It happens because trees and high obstructions, lift the wind up and off the beach, thereby impairing the beach self repair mechanism. It is a process that happens so slowly that people fail to recognise it. Reacting to the problem, people densely plant the remaining foreshore in the hope that it will trap more sand. And indeed this appears to help at first when some seawind remains. But as the shelter belt grows taller, the situation worsens.

People often plant trees close to the water's edge in the hope that their roots will prevent erosion. But the trees grow up, lifting the seawind off the beach, thus worsening the situation.

People also plant on the fore dunes to prevent sand from entering their houses. It is a very effective method because the vegetation lifts the wind from the bottom, preventing the sand grains from saltating (jumping) into properties. It also invites sand to accumulate among the vegetation. As a consequence, the fore dune grows tall, eventually lifting the sea wind from the beach and permanent beach erosion sets in.

From experience with shelter belts on farms, shelter belts have proved to provide shelter to a distance of at least thirty times their height. Towards the windward side, they also provide shelter, although perhaps not to the same distance, thus a 10m tall shelter belt should not be placed closer than 300m from the beach. A similar rule should apply to buildings, making allowance for their less dense wind profile. When visiting coastal settlements all over the world, one can see that obstruction of the sea wind causes major problems.
 

The coastal holiday park Pauanui in Coromandel, New Zealand, is protected from the sea wind by a shelter belt of pine trees. The fore dunes were planted to prevent the sand from blowing into the development. But both the trees and the tall fore dune, lift the wind off the otherwise healthy beach, impairing its self repair mechanism. Nothing can stop the sea now from marching up. The situation can be reversed by felling the trees and bulldozing the fore dune.

At the Long Bay marine reserve in north Auckland, New Zealand, a grove of tall pine trees obstructs the wind, causing the sea to erode the beach in front of it. The beach would otherwise have ended at the very right hand side in this photo. The situation can be remedied by cutting the trees before renourishing the gap with new sand. Note that the dunes here have been flattened and capped with a thin layer of clay loam. It allows grass and shrubs to grow while covering the sand. Where people tread, the edges of this clay capping are removed, resulting in the kind of wear shown here.

At Whangamata in the Coromandel Peninsula, New Zealand, a shelter belt and a surf lifesavers club house obstruct the beach which is eroding right here because the tall structures obstruct the sea wind from an otherwise healthy beach. To remedy the situation, the shelter belt should be felled and the clubhouse placed further back with the least possible frontage facing the sea. The beach in front of such club houses is usually eroding.

Near Te Arai Point, north of Auckland, New Zealand, a large pine forest has been planted extending to 30m from the fore dunes. Now that the trees mature, they lift the seaweed from the beach, which causes beach erosion. The pine trees also effectively trapped sand, which has made the rear dunes too tall. This beach can be saved by clear felling the forest and burning its remains. Notice the various zones: water, wet beach, dry beach, fore dune, forest.

Engineers have discovered that coarse sand is more stable and is moved by waves and wind less easily. (See sedimentation diagram) It appears to stay longer than fine sand. But coarse sand is much more scarce in nature and known resources are not likely to last a century. What will happen then?
 

High-rise hotels and apartments obstruct the seawind which causes beach erosion. Everywhere in the world, resorts such as this, have serious beach erosion problems. Were authorities really unaware of the consequences of high buildings so close to the beach?

Pukehina Beach, Bay Of Plenty, New Zealand, is a typical example of people building too close to the beach. Little do people realise that their houses and plantings are often the single cause of beach erosion. A 4m high roof affects the wind to a distance of perhaps 120m.

