The drainage basin hydrological cycle
The drainage basin hydrological cycle is an open system with inputs, outputs and stores.
- Inputs - The energy from the sun needed for evaporation, and the volume of precipitation.
- Outputs - Evaporation and transpration (evapotranspiration), water percolating into underground stores where it is effectively lost from the system, and runoff water into the sea.
- Stores - Water is stored within the drainage basin, e.g, rivers, lakes, etc. Vegetation also stores water through interception and plants also contain water. In addition to this, soil stores water held in groundwater by permeable rocks.
The main processes of river erosion:
There are four main types of river erosion, these are; Abrasion (also known as corrasion), Hydraulic action, Corrosion, Attrition.
Abrasion/Corrasion: Abrasion is when materials which are carried by the river rub away at the bed and banks of the river.
Hydraulic action: This is the movement of material due to the frictional drag of the moving water on sediment lying on the channel bed.
Corrosion: This is when minerals within the rock are dissolved by acids in the river and are carried away through the transport process solution.
Attrition: Attrition is when materials hit eachother as well as the river bed causing them to become smoother, smaller and more rounded as they move along the river's channel.
The ways in which a river transports its load:
There are four main ways in which a river transports it load; Traction, Saltation, Suspension, and Solution.
Traction: Large stones and boulders roll along the river bed.
Saltation: Small particles are thrust up from the rivers bed and fall back to the bottom again further downstream.
Suspension: Very small particles of sand and silt are carried along by the flow of the river.
Solution: This is when dissolved minerals are transported within the mass of the moving water.
Deposition is where the river deposits its load, it usually occurs when;
- There is a reduction in the gradient of the river (e.g. when it enters a lake).
- Discharge is reduced. (e.g. due to dry weather).
- There is shallower water (e.g. on the inside of a meander).
- There is an increase in the size of the load.
- The river floods and overtops its banks, resulting in a reduced velocity on the floodplain outside the main channel.
Meanders are a bend in the rivers channel, they are formed because rivers have sinuousity, this means that they do not flow straight. Alternating bars of sediment on their beds and the moving water is forced to weave around these bars. This creates alternating shallow sections (known as riffles), and deeper sections (known as pools). The flow of the river directs maximum velocity towards the outside of the bend of the meander, and results in erosion, and the deposition is on the inside of the bend where there is low velocity.
Diagram of a meander:
Above is a diagram of a river meander, it shows where deposition, erosion, pools, riffles, and thalweg (fast flow) occurs.
An oxbow lake is created by the increasing sinuosity of a river meander. Erosion is greatest on the outside of the river bend and deposition occurs on the inside of the meander. The gap between the two bends becomes progressively narrower due to this rapid erosion and deposition. In times of high discharge (e.g. floods) the river cuts through the neck and this becomes the main channel of the river. The former channel has been cut off by deposition and becomes an oxbow lake.
Rejuvenation occurs when there is a fall in sea level relative to the land or a rise of land relative to the sea. The river adjusts to its new base level starting at the sea and gradually works its way up the river. A number of landforms are created as a result of rejuventation;
- Knick points: A sudden break/ irregularity in a rivers channel where the old long profile meets the new.
- Waterfalls and rapids: A rapid change in gradient.
- River terraces: A former flood plain which has been left at a higher level after rejuvenation.
- Incised meander:
Physical causes of flooding:
- Excessive levels of precipitation over a long period of time, this leads to the soil becoming saturated and therefore there is increases overland flow or runoff.
- Intensive precipitation over a short period of time, particularly after periods of dry weather. Infiltration can not soak up the water quickly enough so the water reaches the river quicker than it previously would have done.
- The melting of snow, as this reduces the capacity of inflitration.
- Climatic hazards which produce large amount of precipitation.
Human causes of flooding:
Flooding is made more likely by human activities, such as:
- Urbanisation - This has led to the demand for space to build housing and for other urban land uses. Many have built within the flood plain because the flat lands are ideal but this means that towns are more vulnerable to flooding. Impermeable surfaces have also been used in urban areas for roads and pavement (tarmac and concrete), this has led to precipiation being unable to infiltrate slowly into the soil as it would have done in a vegetated area. This means that the precipitation reaches the river quicker due to increased surface run off. In addition, surface water is directly channelled into drains and sewers to the rivers, increasing the discharge. Lastly, bridges are built which can slow down discharge and can reduce a rivers capacity which may increase the likelihood of flooding.
- Deforestation - In some countries there is rapid deforestation which has led to a greater risk of soil erosion and sediment finds its way into rivers, obstructing them and adding to the flood risk. In areas of deforestation there is less interception and so there is increase runoff - water reaches the channel quicker.
Evaporation: The process where water is transformed into water vapor. The energy needed for evaporation is provided through heat from the sun.
Evapotranspiration: The total amount of moisute removed by evaporation and tranpiration from the drainage basin.
Groundwater flow: The water that flows through bands of sedimentary rocks.
Infiltration: Water soaks into the soil, as the soil becomes saturated the rates of infliltration fall rapidly.
Interception: Raindrops are prevented from reaching the soils surface through trees and plants.
Percolation: The movement of water within the rock under the soil surface.
Percipitation: Water in any from that falls from the atmosphere to the earths surface.
Runoff: All the water that enters a river and eventually flows out of the draiange basin.
Throughflow: Water that moves down through the subsoil pulled by gravity.