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Chapter 7 - Substrates
7.1 Watertight linings
1) they dictate permeability and sub-surface water movements (see 6.3(Chapter 6 - Water: the physical environment) and 7.1);
2) they provide both a medium in which benthic invertebrates can live, and a source of organic matter on which they can feed (see 7.2);
3) they provide a source of plant nutrients and a medium in which plants can root (see 7.2 and 4.6(Chapter 4 - Wetland wildlife));
4) they influence the chemical nature of the overlying water (see(Chapter 5 - Water: chemistry and quality)).
When planning a new wetland one needs to decide how, ideally, the substrate should perform in respect of these various functions. Is there, for instance, a need for an impermeable substrate to create a perched water-table? Is it hoped that plants can be encouraged to grow right across the wetland? Is a nutrient-rich substrate likely to lead to eutrophication in the wetland? Having asked such questions, it is then necessary to find out whether the site offers the right materials to create the required substrate(s). If it does not, then consideration needs to be given to bringing in additional materials to replace or modify what is on site.Return to top of page
If the water-table does not lie close to the surface then an impermeable liner will be needed to create a wetland. The permeability of natural substrates varies tremendously, the finest clays (consisting of particles <0.002 mm) being entirely impermeable. The cheapest linings will be provided by clays available on site. The pros and cons of the commonly used liners are considered below.Return to top of page
The performance of clay as a liner is related to how fine and pure the particles are, while its price will be largely determined by transportation costs. High grade clay is still excavated commercially in a number of places. However, small quantities of suitable material may be generated on development sites and by other excavation works. In most situations clay should be applied in a layer at least 300 mm thick and carefully puddle (compacted) to eliminate cracks. It is best applied using a hydraulic excavator, which has the advantages of not needing to track over the material (which increases the risk of contamination by coarser materials) and of being able to exert considerable pressure. On larger sites a roller can be used to puddle the clay further. A roller can also be used to increase the impermeability of less pure clays occurring on site such that at least seasonal pools can be retained. To achieve maximum compaction, the clay should be slightly moist when applied; working in very wet or very dry conditions is likely to compromise the quality of the finish in terms of impermeability and resistance to erosion. Clay can be worked to create most contours and gradients and leaves a natural finish. However, a superficial lining of clay can spring a leak if it is allowed to dry out or if deep-rooted plants grow through it. Covering the clay with a material such as an organic soil which is good at retaining moisture, will help to reduce the threat of it drying out. However, it is better to avoid using clay to line wetlands that are likely to have highly fluctuating water levels, unless a considerable depth (>600 mm) can be applied.Return to top of page
Sheet liners have the advantage of being easily transported and handled. They vary in their tear resistance and lifespan according to, among other things, their thickness. Most types are likely to work satisfactorily for 25 to 50 years if fitted and treated properly. Polythene liners may be suitable for the smallest ponds where they can last up to 10 years or so, although they offer limited durability and perish rapidly when exposed to sunlight. Small ponds can be lined by a small work team while larger wetlands are normally lined by the suppliers. The main concern regarding artificial liners is their vulnerability to puncture. It is unwise to use vehicles or heavy machinery over butyl-lined sites unless the liner is protected by a considerable depth of soil and/or geotextile matting. The spreading of a capping material must be carried out with considerable care. Liners should be laid out over a layer of sand or matting 150 mm deep and protected by at least 150 mm of protective materials and soil. A deeper capping of at least 400 mm should accommodate the root growth from most wetland plants. Gradients should not be steeper than I in 3 in order to ensure that a stable capping can be maintained. It is difficult, therefore, to use such synthetic materials to line ditches and other steep banks without leaving the lining exposed.