Doomed to destroy the very places we love People realise little that sandy beaches and dunes are a freak phenomenon of nature, a miracle if you like. There happens to be a grain of substance that has the opposing properties of being moved easily while also staying put. It happens to be moved by fairweather waves but also by fairweather winds, even though air is 800 times lighter than water. It also happens that grains between 0.1 and 0.8mm in diameter, all behave the same. These grains are almost as hard as diamond, and may live millions of years without wearing much. The sand in your area has seen many ice ages and is still there, even though the abyss is not far away and many metres of heavy and light sediment have been dropped on it. Remember that most shores in the world do NOT have sandy (quartz) beaches.  In the above aerial photograph of South Beach in Florida (courtesy of Nelly Spengler) one can see that people have been tempted to build on the fragile barrier islands of America's east coast. After the ice load disappeared in the north, the northern part of the continent is rising while the barrier islands of the south are sinking. These islands were able to stay above water because the sea sand was free to blow. Now this is no longer the case and protective beaches are eroding continually. People came here for its wilderness, its natural beaches and its ocean bounties, which have all but disappeared.

Tall dunes Instinctively, one would think that tall dunes offer more protection than low ones. Indeed, tall dunes contain more sand so that it takes longer to wear them away. But if the beach/dune system's self repair mechanism is impaired, the coastline moves only one way: eating the dunes away. Dunes grow tall mainly from human intervention, stabilisation, which is critically evaluated in the next chapter

Headlands Although the beaches near headlands are surrounded by tall hills that lift the sea wind from the beach, they paradoxically appear healthy while more central parts of the beach may show serious erosion. The drawing shows two headlands and a 'pocket' beach strung in between. The wind is blowing from the top right, producing wave fronts as shown.  Waves washing up and down the beach, transport the sand towards the left headland (the lee side) and away from the right headland (the luff side). Winds similarly blow dry sand in the same direction. Because of the shelter offered by the righthand headland (both to waves and wind), the sand transport increases from right to left and decreases sharply in the shelter of the lee headland. The waves here arrive perpendicularly (not on an angle) and the sea wind is lifted from the beach.

As winds turn, arriving from random directions, the total sand movement is away from the middle of the beach and towards both ends, as shown by the yellow arrows. It may explain why pocket beaches are always crescent-shaped (curved).

The apparent paradox becomes stronger where beaches erode due to steep banks or tall shelter belts or buildings. These prevent the seawind from blowing the sand onto the dunes. But winds running parallel with the beach, are still capable of shifting the sand towards the capes, where it accumulates. A steep bank behind the beach promotes this. Such beaches may show severe erosion in the middle, while appearing perfectly healthy and capable of self-repair, towards the ends.

A corollary from these observations, and a guide for planning beach resorts,  is that tall buildings such as hotels and appartment flats, can best be placed near headlands. The middle of the beach is to be reserved for spare dunes and recreation. Shelter belts can be tolerated near headlands but not in the middle of the beach.

If the wind in the drawn situation were prevailing, sand transport to the right of the beach would be minimal, rendering that part of the beach susceptible to erosion too.
 

Sunshine Beach, immediately south of Noosa, Australia, during an overcast day. In the distance the promontory and at left high hills and shelter belts. The beach here has been eroded by a recent storm but shows all signs of being relatively healthy: a steep beach, a flat dry beach, a gradual wind profile and sparse vegetation. Yet, the sand suffers from crusting.

Ohope Beach, south of Whakatane, New Zealand, is sheltered by the promontory and high hills in the background of the photo, looking north. Dunes cannot form here and the beach depends on its sand supply from the dunes further south. However, this beach is in a serious state of ill health: a very flat beach, polluted sand that won't dry but crusts and built-up dunes and sheltering trees further south.

Once washed up on the beach, waves push driftwood as far as they can reach, high up onto the beach. Here the stems and branches interfere with the wind, inviting sand to settle behind the trunks and in between the branches. Also seaweed is trapped here. Driftwood occurs in a strip along the beach which is out of reach of dune plants. High on the dry beach, it performs the dune plant's function, that of slowing the wind and trapping sand. As a result, a fore-foredune is formed which slowly migrates onto the foredune. Steep scarps carved by storms are repaired more quickly with driftwood than without.

Now that driftwood has become a rare commodity, we can expect that the dune-beach system is adversely affected where driftwood once was common.  Some sick beaches may well be salvageable if their driftwood was left in place.

