How hard and soft engineering can be used in flood management
Hard engineering describes the installation of measures to limit or stop natural processes. While hard engineering can be very effective to deal with a specific problem, interrupting the natural processes of the environment can also have negative impacts on an area.
Hard engineering techniques may be used to stop or disrupt natural coastal erosion and high sea levels from advancing inland. Sea walls, revetments, groynes and breakwaters can absorb and dissipate wave energy and stop the transfer of beach material along the shoreline. They prevent high tides and storm surges from inundating our coastal towns and vital infrastructure. There can also be negative impacts too, including greater erosion adjacent to the intervention as wave energy is redirected and the loss of beach material as ‘long shore drift’ is interrupted.
Flooding from watercourses occurs when channel capacity is exceeded or flow is disrupted by in-stream structures or blockages. Hard engineering can involve raising the height of river banks by installing flood walls, bunds and levees which attempt to fix the river in place, stopping the natural process of the river overtopping and flowing onto the floodplain. However, schemes to keep water in channel can be expensive and may also speed up flow, leading to a greater risk of flooding and erosion further downstream.
Historically, hard engineering was seen as damaging to wildlife and natural habitats, but schemes now have to go through a rigorous appraisal process to select the most economically viable, publicly agreeable and environmentally acceptable solution and credit is given for creating wildlife habitat, public amenity and opportunities for biodiversity.
Soft engineered interventions can reduce the effects of coastal and river flooding through the use of more natural methods of flood management. Which rather than stopping events aim to slow water in a more natural way.
On the coast, beach replenishment is the reprofiling of the beach to dissipate wave energy using material which may have been dredged out at sea or trucked in. However, this method is expensive and requires regular maintenance. Restoring sand dunes which may have been eroded by planting vegetation such as Maram Grass to trap sand and stabilise the dune, creating a natural defence to absorb the wave energy of storm surges and high tides. Where the cost of maintaining traditional flood defences or holding the line may be high, or land is no longer considered of value, managed realignment (the removal of coastal protection to allow an area to be inundated by the sea) can help to restore intertidal habitat which can naturally reduce the impacts of wave energy.
Instead of trying to keep water in channel over a whole river catchment, natural flood management (NFM) can be used to slow the flow of water entering river systems, before it reaches homes businesses and infrastructure where it would cause flooding. Tree planting, installing leaky dams, reintroducing meanders to the channel can all contribute to slowing down the flow of water.
Offline storage or restoring the rivers natural flood plain allows the river to flow out onto land adjacent to the river, and can store large amounts of water which might otherwise cause flooding. Using natural materials such as willow instead of concrete to protect river banks can slow the effects of erosion and encourage naturally vegetated buffer strips to form. Currently there are pilot schemes looking at the impacts that beaver dams can have on watercourses, as they can offer natural online storage. Soft engineered schemes such as these may be beneficial in reducing peak flows in areas where more expensive hard engineered schemes are not viable.
The effect that using softer more natural processes has on large scale flood events is up for debate. It is not always possible to implement these schemes in areas which are densely populated, as they can require large areas of land and take time to be effective.
Traditional, hard engineered schemes may be more appropriate to hold back flood water and protect homes, businesses and infrastructure, but the implementation of soft engineering further up the catchment may decrease the scale of hard engineered flood alleviation schemes where they are needed.
Rivers are best managed with a combination of both hard and soft engineering which can work together to reduce flood risk.