|The factors and processes affecting the life of a beach and dune system are so slow and random that they may be too complicated for computer models. Coastal science is still subject to conflicting opinions.|
|By looking at the beach/dune system as if it were a living organism, powered by waves, tides, wind and sun, and having its own self repair and defence mechanism, new insights could be gained.|
|The self repair mechanism, discussed in more detail in oceanography/beach, is summarised here because it plays a central role in the concept of a beach as a living organism.|
|Using the force of the water, the beach appears to fight high waves and storm surges. Like a living organism it defends itself where it is being attacked.|
|The beach's self repair mechanism is a kind of sand pump that pumps water out of the sea, onto the dunes and further inland.|
When slow trends are accompanied by larger cyclical changes or sudden extreme events (such as large storms), the situation becomes even more confusing. We do observe a sudden dune erosion after an unusual storm, but not the slow decay preceding it. For the beach/dune system, extreme storms have dramatic effects, both good and bad. The surface waves carve sand from the dunes into the sea, but the deep waves drag new sand from deep waters towards the beach. Sand banks can suddenly appear.
Now that beaches all over the world are causing concern, scientists and coastal engineers are motivated to research the processes that affect them. Much of this effort is directed toward measuring quantities of sand and how they move under the influence of waves and currents. Computer models are used to bring clarity to the maze of influential factors. Time is needed to evaluate the assumptions that feed these models and to compare models with actual situations. The results obtained are not much better than educated guesses and we may eventually have to admit that the factors involved and the unpredictability of extreme events are too complicated for exact computer modelling. But above all, their underlying assumptions are wrong.
Coastal engineers and surveyors are monitoring the sand balance of many beaches in the hope of being able to predict their future and the risk posed to properties. But a beach/dune system experiences long periods of almost static equilibrium, interrupted by sudden, decisive events. Most studies concentrate on the 'wet beach', the sand that is moved around by the water, ignoring the 'dry beach' which is part of the beach's self repair mechanism. As a consequence, their advice is of little value in the long term.
Beaches and dunes have been influenced by humans for a period too long to remember. Dunes have been built over or stabilised. Erosion from clearing the land, has had and still has a major effect. It has become nearly impossible to find a beach which is still natural, unaffected by humans and which can be studied. Here in New Zealand, fortunately some natural beaches can still be found.
Having studied a good number of scientific publications and books, my impression is that there are too many conflicting opinions to make coastal knowledge a science. There appears to be room for a novel approach. During my extensive observations I discovered how the beach-dune system works and which natural laws are behind these processes.
|The living beach
Traditionally, scientists and engineers treat the beach the way an accountant treats a balance sheet. They talk of accounts (compartments), debits and credits (inputs and outputs) and profit or loss (accretion or erosion, growing or shrinking). Sand can be found in various compartments, between which it shifts around: rivers, wet beach, dry beach and dunes. (click here to view the sand balance diagram)
Our new approach treats beaches and dunes as living organisms that use
the energy from waves and wind for living. As organisms they can grow or
shrink, adapting to changing conditions. They repair damage and they defend
themselves where they are being attacked. They have likes and dislikes,
can get sick and eventually they can also die. Central to this concept
stands the self repair mechanism which is summarised below.
The diagram on the right shows the dune and beach building process. Because water is about 800 times denser than air, it transports sand hundreds of times more easily. Where waves break, particularly over the wet beach in the intertidal zone, sand transport can be massive. Deeper down, sand moves down-hill away from the beach, but occasional storms drag it back towards the beach. Without wave action, this transport would stop and coastal sand would gravitate to deeper water.
My own observations showed that the sand down to a depth of 40m (often several sea miles away) may still be part of the dune/beach system, although it gets stirred rarely by sufficiently large waves. Scientists agree that sand found at 20m depth is definitely still part of the dune/beach system.
