Rivers, floods and management Flashcards
What is the difference between a local and global hydrological cycle?
The global hydrological cycle is continuously cycled between the oceans and atmosphere, returning to the oceans when it falls as precipitation. It is a closed system as there are no inputs and outputs. A local (drainage basin) hydrological cycle is an open system: there are inputs and outputs.
What is a river’s drainage basin?
The area surrounding the river where the rain falling on the land flows into that river. It is also called the river’s catchment.
What is the watershed?
The boundary of the drainage basin. Any precipitation falling beyond this point enters a different drainage basin.
What are the inputs of a drainage basin?
Precipitation.
What is precipitation?
Mainly rain, but also snow, hail, dew and frost.
What are the 5 ways water can be stored in a drainage basin?
Interception, vegetation storage, surface storage, groundwater storage and channel storage.
What is interception?
When some precipitation lands on vegetation or other structures before it reaches the soil.
What is vegetation storage?
Water that’s been taken up by plants.
What is surface storage?
Water in puddles, ponds and lakes.
What is groundwater storage?
Water stored in the ground, either on the soil or in rocks.
What is channel storage?
The water held in a river or stream channel.
What are the 10 flows and processes?
Surface runoff, throughfall, stemflow, throughflow, infiltration, percolation, groundwater flow, baseflow, interflow and channel flow.
What is surface runoff?
Water flowing over the land.
What is throughfall?
Water dripping from one leaf (or other plant part) to another.
What is stemflow?
Water running down a plant stem or tree trunk.
What is throughflow?
Water moving downhill through the soil.
What is infiltration?
Water soaking into the soil.
What is percolation?
Water seeping down through soil into the water table.
What is groundwater flow?
Water flowing below the water table through permeable rock.
What is baseflow?
Groundwater flow that feeds into rivers through river banks and beds.
What is interflow?
Water flowing downhill through permeable rock above the water table.
What is channel flow?
Water flowing in the river or stream. Also called the river’s discharge.
What are the outputs of a drainage basin?
Evaporation, transpiration, evapotranspiration and river discharge.
What is evaporation?
Water turning into water vapour (from a liquid to a gas).
What is transpiration?
Evaporation from plant leaves.
What is evapotranspiration?
Evaporation and transpiration together.
What is potential evapotranspiration?
The amount of water that could be lost by evapotranspiration.
What is the water balance?
The water balance shows the balance between inputs and outputs.
What is the water balance like in wet seasons?
Precipitation exceeds evapotranspiration, creating a water surplus.
What is the water balance like in dry seasons?
Precipitation is lower than evapotranspiration, creating a water deficit.
What is river discharge?
The volume of water that flows in a river per second (measured in cumecs).
What is river discharge affected by and how?
Precipitation (the more precipitation, the higher the discharge), hot weather (the higher the temperature, the lower the discharge because of the increased rate of evaporation), removal of water from the river/abstraction (reduces the discharge.
What do hydrographs show?
River discharge over time.
What do storm hydrographs show?
River discharge around the time of a storm event.
What is the peak discharge on a hydrograph?
The highest point on the graph, where the river discharge is at its greatest.
What is the lag time on a hydrograph?
The delay between the peak rainfall and peak discharge.
What is the rising limb on a hydrograph?
The part of the graph up to peak discharge.
What is the falling limb on a hydrograph?
The part of the graph after peak discharge.
What is a flashy hydrograph?
A hydrograph that has steep, roughly symmetrical rising and falling limbs.
How do drainage basin characteristics affect lag time and peak discharge?
Larger drainage basins can catch more precipitation, so they have a higher peak discharge compared to smaller basins. Smaller basins generally have shorter lag times because precipitation has less distance to travel, so reaches the main channel quicker. Steep-sided drainage basins have shorter lag times than shallower basins - water flows more quickly downhill into the river on steep slopes. This can also increase peak discharge. Circular basins are more likely to have a flashy hydrograph than long, narrow basins. This is because all points on the watershed are roughly the same distance from the point of discharge measurement. This means lots of water will reach the measuring point at the same time, increasing peak discharge. Basins with lots of streams (high drainage density) drain quickly, so have shorter lag times.
How does the amount of water already present in the drainage basin (antecedent moisture) affect lag time?
If the ground’s already waterlogged (the soil can’t absorb any more water) then infiltration is reduced and surface runoff increases. Surface runoff is much faster than throughflow or baseflow, so rainwater reaches the river more quickly, reducing lag time.
How does the rock type affect lag time and peak discharge?
Impermeable rocks don’t store water or let water flow through them. This reduces infiltration and increases surface runoff, reducing lag time. Peak discharge also increases as more water reaches the river in a shorter period.
How does the soil type affect lag time and peak discharge?
Sandy soils allow a lot of infiltration, but clay soils have very low infiltration rates. Low infiltration rates increase surface runoff, reducing lag time and increasing peak discharge.
How does vegetation affect lag time and peak discharge?
