Erosion The power of the water and the material that is carried will continually shape and wear away the bed and banks of a river channel. There are four main processes important in fluvial (water) environments: •
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Hydraulic action is the force of the water pushing into cracks and hitting against the river’s banks. This repeated action weakens the riverbank as air in the cracks is compressed and pressure builds up. Collapsing air bubbles create small shock waves in a type of hydraulic action known as cavitation. Unlike coastal environments where waves may be large and powerful, hydraulic action is a slow and ineffective process of erosion. Corrasion occurs when sediment in the river is thrown into or scraped along (abrasion) the banks and bed of the river. This process is extremely common and is the main form of erosion within a river. During times of high flow or flood the river has a greater capacity to transport larger material, which results in the greatest amount of damage. Potholes may form as stones become trapped in depressions and hollows and are continually swirled around by eddies in the turbulent flow. Attrition is the process by which stones and sediment within the river become increasingly rounded. As material is transported it collides with other objects in the river. The collisions cause the stones to break into smaller pieces and the edges and points of the stones to break off. Corrosion or solution is a continuous chemical process that occurs independently from river flow. Water that has slightly acidic properties, for example as a result of decomposing organic material (humic acid) or acid rain (carbonic acid), will chemically dissolve and weaken certain types of rock. Limestone is composed of calcium carbonate and is particularly vulnerable to corrosion.
Transport In addition to the movement of water, rivers also become important conduits for the transport of sediment. Rivers transport sediment in a number of ways. The mode by which sediment is transported is related to the speed of flow and its size. Unsurprisingly, faster flows can transport larger material. This is perhaps most noticeable in times of flood when large boulders, trees and even cars may be carried by a river. Material carried by a river is referred to as its load. Rivers can only carry so much load depending on their energy. Capacity is the name given to the total load of material actually transported. Competence is the name given for the maximum size of material that a river is capable of transporting. The load is transported by four main processes: •
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Traction is when the largest stones, boulders and cobbles are rolled along the riverbed by strong turbulent flow. Often these sediments will lie undisturbed on the riverbed until sufficient discharge is reached to displace them. Saltation is where smaller bedload such as pebbles, stones and gravel are lifted and carried temporarily in the flow in a hopping or bouncing motion. As turbulent flow is not constant the river will have varying amounts of energy to lift and carry the load. Suspended load is when very fine particles of sand and silt are carried in suspension in fast flowing water. The faster and more turbulent the water, the greater the amount and size of material that can be transported. Suspended load is easier to see in the lower reaches of a river or after a rainfall event where the water has a muddy brown appearance. Dissolved load or solution is the process by which small dissolved sediments and minerals are transported within the river. They form just a small proportion of the total load but are significant as corrosion (or solution) is constantly occurring.
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