< > Glossary & References / Book List / B93 Wetlands, Industry & Wildlife - a manual of principles and practices / Text Sections:

Wetlands, Industry & Wildlife - a manual of principles and practices
Click here for CONTENTS Page - or use the arrows above to flick through the book

No part of this publication may be reproduced without the prior written permission of the publisher;
for contact details see Title Page -

Chapter 11 - Water storage reservoirs

11.1 Potential constraints associated with reservoirs
11.1.1 Concrete shorelines
11.1.2 Salinity
11.1.3 High silt loads
11.1.4 Fluctuating water levels
11.2 Attracting wildlife to reservoirs
11.2.1 Concrete-banked reservoirs
11.2.2 Saltwater reservoirs
11.2.3 Reservoirs with highly fluctuating water levels
 Feature 11.1
Feature 11.2
Feature 11.3
 Feature 11.4

In addition to the municipal reservoirs run by the various water companies (regional councils in Scotland), many other industries possess their own reservoirs. For industries that require large quantities of water on a regular basis it can be cost-effective to have their own supply. This is particularly the case where the water does not need to be absolutely pure, for instance when it is merely being used for cooling. Power stations are the main water consumers; in 1990 they utilised 36% of- all abstracted water, while other industries consumed a further 11% in the same year (28 (Appendix 11 - Selected references and further reading)). Although cooling systems may use vast quantities of water, it is often possible, and preferable in environmental terms, to recycle much of the water; relatively small amounts might be required at any one time to top up a continuous process.

Traditionally, industries requiring a lot of water have often taken their supply direct from the sea or from large rivers. Where the corrosive properties of saline water are not considered a particular problem then sea water has offered the cheapest and most reliable source. In estuarine situations it may be necessary to have a reservoir to cover the low tide periods. Estuarine waters have particularly high levels of suspended solids, which are likely to impact upon water management, plant and reservoir designs. The power stations at Fiddlers Ferry and Oldbury (Features 11.3 and 12.2 (Chapter 12 - Silt storage lagoons)) both utilise estuarine water. Silt can also be a problem, at least seasonally, in river supplies; such a situation is found at the Courtaulds Factory at Spondon (Feature 11.1).

With the increasing concern about the consequences of water abstraction and low river flows, it seems unlikely that industry would be offered new abstraction licences to take large quantities of water direct from a river in the future. Reservoirs which draw largely upon flood flows are far more acceptable (see also 15.2.2 (Chapter 15 - Wetlands for drainage)).

Reservoirs are broadly comparable with lakes and consequently are often of great value to wildlife, especially birds. In the early 1980s almost 12% of all the winter wildfowl counted in Britain were to be found on reservoirs (72 (Appendix 11 - Selected references and further reading)). The best of these reservoirs, in terms of their value to waterbirds, have been formed by flooding shallow, lowland valleys. This is amply demonstrated by some of the large, eutrophic municipal reservoirs such as Chew Valley Lake (Bristol Water) which has attracted over 260 species of bird, and Rutland Water (Anglian Water) which, only 15 years after completion, was recognised as being of international importance for waterfowl (under the terms of the Ramsar Convention). Most reservoirs attached to factories, however, are enclosed by artificial concrete or earth bunds, but none the less can have considerable value for wildlife. The few upland reservoirs, such as the one associated with Trawsfynnyd Power Station (Nuclear Electric), are likely to be of limited wildlife value owing to the great depth of water they contain and the harsh winter climate of the areas concerned.

Rutland Water, a lowland valley reservoir of exceptional value to waterbirds.
Click on Illustration for full-page view

Return to top of page

11.1 Potential Constraints associated with reservoirs

11.1.1 Concrete shorelines

Concrete is often used to line steep banked reservoirs, because of its resilience to wave erosion. Such reservoirs are rarely shallow (except when being drained) and may be consistently deep, in which case they are only likely to be of value to diving birds and roosting waterbirds. Deeper reservoirs may assume much greater importance, however, during periods of cold weather when they may be the last waters to freeze. Freezing will also be inhibited by a rapid through-put of water, a feature of some reservoirs such as those at Spondon (see Feature 11.1 ). Most concrete reservoirs have simple round or oval shapes, owing to the difficulties of incorporating irregularities into a concrete shoreline, and lack marginal plants. The exposed nature of such reservoirs restricts their attractiveness to many species.

