Hydrology.

Hydrology.

1. Factors affecting disharge.

   • Antecedent rainfall- heavy rainfall falling on a soil which is saturated from a previous period of wet weather.
   • Snow melt- Water levels may fall in a period of snowfall and cold weather, but when temperatures rise, melting occurs and massive amounts of water is released and increases the discharge.
   • Porous soil/permeable rock- Eg, limestone, where water is regulated more slowly through the natural systems, so creates a lower amount of discharge.
   • Impermeable rock- Eg, granite and clay tend to have higher drainage densities, so the higher the density, the bigger increase in discharge.
   • Slope angle- The steeper the gradient of the slope, the faster the water reaches the river, so the higher the density.
   • Temperature- Cold temperatures mean that the ground may freeze, which restricts the infiltration, increases overland flow and therefore increases river discharge.
   • Vegetation/Deforestation- If there is a lack of vegetation and trees have been removed, there is less interception, so the discharge increases.
   • Land use- Water runs off land that has been compressed, such as farmland that has been trampled on by cattle and tractors, so this means that less water can be stored, which leads to a higher discharge.
   • Urbanisation- For example, the building of roads which increases the amount of impermeable rock and the removal of topsoil and compaction of the ground due to machinery during urbanisation all increase discharge.

2. Factors affecting transport.

   • Any energy remaining after the river has overcome friction can be used to transport sediment.
   • The amount of energy available to transport sediment increases as the discharge, velocity and turbulence increases until the river reaches flood levels.
   • Sediment can be transported in 3 ways:
     • Solution- The dissolved load is carried by moving water, but it only makes up a small part of the load in the river, except in limestone areas.
     • Suspension- Fine particles such as clay and silt are carried by the turbulence of the water in a fast flowing river and the suspended load is usually the largest amount of load.
     • Movement along the riverbed- This can be done by rolling (traction) or by bouncing (saltation). The large particles are rolled along the river bed, but through times of high discharge, and slightly smaller particles are bounced up from the bed of the river by turbulence and fall back on the bottom further downstream. As they land on the river bed, they may loosen other particles and cause them to bounce.
   • Capacity- This refers to the amount of material that a river can carry, so the total load. If the rivers velocity doubles, then the capacity increases by 8 times.
   • Competence- This refers to the diameter of the largest particle that a river can carry for a given velocity. If a river's velocity doubles then its competence increases by 64 times, this is because faster flowing rivers have greater turbulence and are able to lift particles from the river bed.
   • Deposition.
     • This occurs when there is a decrease in energy levels and the river is no longer competent to transport the load.
     • It occurs when; there is a reduction in the gradient of the slope, the discharge is reduced, for example after a dry spell of weather, there is shallow water, there is an increase in the size of the load, or the river floods, so velocity outside of the channel is reduced.
     • Generally, the largest material will be deposited first, followed by smaller particles, but the fine particles that are carried in solution, may never be deposited.

3. Factors affecting erosion.

   • The river erodes because it possesses energy and the total energy depends upon 3 factors.
     • The weight of the water- So the greater the mass of the water, the more energy it will possess, so more erosion.
     • The height of the river above base level- This gives it a source of potential energy, so the higher the source of the river, the more energy it has, so more erosion.
     • The steepness of the channel- This controls the speed of the river which determines the amount of kinetic energy. The more kinetic energy, the more erosion
   • Much of the energy is lost through friction, whether it is internally due to turbulence within the flow of the river, or externally through contact with the bed and banks of the river.
   • 4 main types of erosion:
     • Abrasion- Where materials scrape against the river bed, causing the erosion of the river bed and banks.
     • Hydraulic Action- Where the force of the water is put on the pressure of the banks of the river and remove loose material. In turbulent water, it can cause a suction affect the dislodges material from the channel bed.
     • Corrosion- Where minerals in rocks are dissolved in water and carried in solution. Minerals such as magnesium and potassium are carried.
     • Attrition- The reduction in size of materials and particles in a river. They collide with other material and become smaller, smoother and rounder.
   • River erosion can be vertical or lateral. Rivers tend to erode vertically in high levels so that they can reach base level quicker.