|This diagram depicts the relationships between water velocity and grain size with relevance to erosion, transport and sedimentation. It defines the behaviour of sediment in water, under influence of currents and waves. Notice that the scales span an enormous range of four decades! Vertically, from bottom to top, the grain size increases, and coloured bands have been drawn to mark clay, silt, sand, granules and pebbles. The diagram could have been continued to include boulders of 100mm diameter. The straight line is the theoretical drag, as defined by Stokes' equation [see comment below], which excludes turbulence. The two curves left and right of it have been derived from actual measurements.|
|Please note that the diagram above was obtained by measuring the actual behaviour of particles of various sizes. The scientist Stokes described how very small particles fall in a liquid, and this corresponds only to the leftmost part of the left curve such that a particle hundred times larger (= 2 divisions horizontally) sinks ten times faster (one division vertically). The curve ends at its top for coarse sand, granules and pebbles where a ten times larger particle falls hundred times faster. The righthand erosion curves bends mysteriously to the right, indicating that caked silt and mud are very hard to dislodge. Ironically, this curve runs vertically precisely for all beach and dune sands, indicating that they are all most easily dislodged, regardless of grain size. This strange property must be kept in mind when studying dunes and beaches.|
A similar diagram would explain how sand is moved and deposited by wind,
but such a diagram is not available. A close approximation would be to
read wind speed on the horizontal scale at km/h instead of the water speed
of 100cm/s. Accordingly, winds exceeding 20-30km/h would start moving sand,
whereas sand drops out of the air at wind speeds of 5-10km/h. Note that
no actual data are available!
|We'll now discuss the properties of the dune/beach
system by defining the essential physical laws that govern their formation
and maintenance. It allows us to focus our thoughts and to make predictions.
Please note that his is entirely my own work, not yet supported by the
scientific community. (Floor Anthoni, 1999)
Beaches do not necessarily need to consist of sand. They can be made
of any material that has the two opposing properties of being able to move
and being able to remain in place, within the wave regime (average wave
conditions) that prevails. Some beaches consist of round boulders; others
of rocks of various sizes. Most beaches consist of sand or broken shell.
For the sand to be able to blow, it needs to be
dry. Wet sand sticks together and cannot be transported by the wind. For
sand to dry on the upper beach, it is necessary that the water recede with
the tide (or that a lake dries up regularly). Wind and sun then dry the
sand, after which it can be transported up the dunes.
|Second beach law: Dunes need tides. No tides, no dunes.|
A corollary of the second beach law is that dunes cannot be expected
around seas and lakes that have no tides, or do not frequently dry out.
Thus the Mediterranean Sea is not likely to have dunes formed by the sea
(but it could have dunes formed by adjacent deserts). Other places where
dunes are unlikely, are large inland lakes and seas.
Although the water can transport the sand as high
up the beach as high tide permits, it cannot transport it into the dry
beach or the dunes. The only physical force able to do so is the sea wind.
Land wind would blow sand back into the sea but because it is much weaker
(due to the shelter provided by the land), the sea wind prevails. If the
sea wind is absent or weak, such as is the case in front of high cliffs
(and sky scrapers), dunes cannot develop. As a consequence, in front of
beach houses, hotels and trees, dunes will disappear and with them the
|Third beach law: Dunes need sea wind. No sea wind, no dunes.|
Even after all three laws apply to a beach, it may
be possible that its sand won't dry in the period between high tides. Very
flat beaches won't dry out, neither do beaches with very fine sand or mud.
Some beaches have their sand crusted by excessive organic activity or salt
concentrations. Such beaches cannot form dunes. Beaches that can no longer
dry their sand, can no longer repair damage caused by storms. They become
ill and will eventually erode away.
|Fourth beach law: The beach must dry out. If the beach can't dry out, it won't be able to repair damage and will eventually disappear, together with its dunes. No drying sand, no dunes.|
Note that in the above discourse, the amount of sand in either dunes
or wet beach is of no importance. The first four laws govern the beach/dune
system's ability to repair damage and to react to extreme events. Indeed
beach/dune systems can be perfectly healthy with only a small sand budget.
Likewise those with large sand budgets can be very ill or dead. In fact,
those with excessive amounts of sand, form sand bars out in sea. These
shelter the beach, lay it flat and prevent it from drying. In most cases
such beaches are 'dead', not being able to repair storm damage.
A separate chapter has been devoted to this important issue.
The most insiduous threat to beaches and dunes comes from sediment runoff
from the land (erosion). The fine mud (silt + clay) particles infiltrate
the pore space between the sand grains, and often with the help of bacteria
feeding on decomposing organic matter, change the properties of the beach
sand dramatically. Ground water can no longer flow freely and the beach
stays wet much longer than a clean beach. Furthermore, through highly increased
capillary action, the groundwater rises higher. Eventually the beach won't
dry out in time before the next tide. The sand stays wet and cannot be
blown into the dunes. Successive storms carve the dunes out until all are
lost. A separate chapter is devoted to this issue.
