Flood storage areas (FSAs) are natural or man-made areas that temporarily fill with water when river levels rise, retaining a volume of water which is released back in to the watercourse when flood risk has passed. The vast majority of FSAs are located either on or adjacent to rivers to provide flood protection to downstream communities.
There are well over 1000 FSAs in the UK and about one fifth of these are built on deciduous woodland. The area targeted for a FSA in Dinas Powys contains ancient and new deciduous habitat. The frequent flooding and draining of such areas has numerous potential impacts on broadleaved trees, including increased infection by soil-borne pathogens, reduced root depth, increased likelihood of wind throw (tree breakage) and loss of specialist species associated with veteran tree habitat (primarily fungi, invertebrates and lichens). According to an Environment Agency report in 2009, only 5.3% of FSAs support high quality habitat – the vast majority only support low value biodiversity. As a response to devastating floods in 2007 in the UK, the Pitt Review (2007) recommended that the
Environment Agency should work with partners to establish a programme of ‘working with natural processes’ (WWNP) to manage flood risk. The WWNP concept is becoming increasingly accepted in flood risk management, but what exactly does it mean? Briefly, it means taking a smarter approach to flood risk, such as enhancing the water storage capacity of natural wetlands and washlands, river widening and river restoration. In other words, it is about restoring and emulating the natural regulating function of rivers, and floodplains. It is worth mentioning here that not all FSAs are bad. FSAs built on natural washlands formalise the natural process of flooding and ensure that if a river floods it will occur in a predetermined location, away from major areas of human habitation.
Let’s take a brief look at the various types of FSA and their standard of flood protection. Some FSAs have a huge shallow footprint, while others are relatively small in area but have a significant depth of water. FSAs must be carefully designed to remove peak flood flows – if they fill before the peak flow arrives, they are completely ineffective. Impounding FSAs, also known as on-line FSAs, are constructed across rivers and restrict flow by means of a dam and a culvert (pipe). In non-impounding, or ‘off-line’, water is diverted from the river, stored temporarily, and then returned once the peak flow subsides. To be clear, most if not all FSAs are ‘hard-engineered’, which means they require the construction of physical structures such as weirs, culverts, channels and control gates. These can be detrimental to the aesthetics and biodiversity of the immediate environment. Surprisingly, Natural Resources Wales (NRW – the organisation responsible for flood management in Wales) has not yet decided on the type of FSA being considered for Dinas Powys. Furthermore, there is little available information on
the standard of protection it will provide or on its frequency of operation. There is no doubt that an area of precious natural beauty is at risk of being destroyed and we eagerly await the outcome of NRWs deliberations.