River Flows Background
Total Page：16
File Type：pdf, Size：1020Kb
5 River flows Background Being groundwater-fed, chalk rivers are reliant on rainfall in the autumn and winter to recharge the aquifer and maintain flows through the year. Natural flows produce and sustain a characteristic diversity of physical channel and riparian habitats. The high winter flows wash out fine sediment and help maintain clean gravels for invertebrates and fish spawning. When flows are artificially reduced this decreases the wetted area and the duration of flow in winterbournes. Low flows also slow the current, increasing the build-up of sediments. Fish habitat is lost, pollutants are less diluted and algae are more likely to choke the channel. Many chalk rivers lie in densely populated parts of south east England, where rainfall is relatively low. In some of these catchments there is no additional summer surface water or groundwater available for abstraction and present abstractions are unsustainable (Environment Agency, 2001a). Intensive agriculture and urbanisation have accelerated runoff, increasing peak flows and possibly affecting groundwater recharge. Extreme flood flows can wash out weed beds and spawning gravels, undermine banks and structures, and lead to extensive flooding. Infrequent heavy summer rainfall has, though, been © Dennis Bright identified as being potentially important for Ranunculus and salmon migration. >> A healthy chalk river habitat Local manipulation of water levels and flows also affects chalk requires good flows river habitats. In the past, water mills and the sluices of water meadow systems controlled the movement of water. Many sluices are still used to control water levels for fisheries purposes. Weed-cutting for fishing or flood defence reduces water depth. These management techniques have different objectives, which need to be balanced with the needs of wildlife. © Dennis Bright >> Hatch control structures significantly affect water levels and flows © Lawrence Talks >> River Cerne, near Cerne Abbas, Dorset 28 Environment Agency The State of England’s Chalk Rivers River flows 29 Low river flows When flows and wetted areas are restored, aquatic and wetland species typically return rapidly (Case study 3). Modelling has Sites where water company abstractions are known or thought been used to assess how habitat availability changes with flow to be causing an unacceptable impact give an indication of the for a range of species. At Briantspuddle on the River Piddle, extent of low flow problems in chalk rivers, but more may be abstraction reduced the estimated habitat area for brown trout affected. Between 2000 and 2005, 37 chalk river sites totalling fry and juveniles from May to October by up to 50 per cent some 240km are, or will be, under scientific investigation. or more when flows were already naturally low (Figure 8, Of these, 12 catchments have plans for low flow alleviation Strevens, 1999). Juvenile trout were correspondingly sparse schemes (Figure 9). The longest affected reaches are on the in the affected reaches. A low flow alleviation scheme has Lark, Hampshire Avon, Darent, Waithe Beck and Itchen. reduced the frequency of low flow events and should allow In some cases the impacts of abstraction are severe. trout populations to recover. Dry weather flows have been greatly reduced, peak flows curtailed and siltation increased (Case study 2). The summer dry period of some winterbournes has been extended by months and longer reaches become dry (Case study 3). Associated wetlands also dry out, as at Great Cressingham >> River Darent – flowing Fen, for example. The ecological impacts include reductions or losses of Ranunculus, fly life (Ephemeroptera), caddis > Figure 9 Water company investment in projects alleviating flies (Trichoptera), salmon, trout, three lamprey species over-abstraction in chalk rivers, 2000–2005 (Lampetra planeri, Petromyzon marinus, Lampetra fluviatilis) and coarse fish such as dace (Leuciscus leuciscus). In the Little Stour and Darent the freshwater limpet (Ancylus fluviatilis) is thought to be extinct. These species favour fast currents and clean gravels. Low flow alleviation schemes have generally been successful in restoring river flows, although monitoring is difficult and has often not quantified the ecological response. In addition, many schemes involve artificial flow support through pumping, rather than the restoration of natural hydrological processes. >> River Darent – dry > Figure 8 Impact of abstraction on juvenile brown trout habitat in the River Piddle, Dorset 30 Environment Agency The State of England’s Chalk Rivers River flows 31 > Case study 2 River Piddle low flow alleviation > Case study 3 Recovery of a winterbourne – the Misbourne low flow alleviation In the 1980s and early 1990s, much