Disappearing beaches: drying in the tide
by Dr J Floor Anthoni (2000)

Dunes grow because the wind blows new sand onto them. But before sand can be moved by the wind, it must first be dry. The wind cannot move wet sand.  The sand must dry in the short time between high tides and this can go wrong very easily. Read why.
the tide's critical timing
In the short period between high tides the sand must first dry before being able to be blown onto the dunes. It is rather critical and can easily go wrong.
drying and beach slope
Before its surface can dry, ground water must first drain away through the sand. In very flat beaches, this is not possible.
Many beaches are impaired by sand forming dry crusts. We don't know exactly why but it may simply be caused by the beach laying flatter.
polluted beaches
Beaches are very easily polluted by mud from eroding land. It changes the way water flows within the beach sand. It makes beaches stay wet and unable to repair damage.
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The tide's critical timing
The tide's critical timingOnce the tide moves out, the sand can begin drying. During a warm day with wind and sunshine, the beach dries much faster than during a calm cool night. The difference may lead us to ignore the drying effect of the night. Before the tide comes in, the sand must have been blown up-hill or it will be wetted again. This rather critical process forms the heart of the beach self repair mechanism. It is easily upset by small changes. If it takes longer to dry the sand, there will be equally less time for blowing it. Thus a beach can become ill very rapidly, unnoticeably and unexpectedly.
In the top drawing the yellow triangles attempt to visually represent this phenomenon for two different situations: left drying time 2-3 hours, right 4-6 hours (double). The yellow triangle becomes less than half in size.

Twice each month the tide reaches a maximum (spring tide). As it moves towards neap tide, the small sand wedge left dry will be able to blow (bottom drawing). So even though the sand won't be able to dry between tides, it still has a chance to dry a very much smaller area in between spring times, provided that waves stay small during that time. It enables beaches with drying times over 12 hours, to gain some dry sand over several days, twice each month, between spring tides.

Drying and beach slope
Drying of wet sandThe rate at which a beach can dry depends very much on its slope. The diagram compares a healthy steep beach with a flat, sick beach. In a steep beach the water table is low. Hydrostatic pressure is high, enabling the water to flow more quickly through the sand. It doesn't need to flow far to reach the sea.

In a flat beach, the flow is low. In order to dry, the beach water needs to travel far to reach the sea. Such beaches won't dry fast enough. Because their self repair mechanism is impaired completely, the beach can be considered dead. Although its dead body of wet sand will continue to slow ocean waves down, its shores will erode further with each major storm event.

It is often thought that exposed beaches always have steep slopes but this is not so. Observations have shown that exposed healthy beaches do, whereas exposed ill beaches don't.

Hard flat beaches are welcome
Ironically, the public welcomes hard, flat beaches so they can drive their cars over them. Nobody seems to be aware that this beach is mortally ill, destined to erode away into adjacent properties. Yet a few km back lies NZ's healthiest beach, Otama Beach with pure white sand.
Opito Beach, Coromandel, NZ.
Hard beaches are good for games
Hard, flat beaches are also preferred for playing games on and for walking on.

Possible cause of crustingMany beaches whose sand dries rapidly, are still impaired because it forms crusts that cannot blow in the wind. This phenomenon is poorly understood and could be caused by:
  • Bacterial activity, where bacteria glue sand particles together
  • Mud or fine grained sediment that dries hard like clay soils do
  • Fine particles and bacteria making the sand act like a wick
  • Excessive phyto plankton blooms decaying by bacterial action
  • Concentrated solubles and salts
  • A combination of the above
The diagram attempts to explain how salt, combined with poor drainage, could cause crusting. In a healthy beach the ground water is able to recede rapidly after each wave and while following the tide down the beach. In such beaches the sand contains air pockets preventing capillary action. As the sand dries, the surface moisture permeates up while the moisture deeper down drains towards the water table. There are enough air passages to let air replace the lost water. The sand dries without crust.

On a sick beach polluted by fine particles or bacterial action (the right hand situation), however, not enough air channels and pockets are available to let air replace the lost water. As the drying sand evaporates moisture, capillary action and the vacuum caused, pull the water table up. As the top layer dries, new water with solubles like salt is drawn up, resulting in a crust with a high content of salts and solubles which cakes the sand grains together.

