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Chapter 11 - Water storage reservoirs
11.1 Potential constraints
associated with reservoirs
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. Return to top of page 11.1 Potential Constraints associated with reservoirs
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.
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)).
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).
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.
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.
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).
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)).
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.
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