Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.

Fluvial processes

Fluvial processes include the motion of sediment and erosion or deposition on the river bed

The main processes of fluvial erosion occur throughout the course of the river. These are outlined below. The river itself, however, will try to erode in different directions, depending on how far down the course you are. Very basically, rivers are trying to erode down to their base level. In most cases this is sea level, but it can also be the level of a lake that the river might be flowing into.

The main processes of fluvial erosion are: Abrasion: The erosion of the river bottom and the riverbank by material being carried by the river itself. Attrition: The rocks and pebbles being carried by the river crash against each other, wearing them down to become smaller, rounded pebbles. Corrasion: see Attrition. Corrosion: The chemical erosion of the rocks of the riverbank by the slightly acidic water. This occurs in streams running through rocks such as chalk and limestone. Hydraulic Action: The water forces air to be trapped and pressured into cracks in the rocks on the bank of the river. This constant pressure eventually causes the rocks to crack and break apart.

Fluvial transportation Once it has been eroded, material in the river is transported down the river.Whilst this is happening, erosion processes such as attrition and abrasion continue to occur.

There are four main processes of fluvial transportation,depending on the size of the material being moved: Traction: The largest rocks in the river are slowly rolled along the bottom of the river by the force of the water.This primarily occurs in the upper reaches of the river. Saltation: Smaller rocks are bounced alongthe river bed. This occurs in the upper and middle sections of the river in general. Suspension: The water carries smaller particles of material. This process occurs throughout the course of the river, but increases the closes you are to the mouth of the river. Solution: Materialis dissolved within the water and carried along by it. Primarily this occurs in the middle and lower reaches of the river.

Fluvial deposition occurs where the river losses energy and therefore cannot continue to carry the material it is transporting. This could happen in an estuary when the river meets the sea and slows down, depositing its load, which may eventually lead to the formation of salt marshes or a delta. Material is also deposited further up the course of the river. For instance the slower moving water on the inside of a bend of a river will have less energy and therefore drop its load, helping to create a meander.

Drainage patterns

All rivers are joined by smaller rivers or streams which are called tributaries. The area drained by the river and its tributaries is known as the river basin. The highland area around the basin is called the watershed. River systems often form a distinct pattern which is due to the structure of the rocks in the drainage basin. Three distinctive patterns can be recognised, dendritic, trellis and radial.

Dendritic: This pattern looks like tree branches. This pattern develops in gently sloping basins with fairly uniform rock type. The tributaries flow into the river at random forming a pattern like the veins of a leaf. Examples of dendritic drainage are in the Caroni River in Trinidad and the Bruce Vale river basin in Barbados. This type of drainage pattern is the most common in the Caribbean region.

Trellis: This drainage pattern has an appearance of a rectangular grid. Rivers and their tributaries flows almost perpendicular to each other with confluence of almost 90o. Trellis drainage takes place where there is an alternate band of hard and soft rock at right angles to the main direction of the slope. The main river has the power to cut though the hard rock while the tributaries cut though the softer rock at more or less right angles.

Radial: Radial drainage patters happens on a dome or volcanic cone. This pattern resembles the spokes of a wheel. The river radiates outwards in all directions from a high central point or dome. The volcanic islands in the eastern Caribbean have radial drainage pattern.