Polluted beaches
A symptom of our time, most beaches are now polluted by mud. Mud comes from the land. It washes down the rivers and ends up in the sea, where it remains close to the shore. A clean beach is easily polluted because the clean sand acts like blotting paper to ink. It absorbs the fine particles readily. Only after repeated rinsing by waves, will the fine particles be dislodged and transported towards deeper areas in the sea. This may take weeks to years. Read soil/erosion for more information.

Not all waves contribute to the accretion of the beach. Depending how strong the water's backwash is, sand will be eroded or deposited. When a wave breaks and spills, it ends up running up to the beach. If the water's velocity is sufficient to dislodge sand, it will be transported higher up, against the force of gravity. As the water slows down, the sand settles out. Now the water starts its backwash, accelerating as it moves down the beach. At some point it starts dislodging the sand, moving it back to sea. On an unpolluted beach, the water soaks easily into the sand, which reduces the wave's speed (both forward and backward), resulting in more sand being deposited, higher up the beach where it dries best. Clean and healthy beaches are characterised by a steep slope and a shoulder at the top of the beach, where the most recent sand was deposited. Although this principle applies to all  beaches, one should distinguish the effect of wave exposure. Exposed beaches tend to have steeper slopes and coarser sand grains than sheltered beaches. Read more about waves in oceanography/waves.

Although the permeability of the beach sand has some beneficial effect on how much sand is deposited in one tidal cycle, the most pressing problem of a polluted beach is that it doesn't dry quickly. The fine mud particles fill the voids between the sand grains, creating resistance to the flow of ground water. The smaller voids also increase the sand's capillary action, which allows the ground water to rise higher up in the beach profile, like lamp oil in a wick. When bacteria thrive on the remains of animals and plants (plankton, sewage and humus), the situation becomes worse still.
Kicking the sand
Pollution from fine mud particles (silt and clay) cannot readily be observed in the sand, or even under a microscope, but it shows up in a simple test. One needs good sunlight, preferably coming from a low angle such as in the early morning or late afternoon, and one needs a little wind. Sand can be scooped up in two hands, crushed slightly and 'winnowed' by pouring it gently into the wind. 
The coarse sand particles fall down nearby, but the polluting silt and clay remain airborne, disappearing as a cloud of dust. Simpler still, is to kick the sand into or across the wind. The embedded pollution shows up as a dust cloud, reluctant to settle out in the wind.
Healthy beaches do not show such dust clouds because their sand is washed by the waves, the finer particles carried away by water currents. Heavy rains with accompanying mud storms, can pollute a beach in a single event, when new sand is laid down while the water carries heavy loads of mud. In this manner, an otherwise healthy beach can suddenly become sick.

Crusted dry sand
Some beaches are impaired because their dried sand cannot blow due to crusting. 
Green foam
Green foam may indicate bacterial activity or excessive amounts of phyto plankton, which could cause crusting.
Beach with high organic pollution
A sand sample from Hatfield's Beach near Orewa contained so much plankton and organic matter that it smelled very bad after one week in a sealed container.
Phil Somerville's cartoon

Broken crust
Where the crust is broken by footsteps or by vehicle tracks, the sand dries freely and is blown by the wind. This photo was taken on Auckland's black western beaches of iron sand, near the mouth of the Waikato River.
Buggies leaving tracks int he crust
Ironically, vehicles and motorbikes hooning over the beach have a beneficial effect: they crack the sand's crust, allowing the sand to blow freely to repair dune damage. But this coast has other problems too.
f212016: a severe case of crusting by salt
A severe case of crusting, after favourable drying conditions on a flat West-Auckland beach containing a large percentage of fine, black tectite sand. The white patches are pure seasalt.
f212031: kicking the sand to see pollution
Because even small amounts of pollution (mud or bacteria) enhance the wick-like properties of sand, beaches can become sick surprisingly rapidly. The fine particles are not easily visible under a microscope, but kicking the sand shows them up in the sunlight. The coarse sand particles fall down rapidly, whereas the fine clay remains airborne in even a light wind.

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