Rivers

Hydraulic Action- the force of water hitting the river bed and banks. Most effective when water is moving fast and high volumes

Abrasion- The load carried by the river hits the banks dislodging particles

Attrition- Stones carried by the river knock against each other becoming smaller/more rounded

Solution- Alkaline rocks e.g limestone are dissolved by slighty acidic river water

Solution- dissoled load

Suspension- small sediment held in the river

Traction- large particles rolled on river bed

Saltation- bouncing particles too heavy to be suspended

Precipitation- torrential rainfall and/or prolonged periods of rain can lead to flooding

Geology- impermeable rocks don't allow water to pass through so it flows over land and causes floods

Relief- steep slopes  mean water flows quickly into river channels

Urbanisation- impermeable surfaces e.g tarmac roads means water flows quickly into drains, sweres and river channels.

Deforestation- when trees are removed, much of the water which had been evaporated from leaves or stored on leaves or branches flows rapidly into river channels

Agriculture- exposed soil can lead to increased surface runoff

• basin size
• drainage density
• rock type and permeability
• land use
• relief
• soil moisture
• rainfall intensity

River straightening and dredging

A dredger working to deepen the channel of the River Maas, Port of Rotterdam, NetherlandsA river dredger in the Netherlands

Straightening the river speeds up the water so high volumes of water can pass through an area quickly. Dredging makes the river deeper so it can hold more water.

• More water can be held in the channel.
• It can be used to reduce flood risk in built-up areas.
• Dredging needs to be done frequently.
• Speeding up the river increases flood risk downstream.

Embankments

Raising the banks of a river means that it can hold more water.

• Cheap with a one-off cost
• Allows for flood water to be contained within the river.
• Looks unnatural.
• Water speeds up and can increase flood risk downstream

Flood relief channels

The floodwater flows into the relief channel and is taken either to an area where it can be absorbed, or re-enters the river further down its course.

• Removes excess water from the river channel to reduce flooding.
• Expensive to build.
• If water levels continue to rise, the relief channel may also flood.

Soft engineering does not involve building artificial structures, but takes a more sustainable and natural approach to managing the potential for river flooding. Each approach has its advantages and disadvantages.

Flood warnings and preparation

The environmental agency monitors rivers and issues warnings via newspapers, TV, radio and the internet when they are likely to flood so people can prepare.

• People have time to protect their properties, eg with sandbags.
• Many possessions can be saved, resulting in fewer insurance claims.
• Some people may not be able to access the warnings.
• Flash floods may happen too quickly for a warning to be effective.
• They do not stop land from flooding - they just warn people that a flood is likely.

Floodplain zoning

Allowing only certain land uses on the floodplain reduces the risk of flooding to houses and important buildings.

• More expensive buildings and land uses are further away from the river, so have a reduced flood risk.
• Less damage is caused, leading to fewer insurance claims.
• Not always possible to change existing land uses.
• Planners have to decide what type of flood to plan for.

Aforestation to increase interception when it rains. It reduces soil erosion and uses up some of the water

WHERE IS BANBURY?

Banbury is located in the Cotswold Hills about 50km north of Oxford. The town has a population of 45,000 people. Much of the town is on the floodplain of the River Cherwell which is a tributary of the River Thames.

How has Banbury been affected by flooding?

Banbury has a history of devastating floods. In 1998 flooding led to the closure of the town's railway station, shut local roads and caused £12.5 million of damage. More than 150 homes and businesses were affected. In 2007 the town was hit again by floods that extended over much of central and Western England. Many more homes were affected as the river burst its banks after very heavy rain.

WHAT HAS BEEN DONE TO REDUCE FLOODING

In 2012 Banbury's new new flood defence was completed. A 2.9KM earth embankment was built parallel to the M40 to create a new flood storage area. The embankment is 4m high and can hold 3 million cubic metres of water

The flood storage area is located mainly on the natural floodplain of the River Cherwell. It collects rainwater that otherwise would have swelled and caused the river to burst its banks.

The specially designed opening controls the rate of flow downstream towards Banbury. Any excess water backs up to the structure filling up the resevoir.

Raising the A361 road in the flood storage area and improvements to drainage beneath the road means flood storage is improved.

New Earth embankments and floodwalls protect property and business

A new pumping station transfers excess water into the river below the town

The creation of a new Biodiversity Action Plan habitat with ponds, trees and hedgerows to absorb and store excess water