This law predicts that around the mouths of large rivers, which have always been dirty due to their huge catchment areas, dunes have never formed. Where dunes are found around dirty rivers, it means that these rivers have once been clean.
This beach named Désert des Agriates on Corsica in the Mediterranean Sea cannot grow dunes because the Mediterranean Sea has no tides. Storm waves have deposited sand here, to a height of nearly 1m, and winds are able to blow the sand further inland. However, erosion by rain, keeps up with the slow accumulation, and dunes cannot form. The black plumes in the sea could well be the dead fronds of a seagrass, growing deeper in the sea. It could also be volcanic black sand. Photo courtesy of R Palomba.
|The beach fights back
During a storm, the waves are larger and able to penetrate the beach defence. Storm surges lift the level of high tide, facilitating this process further. But the beach fights back. It borrows sand from the dune, forming a new beach at a higher level (top drawing). Often a temporary sand bank is laid to protect the beach further. The physical forces doing this are inanimate, but their combined result seems life-like. The whole dune is held in reserve for even bigger storms or rare sequences of big storms.
After the storm, a great quantity of sand has ended up in the water. The beach repair mechanism starts to repair the dunes (bottom drawing). If this mechanism is absent, the wet sand remains in the water, forming off-shore sand banks that shelter the beach. The beach becomes flat and eventually unable to dry out. Successive storms erode the dunes further until all dry sand is lost.
|In recent years scientists have made much ado about rising sea levels, thought to have been caused by global warming. A rise in sea leavel would push the shore line back according to the slope of the continental shelf. It is thus reasoned that dune erosion is inevitable. We seriously question this reasoning because it is based on bad science. Sick beaches erode but healthy beaches don't. The implied physical law: "all dunes around the world erode because of rising sea levels" is untenable because of its many exceptions. For healthy beaches a rising sea level would simply raise the level of the beach using sand borrowed from the sea. The beach's defence against a sudden storm surge is to immediately borrow sand from the dunes for a higher beach.|
|Fifth beach law: Only self-repair protects. No amount of dry or wet sand will stop erosion if the beach self repair mechanism is damaged. Such beaches and their dunes will eventually disappear.|
The sand that is capable of forming dunes must have
a number of conflicting properties: It must be transported easily in the
water, yet stay put when the water drains away. It must be transported
easily by wind, yet in the absence of wind it must stay put on dune slopes.
Only a limited size range of sand grains is able to do so (0.3-1 mm).
These grains do not become air-borne by the wind, but saltate about 5-10cm above the stationary sand. After having been blown up a dune slope, they drop down on the lee slope. Dunes are thus shifted by the wind in a peristaltic rolling motion. The dry beach rolls onto the fore dune, which rolls onto the second dune and so on.
If dunes can't roll, the sand is trapped and cannot move. The dune grows in size, eventually lifting the sea wind off the beach. The self repair mechanism gets damaged. The beach and dune will eventually disappear. A separate chapter is devoted to this important matter.
|Sixth beach law: If dunes can't roll, the beach will die.|
The sixth beach law has important consequences for how we have been treating the dunes. By planting trees and attempting to stabilise the sand with various dune grasses, we have also sealed the fate of the dune/beach system. A separate chapter is devoted to this important issue.
|The sand pump
The self-repair mechanism of a beach/dune system acts like a sand pump, pumping sand out of the sea and onto the dunes and then further inland. Once the sand is out of the sea, it cannot be lost to coastal drift and it becomes available for immediate deployment in case of bad storms. The sand pump's capacity is highest in 'good' years with strong sea winds, dry weather and sunshine and lowest in 'bad' years with opposing properties.
A healthy sand pump is limited by how much sand lies within its reach in the sea. Its over-capacity keeps the shoreline stable in both good and bad years. A weakened sand pump lacks over-capacity. Its shoreline recedes in bad years and recovers in good years. When the sand pump won't work adequately, the shoreline recedes permanently. Many beaches are in this state.
Note that scientists and engineers invariably blame the along-shore transportation of sand for a beach's erosion. While this cannot be neglected entirely, a healthy beach quickly pumps its lost sand back onto the dry beach, out of reach of coastal drift. It explains why these beaches used to be insensitive to coastal drift. Once they become sick, their lost sand remains a playball of waves and currents for too long, and they become sensitive to coastal sand drift.