Rivers Flashcards
What is the drainage basin?
The area of land drained by a river and its tributaries. They are separated by ridges known as watersheds.
What is an open system?
Where there are both inputs and outputs of both energy and mass
What is a closed system?
When there are both inputs and outputs of just energy
What is interception?
When precipitation lands on vegetation before it reaches the soil.
What is vegetation storage?
All of the water that has been taken up by plants at any one time
What is surface storage?
Water stored above ground like puddles and lakes
What is ground water storage?
Water stored in rocks or in soil in the water table (zone of saturation)
What is channel storage?
Water held in a river or stream
What is surface runoff?
water that flows over the land as a mass or in little channels
What is throughfall?
Water dripping from one leaf to another
What is stem flow?
Water running down the stem of a plant or the trunk of a tree
What is throughflow?
Water moving slowly downhill through the soil
What is infiltration?
Water soaking into the ground
How do infiltration rates vary?
It is influenced by the soil type and structure and how much water is in the soil. when the soil is saturated, water does not infiltrate quickly and so flows over land
What is percolation?
Water soaking into the watertable. It is deeper than infiltration
What is groundwater flow?
Water that flows slowly below the water table
What is base flow?
Groundwater flowing into rivers via the banks and bed
What is interflow?
Water flowing downhill through permeable rock above the water table
What is a water budget/ balance?
It can predict water shortages and flooding
What is soil moisture store?
The water stored in the soil
What is meant by saturated?
When the ground cannot hold any more water
What is field capacity?
The point where the soil becomes saturated
What is soil moisture utilisation?
When the plants take up the water through their roots
What is potential evapotranspiration (PET)?
The amount of water that could be lost by evaporation/ transpiration assuming an unlimited water supply
What is soil moisture discharge?
When the soil moisture is replenished with water from precipitation
What is river discharge?
The amount of water passing through a gauging station at a point in time. It is measured in cumecs (cubic meters per second)
What does a flood hydrograph do?
Shows how a rivers discharge changes following a period of rainfall
What are the main features on a flood hydrograph?
- Peak rainfall- Hour of greatest rainfall during storm
- Peak discharge- The point where there is maximum discharge due to the storm
- Lag time- The amount of time between peak rainfall and peak discharge
- Rising limb- Discharge of the river is rising
- Falling limb- Discharge is falling after reaching its peak
- Storm runoff- The area above the base flow on the graph
- Base flow- The average discharge of the river
- Storm event- The period of rainfall
What are the 7 factors affecting the discharge of a river?
Rock type, soil type, land use, drainage basin, drainage density, temperature, precipitation
How does the drainage basin affect the discharge of a river?
- Size- If the basin is larger then the lag time is longer as there is further for the water to travel.
- Shape- circular basins have shorter lag times and higher peak flows than elongated basins.
- Relief- Steep sided river basins reduce lag time as the water has more velocity
How does rock type affect discharge?
Permeable rocks allow water to drain through it as it is pourous so it speeds up the rate of infiltration
Impermeable rocks don’t let water pass through and so there is more surface run off so the lag time decreases
How does soil type affect the discharge?
Soils with high rates of infiltration (sand) will mean less surface runoff so a longer lag time
Soils with low rates of infiltration (clay) will mean more surface runoff so a short lag time
How does the drainage density (number of surface streams in a basin) affect discharge?
A high drainage density means there is a greater volume of water in the channel and so the lag time is shorter.
How does temperature affect discharge?
- High temps- More evapotranspiration so less discharge. OR If the ground is baked it means less infiltration so more Surface run off and discharge increases
- Freezing- The fresh precipitation will runoff into the river increasing discharge
How does land use affect discharge?
Vegetation
More plants and trees increase interception which decreases surface runoff
Urbanisation
Impermeable surfaces have an infiltration rate of 0.
Drains transport water efficiently and that decreases lag time.
How does precipitation affect river discharge?
Amount- Long periods of rain saturate the soil so surface run off increases decreacing lag time
Type- Snow melts slowly and rain drains slowly so lag time increases
Intensity- Intense rain means that the ground can’t hold as much water so more surface run off occurs so lag time decreases
What is transportation?
The process of eroded load being carried by a river. Needs a high velocity
What is solution?
Some materials will be dissolved in the water and will be carried in a solution.
What are the three types of load that a river can transport?
Bed load
Dissolved load
Suspended load
What is saltation?
Sand sized particles are bounced along the bed as they are too heavy to be carried by suspension.
What is suspension?
Fine material (sand, clay and silt) is carried in the turbulant flow of the water. This is the most common type of transportation.
What is traction?
This happens with the heaviest material where material is rolled along the river bed. It requires the most energy.
What is hydraulic action?
The movement of sediment by the frictional drag of the moving water. The flow lifts loose sediment as the velocity increases
What is abrasion?
The rubbing of the bed and the banks the by sediment carried by the river. The load can range from fine sediment to boulders.
What is corrosion?
When rocks dissolve in the water. Carbon dioxide dissolves in the water to form a weak acid reacts with carbonate rocks.
What is attrition?
The reduction in the size of particles as they collide with each other. Sediment becomes smaller and rounder.
