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10. Water resources

Who does what?

o. The Department of the Environment and the Welsh Office are responsible for Government policy.

o. Within the area, water supply is the responsibility of water companies. These include Dwr Cymru Welsh Water , Severn Trent Water Ltd, Wessex Water and Bristol Water Company.

o. The Environment Agency is responsible for ensuring the sustainable management of water resources. They licence most impoundments of and abstractions from rivers, watercourses and defined areas of the estuary.

o. Harbour authorities, internal drainage boards, water companies, English Nature and the Countryside Council for Wales are consultees on licence applications.

Stated Government aims


The major abstractions direct from the estuary are for cooling water for power stations. Water is also abstracted from the Severn Tunnel for industrial use and there is some interest in the development of groundwater resources beneath the estuary. Minor abstractions are also taken for agriculture from the tidal rivers feeding into the Severn. There is a major abstraction from the Severn at Gloucester which feeds the Gloucester - Sharpness Canal. This water is used for lockage at Sharpness Docks and drinking water. Bristol Water's intake at Purton provides over 50% of Bristol's water in summer months. Abstractions near the tidal limit can be affected by estuary processes such as salt water intrusion. Major abstractions require the use of pumps and they inevitably draw fish through them. This can be minimised by careful design operation and screening but there is still some impact. The knowledge of fish species in the estuary has been much improved by analysis of fish from the screens of power stations.

Local abstractions may also have an impact on the estuary or the surrounding area. Sediment distribution in local creeks used by yachts may be affected by river control and abstractions in the adjacent wetlands can have an impact on the estuary habitats.

Probably the main impact of water resources management on the estuary, however, concerns flows which are left in rivers passing to the estuary. Table 10.1 below describes the flows below which possible effects on the estuary or users may occur for five of the main rivers and the reasons for those flows being protected. The final column gives an indication of how close the river is to its natural flow under these conditions. To avoid additional problems flows in rivers should be as near as possible to natural below the identified flow requirement. This requires management of releases from support reservoirs such as those on the Wye and Severn or control of abstractions. The use of water at the tidal limit could become a major conflict between water suppliers and environmental groups as the costs involved in replacing the possible resource are very large. There is already considerable interest in the possible use of Severn or Wye water for transfer to the Thames catchment to address possible shortages in the South East of England. This conflict could be even more significant if global warming reduces normal summer flows. The effects are also difficult to quantify and the area requires significant scientific investigation to aid informed decision making.

Table 10.1: Suggested residual flow levels for the Rivers Severn, Avon, Wye, Usk and Parrett.





Reasons for Flow




1200 (neaps)




1800 (springs)



















Key: RF=Residual Flow Q95= Flow exceeded 95% of time.

Reasons for setting residual flow
Am = Amenity
D = Dilution
E = Ecology
F = Fisheries
Q = Quality
S = Saline intrusion
St = Silt/ suspended solids re-suspension
N = Navigation requirements

Flow factor:
A = Potentially balanced abstraction and releases
B = Flows potentially below natural
C = Flows significantly below natural.

For the Severn, flows are required to maintain dissolved oxygen levels, acceptable suspended solids level at the Gloucester abstraction, to prevent saline intrusion and to provide dilution for effluent discharges. Flows are also required for migratory fisheries.

The flow on the Avon was suggested to safeguard the estuary conservation area, navigation requirements in Bristol floating harbour and for amenity and landscape considerations within the Avon Gorge.

Uses that need protection in the Wye include fisheries navigation, amenity and recreation. The most significant issue relating to the residual flow is the major salmonid fishery.

The reason for setting residual flows in the Usk are to protect fisheries and water quality in the estuary which receives significant sewage effluent and is also affected by the sediment oxygen demand of high suspended solids.

The Parrett requires freshwater flow to the estuary to maintain dilution for sewage works, for the elver fishery and to prevent saline intrusion into the moors. In addition, the fresh water flows flush out sediment brought in from Bridgwater Bay by high tides. Good river flows are therefore needed to prevent siltation and maintain navigation.

Studies on the required flow to estuaries for environmental reasons suggest that different flows may be needed at different times of year, for example to meet the needs of migratory fish. Different flows may also be needed for spring tides and neap tides- for example to keep saline water away from water supply intakes or to prevent landward movement of marine silt.

Water supply

Water supply in the area is provided by three Regional Water Companies who also provide sewage treatment (Severn Trent Water, D_r Cymru Welsh Water and Wessex Water) and Bristol Water who provide the water to Bristol and surrounding area. An indication of water usage in the estuary area is given by Bristol Water's figures which are shown in Table 10.2.