Return to top of page

11.1.2 Salinity

Some industrial reservoirs store salt water which will have a major bearing on the plants and animals that it will support (see 5.5 (Chapter 5 - Water: chemistry and quality)).

Return to top of page

11.1.3 High silt loads

It is not unusual for reservoirs to show high levels of silt deposition. This is particularly true if the reservoir is supplied either by watercourses passing through clay substrates or direct from an estuary. Silt deposition tends to occur wherever the water flow slows down. While patches of silt can add to the diversity of a reservoir bed, uniform deep deposits can significantly limit the development of communities of benthic invertebrates and submerged plants (see 7.2 (Chapter 7 - Substrates)). The two main strategies for increasing the wildlife value of reservoirs used to store water with a high silt loading are: 1) the use of silt traps, and 2) regular aeration of silt deposits. In order to limit the potential for the resuspension of the silt, silt traps should be dredged on a regular basis preferably before the winter rains in the case of stream-fed reservoirs. Drying out silt deposits helps to consolidate them, reducing re-suspension and improving the consistency of the substrate, but is obviously very disruptive and should not be undertaken without consideration of the potential implications (see 6.5 (Chapter 6 - Water: the physical environment)). Further guidance on how to increase the wildlife value of wetlands receiving water with high levels of suspended solids can be found in 5.8.1 (Chapter 5 - Water: chemistry and quality), 12.2 (Chapter 12 - Silt storage lagoons) and 15.2.1.(Chapter 15 - Wetlands for drainage).

Return to top of page

11.1.4 Fluctuating water levels

Compared with natural lakes, reservoirs often show extreme seasonal fluctuations in water level. This is due to an exaggerated net loss of water in late summer when the combination of abstraction, evaporation and evapotranspiration can far outweigh stream-fed inflows. The significant through-put of water observed in most reservoirs creates physical, chemical, and biological gradients not normally observed in natural lakes. This typical scenario may not apply to all reservoirs; the sugar industry, for example, actually uses less water during the summer months (see Feature 14.5 (Chapter 14 - Water treatment systems)). Reservoirs typically need to be drained every few years in order to carry out inspections or maintenance works; this again contributes to the stressful environment that reservoirs pose for many aquatic species.

Return to top of page

11.2 Attracting wildlife to reservoirs

Most reservoirs can offer a broadly similar environment to a lake: large areas of standing water potentially suitable for still-water fish and waterbirds. Where new reservoirs are proposed there is likely to be tremendous scope for enhancement as wildlife habitat. Many appropriate ideas are to be found in Part II of the manual. There are many good examples of reservoirs that have been enhanced for wildlife on sites owned by the water companies, most notably on the reserves at Rutland Water managed by the Leicestershire & Rutland Trust for Nature Conservation. This chapter, however, will focus on how to make the most of some of the more awkward designs and characteristics that may be associated with industrial reservoirs.

Return to top of page

11.2.1 Concrete-banked reservoirs

Although very artificial in appearance, concrete-banked reservoirs can be attractive to some species of waterbird. The concrete offers bare embankments, which require virtually no management and may be used as safe loafing sites by waterbirds (as long as there is little disturbance). These banks can even attract waders during the migration periods, especially Common Sandpipers, which feed on invertebrates associated with the debris along the water's edge. Some specific improvements that could be added to this type of reservoir are outlined below and indicated in Figure 11.2.

a) Improvements to the bed of the reservoir. Most reservoirs of this type use the native substrate for the bed material, especially where this is clay. In such situations it is helpful to contour the bed, adding raised mouiids and excavating channels. To be of benefit to diving ducks it is important that there are some areas of less (preferably much less) than 8 in depth. If the water level is likely to fluctuate significantly then it is helpful to add in shelves at a variety of depths (see 6.1 (Chapter 6 - Water: the physical environment)). Where the bed has been lined with concrete or a synthetic liner, then improvement works are likely to be more complicated (see 7.1 (Chapter 7 - Substrates)). The addition of submerged structures to act as reefs will be of particular value (see 7.3.3 (Chapter 7 - Substrates)).

