THE ROLE OF MONITORING IN NATURAL FLOOD MANAGEMENT


Flooding is a natural process, but it endangers lives and causes heavy economic loss. Furthermore, fl ood risk is expected to increase with climate change and increased urbanisation, so a heavy responsibility lies with those that allocate funding and formulate fl ood management strategy. In the following article, Nigel Grimsley from OTT Hydromet explains how the success of such plans (both the design and implementation) depend on the accuracy and reliability of the monitoring data upon which they depend.


Climate projections for the UK suggest that rainfall will increase in winter and decrease in summer, and that individual rainfall events may increase in intensity, especially in winter. This paradigm predicates an increased risk of fl ooding.


Emphasising the urgent need for action on fl ood risk, Environment Agency chairwoman Emma Howard Boyd, has said that on current trends, global temperature could rise between 2 Deg C and 4 Deg C by 2100 and some communities may even need to move because of the risk of fl oods. Launching a consultation on the agency’s fl ood strategy, she said: “We can’t win a war against water by building away climate change with infi nitely high fl ood defences.” In response, Mike Childs, head of science at Friends of the Earth, said: “Smarter adaptation and resilience building – including natural fl ood management measures like tree- planting – is undeniably important but the focus must fi rst and foremost be on slashing emissions so that we can avoid the worst consequences of climate chaos in the fi rst place.”


Historically, fl oodplains have been exploited for agricultural and urban development, which has increased the exposure of people, property and other infrastructure to fl oods. Flood risk management therefore focused on measures to protect communities and industry in affected areas. However, fl ood risk is now addressed on a wider catchment scale so that initiatives in one part of a catchment do not have negative effects further downstream. This catchment based approach is embodied within the EU Floods Directive 2007/60/EC, and in recent years, those responsible for fl ood management have increasingly looked for solutions that employ techniques which work with natural hydrological and morphological processes, features and characteristics to manage the sources and pathways of fl ood waters. These techniques are known as natural fl ood management (NFM) and include the restoration, enhancement and alteration of natural features but exclude traditional fl ood defence engineering that effectively disrupts these natural processes.


NFM seeks to create effi ciency and sustainability in the way the environment is managed by recognising that when land and water are managed together at the catchment scale it is possible to generate whole catchment improvements with multiple benefi ts.


Almost all NFM techniques aim to slow the fl ow of water and whilst closely connected, can be broadly categorised as infi ltration, conveyance and storage.


Infi ltration


Land use changes such as set-aside, switching arable to grassland or restricted hillside cropping, can improve infi ltration and increase water retention. In addition, direct drilling, ‘no-till’ techniques and cross slope ploughing can have a similar effect. These land use techniques are designed to reduce the soil compaction which increases run-off. Livestock practices such as lower stocking rates and shorter grazing seasons can also help. Field drainage can be designed to increase storage and reduce impermeability, which is also aided by low ground pressure vehicles. The planting of shrubs and trees also helps infi ltration and retention by generating a demand for soil moisture, so that soils have a greater capacity to absorb water. Plants also help to bind soil particles, resulting in less erosion – the cause of fertility loss and sedimentation in streams and rivers.


Conveyance


Ditches and moorland grips can be blocked to reduce conveyance, and river profi les can be restored to slow the fl ow. In the past, peats and bogs have been drained to increase cropping areas, but this damages peatlands and reduces their capacity to retain water and store carbon. The restoration of peatland therefore relies on techniques to restore moisture levels. Pumping and drainage regimes can be modifi ed, and landowners can create strategically positioned hedges, shelter belts and buffer strips to reduce water conveyance.


Storage


Rivers can be reconnected with restored fl oodplains and river re- profi ling, leaky dams, channel works and riparian improvements can all contribute to improved storage capability. In urban areas permeable surfaces and underground storage can be implemented, and washlands and retention ponds can be created in all areas. As mentioned above, the re-wetting of peatland and bogs helps to increase storage capacity.


Many of the effects of NFM might be achieved with the re- introduction of beavers, which build dams that reduce peak fl ows, create pools and saturate soil above their dams. The dams also


The new OTT ecoN is an optical nitrate sensor, designed to lower costs


help to remove pollutants such as phosphates. Beavers do not eat fi sh, instead preferring aquatic plants, grasses and shrubs during the summer and woody plants in winter. Beavers are now being introduced in a number of areas in trials to determine their value in the implementation of NFM. One of the key benefi ts offered by beavers is their ability to quickly repair and rebuild dams that are damaged during extreme weather. However, whilst the potential benefi ts of beavers are well known, several groups have expressed concern with the prospect of their widespread introduction. For example, farmers and landowners may fi nd increased areas of waterlogged land due to blocked drainage channels. In addition, dams present a threat to migratory fi sh such as salmon and sea trout.


Beavers are native to the UK and used to be widespread, but they were hunted to extinction during the 17th century. However, other non-native species such as signal crayfi sh can have a detrimental effect on fl ood protection because they burrow into river banks causing erosion, bank collapse and sediment pollution. Signal crayfi sh are bigger, grow faster, reproduce more quickly and tolerate a wider range of conditions than the native white-clawed crayfi sh. Signal crayfi sh are also voracious predators, feeding on fi sh, frogs, invertebrates and plants, and as such can create signifi cant negative ecological effects.


www.envirotech-online.com IET September / October 2019


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