Water and Effluent Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/411335
16 WET NEWS NOVEMBER 2014 There have been many debates about the pros and cons of dredging rivers to prevent future flooding but water quality needs to be considered too. The challenge: Dredging up poor water quality The aim of the dredging operation has been to restore the rivers to their 1960s profile and improve their drainage capability L ast winter, the Somerset Levels was le underwater in many places as the re- gion was one of the worst affect- ed by the worst storms experi- enced in the UK for a very long time. The flood caused extensive damage to properties in the area and many residents had to leave their homes. Approximately 170 homes and businesses were af- fected. The Environment Agency (EA) estimated there were more than 65Mm 3 of floodwater cover- ing an area of 65km 2 . Inevitably, in the aermath the questions started with the then environment secretary Owen Paterson calling for a 20- year Action Plan to be developed to look at the various options for the sustainable management of flood risk on the Somerset Levels and Moors. The plan, supported by a £10M investment from the Department for Transport with a further £500k from the Depart- ment for Communities and Local Government, on top of the £10M announced by the government, centred on six key objectives mainly to reduce the frequency, depth and duration of flooding, and increase resilience to flooding. The dredging of rivers, the pros and cons of which had been debated every time there was a major flooding event, was back on the agenda for the Somerset Levels with such an operation becoming a reality once it was safe to do so as the flood waters receded. Two rivers, the Tone and the Parrett, that run through the Levels had burst their banks and were earmarked for the dredging operation. It would be the first time they had been dredged in 20 years. Drainage capability Back in March, three months aer the flooding began, dredg- ing work started on the banks of the river Parrett between Bur- rowbridge and Moorland, just a few minutes from J24 of the M5 in the South-west. Costing £1M a mile, five miles of river bank was to be dredged (three miles of the river Parrett and two miles of the river Tone), based on restoring the river channels to their 1960s profile "The monitors are quick and easy to deploy, and have performed very well; however, portability is extremely important because the instruments have to be moved and redeployed as the dredging work proceeds." Simon Browning, Wavelength Environmental THE CONCEPT • The monitors comprise Hydrolab water quality sondes and Adcon telemetry systems that transmit near-live data • Each sonde is fitted with sensors for the measurement of dissolved oxygen (DO), ammonium, temperature, pH, conductivity and turbidity • The monitors are being installed approximately 50m upstream and 100-150m downstream of the dredgers to allow sufficient mixing NEED TO KNOW 1 Poor water quality can be caused by low flow levels and high nutrient levels arising from many sources in the area. 2 Dredgings are frequently deposited close to the river bank – from where they can be carried by rain straight back into the river 3 Internal Drainage Boards report the need to dredge material from channels about every five to ten years, depending on local circumstances. THE VERDICT "The water quality monitoring undertaken by the mobile monitors and by our own static monitors will help provide assurance that the environment is not compromised by this work" Graham Quarrier, for the EA • INSIGHT WATER & SEWAGE MONITORING and improving their drainage capability. According to CIWEM, dredg- ing is rarely a one-off activity. Rivers carry runoff and silt from the catchment to estuaries, so as soon as dredging is complete, material will begin to re-accu- mulate. Moreover, the river will seek to re-adjust itself to a more natural form in terms of cross- sectional area, gradient and meanders, with potentially seri- ous unintended consequences including bank failure and erosion. As a result, dredged channels require long-term maintenance. Internal Drainage Boards report the need to dredge material from channels about every five to ten years, depending on local circumstances. Dredgings are frequently deposited close to the river bank – from where they can be carried by rain straight back into the river – or on the floodplain itself. This inevitably reduces the storage capacity of the flood- plain and hence its ability to reduce flood peaks. Sediments can also accumulate pollutants over decades. n