Table 10.2: Use of water in Bristol Water's Supply Area 1995-96






Distribution input


Household %

Non household


Household %

Non household %









While leakage rates for Bristol Water are better than the national average there may be some scope for further reductions. Water companies now have a general duty to promote water conservation and waste management initiatives in industry should reduce demand. Such reductions could save companies money, reduce energy requirements and leave more water for the environment.



Many issues in this report are related to one another. Issues raised in this chapter have particular links with those in chapters 2, 4, 8, 11, 12, 13, and 15.


X1 Flows required for migratory fish

Freshwater flow down the rivers into the estuary is required for migratory fish:
a. to maintain sufficient attractive flow so that the salmon can identify the river, and
b. to induce migration.

Maintaining sufficient attractive flow is dependent on sustaining flows during dry periods. This requires control of abstractions under low flow conditions or compensation by releases from upstream reservoirs if this is possible. Inducing migration depends on flows being available when the fish are ready to migrate. It may require a relatively large flow, possibly twice the dry weather flow. It is therefore important to control abstractions from the small floods which occur during the migration periods. Regulation from reservoirs is unlikely to be of significant impact under these situations.

Who is involved: Environment Agency, water service utilities, scientific community, British Waterways.

What is happening: The Environment Agency are undertaking a research project to investigate the relationship between fish migration and flows. Within the LEAP process the Agency is also reviewing the required flow in each major sub-estuary.

Some suggestions: Where new reservoir capacity or licensing changes may be needed the Environment Agency will need to discuss the issue with major users of the river. For all aspects of residual flow requirement consideration must be given to possible water transfers to the River Thames from the Severn, or its major tributaries and the possible impact of global warming. If investment is required by water companies this will need to be discussed with the water regulator the Office of Water Services. The Environment Agency investigations are only part of a much larger requirement for scientific work in this field.

X2 Maintenance of flow to ensure quality

Studies by Birmingham University and the Environment Agency in the upper estuary have shown that water quality is dependent on river flow. The extent of sediment re-suspension during spring tide bores varies with river flow, being highest when river flow is lowest. As the sediment has an associated oxygen demand this can sometimes put additional stress on migratory fish when river flows are less than their natural level. Flow to the estuary is also needed to maintain adequate dilution for sewage works below the tidal limit.

Who is involved: Environment Agency, water service utilities, British Waterways.

What is happening: Some licences have recently been issued which allow less water to be abstracted when spring tides coincide with low flows. An agreement with British Waterways is pending which will also regulate the abstractions to the Gloucester Sharpness Canal when there are spring tides and low flows.

Some suggestions: as X1

X3 Maintenance of flow to prevent saline intrusion

During low flows and high tides, salty water may occasionally penetrate above the British Waterways abstraction point at Gloucester which feeds the Gloucester - Sharpness Canal. Water is abstracted in turn from the Canal for Bristol's water supply and can provide more than 50% of the City's needs. Even small amounts of salt can cause problems for water suppliers. The penetration of the saline water is very dependent on the prevailing freshwater flow, being greatest when river flows are lowest. The method of pumping during tide at Gloucester is also important. This factor may become more important if sea level rise due to global warming affects the extent of saline intrusion.

Who is involved: Environment Agency, British Waterways, Bristol Water.

What is happening: British Waterways are investigating new pumps at Gloucester Docks so that enough water can be abstracted during the part of the tide when salt and silt levels are at their lowest. British Waterways in collaboration with the Environment Agency are also looking at best use of water in the canal. Bristol Water are investigating ways of using less water from the canal during high tide and low flow periods. The Environment Agency are investigating several options to increase flow in the River Severn during high tides and low flows.

Some suggestions: As X1.

X4 Maintenance of flow to prevent landward movement of sediment and increased sediment re-suspension

When low flows correspond with high tides sediment from the estuary is moved inland. The level of suspended solids in the vicinity of the Gloucester abstraction can also become very high reaching almost 10 % solids under extreme circumstances. These problems can be exacerbated by low river flows. The landward migration of silt has caused problems for craft trying to navigate between Gloucester and Tewkesbury and also reduces the bottom feeding areas available to fish. There are many other factors which may contribute to this problem such as changes in boat traffic, lack of dredging and weirs. The abstraction of water with a high silt content into the Gloucester Sharpness Canal leads to high dredging costs and environmental concerns in the disposal of dredgings.

Who is involved: Environment Agency, British Waterways, Bristol Water, Gloucester City Council, Gloucester Harbour Trustees.

What is happening: As X3. Those involved have undertaken a feasibility study for a river control structure at Gloucester which would exclude some spring tides and most of the silt.

