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Rain is undoubtedly the largest cause of erosion. Water is about 800 times heavier than air, half to one third the weight of rock and about equal in weight to loose topsoil. When it flows, it can move loose substances with ease. Surprisingly, rain's most damaging moment is when a water drop hits the ground.
|This diagram shows the effect of raindrop impact. Only recently its unexpected destructive power has been discovered. Previously it was thought that sheetwash, the flow of water over the soil, was the most destructive. The kinetic energy of a moving object is equal to half its mass multiplied by its speed squared: e=mv2/2. As water droplets grow in size, both their speed and mass increase. The mass of a 5 mm raindrop is 5x5x5=125 times that of a 1 mm drop and its 'terminal' speed doubles, resulting in a destructive energy 500 times larger! Thus the destructive power of rain increases dramatically as the rainstorm produces larger drops, which is relatively rare. But when it occurs, its effect is profoundly destructive. In the past ten years, starting around 1987, rains have become heavier everywhere in the world, and with it, erosion from raindrop impact.
To fight this kind of erosion, it is only necessary to keep the soil
covered, either by dense foliage or by a ground mulch. Hence the importance
of growing a cover crop under horticulture (fruits, wine), keeping the
soil covered after harvesting (stubble on the field), and not overgrazing
pastures. On building sites, a mulch should be spread or a plastic cover,
or a temporary grass cover. Road sides should at all times be vegetated
and not mowed to show the dirt. Fertilisation helps to make foliage denser
and to produce more leaf litter. Everyone could play a role here.
rain drops hit the soil, they loosen its structure, freeing up fine clay
particles, which do not settle down easily, and which are transported down-hill
in the sheetwash. The diagram on right shows how this happens . It is
a very important diagram, since it shows how particles are transported,
and it applies to wind, dunes, beaches, coasts, rivers and estuaries. It
has to do with the friction a particle experiences when moving through
a medium (water, air). The scientist Stokes formulated this mathematically,
and it is shown in the three red curves. The left one gives the speed at
which particles settle out; the rightmost one the speed to erode cohesive
material; the middle one to erode loose material.
Horizontally: the water speed from 0.01cm/s to 1000cm/s (36 km/hr or 20 knots). Vertically the grain size from 1 micron (clay) to 10mm (pebbles). The coloured bands show where clay becomes silt, then sand, then granules, and pebbles.
Look at pebbles first (top grey band) and assume they form a loose river bed. Nothing happens till the speed exceeds 20cm/s. Even then the pebbles won't move. It needs a water speed of close to 100cm/s (2 knot) to start moving the bed. Once pebbles are on the move, they won't settle out until the speed has decreased below 20cm/s. This transportation gap between erosion and sedimentation, widens as the particles become smaller. Thus silt and clay particles reluctantly settle out. It takes clay weeks in fresh water.
The right-hand curve shows how cohesive material erodes. Sand and gravel do not pack very well and even cohesive sand or pebbles, are easily eroded. But the situation changes for silt and clay. Once packed, they need as much water velocity as a packed bed of pebbles! It explains how easily clay can suffocate water organisms and how a packed cake of clay stays put for years. (For instance, the 7 cm clay bed deposited in 15m deep water by cyclone Bola, took 7 years to disappear.) It also explains how rain drops (see previous subchapter) have practically no effect, until their energy becomes sufficient to erode the packed silt and clay. It is a very sudden transition, explaining why soil erosion happens only rarely, but when it happens, it does so in grand style. Another steep effect is that water, laden with soil, becomes up to twice as heavy, exerting more pressure and scouring more readily. It is as if all three red curves are moved to the left by a factor of two. Being heavier, water also flows more rapidly down-hill, causing even more destruction. Its erosive energy mv2 can increase up to 8 times! In addition, both its pressure and friction increase, making it even more effective at scouring river banks.
Using this knowledge, one can understand that particles settle out in
different places, as a river slows down in its lower reaches. Follow the
leftmost curve: first the pebbles; further down clean sand and perhaps
near the sea the silt. But the clay travels all the way to the sea. Here
something else happens.
simple but very important experiment has shown that in salt water fine
particles and organic matter settle out considerably faster than in fresh
water (4-200 times, depending on particle size). It is thought that the
salt water encourages particles to join together, thus making them larger
and sinking faster, but this goes against Bernouilli's law and the sedimentation/transport
diagram above. 
Reader please note, that no scientific work has been done to explain this phenomenon. Water is an excellent solvent for charged particles due to its polarity and charged ions OH- and H+. Salt consisting of Na+ and Cl- may neutralise much of water's capacity to dissolve substances. Note that soap won't lather well in salt water, presumably for the same reason.