This is a very fine powder of clay particles. Rolls consisting of bentonite sandwiched between layers of geotextile fabric are available, making its use much easier. By adding water, the powder is transformed into a very pure clay. Two basic forms are available: sodium bentonite and calcium bentonite. Sodium bentonite is imported from USA and is more expensive, but it is a higher quality material than calcium bentonite: it absorbs considerably more water (up to 5 times more, weight for weight), expands to a greater extent (4 to 5 times more) and is consequently more resistant to desiccation and the associated potential for cracking. 'Activated Bentonite' (calcium bentonite treated with soda ash) is also highly absorbent, but if it dries out and cracks it does not reseal and is more easily denatured by contaminants. The geotextile sandwich can support heavy machinery, and if cracked can be mended relatively easily. These rolls are much bulkier than the equivalent length of synthetic liner, but can be fitted around most contours. The relatively rough surface offered by these rolls helps soil to adhere and steep slopes can develop a more natural finish compared with the results obtained with synthetic liners.Return to top of page
Concrete has been used widely to create watertight tanks and ponds. It has the advantage that it can be mixed on site. However, it is rather difficult to contour, although, theoretically, shuttering and moulds can be used to make almost any shape. An engineer should be consulted over the concrete required to line all but the smallest ponds; for areas larger than 3 mē, metal or PVC joints should be incorporated into the structure (see Ref. 13(Appendix 11 - Selected references and further reading)). Despite being a very robust material concrete will crack if, for instance, the ground underneath subsides. As with other liners, leaks can be surprisingly difficult to trace and repair. Concrete can be made with a high percentage of gravel, or stones can be embedded in it, to give it a more natural finish. Fresh concrete is toxic, although this problem is easily remedied by washing or allowing it to weather. Return to top of page
The material lying on the surface of a wetland will strongly influence the community of benthic invertebrates and the growth of rooted plants. Broadly speaking, the more complex the surface in terms of cracks and crevices, the more niches it is likely to offer to aquatic life. The presence of leaf litter, gravel shoals and tree stumps all offer cover ('refugia') which may be fundamental to the survival of some aquatic species.
Wetlands are characterised by waterlogged soils. Because the interstices of soil particles are filled with water, only a very thin surface layer (typically 1-5 mm) has sufficient oxygen to maintain aerobic/oxidising conditions. Consequently chemical processes, including the cycling of organic matter, take place at much slower rates (often less than one tenth the rate) than occur in terrestrial soils. The chemical nature of the substrate may affect plant growth and the chemical attributes, including pH, of the overlying water. Rooted plants will be particularly influenced by phosphate availability and the presence of toxic substances (see 5.6(Chapter 5 - Water: chemistry and quality)). In clay soils, for instance, many substances are chemically bound to the particles and therefore unavailable to plants. Many invertebrates will depend upon the presence of organic matter in the substrate.
There will usually be some scope for influencing the nature of the surface substrate in created wetlands. The value of different substrates to wildlife are outlined below.
1) Bare rock and concrete. Rooted plants are generally absent. Many invertebrates of fast streams have adaptations that allow them to attach themselves to bare rock surfaces and capture detritus carried in the water current (see 6.2(Chapter 6 - Water: the physical environment)). By contrast, there are few invertebrate species in bare stillwaters. Algal films form on submerged surfaces, while mosses colonise the fastest streams and splash zones. Many waterbirds like to loaf on bare, fairly flat surfaces by the water's edge.
2) Gravel and pebbles. The species composition and density of rooted plants are influenced by the size of the stones and the extent to which the interstices fill with silt and debris. Where there are strong currents or wave action, gravel banks will tend to shift around, inhibiting the establishment of plants. Many invertebrates can find shelter in the cracks between stones; however, the porous nature of such materials means that detritus is easily washed out and, therefore, the food supply for subsurface feeders is likely to be unreliable. Salmon and Brown Trout spawn in gravel beds in fast-flowing streams. Shifting gravel banks above the water-line offer a home to some specialist invertebrates that live off the debris washed ashore. A few birds, such as the Little Ringed Plover, select such banks on which to nest. In flowing waters, occasional larger stones and boulders provide important structural diversity and create pockets of still water in their lee.
3) Bare clay. Very pure, compacted clays present an inhospitable environment for most forms of life; under water it can remain bare for several years and even algae can have difficulty adhering to the smooth surfaces. Only the roots of the most robust plants, such as Common Reed and docks, are likely to be able to penetrate compacted clay.
4) Loamy soils. A well textured, reasonably organic soil provides an appropriate substrate in many wetlands; it offers a stable, well-structured medium capable of supporting high densities of benthic invertebrates and offers the best medium for rooted plants. The level of organic matter in the soil will influence the composition and abundance of invertebrates and the moisture retention properties of the soil when it is exposed to the atmosphere. These soils also support the highest densities of earthworms and are, therefore, preferred by waders of wet meadows. They aye less resistant to erosion by water than the substrates described above, although more likely to be colonised by protective vegetation. Such soils may have high concentrations of plant nutrients which can lead to eutrophication of waters and the growth of dense, rank vegetation above water.
