NRA South West 48 RIVER MANAGEMENT STUDIES THE RIVER AVON AND ITS----- TRIBUTARIES NEAR MALMESBURY NRA National Rivers Authority South Western Region AUGUST 1994 BACKGROUND The hydrogeology of the area is complex. To the north and west there are The NRA was formed in September 1989 outcrops of Great and Inferior Oolitic to safeguard and improve the water Limestone. These limestones readily environment. Public concern had been absorb winter rainfall and when fully aroused prior to this about declining replenished provide a source of springs conditions in the Avon and its tributaries to feed the tributaries. near Malmesbury, Wiltshire. The Great and Inferior Oolites are not The NRA first looked in 1990 at the only separated by a layer of clays (Fullers impact of groundwater abstractions on Earth) but are also overlain by clayey river flows. These investigations gave deposits to the south and east. These clear evidence of the effect of abstrac­ clays are not total barriers to water tions but, at a time of natural drought, movement. General seepage and flow were frustrated from reaching clear through fault zones present difficulties recommendations. Evident leakage from in predicting groundwater movement. river beds was inconsistent with inter­ pretations of the geology from published Abstraction of groundwater for public maps. Following this the NRA put down water supplies began at Shipton Moyne exploratory boreholes to re-interpret the and Tetbury more than 50 years ago by geology and resolved to conduct a Bristol Water, who later (1961) added the thorough review of all other data to be Long Newnton source. Boreholes at used in a future computer model of the Milbourne, Charlton, Rodbourne and water resources of the catchment. Park Road were constructed in 1963 and are now operated by Wessex Water. In December 1991 consultants WS Atkins Ltd were appointed to undertake Wessex Water also operates a borehole at a River Management Study, ‘to report on Cowbridge together with five indepen­ sSS the evidence of declining conditions of dent boreholes from which water may Shers Avon at Silk Mills, Malmesbury the river environment with respect to the be pumped directly into the river for habitat and amenity value arising from ‘streamflow support’. These were con­ water and land management practices’. structed in the late 1970s. In aggregate (COVER PHOTO) The work has been recently reported and 56 million litres per day (Ml/d) is The Sherston Avon, with Malmesbury in the the following is a summary. licensed for abstraction for public water background supplies and 26 Ml/d is available for streamflow support. THE STUDY AREA / The upper reaches of the River Bristol Legend Avon are fed from a network of tribu­ taries converging near Malmesbury to Oxford Clay, Cornbrash, form what is commonly referred to as Forest Marble the Malmesbury Avon. The two main ■ tributaries, the Sherston and Tetbury □ Great Oolite Avons, meet at Malmesbury, and all flow from an area of 303 km2 is recorded at 1 1 Fullers Earth Great Somerford gauging station. Inferior Oolite Cotteswold Sand, Lias Clays Great Somerford Gauging Station NW SE Indicative NW-SE Section CATCHMENT GEOLOGY E n v ir o n m e n t Ag e n c y NATIONAL LIBRARY & INFORMATION SERVICE HEAD OFFICE Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol BS32 4UD • copies can be obtained from Public Relations Section, NRA South western Region, Exeter. Telephone the emergency hotline to report all environmental incidents, such as pollution, poaching and flooding, or any signs of damage or danger to our rivers, lakes and coastal waters. Your prompt action will help the NRA to protect water, wildlife, people and property. NRA emergency hotline 0800 80 70 60 24 hour free emergency telephone line NRA SOUTH WESTERN REGION ADDRESSES HEADQUARTERS NORTH WESSEX AREA OFFICE National Rivers Authority National Rivers Authority South Western Region Rivers House Manley House, Kestrel Way East Quay Exeter EX2 7LQ Bridgwater TA6 4YS Tel: Exeter (0392) 444000 Tel: Bridgwater (0278) 457333 Fax: Exeter (0392) 444238 Fax: Bridgwater (0278) 452985 The NRA is committed to the principles of stewardship and sustainability. In addition to pursuing its statutory responsibilities as Guardians of the Water Environment, the NRA will aim to establish and environmental practice throughout all its functions. I M ^ ’!q r j q | Pjv©TS AuH^Oflty jrmation Centre SW-8/94-1 k-E-AKVJ oo Office ENVIRONMENT AGENCY * 'oss N o ____ _________ /-vocesston N o 042032 — KEY Surface Catchment -------- Outcrop Boundary (Great Oolite) ------- Town • Flow Gauging Stations Water Supply Boreholes ♦ Stream Support Boreholes ♦ CATCHMENT DIAGRAM— MALMESBURY AVON OBJECTIVES OF STUDY system was chosen as the computer NATURE OF THE STUDY model on the basis of its scope and The brief to WS Atkins Ltd was wide To achieve a clear understanding of the completeness. The catchment area was ranging. Several of the concerns publicly concerns a local liaison group was represented in the model by some 3,000 expressed focussed