Read Michael Smithers' Sand essay on his observations of the dynamics of the beaches he grew up with. Such accounts are very rare and we are delighted to be able to include it here.
 

Close to rivers which still run through tall forests, one may still find driftwood. Notice how the sand becomes trapped between the tangle of branches. In the distance a gradually sloping foredune.

On its journey to the sea, much driftwood becomes trapped by bridges, where it gets removed, only to be dumped in local land fills. It reduces the amount reaching the sea.

Shells on beaches This diagram shows a cross-section through a beach with shells at various depths. Healthy beaches of clean sand are usually covered with only few shells (a) because they grew by repairing themselves after the last small storm (b). During a storm, the sand is eaten away and all shells above the lowest part of the beach during storm, show for a few days. The number of shells found on a beach gives us an indication of the severity of last storm and how fast the beach is repairing itself.

• Most shells are smooth with rounded edges, indicating a long period of wear.
• Many shells are broken and there are many fragments.
• Many shells are brown, caused by rust compounds (iron hydroxides) leaching into the beach from ground water or from a pollluted beach above.
• Many shells are black, caused by hydrogen sulphide from being buried under layers of anoxic (lacking oxygen) eutrophied (rich in nutrients or life) deposits. This is particularly noticeable on sick beaches.

The story of shells on the beach would end here, were it not for their capacity to make a sick beach sicker. Shells have a form and size suitable to wash up along the top of the beach. Where a sick beach lies too flat, normal waves may not have enough energy to do so, resulting in a long time before the shells are buried again. Often a sand bar forms in front of the beach.
While lying on top of the sand, the shells prevent the sand from drying, and the pockets of dried sand from being blown towards the dunes. It impairs the sand pump, making the beach sicker.

Finding a large number of shells on the beach could mean any of the following:

• There has been a very large storm, larger than many others before it.
• New shells and other sea life washed up.
• The shells are old, indicating that much sand has been carved away.
• The beach does not repair itself quickly enough. Evidence of last storm remains for too long. The beach is very sick.
• The numbers of shells prevent the beach from repairing storm damage.
• Shells have been washed from the entire beach into a corner of the beach.
• The beach has always had a large number of shells, because it is located near a shell bed.

Omana Beach near Auckland, New Zealand, has a cockle bed out in the sea. Cockle shells wash up during storms, taking the top end of the beach. The sand lies underneath.

In this cross section of a beach, one can clearly see the layers of shells (and stones) corresponding to major storms.

In 1995 a major wash-up occurred on Pakiri Beach, north of Auckland. Te Arai Point in the background. These are young fan shells (Atrina pectinata zelandica) that established themselves in shallow water where a moderate storm could dig them up. First washed up over the entire length of the beach, they were then washed to a corner of the beach, stacking up to one metre high and hundreds of metres wide. Such natural disasters are an indication of healthy sea life and recent massive recruitment of Atrina.

A major storm in 1996 washed these marine organisms up on Omaha Beach, north of Auckland. Amongst the large Atrina fan shells (horse mussels), a representative collection of the entire seabed fauna to a depth of 20m was found, including many scallops. Beach and dune erosion was substantive too. After the storm, (one hopes) the seabed will recover slowly, first with young animals that are washed up easily. It is amazing that a storm can wipe out an entire habitat down to a precise depth. It was worrying that no young animals were amongst the debris.

These shells have recently been washed up by a moderate storm. The two main species are dog cockle (Tucetona laticostata) and ringed dosinia (Dosinia anus). The shells have sharp edges, and most are still hinged together. Note the brown colour of the dog cockles, indicating that the sand is polluted by mud.

These tuatua (Paphies australis) shells appeared 'washed up' after a large storm which carved out much of the fore dunes. On closer inspection, most of these are old shells with rounded edges and coloured by iron oxides (brown) and iron sulfides (black and grey). The whiter shells come from a buried layer of a previous, lesser storm. The number of white new shells with sharp edges, washed up during the last storm, is very low. This may be worrying, because many shellfish beds in the sea are disappearing.

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