Some suggestions: As X1. Further investigations into river control structure.

X5 Maintenance of flow to preserve estuarine ecology

Some species in the estuary rely on the input of fresh water to provide their best habitat. The Baltic tellin, for example is an important source of food for birds and is noted for adaptation to brackish waters. The effect of river regulation and abstraction on such species in the estuary is not well understood. Lower flows in summer may also mean less nutrient or organic input to the estuary. The effect on the food chain in the estuary has again not been extensively studied.

Who is involved: English Nature, Countryside Council for Wales, Environment Agency.

Some suggestions: Investigate the impacts of abstractions on the ecology of the estuary by use of models.


X6 The effect of water levels on wetlands

In the upper Severn Estuary and tidal river there are important wetland areas where the water level can influence the drainage pattern and by implication the ecology. Studies by the RSPB and Environment Agency have indicated that there are fewer breeding waders in the area than would be expected because of reduced suitable habitat area. The effects of water abstraction and river regulation are likely to be less than those of land drainage, flood control works and weirs. However, these factors have not been fully evaluated.

Who is involved: Environment Agency, English Nature, Countryside Council For Wales, Internal Drainage Boards.

What is happening: Water Level Management Plans have been produced or are being produced for the most critical areas.

Some suggestions: A further specific study to look at the effect of River Severn levels on wetlands areas is under consideration

X7 The effect of abstractions on the coastal zone

Some abstractions take place in levels close to the estuary. One such site is Walmore Common which is a RAMSAR site just below Gloucester. RSPB and English Nature have expressed concern that this abstraction may be contributing to the low groundwater table during the wading bird breeding season. RSPB and the Environment Agency have undertaken groundwater level monitoring to investigate the effects of the abstraction.

Who is involved: Environment Agency, English Nature, Countryside Council for Wales, Royal Society for the Protection of Birds, internal drainage boards.

What is happening: As X6.

Some suggestions: Actions from the Water Level Management Plans may require farmers to change the use of land or for existing abstractions to be changed. Compensation and agreed operating methods will be important for progress.

X8 The effect of a bankside storage reservoir near Purton on the Gloucester-Sharpness Canal

One option for addressing some of the concerns discussed previously in this chapter is to consider a bankside storage reservoir near Purton on the Gloucester - Sharpness Canal. This could reduce the requirement for pumping to the Gloucester - Sharpness Canal during spring tides and provide additional protection against loss of public water supply through pollution to Bristol Water. Any such storage may affect the feeding grounds of wildfowl or the neighbouring Wildfowl and Wetlands Trust at Slimbridge.

Who is involved: Environment Agency, Bristol Water, English Nature, Wildfowl and Wetlands Trust, planning authorities.

What is happening: Preliminary discussions have taken place between the Environment Agency and the Wildfowl and Wetlands Trust on this option and a preliminary feasibility study has been undertaken.

Some suggestions: Some sites are obviously problematic but there is a possible option which could be beneficial to the Wildfowl and Wetlands Trust . Further discussions on this option should be held.

X9 The effect of large pumped abstractions on fisheries

Although turbines and pumps are usually screened, major abstractions kill fish that are sucked through pumps or are caught on the screens. This is potentially a larger problem in tidal rivers rather than the more coastal parts of the estuary where the actual numbers of fish caught at sites have been demonstrated to be small in proportion to estuarine population of fish. In these situations large influxes of fish and weed during seasonal storm conditions can cause major operational problems for abstractors when screens get clogged. Much of the information on the fish species in the estuary derives from analysis of fish caught on intake screens.

Who is involved: Major abstractors, Environment Agency, MAFF, Sea Fisheries Committee.

What is happening: Discussions have recently been held with British Waterways in regard to the installation of new pumps at Gloucester.

Some suggestions: Abstractions to be designed to minimise impact on fish. Operating agreements may also mitigate the effect. Fish deterrents such as acoustic deflectors may also help.


X10 Water Saving initiatives round the estuary

With the drought of the last few years there has been renewed interest in initiatives to save water. These are being promoted both to industry and householders. Reducing water use not only leaves more water in the rivers and estuary but can also reduce energy demands and financial costs.

Who is involved: Water service utilities, large industrial water users, Environment Agency, domestic households.

What is happening: Some water service utilities are licensing or charging more for garden sprinklers to control demand. Industrial waste reduction schemes are being considered or are already in progress round the estuary (e.g. the Sabina project). Household reductions are being encouraged by a Global Action Plan Scheme run by Vision 21 in Gloucestershire and sponsored by the Environment Agency.


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Created: 10/28/99 Updated: 10/28/99