Disappearing beaches: missing knowledge

by Dr J Floor Anthoni (2001)

Despite the fact that beaches are disappearing everywhere in the world, they remain poorly understood. This is not because of lack of funding or people but because some fundamental processes have not been investigated. As we have shown conclusively, scientists are looking in the wrong direction. The beach-dune process does not work as thought. In this chapter we'll indicate the missing science.

The study of living ecosystems is hampered by not being able to subject them to controlled experimentation. Likewise, beaches cannot be studied in the classical way because they are too vast, and the forces acting on them too large. Instead, one needs to observe them closely, and draw cautious conclusions from studying many. In the end, nature has done its own experimentation, subjecting one beach to a severe storm, another to a tsunami, yet another to the muddy waters of a river. All experiments one would ever like to do, have in fact already been performed by nature. It now becomes a matter of finding those places. Unfortunately, most beaches of the world have become ill in one way or another, and perfectly healthy beaches are hard to find. In this respect, New Zealand may prove to be fortunate, providing scientists with a rich variety of beach, within easy reach.

The reason I wrote this section about our disappearing beaches and how they really work, is because what I found, disagreed with accepted knowledge. A number of important physical processes have simply been overlooked. When talking with coastal scientists, I furthermore discovered, that they failed to understand what was missing in our knowledge of beach dynamics, even though the previous chapters spelled this out in detail. It is for this reason that the gaps in our knowledge are summarised here again, so that scientists may be motivated to provide the missing data. If you are not a coastal scientist, you may wish to skip this chapter entirely.

The main challenge to coastal scientists is the set of six beach laws I have defined, governing all beaches of the world. It is up to them to prove these either right or wrong.

I hope that scientists will rise to the challenge, and provide the necessary feed-back to keep this section on beach dynamics uptodate.

-- home -- back -- oceanography -- Revised: 20010410,20051210,

Sand transport by waves

Sand erosion and deposition: The behaviour of waves has been studied in great detail, and can be described in mathematical ways. Using computer models, the movement of water particles can be simulated with great precision. Where water skims the bottom, friction and turbulence detach sand particles, causing them to be water-borne. It is in this area that computer models often fail, requiring ad-hoc adjustments to make them fit reality. The sand-water interface needs further study, particularly in relation to the size of the sand particle. A sandy bottom is not entirely solid, but capable of conveying wave motion through its pore space. The rules describing when a sand particle becomes water-borne and when it settles out again, are due to be complicated. Computer models fail in this respect.

How wide is the beach? It is important to know how wide the beach system is, reaching from the back dunes to the wet beach, and in sea to a depth depending on extreme exposure. Cores drilled in the sea bottom have revealed that many beach systems extend down to depths of 40m or more, and that even here, the sand still moves towards the beach. More research is needed world-wide, to establish the dynamics of sand at these depths.

Along-shore sand transport: the transport of sand along the shore, mostly in the littoral zone, is often blamed for beach erosion. Yet why has it become a problem only recently? Winds and currents have always been there, but only now have they become a problem. This requires explanation. It could well be that the sand stays too long in the water, rather than being put back into the dunes soon after it was lost from there. As long as it stays in the water, it will be transported by littoral drift, but as soon as it lies beyond reach of the water, it will stay put. Sick beaches increase along-shore sand transport.

Rips: rips move water and sand from the wet beach out to sea. It is thought that rips move away from the beach perpendicularly, but it may well be that they form circular cells, moving the sand upwind in deeper water, thus counteracting littoral sand drift. Practical measurements are needed.

Drying of the beach

The capacity of a beach of repairing storm damage, depends largely on how rapidly the sand will dry. As far as I know, no work has been done in this area. The effects of beach slope, solar radiation, wind and pollution have not been measured empirically. There is no standard test setup for doing comparative in-situ measurements of beaches.

Beach slope: the effect of beach slope on the drying of the beach needs to be studied. Hydrology of beach groundwater, in relation to slope, salinity and particle size.

Solar radiation: the effect of solar radiation and heat absorption needs to be studied. In some places of the world, solar radiation is high, in others low. How does it affect beach dynamics?

Wind: the effect of wind on the drying of beach sand, needs to be studied. Some places in the world have dry winds, others moist winds. How does it affect beach dynamics?

Tides: in the period between high tides, the sand must dry, and then it must be blown out of the intertidal zone. The effect of tides on the drying of the sand has never been investigated.

Pollution and particle size: the effect of sand particle size and that of pollution from fine particles, bacteria or plankton, needs to be studied. How is the wick like property of sand affected by various forms of pollution? Many beaches have up to 10% of mud mixed in with their sands. What is the level of mud tolerable for a healthy beach?

