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  • Subject area(s): Engineering
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  • Published on: 7th September 2019
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Flooding, as defined by the US Geological Survey is “An overflow or inundation that comes from a river or other body of water and causes or threatens damage” (USGS, 2015), and can be caused by physical factors such as storm rainfall events and ground saturation. Anthropogenic impacts can also have a large impact on flooding such as urbanisation -which increases surface run off, or deforestation – which removes a source of interception. Flood events can cause catastrophic and often fatal damage to communities and the environment across the world in both MEDCs and LEDCs, for example by destroying homes and agricultural land that many rely upon. It is for these reasons (and many others) that efforts must be made to protect vulnerable areas from flood events.

In many MEDCs, rivers and their drainage basins are engineered to lessen the impacts, and can be divided into 2 categories. Hard engineering is a set of flood management techniques that use structural means to control water – such as concrete levees along the banks. This essay however will focus on soft engineering strategies, techniques of reducing flooding that use natural sources and processes to hold back flood waters.

One major source of flooding across the world is coastal flooding, flood events that takes place in areas along the shoreline which sit on or below sea level. This type of flooding can be caused by a range of factors such as storm surge events, low pressure systems which can cause high winds and large waves to make landfall and inundate the coastal environment (NOAA, 2016).

It has been shown that depositional environments such as beaches, salt marshes and mudflats are the most efficient ways of reducing the erosional power of waves, and so are often utilised to provide protection against coastal flooding. Beach feeding is the process that uses this principle, and uses extra sediment such as sand and shingle are added to the shoreline to absorb this wave energy, and so reduce incoming wave power to prevent inundation of the land. This by far is the most popular method of the last decade (Han, 2011), and is being used around the world to protect vulnerable areas.

Alongside this, previously eroded sand dunes can also be restored, particularly for low lying areas set behind the beach. This can be done in a variety of ways, including using large sand bags, and permeable barriers with marram grasses to encourage sand dune development (Smith et al., 1998). This method however is less effective than beach nourishment, as restoration requires precise abiotic conditions such as temperature and wind direction, which can often unreliable – for example in 1969, a trial was run in N. Ireland testing different methods of sand dune restoration. Many of these failed because of abiotic factors such as sediment supply and wind strength (Wilcock et al., 1977).

Dawlish Warren, Devon

Dawlish Warren is a tourist settlement found in South Devon, and is built upon a sand spit in the Exe Estuary. This natural feature is crucial in protecting the estuary to the city of Exeter in the north, reducing incoming wave height by around 3m. After the 2013-14 winter flood events, this area was close to breach point at the Neck, and along with sea level rises threatened settlements such as Starcross with estuarine flooding (Teinbridge District Council, 2014). Decisions were taken to increase the proportion of more natural defences, and included ‘recycling’ sand through dredging and ‘feeding’ the beach, as well as sand dune restoration. This area has increased 2m in height and now sits at 40m wide (high tide) (Environment Agency, 2015).

Managed realignment is another somewhat controversial way of protecting the coastline, and works to protect high value spaces. Areas along the coastline are left unprotected from the sea, and existing defences (often structural such as sea walls) are realigned to protect higher value land (Han, 2011). This method is increasingly being used by planners, possibly due to financial constraints. However, moral questions are then raised about the value of coastal communities and how to select those to be saved.

Away from the coastline, rivers are also extremely prone to flooding and have a range of defences that can be used. Most major settlements tend to lie in the middle or lower courses of rivers because of historic trade and navigation, and are usually accompanied by hard engineering strategies such as diversion channels or sluice gates to protect these. Therefore, many soft engineering on a river’s course are focused within rural upper course areas.

Afforestation is one technique used in the upper course of a river system, and aims to slow the movement of water into a river system by interception. This involves planting trees and other vegetation within the catchment area, which will slow the run off rate into the system. One study completed between 2007-2011 (Nadal-Romero et al., 2016) concluded that afforestation significantly reduces the water yields, and that the study area (Araguás, Spanish Pyranees) was significantly less likely to flood when compared to non- vegetated areas. Benefits to this type of scheme are aplenty, acting as carbon sinks to climate change, promoting local biodiversity and being more economical in the long run – aswell as preventing flooding further downstream. However, schemes such as this one are often not entirely practical, particularly in dense urbanised areas where space is limited.

In many areas, planners and decision makers are now implementing a zonation system, whereby high value properties such as office buildings and schools are set back from away from the river corridor – ‘the area of land required for the river to achieve a natural meandering course with associated riparian habits’ (Gardiner et al., 1992). This helps to prevent inundation by floodwaters, which could prove very economically and socially damaging. Instead, lower value assets such as playing fields and playgrounds line the banks of the river (Wharton, 2000). In a similar fashion to other soft engineering strategies, land use zonation is often not appropriate for historic urbanised areas, which are based on or very near to the rivers themselves, but instead can be used in the development of newer settlements across the world to reduce the need for harder strategies in the future.

Aswell as conventional engineering, there are other more unique strategies that can be implemented, including one based along the River Otter in Devon which is using ecological techniques to manage flooding.

River Otter, Devon

The River Otter in Devon travels from its source in the Blackdown Hills in Somerset, 32km southwards to its mouth in Lyme Bay, Devon. After beaver sightings along the river (some stretching back until 2007), an official 5 year trial between the Devon Wildlife Trust and the University of Exeter was set up to monitor the beaver’s impacts on the local rivers – focussing on water levels and quality, pollution filtration, aswell as the local community and businesses (DWT, 2016).

Since this project began in 2015, 13 barriers have been spotted along 150 metres of small tributaries of the river, where beavers have dammed the area to create safe spaces to live. This has evened out water flow, and is now “a small stream that, before, would only have held a few hundred litres. Now it can hold 65,000,” (Professor Richard Brazier, Hydrologist at the University of Exeter). This extra capacity of the river means that more water can be held in the system instead of flooding onto the flooplain and the surrounding area, and damming also creates a ‘cascade’ effect, whereby water is held back in stages instead of travelling straight down the channel, reducing the convergence effect seen at tributaries (The Guardian, 2016).  This scheme here has many other benefits too, increasing the biodiversity of the area, and is much cheaper than other soft techniques such as afforestation. Unexpectedly, this scheme as also had many economic benefits too, becoming a tourist site in their own right, bringing money into the local economy.

However, according to Paul Cottington (2016), a member of the National Farmers Union, in the long term this scheme could become detrimental to many anthropogenic features of the environment such as flooding of agricultural cropland and damage to infrastructure.

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