http://www.geography-fieldwork.org/coast/coastal- processes/5-review.aspx Coastlines can be broadly categorised into two different types. Low energy coasts stretches of the coastline where waves are not powerful often the rate of deposition exceeds the rate of erosion landforms include beaches and spits High energy coasts stretches of the coastline where waves are powerful for a significant part of the year often the rate of erosion exceeds the rate of deposition landforms include headlands, cliffs and wave-cut platforms Waves Waves are created by the action of wind blowing over the surface of the sea. Wave energy depends on wind strength wind duration (how long the wind is blowing) water depth the fetch of the wave (the maximum distance of open sea a wave can travel before it hits land)
Coastlines can be broadly categorised into two different types.
Low energy coasts
stretches of the coastline where waves are not powerful often the rate of deposition exceeds the rate of erosion landforms include beaches and spits
High energy coasts
stretches of the coastline where waves are powerful for a significant part of the year
often the rate of erosion exceeds the rate of deposition landforms include headlands, cliffs and wave-cut platforms
Waves
Waves are created by the action of wind blowing over the surface of the sea. Wave energy depends on
wind strength wind duration (how long the wind is blowing) water depth the fetch of the wave (the maximum distance of open sea a
wave can travel before it hits land)
The highest part of a wave is the crest and the lowest point is the trough. The difference between crest and trough is the wave height. The distance between one crest and the next is the wavelength.
When a wave breaks, water washes forward onto the shore. This part of the wave is called the swash. The swash transfers energy up the beach. The backwash is the opposite action that returns water and energy down the beach.
Constructive waves and destructive waves
There are two types of wave: constructive waves and destructive waves.
Constructive waves have limited energy. They have a strong swash that transports material up the beach increasing the amount of beach material and creating a shallow, longer beach. Constructive waves appear lower in height and are less frequent (about 6-8 waves per minute).
Destructive waves have much more energy. They have a strong backwash that transports material back down the beach reducing the amount of beach material and creating a steeper, shorter beach. Destructive waves appear to be higher and more frequent (about 12-14 waves per minute).
Wave refraction
The direction in which a wave moves may be altered by the shape of the coastline. Waves travel faster in deeper water. If a wave approaches the coast at an angle the side nearer the coast, in shallower water, loses more energy to friction so slows down. This causes the wave to refract (change direction).
The direction of the waves is affected by features such as coastal defences, bays and headlands. Refraction around a headland can result in erosional formations on each side of the headland.
Wave transport
Waves transport material in the same ways as rivers transport material e.g. traction, saltation, suspension and solution. The
energy of the waves dictates the type of material carried. The load is the total amount of material carried by a wave. The competence of a wave is the maximum size of particle that the wave can transport. Waves need more energy to carry larger particles so only the waves with the highest energy can transport rocks and boulders. The weakest waves can only transport sand and clay.
Longshore drift
Longshore drift is the movement of material parallel to the coast. Longshore drift occurs when waves approach a beach at an angle due to the direction of the wind. The swash, produced by breaking waves, moves material diagonally up the beach at the same angle as the wave. In contrast, the backwash moves material down the beach perpendicular to the shoreline. This results in a zig zag movement of material along the coast.
Investigating longshore drift
For investigations looking at longshore drift along the shoreline, you may choose to establish a systematic sample, using equally spaced intervals along your beach. Quantitative evidence for longshore drift can be collected in three main ways.
1. Beach profiles
Beach profiles use a combination of distance and angle measurements to investigate the shape of the beach. They also allow for calculation of cross-sectional area as a measure of the amount of beach material present at a location.
If you intend to statistically analyse this data, a robust test will require at least 10 sites.
