rivers 2 Flashcards
What is a hydrograph?
A graph showing how a river responds to a storm, showing the rainfall and discharge over time
What is a ‘flashy’ hydrograph?
A hydrograph with a short lag time and a high peak discharge
What is a ‘flat’ hydrograph?
A hydrograph with a long lag time and a low peak discharge
Which factors influence the shape of a flood hydrograph?
Basin size Drainage density Rock type Land use Relief Rainfall intensity
What is ‘lag time’?
The time between peak rainfall and peak discharge on a hydrograph
What is antecedent rainfall?
The amount of rain that has already fallen prior to a specific event
Define ‘interception’
Leaves and tree branches catch rainfall and prevent it from running into the river
Define ‘infiltration’
Rain that soaks into the top layers of the soil and rock below
What is impermeable rock?
Rock which water is not able to flow through
What is permeable rock?
Rock which water is able to flow through
Name a range of hard engineering stategies used to manage river flooding?
Reservoirs and dams
Channel straightening
Embankments
Flood relief channels
Name a range of soft engineering strategies used to manage river flooding?
Afforestation Land use zoning River restoration Wetland restoration Flood warnings
How do dams regulate water flow?
Durings periods of high rainfall, water is stored in the dam.
Water is released when rainfall is low
Why is building dams controversial?
They are very expensive
Land is flooded to create the reservoir
People are relocated to make way for the dam
What are the benefits of dams?
They are multi purpose - so they can be used to generate HEP as well as manage river flow.
They are affective holding back water in times of potential flood
They are long lasting
Most dam schemes are multi-purpose. What are their main functions?
Flood prevention Irrigation Water supply Hydro-electric power generation Recreation e.g. sailing
What is channel straightening?
Cutting through meanders to create a straight channel to speed up water flow
What are the advantages of channel straightening?
It moves water out of an area quickly as water passes through a straight channel quicker than a meandering channel.
Why is channel straightening problematic?
It may increase the risk of flooding further downstream
What are river embankments?
Raised riverbanks which allow the river channel to hold more water and prevent flooding
What are the advantages of river embankments?
Embankments increase the volume of water that the river can hold, reducing the risk of flooding.
What are the problems with embankments?
Not always effective. Water can rise above the embankments, causing flooding.
What are flood relief channels?
A man-made river channel constructed to by-pass an urban area
What are the benefits of flood relief channels?
River discharge can be controlled by using gates to control water levels.
What are the disadvantages of flood relief channels?
How does floodplain zoning work?
How does floodplain zoning work?
It restricts different land uses to certain locations on the floodplain.
Expensive/important land uses are placed further away from the river e.g. housing and industry
Low-value uses are placed close to the river e.g. pasture and parkland.
What are the advantages of flood plain zoning?
Consideration into the landuse around the river is taken which means concrete surfaces are not used, reducing impermeable surfaces close to river. Less damage occurs as important facilities are located futher away.
What are the disadvantages of flood plain zoning?
It limits the uses of land closes to a river
It is almost impossible to use flood plain zoning on land that has already been built on
What is river restoration?
Making the river more natural. For example:
When rivers have previously been artificially starightened they are returned to their original course.
Removing levees so that the river floods naturally
The purpose of this is to prevent flooding further downstream.
What are the benefits of river restoration?
Less risk of flooding downstream
Limited maintenance needed
Creates habitats for birds
What are the problems with river restoration?
Flooding can increase
Major floods are more likely unless planning takes place.
How does afforestation help manage river flooding?
Planting trees to intercept and absorb water reduces surface run off.
Who issues flood warnings in England and Wales?
The Environment Agency
What are the 3 levels of flood warnings?
- Flood Watch
- Flood Warning
- Severe Flood Warning
What would people be encouraged to do by the Environment Agency to prepare for a flood?
- Plan what to do eg. move possessions upstairs
- Use flood gates to prevent water entering their property
- Use sandbags to keep floodwater away from buildings
give flood relief channel example
Jubilee River – River Thames
Constructed to reduce flood risk in a number of towns, this stretch of the river is highly developed with extensive housing developments and infrastructure
Completed in 2002, cost £110 million
New river is 12km in length and up to 50m wide. Maintained in low flow conditions, takes excess water from the River Thames
what were the social issues of the flood relief channel in jubilee river
Protection of wealthy properties in Maidenhead and Eton at the expense of less affluent areas further downstream at Wraysbury.
Paddle boaters promised a fully navigable river are disrupted by weirs
what were the economic issues of flood relief channels in jubilee river
it was the most expensive flood relief scheme in the UK.
Floods in 2003 damaged weirs – the cost of maintenance is quite high.
Additional local relief measures are required downstream; the scheme has not so much solved the problem but shifted it elsewhere – serious flooding in 2014.
environmental issues of flood relief channels in jubilee river
Extensive flooding in 2014 below the confluence of the River Thames and the flood relief channel damaged habitats
Concrete weirs are unattractive.