b) Floating islands. Islands can make a very valuable contribution, providing secure nesting and loafing sites for waterbirds. Where the bed of the reservoir cannot be modified, or it is uniformly deep, then floating islands may be the only realistic option. In larger and deeper waters, both the system of anchorage and the island itself will need to be of stout construction and the chosen design should preferably be checked by a structural engineer. They should be sited reasonably close to the shore that offers most protection from the prevailing wind. If a sheltered site cannot be found then rafts are unlikely to offer safe nest sites; high waves may wet or wash away nests and any materials used to cover an island. Floating buoys offer valuable loafing sites for terns. See 8.1.2.(Chapter 8 - Other aspects of wetland design).

c) Reducing wave action. There is often a lack of sheltered water on such sites, a feature that would aid flying insects and invertebrates preferring calmer waters, e.g. pond skater, whirligig beetle and water measurer, which in turn are preyed upon by various waterbirds. This can be partially overcome by judicious planting of trees and shrubs (see 6.8.1 (Chapter 6 - Water: the physical environment)). An alternative, which is less likely to impact upon the open aspect of such waters preferred by many waterbirds, is to use floating booms. Such booms, however, need to be substantial in order to dampen the height of waves significantly.

d) Introducing emergent plants. Although lined embankments are usually kept free of vegetation, it may occasionally be possible to introduce plants on top of a concrete liner, thus adding considerably to the structural diversity of the reservoir. However, emergent plants are likely to be of value only if the reservoir has a fairly constant water level, at least during the bird breeding season. Plants such as Common Reed can be established in large, concrete-sided boxes on the side of the reservoir, their tops lying 150-450 mm below the typical water level. The boxes should be filled with suitable earth and ample transplanted reeds before flooding (see 9.2.1 (Chapter 9 - The establishment and management of wetland plants and animals)). The opportunities for using this technique will be dictated by the steepness of the banks; the presence of flat or shallow shelves will help greatly. Stands of emergents less than a metre wide will be of limited value.

e) Recreation. Owing to the uniform shape of such reservoirs it is virtually impossible to screen off areas for watersports (see 8.5 (Chapter 8 - Other aspects of wetland design)). The only way such a reservoir can maintain a significant bird interest and accommodate recreation is if there are other undisturbed waters nearby where birds can take refuge during periods of activity (see Feature 11.2).

Figure 11.2 Cross-section through an idealised concrete-banked reservoir showing features that could be used to enhance the design for wildlife.
Click Illustration for full-page view

Return to top of page

11.2.2 Saltwater reservoirs

in general, such reservoirs seem to be of less value to wildlife than freshwater reservoirs, presumably because they do not replicate any naturally occurring habitats. They offer few advantages over adjacent areas of sea where a wider selection of life is likely to be present. However, they may be attractive to waterbirds as a safe area to loaf or roost, or as a bathing area for gulls. During stormy weather they can provide a temporary haven for coastal birds. They can support a variety of fish species, although it seems unlikely that many would breed in such waters. Such reservoirs may be used as fishing areas by birds such as Cormorants and grebes, owing to the relatively clear water they are likely to offer compared with most estuaries.

Concrete-banked, saltwater reservoirs can be modified as suggested above. It should be recognised that the main wildlife value of such reservoirs is likely to be as a roost site for birds and, therefore, the most valuable improvements that could be made are the addition of islands and perches and the creation of areas of calm water.

Reservoirs with soil banks will offer more opportunities for improvement. If they have shallow margins, saltmarsh plants are likely to establish and will in turn offer some food and shelter for a range of animals. Where water levels fluctuate and substrates permit, estuarine invertebrate communities may develop. These communities will be killed off by anything more than a few days of draw-down, especially in warm or freezing weather. However, if sea water is regularly being pumped in then such communities should quickly re-establish (see also 12.2.1 (Chapter 12 - Silt storage lagoons)).