At each location, students will follow a straight transect line from the edge of the sea to the end of the active beach (this may be marked by a defence or the presence of vegetation etc.). The transect is split into smaller measureable segments. Taking measurements at equal intervals up the beach is more straightforward, but it tends to hide the small variations in slope which can be important in showing beach shape. Instead, you may wish to divide your transect according to where you estiamte the slope angle changes (from break of slope to break of slope). This means that you normally end up taking more slope readings, but the profile that you draw is more accurate. A step-by-step method for beach profiling is as follows:
Person A stands at a safe distance from the edge of the sea holding a ranging pole
Person B stands holding a second ranging pole further up the beach where there is a break of slope
The distance between the two ranging poles is measured using a tape measure
The angle between matching markers on each ranging pole is measured using a clinometer
Repeat this process at each break of slope until the top of the beach is reached
Beach profiles can also be used to investigate the effects of coastal management.
Data Presentation
Beach profile
A beach profile is a cross section of the beach from the top of the beach to the seashore. It shows distance on the x-axis and height above the seashore on the y-axis.
The distance and angle information for each facet of the beach can be plotted by hand or using a spreadsheet program to create a beach profile.
2. Pebble measurements
Pebbles can be selected using a variety of sampling strategies and methods.
If you are looking for a difference between the two ends of your beach, you should use stratified sampling, and collect a sample of at least 10 pebbles from either end.
If you are looking for a correlation between distance along the beach and a pebble characteristic, pebbles should be sampled at systematic intervals along the beach. If you wish to carry out robust statistical analysis of this data, you you should establish at least 10 sample sites.
At each sampling location, pebbles can be selected in a variety of ways e.g. using a 10m tape measure laid out parallel to the water,
and using a random number chart to choose points along this tape to collect pebbles from. Students should be aware that sediment size is likely to change with distance up the beach and take this into account, either by selecting pebbles from an equal distance up the beach at each location, or by collecting a sample which represents all distances up the beach.
Once pebbles have been collected, several measurements can be taken:
(a) Pebble size
Pebble size measurements allow you to investigate whether pebbles appear to have been moved along your coast, experiencing erosion, and therefore becoming smaller, as they travel.
Pebble size can be measured using a 30cm ruler or using calipers for greater accuracy. A single axis of each pebble can be measured e.g. the longest or 'a' axis (see diagram) or students may chose to measure multiple axes and calculate an average.
Alternatively a set of graduated sieves can be used to sort sediment samples into different size categories (in millimetres or as phi sizes). The sieves are arranged in decreasing mesh diameter with the largest at the top. The sediment sample is placed in the top sieve then the sieves are shaken to sort the sediment into the various sieves. The mass of sediment in each sieve is measured using scales and the percentage of the total sample can be calculated.
(b) Pebble shape/roundness
The simplest way to record pebble shape is to classify the stone as very angular, angular, sub-angular, sub-rounded, rounded or very rounded using a Power's Scale of Roundness.
very angular angular sub-
angularsub-
rounded rounded very rounded
Alternatively, for more precise shape data, Cailleux's Flatness Index can be used to obtain a numerical and reasonably objective value for roundness. The raw data needed for each pebble is as follows.
1. The length of the longest axis (called l)2. The radius of the sharpest angle (called r). The radius can be
measured using the Cailleux Roundness Chart with accompanying instrutions. To calculate the Cailluex's Index from this data, see Stage 4.
The a, b and c axes can also be used to calculate Krumbein's Index of Sphericity and for Zingg's shape classes (see Stage 4). Zingg's shape classes can also be visually estimated, by seperating pebbles into the following categories by eye:
Type of pebble Characteristics Exampl
eSphere a, b and c axes roughly equal Ball
Disc a and b axes roughly equal, c axis much shorter CD
Rod relatively long a axis, with b and c axes shorter and roughly equal Tube
Blade relatively long a axis, with a shorter b axis and much shorter c axis Knife
Pebble measurement
(a) Pebble size
If you have measured pebble size using calipers or a ruler, you could calculate the mean pebble size for each sample site on the beach. The data can be presented in a graph, such as a bar chart.
f you have sieved the sediment, you can calculate phi sizes. Use the conversion table if you do not have the phi sizes already.