Beach sand quality monitoring: it could well be that the beach acts as a trap for fine particles, and as such may prove an indicator of the pollution of the coastal ecosystems. A natural sediment trap?

Blowing in the wind

Wind transport of sand particles: erosion and deposition by wind of sand particles ranging from mud to coarse gravel, needs to be established empirically, both for dry and wet sand. See also the graph of sediment transport by water.

Saltating up a slope: the sand particles of which dunes are made, saltate in the wind. They cannot saltate up steep scarps. It is important to know the relationship between slope angle, wind speed and sand transport.

Wind shelter: the effect of wind shelters, both down-wind and upwind. How much is the wind lifted by obstacles down-wind? How much do houses and trees affect the wind on the beach? How far away from the beach can houses be allowed? Trees? Hotels? How much does a steep scarp in the foredune affect the wind on the dry beach?

Dune blow-outs: these are considered harmful, but may well be the natural response of a beach system to overly dense vegetation. A blow-out becomes a natural escalator of excess sand away from the beach. How much does a blow-out affect the wind on the beach? How much sand is transported inside a blow-out, relative to the foredune elsewhere? How do blow-outs affect dune vegetation?

Driftwood: driftwood is washed up as high as waves can reach. It is not shifted by wind, but it traps sand, building a dry fore-foredune, which migrates onto the foredune, thus enhancing beach self-repair. This mechanism has never been studied, and we do not know by how much our beaches and dunes have been affected by the lack of driftwood. Driftwood must be present in the deeper layers underneath some dunes, possible providing more clues.

Biotic factors

Dune planting: much is being done about planting dunes, and the effect of densely planted dunes has become visible and measurable over time. The effects of dune plantings must be studied in the new light shed on beach dynamics. My contention is that planted foredunes form steep scarps that cannot collapse. It blocks sand transport and the natural repair mechanism. The beach dies.

Littoral rock cover: microalgae (particularly Corallina and Lithothamnia), grazing animals and large algae protect rocks from the pounding of waves and abrasion by water-borne particles. The living film repairs damage to itself. As a result, live rock cover reduces wear considerably, but by how much is not known. Too little attention is paid to the importance of rock cover. It may well be that the well-published coastal erosion at Cape Hatteras (USA) is mainly caused by the disappearance of rock cover. Muddy deposition and muddy water cause algae to disappear.

Lichens: where lichens cover coastal rocks, erosion appears to proceed more slowly. But lichens are disappearing or at least not growing as fast as they used to be, thus increasing coastal erosion. More research is needed in this field.

The overall system

The sand pump: the forces of waves, wind and sunshine combined, give the beach system its capacity for self repair. But how large is this capacity? Healthy beaches have an overcapacity, but what is the ultimate limiting condition? How are the beach slopes above and under water related to overcapacity?

Sand banks: sand banks in the sea have always been assumed to be natural features. But they may well be indicators of excess sand and undercapacity of the natural sand pump. Sand banks have a bad effect on beaches, because they shelter them. As a result, the beach flattens and the sand pump dies.

Soil erosion: soil erosion has increased everywhere on the planet, by considerable amounts, between 5 and 100 times what used to be 'normal' when the land was covered in its natural vegetation. The effects of soil erosion on beaches has to be considered, because the mud is not able to be washed away fast enough. There appears to be a cumulative effect as well, where soil lost a hundred years ago, is still affecting estuaries and coasts today.

Mud breakdown: as soil reaches the sea, it is segregated by grain size. The coarse sand stays near the shore, fine silt goes further and deeper and clay further still. As clay interacts wit salt water, it changes its properties. How precisely, has not been studied, but is important to understand how nutrients are released. Clay, consisting of silica, alumina and iron hydroxides, may fall apart, providing the much needed silica and iron compounds to feed plankton blooms.

Sticky mud: as mud (silt and clay) enters the sea, it provides attachment for bacteria and planktonic algae. These combined, make it stick better to the substrate and organisms. How this works precisely, has not been studied, but is important to understand how biota disappear. It is not clearly understood either why clay particles settle out much more quickly in sea water than in fresh water. However, once coated with planktonic creatures and 'snot', these particles can stay afloat indefinitely, to the detriment of filter-feeding animals. This phenomenon needs further study.

Sand renourishment register: in New Zealand and other places in the world, sand renourishment projects are done on a local scale, by local government and private interests. If one does not know the history of a beach, it becomes impossible to make valid predictions from the present situation. A national register of beach sand renourishments is therefore urgently required. It could also register other engineered sea defences.