on aspects of river formed, comprised of representatives of land elements; each allocated elevation, management not entirely influenced by civic, countryside, nature conservation, geology, abstraction, river channel and groundwater abstractions. riparian and fisheries interests. Through rainfall data. The model as set-up was There are many factors at work which this group an open day was organised in then tested against a 12 year period of contribute to changes in river flow, water the summer of 1992 to which the general observed streamflows. Simulated stream- quality, river biology and fisheries, and public was invited to share with us their flows could then be produced from the these are common to many river catch­ experiences, past and present, of the computer model for various theoretical ments where land development pressures river system. From these sources the changes: no abstraction; some abstrac­ have been high. They range from changes various concerns were distilled into 15 tion; different stream support arrange­ to field drainage efficiency, urban cover, river management issues. ments. increased use of agrochemicals, lack of Through the open day event a number of channel maintenance by riparian owners volunteers were recruited to keep a and river channel engineering for flood ‘River Diary’. These people were regular GENERAL FINDINGS protection. It was not expected that a visitors to, or users of, the river and they From the public open day the 15 river perfect and whole answer could be were asked to record at weekly intervals management issues not entirely attribut­ provided ‘at a stroke’ but that problems their personal views of the adequacy of able to water abstractions have been and their causes should be identified and river conditions. Records supplied were grouped within a set of environmental potential remedies classified within a checked by the NRA against measured considerations as follows. practical timescale and with reference to flows obtained at the time to assess the In the main these aspects are to be a scale of costs. diarists’ perception of what flows were addressed within the Upper Bristol Avon Fundamental to the study was the need acceptable. Catchment Management Plan. Copies of for a thorough analysis of the effects of At the core of the study lay the need to the initial consultation report are avail­ groundwater abstraction and for a provide, test and use a computer model able free of charge from the NRA South reliable computer model for use in of the catchment drainage processes in Western Region. Several aspects have the examining the options for changes. order to evaluate what changes had potential for early resolution; others will occurred and why. The MIKE-SHE take longer. River Water Quality Augmentation Flows Although in general river quality is good, Water pumped from a borehole at there are localised problems caused by Hullavington as streamflow support may poorly diluted discharges from Tetbury be lost to a sink hole a few metres from sewage treatment works and by the the source. The sink hole could be offensive product of unscreened storm isolated from the channel. In general overflows at Malmesbury. Dissolved stream support from the eight oper­ oxygen levels, particularly in the upper ational boreholes used for this purpose reaches, might be improved despite the is not provided instantaneously when natural low level in spring outflows. measured river flows decline to critical levels. There is a delay of seven days for Channel Engineering assessment before support is provided. At Malmesbury a number of weirs encourage the build-up of silt and debris and some crest levels and hence water EFFECTS OF ABSTRACTIONS levels may not be ideal. Downstream of Sherston Avon Malmesbury there has been an element At the Fosseway Gauging Station aver­ of re-sectioning for flood defence that is age flows are reduced by abstraction, but unsympathetic to wildlife. during periods of genuine drought they are now artificially enhanced by water Flora and Fauna from stream support boreholes. The encroachment on river channels by aquatic or semi-aquatic plants is widely Tetbury Avon evident and requires action where bene­ Similarly, at the Brokenborough Gauging ficial, not only by the NRA but also by Station there is a reduction in average riparian owners who should be advised flows from the natural condition but an by the NRA. increase in minimum flows. Particular concern is warranted for Avon at Malmesbury increasingly common algal blooms. Nutrient concentrations and their en­ Both the Sherston and Tetbury tribu­ trapment in sediments are a probable taries suffer major losses of flow due to major influence. River corridor habitat is river bed leakage caused by depressed diminished downstream of Malmesbury groundwater levels around the abstrac­ where arable land has replaced meadows. tion boreholes near Malmesbury. At the confluence of these tributaries it is esti­ Invasive weed growth mated that flows are now reduced to Aesthetics around 40% of the natural flow.