What is cavitation?
Air bubbles in turbulant water implode causing shockwaves that break peices of rock off the bed and banks.
What are the three types of erosion that occur in a river?
Vertical- Makes the river deeper. Happens in upper course when river has more energy
Lateral- Makes the river wider. Happens in the lower and mid stages
Headward- Makes the river longer. Happens near the source as surface run off and throughflow cause erosion
What is meant by the competance of a river?
The size of the largest material it is capable of transporting
What is meant by the capacitance of a river?
The total volume of material the river is able to transport
What does the hjulström curve do?
It illustrates the relationship between the velocity and competance. It shows at what velocity different types of material will be transported, eroded and deposited
When does a river deposit?
When it is no longer competant or lacks the capacity to carry load. Competance is reduced when the velocity is reduced as there is less energy
When does deposition occur frequently?
- When a river enters shallow water
- Reduction in gradient- energy loss
- When a river floods- Friction with the flood plain
- When a river enters a lake- Merging currents
- Discharge of a river is reduced
- Sudden increase in sediment volume
What is the long profile?
It represents how the height of the river channel changes as it goes from source to mouth
What is the hydraulic radius formula?
cross sectional area/ wetted perimeter
if <1 it is inefficient
if >1 it is efficient
How does the cross sectional profile change from upper to lower course?
The upper course of a river consists of a steep V shaped valley and a thin channel. Here the river has a low velocity due to friction and erosion is vertical due to it being high above base level.
In the middle course there is a mix of both vertical and lateral erosion which results in a wider river valley. A flood plain starts to develop due to the lateral erosion.
In the lower course of the river the dominant erosion type is lateral. This results in a large flood plain and a river channel with shallow sides. The river is fastest and most efficient at this point due to it having a large hydraulic radius and the most kinetic energy.
What effects a rivers velocity/ efficiency?
Channel shape
Channel Gradient
River level
Roughness of Bed and Banks
How are rapids formed?
Riffles are areas of shallower water due to elevation from hard resistant rocks. Pools are created when the less resistant rock is eroded creating an area of deeper water
How do waterfalls form?
It is created due to a sudden change in the gradient of a river. The water enters an area where there is resistant rock above less resistant rock. The less resistant rock is eroded due to it being undercutted and the process of hydraulic action. A plunge pool is created. The overhang left due to the undercutting collapses due to gravity and leaves a waterfall. The waterfall keeps retreating to form a gorge
How are potholes created?
Potholes are formed when abrasion along the river bed produces craters especially in faster flowing rivers where there is more eddying (whirlpools). The potholes may get filled with material that swirl, making it wider and deeper.
How are meanders formed?
The river has high energy in the middle/lower course. When the discharge decreases, the hydraulic radius decreases which results in sediment being deposited as areas of pools and riffles. The fastest flow (thalweg) is diverted to one side where it undercuts to form a concave river cliff. The concave shape diverts the thalweg down which deepens the meander and causes more to form upstream. It causes a helicoidal flow which results in sediment being deposited as a slip off slope.
How do meanders turn into oxbow lakes?
When the meander is created it doesn’t stop bending. Its sinuosity will increase over time until the neck of the meander bends inwards. As the river will take the path of least resistance, it wants to go straight and so it cuts off the meander and creates an oxbow lake
How is a delta formed?
When a river meets a body of water with a lower velocity (sea) the material is deposited as the river loses energy and therefore will lack the competance to carry. If the tidal range is low then sediment will not be washed out to sea so large areas of deposited sediment form.
How is a floodplain formed?
When a river floods its banks it immediatley loses some energy due to friction with the ground. This means it lacks the competance to carry material and so layers of alluvium are formed
How are levees formed?
When a river floods, it bursts its banks and so it loses energy and so can’t carry large sediment as it require the most energy. This results in large mounds being formed by the river bank.
What is braiding?
When a river erodes through sand and gravel, it creates wide and shallow channels which have a small hydraulic radius.
What is rejuvenation?
River rejuvenation is when the base level of a river is lowered. This allows the river to resume vertical erosion due to more potential and kinetic energy. The base level is generally considered to the sea level.
At the source of a river, there is more potential (stored) energy to erode as the river is at its highest above base level. There is also more kinetic energy as the river has a steep gradient and so vertical erosion occurs.
Why does a rivers base level fall?
Eustatic (global) fall in sea level that occurs in colder periods where the water contracts and so there is more ice which results in a fall in sea level.
An isostatic (local) fall in sea level can also result in a fall in base level. This is because the glaciers that were formed during the ice age slowly melt and the land begins to rise as there is less pressure on it. This is known as isostatic rebound.
What are knick points?
They occur when a river attempts to regrade the long profile to a concave shape. Vertical erosion occurs and slowly cuts towards the base level. Many falls in sea level during many years may result in several knickpoints being formed. They are often waterfalls.
What are river terraces?
They are where vertical erosion happens on floodplains. The river cuts downwards and abandons the old floodplain. Over time the river regrades itself and may resume lateral erosion.
What are incised meanders?
When rejuvenation occurs, and the river meanders, there will be vertical erosion so the river will downcut.
