How it works
Example - Mississippi
Embankments on either side of the channels to increase capacity.
Therefore the river can hold more water without overflowing so it floods less often.
They allow a flood plain to be built on.
Dretching, raises the height of riverbanks can be done quite easily and cheaply – which is sustainable.
Alternative measures involving built structures are more-effect forms of defence
Alternative measures involving built structures are more expensive.
Potentially hazardous if the water level is above the floodplain.
Unless set back from the channel, levees raise water levels and cause a severe flood risk.
Example – Mississippi, redued length by 240km
Removing meanders steepens the average gradient. Increasing flow velocity.
Water drains downstream quickly and doesn’t build up to a point where the channel overflows.
It takes less time to navigate the river because it has been made shorter.
Channelisation improves the rate of flow and also benfefits transportation.
The problem is pushed downstream –flooding will happen in other areas where the river flows faster.
More erosion occurs downstream because of increased velocity.
Damages wetland and wildlife habitats.
Example Sinapore river
Diversion spillways divert water if the base level is too high.
Water is normally diverted around an important area
During high-flow conditions sluice gates are opened to allow the excess water to flow along the new channel.
Spillways can have vegatation built on them, which has a positive impact on the environment – creating new wetlands.
Popular for recreational activities, like walking or fishing.
An increase in discharge when the diverted water joins another river (or rejoins with the same one) could cause flooding further downstream of that point.
If spillways are over-whelmed, water will flood areas that are not used to flooding, which could cause even bigger impacts/problems.
Example – Three Gorges Dam in China
Storage of floodwaters in reservoirs or flood storage basins.
A reservoir (artificial lake) is formed behind the dam, which prevents flooding downstream.
Water is released slowly during low-flow conditions. Allowing a steady flow of water to be maintained throughout the year.
Dams are multi-purpose, which means they can perform several functions (e.g. water supply, hydroelectric power, recreational use).
It is the only hard engineering technique that acts as a basin management - natural processes across the basin are not affected.
They’re very expensive.
Usable land is flooded (farmland) often resulting in people being forcibly moved)
They affect wildlife (e.g. they can prevent salmon migrating upstream for breeding.)
Sediment that is normally transported will deposited and trapped – causing the damn to fail.
How it works
Wetland and River Bank Conservation – Thames Barrier in London
Wetlands store flood water and slow it. Reduces flooding downstream.
Conserving or re-establishing wetlands gives natural protection by preventing their erosion and collaspe.
Planting vegetation along bank increases interception and lag and – reducing discharge
There are no new buildings or roads on the flood plain to be damaged, so impact of any flooding is reduced.
It provides recreational opportunities (e.g. football fields)
Less land is available for flooding – this is especially bad for countries with a low food security or have periodic famine.
Weather Forecasts and Flood Warnings – Glasgow, Scotland
Forecasting a possible flood event and warning people is a behavioural responce to flood risk.
There are three levels of warning: flood alert, flood warning and severe flood warning.
The system encourages people and organisations at risk to take action to mitigate the worst effects.