hydrology and fluvial geomorphology

• What is a drainage basin?


  A drainage basin can be classified as a natural system. A drainage basin is the area that is drained by a river and any tributaries (a tributary is a smaller stream that flows into and contributes to a larger river). The boundary of the drainage basin is called the watershed ; if any water drains outside the watershed, it is classified as being within another drainage basin.
• What are the different outputs within a drainage basin?


  Evaporation: The change in state of water from liquid to gas . Evaporation occurs in the drainage basin system when water is heated by solar energy , causing it to evaporate into a gas and rise into the atmosphere. Water is leaving the system here, which is why evaporation is an output .

  
  

  Evapotranspiration: Compromised of evaporation and transpiration . Evaporation occurs when water is heated by the sun, causing it to become a gas and rise into the atmosphere. Transpiration occurs in plants when they respire through their leaves, releasing water they absorb through their roots, which then evaporates due to heating by the sun.

  
  

  River discharge: The volume of water passing through a cross-sectional point of the river at any one point in time, measured in Cubic Metres Per Second (Cumecs) . Water leaves the basin through streams which drain the basin. These may flow as tributaries into other rivers or directly into lakes and oceans.
• What are the different stores within a drainage basin?


  Interception: Water (e.g. precipitation ) that is intercepted by plants' branches and leaves before reaching the ground. This water is stored short-term .

  
  

  Soil Water: Water that is stored in the upper levels of the soil , which is utilised by plants .

  
  

  Surface Water: Water stored in puddles, ponds, lakes etc . Dependent on the size of these stores, they can last from hours to millions of years.

  
  

  Groundwater: Water that is stored in the pore spaces of rock or lower soil. Channel Storage: Water that is stored in a river's channel .
• What are the different above ground flows within a drainage basin?


  Throughfall: Water flows from leaves and foliage onto the ground . This is especially prominent in areas with a large canopy that receives a lot of rainfall , like a rainforest, but will occur whenever precipitation falls onto plants and trees.

  
  

  Stemflow: Intercepted water stored on plants and trees flows down a stem onto the ground .

  
  

  Overland flow: Water flows above the ground, as sheetflow (lots of water flowing over a large area), or in rills (small channels similar to streams, that are unlikely to carry water during periods where there is no rainfall). Overland flow will occur when water cannot infiltrate the soil.

  
  

  Channel flow: Water that moves through established channels , like streams or rivers. Naturally, channel flow is fed by two sources : overland flow (i.e. precipitation that flows over the ground, rather than infiltrates the soil), and groundwater flow (groundwater flow contributes to channel flow where a channel intersects the water table - this will be explored further).
• What are the different below ground flows within a drainage basin?

  Infiltration: The movement of water from above ground into the soil . Infiltration is affected by multiple factors, including: Permeability of soil, Relief, Rate of precipitation, Saturation levels of soil

  
  

  Percolation: Water flows from the ground or soil into porous rock or rock fractures . The percolation rate is dependent on the fractures that may be present in the rock and the permeability of the rock.

  
  

  Throughflow: Water flows through the soil and into streams or rivers . Speed of flow is dependent on the type of soil , e.g. clay soils will have a slower flow rate than sandy soils .

  
  

  Groundwater Flow: Water flows through permeable rocks , below the water table , and may also flow as springs. Groundwater maintains water levels elsewhere , which will be discussed further in the 'Underground Water' section.
• What is groundwater?


  Groundwater is classified as any water underground, meaning it can be held in soils or in between rock spaces. Water that percolates into the bedrock.
• What are aquifers?


  Aquifers are underground water stores. On a global scale they are unevenly distributed. Shallow groundwater aquifers can store water for up to 200 years , but deeper fossil aquifers , formed during wetter climatic periods, may last for 10,000 years .
• What is a water table?


  The water table is the level at which the pore spaces and fractures in the ground become saturated , meaning above the water table is unsaturated soil , and below the water table is saturated soil
• What does a hydrograph show?


  A hydrograph is a graph that shows how river discharge changes over time. Discharge is the volume of water in a river at a given point , measured in cubic metres per second . This combines the speed of the river in a given direction (velocity) with the area of a cross sectional point (area) to show how much water is travelling through an area, and how fast it's doing so.

  
  

  A hydrograph may show how a river channel changes in response to a storm event (storm hydrograph) or it may show the changes of a river channel over a year (annual hydrograph).
• What is a storm hydrograph?


  Storm hydrographs show how a storm event (with consequent higher precipitation levels in the drainage basin) alters the discharge of a river. The hydrograph can be used to identify several components of a flood.
• What are the different components of a hydrograph?


  Discharge : The volume of water passing through a cross-sectional point of the river at any one point in time, measured in Cubic Metres Per Second (Cumecs) . Made up of baseflow and stormflow.

  
  

  Rising Limb: The line on the graph that represents the discharge increasing .