Entrentched meanders are symmetrical as they are formed quickly and so there is no time for any lateral erosion
Ingown meanders are formed slowly and so are asymmetrical as the river has time to erode laterally
What are the general causes of flooding?
Prolonged rainfall- The ground becomes saturated and no infiltration occurs
Heavy rainfall- Leads to rapid surface runoff so no infiltration can occur
Melting snow/ice - Results in a large increase in discharge
What are the physical factors that increase the chance of flooding?
- Impermeable surfaces
- Circular drainage basin
- High relief
- Lack of vegetation
- High drainage density
What are the human factors that increase the chance of flooding?
- Deforestation
- Impermeable surfaces
- Overgrazing
- Flood management going wrong
- Climate change
What are the general SOCIAL impacts of flooding?
Death
Contaminated floodwater due to sewage
Lack of clean drinking water
Diseases such as diarrhoea
Possesions are damaged
People made homeless
What are the general ECONOMIC impacts of flooding?
Businesses shut down as their premises are inundated and power supplies affected
Rescue work and repairs can be costly
High insurance premiums
Unemployment as business may be shut for long periods of time
Public transport and many vital roads may be damaged
Crops may be destroyed so the price of food may rise
What are the general ENVIRONMENTAL impacts of flooding?
Flood water contaminated with sewage which may pollute rivers
The erosion of river banks
River sediment is deposited on flood plains which makes the land more fertile
Wetlands may be created e.g- marshes which provide habitats for wildlife species
What is flood frequency?
Records of floods are kept in order to see when a flood of the same magnitude will occur again (flood return interval). RI
Time = n+1/m
N years of observation
M is the rank order
What is the Catchment Flood Management Plan?
Written by the environment agency and recommed ways of magaging risks now and over the next 50-100 years
They look at a range of structual or river basin methods
What are the River basin management techniques?
- Flood abatement- Managing the land use by aforestation, farming, contour ploughing and reducing the amount of earth to limit runoff
- Flood proofing- Can include sandbags, keeping high value areas above low value facilities
- Flood plain zoning- Records historic floods and based on that, sections of the flood plain are used for building houses, car parks and farmland based on the RI of the flood and the value of the activity
- Flood prediction/ warning- Having measures in place that will speed up evacuation. Text messages are sent to local residents
What are the structual methods of river management?
- Flood walls, embankments and levees- Designed to raise the height of the river channel
- Channel improvement- Create a smoother, wider, deeper river channel for a faster flow
- Relief channel- Give the river an alternative route by creating an overspill channel for when the discharge is too high
- Flood stroage reservoirs- Store the excess water in the upper areas of a catchment
- Flood interception schemes- Re routing a river to avoid higher value areas
What are the pros and cons of Flood Abatement?
PROS
Increases interception storage and evapotranspiration so it reduces surface runoff and holds the soil together
CONS
Trees take along time to mature and the soil distrubance leads to more run off in the start
What are the pros and cons of Flood Proofing?
PROS
By keeping higher value facilities above lower value facilities only these lower value areas will be flooded. Reducing the potential damage
CONS
Sand bags are only temporary and can only be used on a small scale.
It does not stop the flooding completley and only reduces the damage
What are the pros and cons of Flood Plain Zoning?
PROS
It restricts development of high risk areas so by reducing the impact of flooding
CONS
It reducues the area of land available to build houses which increases the total amount of land shortage
What are the pros and cons of Flood Prediction and Warning?
PROS
People are warned about floods before they happen which speeds up evacuation and preparation
CONS
Not accurate enough to predict flooding 100% of the time
Not accessible to everyone
What are the pros and cons of Levees, Flood Walls and Embankments?
PROS
They reduce flooding on valuable land
They allow the floodplain to be built on
CONS
They are expensive and create an eyesore
They can cause a severe flood if they are breached
What are the pros and cons of Channel Improvement?
PROS
Makes the river more efficient
Decrease the flood risk
CONS
The risk downstream is greatly increased due to the increased velocity and discharge
Require alot of maintanace
What are the pros and cons of Relief Channels?
PROS
As the discharge is reduced, it prevents flooding
The release of water from the relief channel can be controlled
CONS
The discharge will eventually increase when the water rejoins the river
If the relief channel is overwhelmed, it may cause flooding in areas that don’t normally flood
What are the pros and cons of Flood Storage Reservoirs?
PROS
Easy to construct in areas of high land with a low value
Enhances the water supply of the UK
CONS
Expensive
Land is flooded when the reservoir is created which forces some people out
Affects the wildlife eg- prevents salmon from migrating upstream to breed
Requires a large area of land
What are the pros and cons of Flood Interception Schemes?
PROS
Provides storage for flood water so discharge is reduced
Shortens the river
Avoids/ by passes high value areas
CONS
Expensive
Eyesore
What are the general advantages of soft engineering?
Cheaper to maintain
Flooding is made more predictable
Can improve opportunities for recreation
More visually attractive
What are the general disadvantages of hard engineering?
Expensive to build
Floods will happen less often but they will be of a larger magnitude
Natural processes are disrupted eg- crops don’t get fertile silt from the river