Its effect is that fine silt and clay settle out, as soon as they reach
salt water, which happens in coastal estuaries. Organic matter does the
same, enriching mud flats and mangroves.
Water causes the following types of erosion:
diagram shows the various remedies to combat erosion on grassland and hilly
terrain. From left to right:
for saving cropland are, from left to right:
Read also how runoff results from the difference between rainfall and
evaporation in geology/evaporation,
and how narrow the margins are for sustainable farming.
 Heezen & Hollister (1964) in Turekian, Karl K (1968): Oceans. Prentice-Hall, p25
 Williams, B L & Rutherford, J C (1983): The flushing of pollutants and nutrients from the Upper Waitemata Harbour. ARA Specialist Report for NWASCO. Ministry of Works and Development Water Quality Centre Report No.1 88pp and Appendices.
When water freezes, it expands suddenly and with tremendous force. When water inside a crack in a rock freezes, its expansive strength may be sufficient to crack the rock and to break parts off it. Frost is tremendously active in snow-covered mountains, particularly along the snow boundary where water repeatedly thaws and freezes. It causes steep cliffs in this region.
A particularly mysterious form of frost damage is frost heave, resulting in damaged roads, buildings and cropland. It appears as if the frost heaved sections of the land upward, by as much as 20cm and usually in very irregular ways. As can be expected, frost heave works with the strength of frost.
Frost heave is not predictable but happens after a deep frost period,
followed by thawing and freezing again, and a few repeats of this sequence.
In permafrost soils of the arctic, it causes engineering headaches that
have to be met with special solutions.
Frost heave can be understood as follows: a deep frost, or permafrost freezes the soil to a certain depth. When this frost thaws incompletely, it leaves a frozen layer behind. Underneath it, the soil may still be thawed but in permafrost places, this frozen bottom is always present. Above it, melting water collects. A repeated frost now freezes it again from the top down, forming a hard layer on top with water in between the two frozen layers. As the frost progresses deeper, the entire top layer is pushed up a few centimetres. The next thawing/freezing cycle repeats this, ratcheting the top layer higher and higher, and always with the same force. Only when the deepest layer is thawed again, will frost heaving stop.
It is not known how much erosion is caused by frost heaving, but it
can damage soil structure.
Memories don't last forever. They disappear when one dies, reason why people are doomed to repeat the mistakes of their grandparents (3 generations = about 90 years). How many people today would remember the days of the dust bowl? This famous photograph in all its sadness, reminds us of an event in the history of agriculture in America, that is remembered for its unavoidability and its sadness. A farmer and his two sons are seeking shelter from the arriving dust storm. A derelict shed speaks of extreme hardship. Nothing on their land is like it was only a few years before.
It happened when a period with prosperity and adequate rainfall, was followed by an unexpected one of continuous droughts. The farming practices which were once appropriate, suddenly became disastrous. With their lands bared, farmers were caught out by the drought, unable to do anything meaningful. The winds did the rest. Sadness followed.
like moving water being able to move soil, moving air can do so too. In
fact in the same way as the curves of the erosion/sedimentation diagram
above (soil51.gif). Fine particles are moved easily
(but not when wet) and when they are as small as clay and silt, they become
airborne, reluctant to settle out again, even though air is 800 times lighter
than water. Sand particles between 0.1 and 1 mm move by saltating (jumping)
over the ground, like a sheet. Heavier particles move by rolling.
Unlike water, wind can move soil over very large distances of thousands of kilometres and over sea to other countries. It can move soil up-hill. Of course, the finest clay particles are transported furthest. The amount of soil moved, must not be underestimated, and once in motion, and the air heavy with dust, its erosive power increases. At least rain runs in gullies, but the wind blows everywhere. In many countries, usually those with arid climates, erosion from wind is considered by far the worst.
Damage is caused in various ways:
avoid wind-blown erosion, arid soils should not be worked, but in order
to survive, people do. The remains of the previous crop (stubble, stalks,
straw) should not be removed until just prior to planting. Strip cropping
shelters the land and so do shelterbelts. Reduced tillage or no-tillage
is preferable. Once the soil has been swept into land dunes (as opposed
to sea dunes), these should be planted and stabilised. Fertiliser is an
important weapon in doing so.
(The photo shows the linear dunes of the Sahara Desert encroaching on Nouakchott, the capital of Mauritania. Note how massive and unstoppable sand blowing can be. Photo by Georg Gerster, courtesy U.S. Geological Survey)
(Image courtesy NASA)