  (Make sure you don’t lose the position that you started from.)  After 10 minutes, if it is safe to rescue your orange, do so.  You can then repeat, starting at a different place on the beach, and see if you get the same result.

  • Is longshore drift happening at your beach?
  • Is it moving in the same direction at each end of the beach?

If your beach is in a bay then the material could be coming into the bay from both directions, as in this picture.

If you are standing on the beach in the foreground, facing the sea, the swash is coming from your right.  If you are on the other side of the bay, facing the sea, the swash is coming from your left.  Material is being moved towards the centre of the bay from both ends, building a nice big beach.

 

Wave Frequency

This is another really easy fieldwork method.  All you need is a stopwatch, or a watch with a second hand, or even your phone – something to time for 1 minute.

Find a safe place from which to watch the waves (deckchairs optional!).

Count the number of waves that break on the beach in 1 minute.  Make a note of your answer.  Repeat this a couple of times and calculate the average (or you could count for a straight 10 minutes and then divide your answer by 10).

Use wave frequency results with your beach profiles and / or your pebble measurements to work out whether these statements are true or false at your locations.

  • Where waves are more frequent the beach is steeper. True or false?
  • Where waves are more frequent the beach material is larger. True or false?

Beach Material

If you’ve been thinking that beach profiles look just a bit too complicated or “I’m not standing on a beach with a garden cane looking like a complete idiot” then here’s a measurement that you can do easily and unobtrusively!

All you need is a ruler and pen and paper to record your results.

Head to a pebbly beach.  You can investigate to see whether the size of the pebbles varies at different places on the beach.

  • Are the pebbles bigger nearer to the sea or nearer the cliffs / dunes / promenade?
  • Are the pebbles bigger at the north / east end of the beach or at the south / west end?

But how do you measure a pebble?

You need to always take the longest measurement.  It is called the long axis.  If your pebble is oval shaped, it is easy to know where to measure, but if the shape is more irregular then you may need to test out a few different directions to find the longest one.

So that’s it?  No, not quite.

You can’t just choose 1 pebble.  That’s not going to prove that the pebbles are bigger at a certain place because you may have picked up the one huge one when all the rest are pretty small.

At each location on the beach you need to take a sample of at least 10 pebbles.  You need to pick them up randomly.  Close your eyes and take the first one that you touch.

Note the long axis measurement for all 10 pebbles.  You can then add the numbers together and divide by 10 to find the average pebble size at each location on the beach.

And if you are measuring the beach profile, you can investigate whether the bigger pebbles form the steeper or gentler parts of the slope.

 

Comparing Beaches

Now that you know how to draw a beach profile, you have a record of the shape of the beach on the day that you made your measurements.  But is it always like that?  Pose some questions and then think about what you need to do to find out the answers.

Here are some suggestions:

  • Is the beach profile the same the morning after a storm?
  • Is the beach profile the same on a sandy beach as on a pebbly beach?
  • Is the beach profile the same at the north / east end of the beach as at the south / west?
  • Does a beach in front of cliffs have the same profile as a beach in front of sand dunes?

To find out the answer to any of these questions you are going to need to do some more fieldwork.  You will either need to visit the same location at two different times or visit two contrasting locations.  You will then have two beach profiles which you can compare.  Which one is steeper?  Which one has a smoother shape?

Can you explain your results?

Can you see any problem with the method?  An alternative way of measuring would be to vary the distance between the poles to position them at the points where the slope angle changes.  You would have to measure the distance between the poles each time and account for this when drawing the profile but it would give you a more accurate diagram.

Beach Profiles

Have you been out to measure a slope yet?  Maybe you’ve been to the beach but if not you can practice these fieldwork skills on any slope.

Here’s my results sheet.

Each measurement is the average slope angle for each 10 metre length.  (We are ignoring any changes in slope that happen between your two poles.)  So the first thing we can do is find an average for the entire slope.

How many measurements did you collect?  I’ve got 8.

Average is all of the measurements added together and divided by the number of measurements involved.

So I’ve got 2+5+3+3+7+10+12+4 = 46

and then 46÷8 (because there were 8 measurements).

So my average is 46÷8 = 5.75

Calculate the average with your results.

Now let’s try something a bit more complicated. We are going to draw the shape of the beach.  It’s called a beach profile.

You will need a sheet of graph paper or squared paper, a protractor, a ruler, a pencil and your results sheet.

Have your graph paper in landscape position and draw a line straight across near the bottom.  This is your base line.

You will need to choose a scale for your base line so that your measurements are evenly spread out but they all fit onto the paper.  Remember your poles were 10 metres apart each time.

 

Line up your protractor with the left hand end of the base line.  My first slope measurement was 2°.  I’ve used the scale on the protractor to mark 2° with a dot.  Do the same for your first measurement.

Now line up your ruler so that it is on the left hand end of the base line (where the centre of the protractor was just now) and also in line with the dot.

Draw a line from the base line towards the dot, but keep an eye on the lines that are printed on the paper and stop when you are at 10 metres on your scale.

You may not get to the dot or you may need to go beyond it, it depends on the scale that you have chosen and the size of your protractor.

This was the position of your second pole and the start of your second measurement.  Use your ruler to draw yourself a new base line.  The printed lines will help you to get it straight and parallel to the first base line. (It won’t go through your dot unless the first slope was actually flat!)

The centre of the protractor now goes at the start of this new line.  Make sure the zero line of the protractor is on the new base line and put a new dot for the second measurement.

Again use your ruler, from the start of the new base line, through the dot, to draw the slope and this time stop at 20 metres.

Have you got the idea?

Keep going until you’ve plotted all of your measurements.

Now you have a diagram which represents the shape of your beach – a beach profile.

Next week I’ll give you some ideas for further investigation.