■ v THE RIVER AVON AND ITS TRIBUTARIES NEAR MALMESBURY Towards a Solution En v ir o n m e n t A g e n c y » BRISTOL** u f t f f t T r n June 1997 Towards, a Solution b a c k g r o u n d The River Avon and its tributaries near Wessex Water and Bristol Water accept that Malmesbury have been the subject of their groundwater abstractions in the detailed investigation since 1990. catchment are having an adverse impact Consultants, WS Atkins, were appointed in on river flows. Both companies are working 1991 to determine the impact of closely with the Environment Agency to abstraction and other management effect early improvements to river flows by practices on river flows, river biology and implementing a combination of measures fisheries. WS Atkins used the MIKE-SHE involving increasing stream support and computer model to represent the drainage changing abstraction arrangements. processes in the catchment and to assess Following the recommendations in the the impact of abstraction. Atkins study all parties have agreed that the trials should be implemented on a The study concluded that abstraction by staged basis, starting with those measures Tetbury Avon, downstream St Leonard's Wessex Water and Bristol Water that may provide an immediate benefit. Bridge significantly reduce river flows throughout the catchment. Both the Sherston and In August 1995 Wessex Water increased The study area includes the Tetbury tributaries suffer a loss of flow due the output from their stream support upper reaches of the River to river bed leakage caused by depressed boreholes at Luckington, Stanbridge and Avon above Great Somerford groundwater levels in the Great Oolite Tetbury. The month long trial showed that gauging station. The two main aquifer around the abstraction boreholes considerably more water could be obtained tributaries, the Sherston and near Malmesbury. Although the Great from the Inferior Oolite aquifer and that Tetbury Avon, meet at Oolite is overlain by clay-like deposits to most of the water added as stream support Malmesbury. These and a the south and east and separated from the remained in the river down at least as far as number of other minor underlying Inferior Oolite by Fullers Earth Great Somerford. Bristol Water voluntarily tributaries are fed by surface these clays are not total barriers to water reduced their average summer abstraction runoff and by springs from the movement. Geological investigations have for public water supply at Shipton Moyne Great Oolite aquifer. Wessex shown that near Malmesbury the clays are and Long Newnton thereby leaving more Water abstract from the Great thin and the Great Oolite aquifer is close to water in the Inferior Oolite aquifer for Oolite for public water supply. the surface. During the winter when water stream support. Bristol Water's supplies mostly levels in the aquifer are high, water flows come from the Inferior Oolite. from the Great Oolite into the river. As Further trials in 1996 were designed to see Wessex Water also abstract water levels in the aquifer drop during the if the benefits observed in 1995 could be from the Inferior Oolite aquifer summer the flow reverses and leakage sustained over a longer period. The and pump the water directly takes place from the river to the aquifer. prescribed flows at Fosseway and into the river for stream Brokenborough gauging stations that support. KEY determine when water should be pumped into the river for stream support were - - Surface Catchment ---- increased. Wessex Water used its stream Outcrop Boundary (CO) support boreholes to maintain new target □ Town CATCHMENT MAP ✓ I flows set by the Environment Agency and Flow Metering Stations SIMPLIFIED Bristol Water continued to abstract at • Stream Support Boreholes reduced rates. 1 Water Supply Boreholes • Bristol Water Co. The results of these and other work aujy • Wessex Water PLC conducted during 1996 are summarised in • N this leaflet. The leaflet also contains an I X Long NewMon ^ m ^ N — % outline of work to be undertaken in 1997 [_Brokenborough C h a rity Shipton and and the issues that need resolving / Backbridge V before full and lasting remedies can be Fosseway lalmes finalised. These remedies will be finalised by the year 2005 at the latest. Sherston Avon Bridge Rodbourne Luckington Hullavington * 1 Ml/d =0.22 Mg/d =0.0116 m3'* Lower ! Q uintin Cover photo: Sherston Avon at Silk Mills, September 1996. I ____ ■ Inset photo: August 1995, prior to stream support trial ^ S H S f a o m 5 15 25 DISTANCE - km from origin CO GREAT OOLITE FE FULLERS EARTH 10 INFERIOR OOLITE FM, CB & OC FOREST MARBLE, CORNBRASH & OXFORD CLAY Stream Support Trials Prescribed Flows To compensate for the reduction in river 8 Ml/d. Depending on the weather this As part of the trial the Agency has flows due to abstraction for public