Return to top of page

11.2.3 Reservoirs with highly fluctuating water levels

It should be recognised that established industries are likely to have little scope for altering either the timing or volume of their water consumption. A large summer draw-down will have many implications as far as the ecology of the wetland is concerned (see 6.3 and 6.5 (Chapter 6 - Water: the physical environment)). If the resultant draw-down occurs before July, then the floating nests of birds such as Coots and grebes will be left high and dry, while dragonflies will be denied the overgrown margins that they prefer. At sites that are otherwise suitable for such species, it may be possible to limit this effect by constructing a bund in order to maintain higher water levels within an appropriate section of the reservoir until later in the summer. Mud exposed by a summer draw-down is likely to be colonised by annual plants such as goosefoots, Marsh Yellow-cress and Marsh Cudweed, which, when reflooded, provide a rich source of seeds which may attract Teal and other waterfowl (see 10.3 (Chapter 10 - Wetland designs for wildlife)). Where new water supplies are being proposed, then consideration should be given to constructing two or more reservoirs that can be used independently of each other, thus enabling one reservoir to be left untapped each summer while the other is rejuvenated through a major draw-down.

Return to top of page

Feature 11.1 East and West Lakes, Spondon, Derbyshire(Courtaulds)

The chemical plant at Spondon requires 45 million gallons of water a day for cooling and processing. Since about 1920 this supply has come from two specially created lakes which were excavated in low-lying land next to the factory. The lakes receive their water direct from the River Derwent which encircles them. The lakes are used principally to settle out sediments from the river water, resulting in a substantial deposition of silt, especially in the first (West) lake. These lakes have been known to attract over 1,000 ducks (principally Teal) during the winter, especially during freezing conditions when the constant through-flow of water ensures that at least the centres of the lakes remain ice-free. Traditionally, the lakes have been dredged every 5 to 7 years; the silt being removed using a suction dredger and deposited off site. However, no dredging took place during the 10 years up to 1993 and the turbid water became devoid of macrophytes. No doubt, as a consequence, bird usage has declined in recent years.

The river water also brings fish with it, including carp and Tench, which can cope with the silty conditions in the lake and which tend to exacerbate the turbidity problem and further inhibit plant establishment. The immigration of fish does, however, provide food for Cormorants, Grey Herons and Great Crested Grebes.

Figure 11.1 Map showing the layout of the East and West Lakes, Courtaulds, Spondon.
Click Illustration for full-page view

Return to top of page

 Feature 11.2 Eglwys Nunydd Reservoir, West Glamorgan (British Steel)

This 102 ha reservoir supplies the British Steel plant at Port Talbot. Although concrete-banked it is one of the most important freshwaters in South Wales for waterfowl. Almost all of the 300-400 waterfowl found there in mid-winter are diving birds (mainly Tufted Duck, Pochard and Coot). The bird population is maintained despite the reservoir being used for both sailing and fishing. There are several factors that enable the birds to co-exist with the recreational activities:

  • The reservoir's large size allows birds to swim to quiet areas when there are low levels of use.

  • It is only 2 km from the freshwater lake at Kenfig Nature Reserve.

  • It is only 2 km from the alternative refuge provided by Swansea Bay.

However, it is likely that higher numbers and a greater diversity of birds would use the reservoir if it was not disturbed by recreational use.

Figure 11.3 Map showing location of Eglwys Nunydd Reservoir in relation to other waters.
Click Illustration for full-page view

Return to top of page

Feature 11.3 Reservoir at Fiddlers Ferry Power station, Cheshire (Powergen)

The Fiddlers Ferry site includes a 16 ha concrete-banked reservoir which is used mainly to top up the supply of cooling water. The power station loses about 2% of its cooling water daily through evaporation while another 2% is returned to the Mersey. The water is abstracted from the Mersey Estuary about a mile upstream of the power station, at a point where the silt load is relatively low (typically c.400 ppm). The main function of the reservoir is to lower the silt load; a considerable proportion of the reservoir's 15 m depth is now filled with deposits of silt. The water is pumped into the reservoir from the year-round source, allowing the water level to be maintained within 0.3 m of its maximum capacity, and the silt beds are therefore rarely exposed. The point of abstraction is close to the head of the estuary, where the water tends to be brackish, and this is reflected in the presence of both freshwater and estuarine fish.