Sediment sizemm phi
1.00 00.50 10.25 20.13 30.06 40.03 50.01 6
Calculate the percentage mass of sediment in each phi size category. For example, if total mass=100g and the mass of material at 5-10mm = 20g, then 20% of the total mass of sediment is 5-10mm in diameter. This can be presented in a number of ways
using a histogram with % mass on the y axis and sediment size on the x-axis
pie charts to show changes along the transect, which might be overlaid on a map or aerial photograph
plot a scattergraph to show how mean sediment size varies with distance along the beach (see below).
If you have collected data showing how a variable changes with distance along the beach, you could use the Spearman’s Rank Test.
If you have collected data showing the difference between two areas of the beach, you could use the Mann Whitney U
irstly describe the trends in each of your data sets referring to your graphs and any statistical results generated. For example:
What trends are shown e.g. what is the relationship between distance along the beach cross sectional area of the beach?
What is the strength of the trend (e.g. do you have a statistically significant result at p=0.05?)
Explain the trends (e.g. why does cross sectional area increase with distance along your beach?) referring to the processes which may have caused them.
Are there any anomalous results? Can you explain them? Some possibilities may include:
a) Cross sectional area is smaller/larger than expected in a location due to the presence of a particular coastal defenceb) Pebble size or shape shows an unexpected result in one area due to input of new material via mass movement
Link your data sets together. For example,
a) Try to link data sets collected along the beach to longshore drift or lack of longshore driftb) Try to link data sets collected up the beach (e.g. phi sediment size) to wave energy and time of year i.e. larger particles, found at the top of the beach, are deposited by the swash of destructive waves in winter.
Conclusion
Create a summary of your findings and answer your investigation question/hypothesis. Secondary data can be used to support your findings (e.g. If you have data on prevailing wind direction, you will be able to discuss the differences between the angle of the swash and longshore drift)
Evaluation
In your evaluation section, discuss the reliability of your data collection techniques. You should discuss the limitations or your study, and suggest improvements and/or extensions. The type of questions that you should address include:
How suitable was your sample site? Was it a good location to carry out your investigation?
Was your sampling strategy appropriate? Did you have an appropriate number of sample sites for robust analysis?
How accurate are your results? Were there any limitations to your method which reduced accuracy of data e.g. how accurate is a clinometer?
How reliable are your results? For example, what are the limitations of using subjective data such as Power's Index?
How robust was your statistical test? Did you have enough data? Are there any limitations inherent to the test?
How robust are your conclusions? You are likely to be limited by only being able to sample at one time of year (perhaps only one day), so you will not have data on seasonal variation in pebble roundness, size and sorting, or beach profile. Your
secondary data on wind strength and direction may indeed indicate that there is seasonal variation in wave strength and direction.
What other data would it have been useful to obtain?
3. Other data
A variety of other data can be collected to investigate longshore drift. Students can create field sketches and/or annotated photos to show evidence of longshore drift e.g. showing changes in the beach on either side of a groyne.
The float method can also be used to investigate longshore drift. A biodegradable float such as an apple or orange is placed in the sea and the time taken for it to travel over a set distance (e.g. 10m) is timed.
Investigating wave type
Wave analysis can also be carried out to allow students to comment on the presence of destructive or constructive waves and to compare areas and/or beaches. Differences in wave type may be be used to infer which coastal processes are occuring.
The simplest indicator of wave type is wave frequency. This can be measured by timing the number of waves breaking on the shore in 1 minute. A low wave frequency (e.g. 6-8 waves per minute) usually indicates constructive waves, whereas a higher wave frequency (e.g. 12-14 waves per minute) usually indicates destructive waves. By itself this method does not produce particularly reliable data, but it can be improved by also considering other wave
characteristics such as wave height and orbit shape, either in text or photo form.
