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Environment
and development in coastal regions and in small islands |
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Coastal region and small island papers 19
10
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Measuring longshore |
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Background
While waves are the most important process
moving sediment particles on a
beach,
currents also have a role to play.
Measuring longshore currents
What to measure
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Figure 20 |
When the waves approach the beach at an angle, they generate a longshore current which moves parallel to the beach (see Figure 20). While this current is not in itself strong enough to pick up sediment particles from the sea bottom, it can move material that has already been stirred up by the waves.
The longshore current is responsible for moving material from one part of the beach to another. When a structure such as a jetty or groyne is built out into the sea, this longshore current results in sand building up on one side of the structure (see Figure 21).
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| Figure 21 Effect of a groyne on longshore transport (adapted from Bush et al., 1995). |
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Groyne at Nisbett
Plantation, Nevis, 1992. The sand has built up on the updrift side of the groyne in the foreground, while the waves reach further inland on the downdrift side as a result of erosion. |
Measurements of longshore currents are best combined with wave measurements. So if longshore currents are being monitored, then waves should also be measured (see Chapter 9). Together, these provide a picture of the processes moving sand around on the beach.
The
longshore current flows in a direction
roughly parallel to the beach, near where the waves break. The
current
speed and direction can be measured. Current speeds are recorded in feet
per second or cm per second. Current direction is recorded in degrees
and is the direction towards which the current is going. So if
a current is going from north to south, the current direction is
recorded as south or south-going; similarly, a current going from east
to west is recorded as west or west-going. (This is opposite to wind and
wave direction, which are recorded as the direction from which the wind
is blowing or the waves are coming.)
How to measure
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Place a stick in the sand near the water’s edge. One observer walks into the
water from the stick and places the dye tablet in the water, as near as possible
to where the waves are breaking. The observers on the beach stand by the stick,
watch the coloured water and observe the direction in which it moves. After one
minute, the maximum distance the coloured water has moved is measured along the
beach starting from the stick. This is recorded. The measurement is made again
after 2 minutes and after 5 minutes.
The distance moved after five minutes is used to determine the current speed in ft/second or cm/second. The direction in which the dye moved must also be recorded.
These measurements can be repeated at several different places along the beach to see if the current speed and direction is the same or whether it varies.
If the dye does not move much, but just remains in
a pool near the stick, then this means there is no
longshore current on
that day.
When to measure
What will the
measurements show
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The measurements will show how the
longshore current varies over time, and how
it changes with the wave height and direction. For instance, if the waves
usually approach a beach from the south, and it is only during winter storms
that the waves come from the north, then monitoring
currents and waves during
the normal southerly wave regime and the less frequent northerly storm wave
regime, will yield some interesting results. It may be possible also to relate
these variations to visual changes in the
sand build-up on the beach or
measurements of beach width (see Chapter 4).
Figure 22 shows current speed and direction based on once/month measurements in 2000. The speed was highest in the winter months when the current direction was south-going. While in the middle months of the year, the current speed was lower and the direction of current movement was north-going.
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| Figure 22 Mixed graph showing current speed and direction. |
Further activities
Relate the direction of the longshore current to the source of beach material (see also Chapter 5); possibly some of the material at the monitored beach originates from an adjacent beach or coral reef.
Discuss the impact of groynes and jetties in your area and the role of longshore currents. Often beachfront home owners build such structures to try and protect their homes, but sometimes homeowners on the other side of the groyne or jetty may experience erosion as a result of the structure. Discuss such measures in the context of the entire beach, not just the affected homeowners.
Suggest the students carry out research into who owns the beach in your country. What does the law say? Are there any particular building restrictions near beaches so as to protect the public’s right to use the beach?