of the River Piddle had very low summer flows that were affecting fish populations and The River Misbourne is a winterbourne for much of its the appearance of the river. Public water supply abstractions length. It flows through the winter, and many reaches dry at Alton Pancras, Dewlish and Briantspuddle were causing low up in the summer as groundwater levels fall. From 1962, flows in the upper Piddle, middle Piddle and Devils Brook (Figure an average of 65 per cent of the available water was 10). A series of solutions to alleviate these problems has been abstracted for public supply. In drought years groundwater developed by Wessex Water and the Environment Agency, levels became extremely low, and the river was severely in consultation with the Office of Water Services and other affected. In the summer of 1997, 23.5km out of 27km interested parties. of river were dry, and flow in the remaining stretches was too low to dilute the sewage effluent adequately. Action was Environmentally acceptable flow targets were set to protect river needed to make abstraction more sustainable so as to habitats, angling and the river’s visual amenity. Initially in 1991, maintain groundwater levels, restore more natural flows © Nigel Holmes abstraction from the middle river was reduced between June to the river and allow wildlife to recover. to October, and flows were augmented during the 1995 drought. The following package of measures has been put in place: The flow alleviation agreed by the Environment Agency >> Misbourne – no flow and Thames Water involved reducing abstraction in the • in the middle river, up to half the licensed borehole abstraction catchment by an average of 15 million litres per day, and is used to support the river during low flows; © Natural Image, Bob Gibbons replacing this with groundwater from near the River Colne • additional water is pumped into the Devils Brook when and at Medmenham on the Thames. This took place over >> River Piddle flows fall below the trigger level; the winter of 1997/98, coinciding with natural recovery after the drought (which prevented a clear assessment of • the water supply shortfall has been made up by investment the ecological response to the alleviation scheme alone). in leakage control and using alternative sources; Flow returned to most of the river in early 1998 restoring • in the upper river, abstraction from Alton Pancras will a range of habitats from occasional pools to clean gravel be reduced by two-thirds during low flows and the river runs. This was followed by a recovery of aquatic life flow augmented; including blunt-fruited water-starwort (Callitriche • groundwater levels and stream flows are being obtusangula) and brook water-crowfoot (Ranunculus closely monitored. penicillatus subsp. pseudofluitans); moorhen (Gallinula However it should be noted that the long-term sustainability of chloropus) and other wetland birds; three-spined flow augmentation is questionable and needs to be set against the stickleback (Gasterosteus aculeatus), bullhead (Cottus ecological objectives of restoring natural hydrological processes. gobio) and stone loach (Noemacheilus barbatulus); and a range of invertebrate species. Further recovery > Figure 10 Abstraction impacts on river flow, the middle Piddle is expected, especially as a continuing dry middle section © Nigel Holmes of river began flowing in 2001. >> Misbourne – flowing Source: Environment Agency, 2002a Source: Environment Agency, 2002a 32 Environment Agency The State of England’s Chalk Rivers River flows 33 Outlook The Environment Agency’s Restoring Sustainable Abstraction Programme is improving the ecology of some rivers by working with water companies and others to tackle unsustainable abstraction on our worst affected rivers. Sustainable water use will require developers and land use planners to take into account water availability when locating and designing new developments. However, the projected growth in numbers of people and households in south east England will increase water demand in some chalk catchments over the next 20 years unless water efficiency measures are put in place to reduce our demand for water. Climate change risks causing drier summers in the south and east which would intensify the pressure on water supplies. Wetter winters may help groundwater recharge, but the rainfall is likely to fall more intensively and be difficult for soils to absorb, leading to more extreme flooding events. Careful management of water resources, allied to greater sensitivity in managing catchment soils to enhance infiltration will be essential to protect chalk rivers (Environment Agency, 2001a). The development of Catchment Abstraction Management Strategies between 2001 and 2007 should provide the information and explore the options needed to balance