Owing to its quiet location at one extremity of the 320 ha operational site and its proximity to the Mersey Estuary (which is itself of international importance for wildfowl), the reservoir is well used by duck, regularly attracting over a thousand birds in winter. Some species, such as Mallard and Teal, use the reservoir mainly as a safe refuge when disturbed from the river by wildfowling and other activities. Diving ducks, such as Tufted Duck and Pochard, also feed there; this suggests that the salinity is both low enough and constant enough to support brackish water invertebrates such as snails and chironomids. In recent years the site has also attracted large numbers of Shoveler (max. 440) which appear to sift through the fine scum made up of the small floating PFA particles (cenospheres) which over-flow from the adjacent lagoons. These observations suggest that this artificial, but inert, scum may provide suitable habitat for invertebrates. Other duck species, however, avoid the scum-covered water (see 13.1.5 (Chapter 13 - Ash (PFA) storage)).

The site assessment carried out by the Mersey Valley Partnership and the Groundwork Trust recognised the limitations of the reservoir design and recommended the addition of floating islands and investigating the establishment of emergent plants around the edges.

View across the concrete-lined reservoir at Fiddlers Ferry Power Station.
Click Photo for full-page view

Return to top of page

Feature 11.4 Belvide Reservoir, Staffordshire (British Waterways Board/West Midland Bird Club)

This reservoir was constructed in about 1834 to feed the Shropshire Union Canal by damming a tributary of the River Penk. It is recognised as the tenth most important reservoir for wildfowl in Britain (72 (Appendix 11 - Selected references and further reading)), regularly holding over 3,000 birds during the moult period and 4,000 in winter. Since 1977, the reservoir has been partly managed as a nature reserve by the West Midland Bird Club, members of which have carried out extensive studies of the birds and other wildlife on the site (82 (Appendix 11 - Selected references and further reading)).

The extent to which water levels have dropped below the top level has been influenced by boat usage on the canal and maintenance works on the reservoir itself; the annual fluctuations have ranged from 0 to 4.1 m. When fully flooded, the surface area of water covers 74 ha, while a drop of 3.8 m reduces this by half, owing to the shallow bank gradients (between 1 in 13 and 1 in 112). Observations have shown that the largest numbers of breeding and, more especially, moulting ducks (including Shoveler, Pochard, and Tufted Duck) have tended to occur in summers with high, stable water levels ( <O.5 m draw-down ); at such times large beds of submerged aquatic plants develop around the edge of the reservoir. On the negative side, these conditions limit the habitat available to waders returning south on their autumn migration, although the numbers involved are never of great significance from a conservation standpoint. However, the value of occasional larger draw-downs was recognised as a means of rejuvenating the marginal shallows ( see 6.5 (Chapter 6 - Water: the physical environment)) and the largest wildfowl counts occurred either as the reservoir reflooded, in the case of dabbling ducks such as Teal, or about a year later, in the case of diving ducks. It was felt that these draw-downs drastically, but temporarily, reduced the populations of freshwater snails which are a preferred food source for Tufted Duck and Goldeneye. An ideal water regime, based on these observations, was proposed for the reservoir in years between major draw-downs (Figure 11.4).

While this regime is not immediately transferable to other sites, as the critical depth for draw-down will be influenced by the bank gradients, the principles might be relevant to other waters with similar bird interest.

Figure 11.4 An ideal water regime proposed for Belvide Reservoir in order to maintain the site's value for moulting and wintering waterfowl and waders on autumn migration. (Source: Ref. 82. Appendix 11 - Selected references and further reading).
Click Illustration for full-page view

Return to top of page