Fending off the floods: how we can keep the water away
Flooding was once again in the headlines over Christmas, with many parts of the UK suffering from the aftermath of extreme wet weather.
8th February 2016 | Shaun Pentlow: Associate Director, Rolton Group
Successive storms in December resulted in record-breaking rainfall totals over the Lake District and double the average rainfall across Scotland, Wales and the north of England. The UK record for the highest volume of rain over a 24-hour period was broken, with Honister Pass in Cumbria receiving 341.4mm (13.4in); Thirlmere also set a new record for two consecutive rain days, receiving 405mm (16in).
It is impossible to count how many times in the winter months we have heard that it is the “wettest”, or “coldest” – or even “warmest” – “since records began”. This now seems to be an annual occurrence, accompanied with the resulting pictures of misery on our televisions.
Incidences of river flooding are growing worse in both frequency and scale. This is caused by changes in river hydrology due to human activity, changes in land management, variations in the intensity of rainfall and increased development in areas at risk. Climate change is expected to increase the risk of coastal and river flooding as a result of sea-level rise and more intense rainfall. Lord Stern’s 2006 publication The Stern Review: The Economics of Climate Change warned that as the global temperature increases from pre-industrial levels, there will be (among other effects) a rising intensity of storms and flooding – a prediction which is proving to be correct. On both a national and global scale, damage from flooding is greater than that from any other natural disaster.
So what can be done to keep these images off our screens in future? Flood prevention systems are already in place across many parts of the UK, including areas affected in December, so the extent of the flooding suggests that the guidelines they previously adhered to are no longer adequate. Carlisle and Cockermouth, in particular, had systems already in use, but they were surpassed following previous flooding to those areas only a few days before.
The UK needs long-term measures in place to deal with run-off: measures which are fully integrated throughout the country’s infrastructure, and are designed to be able to cope with more and more intense bouts of rainfall in the future. Rather than measures to purely keep flood water out, we need solutions which will stop these floods from happening in the first place. By dealing with issues such as the source of flow from rivers and brooks heading downstream towards urban areas, and the inadequate drainage in these areas, the threat of flooding will be limited, and the risk of property damage minimised.
Sustainable Drainage Systems (SuDS) are an integral part of the solution to improving the reduction of storm flows, particularly around urban areas, and they are currently being widely introduced on all new developments. Features such as such as green roofs, permeable paving, swales, filter strips/drains, bioretention systems, wetlands, balancing ponds and storage tanks will become even more prevalent, with further emphasis made on the quality of run-off water as well as reducing the quantity of run-off.
The Local Planning Authority is required to ‘ensure that sustainable drainage systems for the management of run-off are put in place, unless demonstrated to be inappropriate’ for major developments – which count from as few as 10 dwellings or more. This lower limit has made developers consider SuDS even for smaller sites, which is resulting in further recommendations for permeable paving where it was unlikely to have previously been used. From 6th April 2015, the Lead Local Flood Authority (LLFA) became a statutory consultee on all major developments (in relation to surface water drainage), which is forcing developers to consider a site’s drainage strategy and its potential effect on developable areas at an early stage of planning. It is encouraging to see the necessary policies being put in place to ensure that SuDS are being fully integrated in the design of development schemes, and not being added as an afterthought, which is how they have been managed historically.
It is unlikely that the widespread implementation of SuDS alone would have completely prevented the property flooding encountered over Christmas, due to the unprecedented level of rainfall encountered. It is also important to note that the implementation of SuDS is not going to occur overnight. Nevertheless, widespread implementation of SuDS would have heavily reduced the property flooding in December. They are a highly effective way of displacing rainwater from built-up areas, and as they are gradually installed over time, urban areas will become increasingly better at coping with increasingly heavy rainfall.
What can be implemented alongside SuDS is source control. Although flooding occurs over low-lying valleys, the floodwater to these catchment areas emanates largely from higher up in the river system, from the smaller tributaries and brooks. With this in mind, we need to create natural flood management techniques in the upstream catchments of river systems, to limit the flow from rivers more naturally and subsequently limit downstream flooding. Features such as natural dams help to slow down the flow and force it to spread over a greater area, while overland flow barriers disconnect flow pathways and temporarily store floodwater; these consequently reduce the amount ending up in towns and cities with limited drainage capabilities. This has proven to be particularly successful in the Stroud valley, where natural dams have been used to spread water over a larger area and limit flow towards the town of Stroud, which suffered during the flooding of 2007.
It is for the Environmental Agency, local authorities and other stakeholders to ensure that these measures are enforced; for maximum effect, natural flood management techniques must be implemented on and around as many tributaries as possible. This will result in far more manageable water levels in downstream urban areas, particularly where SuDS have been put in place alongside the existing drainage systems.
The UK’s problem with flooding is not going to be an easy one to fix. Effective measures are needed now in order to avoid a repeat of the extensive floods we saw over Christmas. Yet these measures must also provide long-term flood prevention, as the threat of intense rainfall is not going to disappear. Widely implemented SuDS, together with natural flood management techniques, have the potential to seriously reduce the risk of flooding, make urban areas better able to cope with intense rainfall and therefore prevent the water’s damage from being done.