Coastal deposition
Where does the material transported by waves come from? There are several sources of sediment at the coast:
sediment deposited by the waves sediment produced by mass movement sediment deposited by rivers entering the sea sediment deposited by human activity
Sediment deposited by the waves has been eroded and transported from elsewhere. Deposition occurs when the waves lose energy and can no longer transport such a large load. As wave energy falls, wave competence falls and the largest particles are deposited first. Wave deposits are rounded by attrition and sorted by particle size
Coastal erosion
The processes of erosion, transport and deposition at the coast are similar to the processes in fluvial environments. There are four types of coastal erosion.
Hydraulic action - air present in joints is trapped and compressed by the pressure of incoming sea-water. Over a period of time, this increase in pressure weakens and breaks off the rock. The rate of hydraulic action is high on coasts where waves are powerful and the coastline is made up of a densely jointed rock.
Abrasion (corrasion) - sand, shingle and boulders, carried by the sea, rub against the surface of cliffs and wear it down. It is the fastest form of coastal erosion.
Attrition - the movement of waves makes rocks and pebbles crash together, so that sharp edges are broken down, and particles become smaller and more rounded. It affects boulders and stones that have already been eroded from the coast.
Solution (corrosion) - rocks are dissolved by acids in seawater.
Factors affecting the rate of coastal erosion
The rate of erosion is affected by the force of the waves (erosivity) and the resistance of the coast to erosion (erodibility).
What determines the force of the waves?
Breaking point of the wave - when a wave breaks it releases a great deal of energy. A wave which breaks at the foot of a cliff releases the most energy and causes fastest erosion, particularly corrasion. A wave which breaks offshore will have lost most of its energy as it travels up a beach.
Type of wave - steep destructive waves have more energy, and power to erode, than shallow constructive waves.
Fetch of the wave - waves tend to become higher and more erosive as their fetch increases.
Shape of coastline - refraction makes waves stronger and more erosive on headlands rather than bays.
Gradient of the seabed - the steeper the gradient of seabed, the more likely it is that the wave will break closer to the shore. Less of the wave's energy is used in overcoming friction with the seabed, so there is more energy to erode.
What determines the resistance of the coast to erosion?
Mechanical strength of rocks - some rocks (e.g. granite) are stronger and more resistant to erosion than others (e.g. unconsolidated sediments such as glacial till). Rocks which can become saturated with water can collapse (e.g. clay).
Jointing - densely jointed or faulted rocks are susceptible to hydraulic action. Faults, joints, cracks and bedding planes can all act as points of weakness.
Chemical composition of rock - some rocks are soluble in water (e.g. chalk is soluble in acidified water) and can be eroded by corrosion.
Vegetation - the foliage and roots of vegetation bind soil and rocks together and reduce the rate of erosion.
Human protection - in many locations, physical structures (e.g. sea walls) have been installed to absorb the energy of waves and so reduce the rate of erosion.
Sub-aerial processes
Sub-aerial processes are those processes which operate at the coast but do not involve direct contact with the sea. Material is loosened and made more vulnerable by sub-aerial weathering and mass movement.
Salt weathering - sea spray enters cracks. Later the water evaporates to leave crystals of salt. Further evaporation enlarges the crystals. The growing crystal exerts force on the rock. The rate of salt weathering is most rapid in well-jointed rocks.
Freeze-thaw weathering - rainwater or seawater enters cracks. Later the water freezes to ice and expands. This exerts extra pressure on the rocks and makes cracks become larger. Thawing of the ice allows the water to trickle into the new cracks. The rate of freeze-thaw weathering is most rapid in well-jointed rocks. It is slower than inland because seawater freezes at a lower temperature than freshwater. Furthermore, frost is less likely at the coast than inland.
Wetting and drying - water enters sediments and causes expansion. The sediment contracts when it dries out. Repeated wetting and drying causes stress fractures in some rocks, such as clay and shale.