  
  

  Falling Limb: The line on the graph that represents the discharge decreasing .

  
  

  Lag Time: The time between peak rainfall and peak discharge.

  
  

  Baseflow: The level of water that the river holds without contributions by overland flow. This is the 'normal' discharge line (without precipitation).

  
  

  Stormflow: This is the additional water in the river bank during a storm, comprised of overland flow and throughflow .

  
  

  Bankfull Discharge: The maximum capacity of the river. If discharge exceeds this then the river will burst its banks and be in flood. When the discharge goes above the line labelled above, it is in flood .
• What are the influences on a storm hydrograph?


  The speed at which the input of rainfall arrives in the river channel is a key influence. It is the balance of overland flow, through flow and base flow that determines the shape and size of the storm hydrograph.

  
  

  Precipitation - Prolonged rainfall can lead to saturated ground and lots of overland flow. Intense rainfall means that infiltration capacity is exceeded, this produces overland flow. Different types of precipitation e.g. snowfall will have an increased lag time because it takes time for the snow to melt.

  
  

  Temperature - If temperatures are warmer evaporation is higher. In winter there is less evaporation so more precipitation will go into the river.

  
  

  Vegetation - Forests can encourage interception, evapotranspiration and infiltration. Forested area have smaller flood peaks.

  
  

  Seasonality - The three factors above show that similar inputs of rainfall can have different effects at different times of the year.

  
  

  Soil and rock type - Permeable soil and rock reduces overland flow and enhance throughflow and baseflow. Impermeable soil and rock enhance surface runoff.

  
  

  Basin relief - Steep slopes and high relief in the drainage basin tend to get water to the river faster and create high flood peaks.

  
  

  Urbanisation - Tarmac and concrete increase overland flow. Gutters and drains speed up through flow.
• How does the size of a drainage basin effect river discharge.


  The size of the drainage basin affects discharge in a river. In larger drainage basins , overland flow has more distance to cover before it reaches a river, potentially leading to a longer lag time . Furthermore, large basins cover more area , therefore there is potentially more water in this area to flow into the river, leading to a higher peak discharge .
• How does the shape of a drainage basin effect river discharge.

  In circular drainage basins, different starting points of overland flow are likely to be similar distances apart, meaning they are more likely to join the river at a similar time . This leads to a large quantity of water concentrating into an area in a short space of time , therefore increasing peak discharge as well as shortening the lag time .

  
  

  In contrast, in an oval drainage basin of a similar area, the starting points of overland flow will be different distances apart, meaning they will arrive at different times . Due to this, the lag time will be longer .
• How does drainage density effect discharge?


  The drainage density is the total length of all rivers and streams in a drainage basin divided by the total area of the drainage basin .

  
  

  In a drainage basin with a high drainage density , there are more streams and rivers carrying water, meaning the peak discharge is high and it is reached quickly , meaning there is a short lag time. As well as this, the amount of rivers mean the water is drained quickly , leading to a steep falling limb .

  
  

  In areas of low stream density , overland flow is more likely to infiltrate rather than make its way into channels, therefore lengthening the lag time and lowering the peak discharge, as there is less water draining into the river.
• What is erosion?


  Erosion is the process of the wearing away of soil and rock . In a river channel, both the force of water as well as fragments of rock cause the river bank to wear away in certain areas over time.
• What are the four main types of erosion in a river channel?


  Abrasion: This is the process of rocks scraping and grinding along the river channel, causing soil and rock to wear away. This usually causes the river bed to deepen and widen, and is especially prominent when a river is flowing with high velocity (like during a flood) as it has enough energy to carry larger rocks.

  
  

  Solution: This is the process of water dissolving parts of rock/soil that makes up the river channel. Solution is most prominent when the river channel is made from water soluble rocks , such as chalk, gypsum, or limestone . Water can slowly wear away at the river channel over time through this process.

  
  

  Hydraulic Action : Hydraulic action is the sheer force of water can cause rocks to fragment off when the channel is hit.

  
  

  Attrition: Particles of sediment in the load of the river (especially the bedload) bump into each other and wear away. As a result, river sediment becomes smaller and more rounded as it is carried downstream.
• What is transportation and what are the different methods in a river?

  A river's load is the sediment and materials transported by a river. The load is transported in different ways, dependent on its size :

  
  

  Traction: The movement of larger rocks and pebbles through water rolling them along the river bed . As the rocks are heavier, they cannot be carried by water within the channel as the river does not have enough energy.

  
  

  Saltation: Pebbles are bounced along the riverbed .

  
  

  Suspension: Small pebbles and material are carried (suspended) within the water, rather than rolling/bouncing along the river bed.

  
  

  Solution: Soluble materials are carried within the water.
• What is deposition and where may it occur?


  Deposition is where the river drops its load when the river no longer has enough energy to carry the load . Deposition may occur when a river becomes shallower or when the volume of water decreases .