It’s nearly a year since severe flooding devastated parts of Cumbria first in December 2015 and then again in January (2016). Perhaps because as autumn deepens and winter sets in we are again reminded of flood risks, the environment agency is once again coming under criticism for its flood prevention strategy. There have been proposals to introduce a dedicated flood defence agency to take over this responsibility from the environment agency in an effort to develop a more focused approach. Whether this happens or not, it is just a relief to finally hear discussion about a wider-scale approach to flood management that considers the entire water basin. A river’s water basin includes all the streams and tributaries that flow into it, and indeed the entire area around them where a raindrop falling on the ground will find its way eventually into that particular river system. It can therefore be a very large area and the knock-on effects of upstream changes (such as deforestation) on the downstream volume and velocity of flood water can be very significant.
For too long flood defences have focused on speeding up river flow away from urban areas by using methods such as channelisation (straightening and smoothing the river channel). However, this method can and has backfired several times as increasing the speed of water through a river system reduces the time available for infiltration (rain seeping into the soil and ground (porous and/or permeable rock) or in other words, reduces the time available for the ground to absorb some of the rain water like a giant sponge. Without this effect, the quantity of water accumulating downstream can actually be much greater. In addition, simple solutions upstream such as increasing forestation/ ground cover can reduce the speed of rainwater entering a system already under stress or at capacity by literally intercepting that rain and slowing it down (both of these methods can also be described as increasing the lag time). For a river downstream that is already about to burst its banks, this extra delay in the arrival of further rainfall from upstream can be critical.
These ideas may still be under discussion but a holistic approach thinking about the water basin as a whole is a very promising start. You can read the article here: flood management strategies.