Biological weathering - boring organisms (e.g. limpets) can drill into the rock and create small depressions. Seaweed attaches itself to rocks and the action of the waves can be enough to cause the swaying seaweed to prise away loose material from the sea bed.
Other processes of weathering - hydration, hydrolysis and carbonation may also occur at the coast.
Mass movement - is particularly active at the coast because undercutting of rocks by the sea makes them unstable. There are two basic types.
Rockfalls occur when the waves undercut the cliffs and weathering loosens pieces of rocks on the cliff face. Rockfalls are most common on cliffed coastlines with resistant rocks such as chalk or limestone
Landslips occur when rocks become saturated with water. The slip is triggered either by the waves undercutting the rocks or following heavy rain. The saturated material flows out from the base of the cliff to form a tongue of mud.
Questions to investigate
A field investigation of a beach can involve a number of working hypotheses, such as
Quantity of beach material will increase in the direction of longshore drift
Pebbles will become smaller in the direction of longshore drift Pebbles will become rounder in the direction of longshore
drift Wave type will be different between two beaches/two areas
Hard Engineering
Hard engineering approaches tend to be expensive, last only a short amount of time, are visually unattractive and unsustainable. They often increase erosion in other places further down the coast.
Hard Engineering Techniques
The table below shows a range of hard engineering techniques.
Groynes Groynes are wooden Cheap, retain wide sandy Beaches to the south £7000 each
barriers constructed at right angles tothe beach to retain material. Material is trapped between these groynes and cannot be transported away by longshore drift. Groynesencourage a wide beach which helps absorb energy from waves, reducingthe rate of cliff erosion.
beaches and do not affect access to the beach.
of the defences are starved of beach material due to their affect on long shore drift.
Sea Walls Sea walls are usually built along the front of cliffs, oftento protect settlements. They are often recurved which means waves are reflected back on themselves. This can cause the erosion of material at the base of the sea wall.
Provide excellent defence where wave energy is high, reassures the public and long life span.
Expensive, can affect beach access, recurved sea walls can increase the erosion of beach material.
£3000-4000/m
Reventments Traditionally these have been wooden slatted barriersconstructed towards the rear of beaches to protect the base of cliffs. Energy from waves is dissipated by them breaking against the reventments. In recent times concrete reventments such as accropodes have been used in places such as Scarborough.
Less beach material is eroded compared to a sea wall. Cheaper and less intrusive than a sea wall.
Short life span and unsuitable where wave energy is high.
£2000/m
Rock armour / boulder barriers
These are often large boulders placed along the base of a cliff to absorb energy from waves.
Cheap and efficient Unattractive, dangerous access to beach, costs increase when rock is imported.
£3000/m
Gabions This is where rocks and boulders are encased in wired mesh. They absorb the energy from waves.
Cheap and efficient. Shorter life span than a sea wall. Visually unattractive.
£100/m
Off-shore breakwater These are large concrete blocks and boulderslocated off shore to change the direction of waves and reduce longshore drift. They also help absorb wave energy.
Beaches retain natural appearance.
Difficult to maintain, unattractive, does not protect the cliffs directly and does not stop beach material from being eroded.
Beach nourishment Beaches are made higher and wider by importing sand and shingle to an area affected by longshore drift.
Cheap, retains the natural appearance of the beach and preserves the natural appearance of the beach.
Off shore dredging of sand and shingle increases erosion in other areas and affects the ecosystem. Large storms will require beach replenishment, increasing costs.
£20 /cu.m
Managed retreat
This is when areas of coast are allowed to erode. This is usually in areas where the land is of low value.
Managed retreat retains the natural balance of the coastal system. Eroded material encourages the development of beaches and salt marshes.
People lose their livelihood e.g. farmers. These people will need to be compensated.
Depends on amount of compensation that needs to be paid to people affected by erosion.
