d) Le gel-dégel
Les conséquences de l'action du gel sont bien connus sur les roches friables et fissurées. La succession des cycles de gel-dégel y provoque des décrochements de diverses dimensions. Plus le nombre de cycles de gel-dégel est élevé, plus le processus est actif et agressif.
e) Les autres agents d'érosion
Parmi les autres agents d'érosion, il faut mentionner la dessiccation (fracturation du sol lors de sécheresse), l'activité sismique (qui provoque des mouvements de masse) et les actions directes de l'homme (déforestation, dévégétalisation,
Whatever the nature of the substrate and the characteristics of the environment, the coast is subject to two groups of agents that contribute to coastal erosion: coastal agents and subaerial agents.
1) Coastal agents
The most frequently encountered littoral agents are waves and currents.
a) The waves
The waves are formed by a process of energy transfer from the wind to the surface of the water. The body of water is then pushed in the direction of the wind forming waves that cause the wear of the cliffs and microfalaisesmeubles. This action is maximized when a storm occurs at the same time as a high tide. Wave action consists of gnawing the shoreline and clearing sediments that have fallen at the foot of the cliffs. In the case of rocky cliffs, the wave has an impact only if it is laden with sediments and debris that cause abrasion of the foot of the cliffs and the surface of rocky foreshores.
The waves, the waves produced by the boats, increase the effectiveness of the natural waves or causes them when there are none or few.
The sediments collected by the waves can be displaced by the current of the coastal drift. This is caused by the waves, directed by the prevailing winds, striking the shore obliquely. A large part of the sediments collected at a given point will then be redistributed along the coast in the direction of the littoral drift. The direction of the littoral drift is likely to be reversed when the winds change direction. However, the coastal drift current has a predominant direction that can be observed in particular by the direction of the coastal arrows.
The tidal stream is not an erosion agent in itself. Being an oscillatory current, its role is to vary the water level so that, according to its amplitude and the slope of the foreshore, the waves will be able to go further on the beach.
2) Subaerial agents
Part of the erosion can be attributed to continental processes, which then become complementary to marine processes. These are the subaerial agents and they fall into five categories: precipitation, gravity, wind, freeze-thaw and other erosion agents.
Heavy rains and spring snowmelt are responsible for runoff and gully. These processes, when acting on portions of land without vegetation, can increase erosion by causing landslides.
Suffering is also a phenomenon due to heavy rainfall. This process is active in sandy cliffs resting on clay. In heavy rain, water penetrates the sand to the impermeable clay. The soil then swells with water and there is an underground drainage channel at the edge of clay and sand. The sediments are then expelled and the soil collapses.
Gravity means mass movements such as recesses, landslides, landslides, collapses and gullying. These processes mainly affect cliffs more than 10 meters high. There are also mixed forms involving several processes, such as the suffusion ravines.
Mass movements often have devastating effects, but they are also part of the phenomena that feed the littoral into sediments. In fact, it has often been observed that sediments slipped on the coastline were rapidly reworked by the waves.
c) The wind
Wind power is a process linked to the action of the wind. Generally negligible, the erosion by the wind activity is carried out on the top of beach exposed to the prevailing winds. The littoral arrows and the recent littoral ridges represent very sensitive places to wind.
The consequences of the action of the gel are well known on friable and cracked rocks. The succession of freeze-thaw cycles there causes recesses of various dimensions. The higher the number of freeze-thaw cycles, the more active and aggressive the process.
e) Other erosion agents
Other erosion agents include desiccation (soil fracturing during drought), seismic activity (which causes mass movements) and direct actions by humans (deforestation, de-vegetation,
rip rap, etc.).
Questions pour valider cette earthcache :
D'après vos observations et à l'aide du desciptif,
Questions to validate this earthcache:
Based on your observations and using the description,
Décrivez les formes d'érosion que vous pouvez observer dans CHAQUE zone 1 à 5.
(NB. La zone 4 correspond à la zone 1 mais plus à l'ouest)
Describe the forms of erosion that you can observe in EACH area 1 to 5.
(NB Zone 4 corresponds to zone 1 but more to the west)
D'après vous, quels agents ont conduit à l'érosion de CHAQUE zone ?
What agents do you think led to the erosion of EACH area?
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