Flooding appears to be a growing issue with flood events predicted to increase in number and severity in the future. In the UK, despite spending around £800 million on defences, we still see approximately £1,400 million of flood damage each year, on top of the associated emotional and social costs. In Cornwall, flood damage can come from river, surface water or tidal influences. In the past, some rivers in Cornwall were straightened or ‘canalised’ to enable transportation in the mining era, but this means that water is flowing downstream much faster than it would have done if rivers contained meanders and open floodplains. While there is not really anything we can do to reduce rainfall or tidal surges, we can have an influence on surface water. Heavy rainfall may increase surface water, but if we are able to slow or store water in key areas, we can spread the release of water downstream over a longer period of time, meaning that river levels can be kept relatively stable.
Different habitats can have the effect of speeding up or slowing down the flow of water overland. It is well-known that impermeable surfaces, such as those found in urban environments, increase surface runoff. Likewise, any exposed, relatively smooth habitats (such as disturbed ground or arable fields) will have a higher runoff. Improved grasslands can often contain a single species of grass which is fast-growing but has shallow roots and does cover the ground as effectively as a diverse wildflower meadow would, so is less able to absorb or slow runoff. Semi-natural habitats on the other hand are ideal, particularly those that have a well-developed ground flora or are able to store large amounts of water, such as bog or fen habitats. Trees not only interrupt rainfall, which lands on the leaves before evaporating back into the atmosphere, but also absorb large amounts of water in the soil.
Farms can reduce the amount of surface runoff through a variety of measures. Arable fields can be over-sown with a cover crop to interrupt rainfall hitting the ground, and soil can be ploughed across the slope rather than down the slope to reduce the speed at which water flows downhill. Livestock farmers can introduce deep-rooting plant species into their pasture and leave buffer strips of long grass or trees to slow water before it reaches the watercourse.
When assessing flood alleviation we considered two aspects:
- The ability of a site to interrupt, slow, absorb or store water or otherwise reduce surface runoff
- The benefit of slowing or reducing water flowing into the watercourse (i.e. if there are any Flood Alert Areas downstream containing homes and/or businesses)
The highest scoring sites will be those that are effective at interrupting surface runoff and are directly upstream of a large conurbation within a Flood Alert Area. Low-scoring sites may not necessarily be poor at preventing surface runoff, but may simply have no or few homes or businesses vulnerable to flooding downstream (this particularly applies to sites which are near the coast).