Vegetation intercepts precipitation and slows its movement to the river channel, increasing lag time. The more vegetation there is in a basin, the more water lost (through transpiration and evaporation directly from the vegetation) before it reaches the river channel, reducing peak discharge.
How does precipitation affect peak discharge?
Intense storms will generate more precipitation and so greater peak discharges than light rain showers. The type of precipitation also affects lag time - e.g. snow that’s fallen in a winter storm can melt and flow into the river in spring, giving it a very long lag time.
How does temperature affect lag time and peak discharge?
Hot, dry conditions and cold, freezing conditions both result in hard ground. This reduces infiltration and increases surface runoff, reducing lag time and increasing peak discharge. High temperatures can increase evapotranspiration, so less water reaches the river channel, reducing peak discharge.
How can human activity affect the hydrograph?
In urban areas, much of the soil is covered with man-made impermeable materials like concrete. Water can’t infiltrate into the soil, which increases surface runoff, so water flows more quickly into the river. This makes the lag time short and increases peak discharge. Man-made drainage systems affect the hydrograph in a similar way. Water flows down drains into the river before it can evaporation or infiltrate into the soil, causing a shorter lag time and increased peak discharge.
What does headward erosion do and where does it happen?
Headward erosion makes a river longer. It happens near a river’s source as throughflow and surface runoff causes erosion at the point the water enters the river channel (the valley head).
What does vertical erosion do and where does it happen?
Vertical erosion deepens the river and happens in the upper stages.
What does lateral erosion do and where does it happen?
Lateral erosion makes the river wider and it happens in the middle and lower stages of a river.
What are the 5 ways in which river erosion happens?
Hydraulic action, abrasion (corrasion), attrition, cavitation and corrosion (solution).
What is hydraulic action?
The pressure of the water breaks rock particles away from the bed and banks.
What is abrasion (corrasion)?
Eroded pieces of rock in the water scrape and rub against the bed and banks, removing material.
What is attrition?
Eroded rocks smash into each other and break into smaller fragments, their edges also get rounded off as they rub together. Attrition doesn’t erode the bed and banks, just makes the particles of rock in the river smaller and more rounded.
What is cavitation?
Air bubbles in turbulent stretches of water implode causing shockwaves that break pieces of rock off the bed and banks.
What is corrosion (solution)?
The dissolving of rock by chemical processes. Carbon Dioxide dissolves in water to form a weak acid, which reacts with rocks like limestone and chalk, breaking them down.
What are the 4 ways that eroded material can be transported in a channel?
Solution, suspension, saltation and traction.
What is solution?
Substances that can dissolve are carried along in the water.
What is suspension?
Very fine material is whipped up by turbulence and carried along in the water.
What is saltation?
Larger particles are too heavy to be carried in suspension. Instead, the force of the water causes them to bounce along the river bed.
What is traction?
Very large particles are pushed along the river bed by the force of the water.
What is the river’s bedload?
Material transported by traction or saltation.
How can the speed and energy of a river can be reduced?
Reduced rainfall causes lower discharge, which means the river slows down and has less energy. Increased evaporation or abstraction also causes lower discharge Friction reduces the speed of the river, reducing its energy. When the river is forced to slow down due to a narrow section of the channel, it loses energy. A lot of energy is lost when the river meets the sea (the sea absorbs the energy).
What is the capacity of a river?
The total load that a river can transport at a given point.
What is the competence of a river?
The maximum particle size that a river is capable of transporting at a given point.
What does the Hjulstrom Curve show?
The relationship between river velocity and competence. It also shows how the process of erosion, deposition and transportation vary with river velocity.
What does the critical erosion velocity curve show?
The minimum velocity needed for the river to pick up (erode) and transport particles of different sizes in suspension or as bedload. It takes a higher velocity to erode material than it does just to transport it.
What does the mean settling velocity curve show?
The velocities at which particles of different sizes are deposited.
Why does the graph show that it requires a higher velocity to erode silt and clay than it does sand if sand is larger?
Silt and clay particles stick together more and are harder to dislodge, so more energy (velocity) is required to erode them.
What is the long profile?
A long profile shows you how the gradient of the river changes from source to mouth by showing the height of the river bed above the base level.
What is the base level?
The lowest point a river can erode to (usually sea level).
What is a graded profile and why would it happen?
Because the total amount of erosion and deposition is balanced, a graded profile is the idea that over time the long profile will change from being uneven to a smooth curve (but it hardly ever happens).
What is the energy like in the upper stage of a river?
Because the river is high above sea level, there is lots of potential energy.
What is the energy like in the middle stage of a river?
Potential energy is converted into kinetic energy and the river guns velocity.
What is the energy like in the lower stage of a river?
There’s very little potential energy, but lots of kinetic energy, and so it flows faster.
How do the velocity and discharge change as you go downstream?
They both increase.
What affects river velocity?
Gradient, discharge and channel characteristics (shape and roughness).