water year these measures may lead to several developed two new target flows to help supply Wessex Water are required under weeks more stream support than would be protect the lower reaches of the Sherston the terms of its licence to pump water normal under the license conditions. Flows Avon and Tetbury Avon in Malmesbury. into the river from its stream support at the gauging stations will be used to The target flows are located in Malmesbury boreholes. The licence stipulates that trigger stream support. Once support has at St Johns Bridge (Sherston Avon) and pumping should start when flows at the begun the support boreholes will be used Back Bridge (Tetbury Avon). During the gauging stations at Fosseway, to try to maintain target flows in summer of 1996 output from the stream Brokenborough or Great Somerford fall Malmesbury and the existing prescribed support boreholes was not limited to 2.5 below prescribed levels. flow (28 Ml/d) at Great Somerford. Ml/d but allowed to increase to maintain the following targets: In 1995, as a trial, the prescribed flow at Fosseway gauging station was increased Target Flows St John's Bridge (Sherston Avon) 7.2 Ml/d from the licensed value of 8 Ml/d to Back Bridge (Tetbury Avon) 6.8 Ml/d 12MI/d. In 1996 the prescribed flow at Malmesbury Fosseway was increased again, this time to The licence limits output from each of the The targets are based on the results of a 14 Ml/d . This increase led to stream support stream support boreholes at Luckington, river diary exercise completed by local being introduced two to three weeks earlier Stanbridge and Tetbury to 2.5 Ml/d. people in 1992 and 1993 and are designed than normal on the Sherston Avon. Pumping at 2.5 Ml/d has generally not to protect the important amenity value of Although the prescribed flow at been sufficient to maintain flows at the the rivers in Malmesbury. The target flows Brokenborough was increased from 6 Ml/d gauging stations at prescribed levels, or were generally met or exceeded, the main to 8 Ml/d in 1996, stream support was prevent the flows in Malmesbury receding exception being due to a pump failure in actually turned on before flows at to very low levels. mid August. Because of rainfall flows were Brokenborough reached 8 Ml/d. Support generally well above target levels. Stream therefore started about four to five weeks In August 1995, for example, prior to the support continued at Luckington, earlier than required under licence start of the stream support trials flow in the Stanbridge and Tetbury until water levels conditions. Sherston Avon at Silk Mills was reduced to increased in the Inferior Oolite in the a trickle (see photo insert on the cover). autumn. In 1997 stream support on the Sherston Now, as part of the trial, Wessex Water are Avon will be started when flows at able to add a total of 24MI/d from the Fosseway gauging station fall below 14 M three stream support boreholes instead of I/d and on the Tetbury Avon when flows at the 7.5MI/d specified on the existing Brokenborough gauging station fall below licence. ENVIRONMENT AGENCY 0 7 9 6 9 0 Towards, a Solution FLOW AT BACKBRIDGE 1996 |UNE |ULY AUGUST SEPTEMBER OCTOBER NOVEMBER AT ST JOHNS BRIDGE 1996 Ml/d JUNE |ULY SEPTEMBER OCTOBER NOVEMBER MEASURED FLOW TARGET FLOW During the trials the Environment Agency, will also be monitoring groundwater levels Wessex Water and Bristol Water have been around their stream support boreholes. actively seeking the views of the public Information on the quantities abstracted about the benefits observed. The for stream support, groundwater levels and Stream support, Luckington Malmesbury Liaison Croup has been river flows will be publicised by the Agency formed to provide a direct link between during the trial. the Agency, the water companies and residents and organisations who have an interest in the river. In general the Great Somerford feedback that we have received has been very positive. Most people considered that In addition to maintaining target flows in flows through Malmesbury were Malmesbury in 1996 the prescribed flow of maintained at an acceptable level in 1996. 28 Ml/d at Great Somerford was exceeded We will therefore again try and maintain for most of the summer. Whilst flows at flows, at, or above these targets in 1997. Great Somerford benefited from the Stream support will start at a relatively low increase in stream support to meet the rate (2.5 Ml/d) but gradually increase as targets in Malmesbury there is still concern, river flows decline during the summer. particularly amongst anglers, that flows there remain too low. The Agency will be regularly monitoring river flows and groundwater levels The Agency is reviewing the prescribed throughout the trial and liaising closely flow at Great Somerford and will identify with both water companies.