5) Sand. In areas of still water, sand can provide a good substrate for benthic invertebrates and some rooted plants. Above water it is free-draining and plant colonisation may therefore be confined to drought tolerant species. The surface of bare sand warms up relatively quickly in the sun compared with vegetated and damper surfaces, and is therefore preferred by heatloving insects such as solitary bees. Pulverised fuel ash (PFA) offers similar properties (see(Chapter 13 - Ash (PFA) storage)).
6) Silt. Characterised by being soft and mobile, wet silt can be too liquid to support the burrows of benthic invertebrates or the roots of plants. Not only do fine silts pose structural problems but they are also readily resuspended by water turbulence. In suspension, sediments can interfere with the respiratory organs of animals and reduce light penetration, affecting plant growth. Silt can contain high levels of organic matter which may be utilised by high densities of some invertebrates or generate high levels of BOD and, ultimately, anaerobic conditions in which only bacteria can survive.
7) Peat. Peat has a large capacity to retain water; it acts somewhat like a sponge, expanding as it absorbs water and drying out very slowly. Many specialised plants and animals are associated with the bogs and mires that can form on peatlands. Every effort should be made to conserve the few peatlands that remain. Attempts are being made to recreate bogs, mires and fens in a number of areas affected by peat extraction. While the importation of peat onto a site is discouraged, very occasionally layers of sub-surface peat are found on a site which can be used in habitat creation (see Refs. 29,76 and 93(Appendix 11 - Selected references and further reading)). Return to top of page
In many situations the substrate that forms the bed or shores of a wetland is not ideal for the target wildlife. When excavating new wetlands or restoring land used for mineral extraction, for example, there may be insufficient top-soil to cover the re-graded site. There are three main options available for improving the wildlife value of a substrate:
1) Cover with an appropriate substrate transferred from elsewhere on the site. More suitable materials may occur elsewhere on a site; piles of soil, rubble or gravel may have been dumped on an industrial site, and areas being excavated may reveal underlying strata that have suitable properties to act as a soil-forming material (see 16.1.1(Chapter 16 - Reclaiming industrial land)).
2) Cover with suitable materials imported from off site. The factor most likely to determine the feasibility of importing materials is the transportation cost involved. If there is a suitable source nearby, or only very small quantities are needed, it may be realistic to import materials. It may, for instance, be possible to obtain top-soil being generated from an adjacent development site for a nominal cost.
3) Ameliorate the substrate by mixing it with other materials. A variety of materials can be added to an existing substrate to modify its structure or increase its organic content.
Some more specific techniques for overcoming certain problems commonly associated with the substrates of artificial wetlands are summarised below.Return to top of page
Sub-soils and lower strata contain little or no organic matter, and hence offer only low levels of plant nutrients and little food for still water benthic invertebrates. A wide range of substances, including manure, compost, grass clippings, hay and straw, have been used to overcome such deficiencies, with varying degrees of success. Some characteristics of potential ameliorating substances are given in Table 7.1. In general, it is better to add organic matter to the floor of a wetland before it is flooded (or while it is kept drained). By giving such materials time to rot down aerobically, the flush of nutrients into the water should be reduced, and the material is less likely to float and/or wash away. To overcome the latter problem, netting can be laid temporarily over the organic layer, ensuring that it is not likely to ensnare birds, or soil used to weight it down. Unlike many sources of organic matter, straw contains relatively low levels of plant nutrients and has the added advantages of releasing chemicals that inhibit the growth of algae and offering good structure as habitat for many invertebrates (see 9.3.6(Chapter 9 - The establishment and management of wetland plants and animals)). However, straw must be soaked for several weeks, or even months to prevent it floating, and therefore considerable care needs to be taken over its introduction. Another good method of introducing a textured organic layer is to sow a I sacrificial crop' or green manure before flooding the wetlands To establish vegetation, even just a grass crop, on a substrate lacking organic matter will normally require cultivation and the addition of fertilisers. The inclusion of leguminous plants, such as clovers, in the seed mix should speed up the development of a for invertebrates a covering deposit need not be very worthwhile sward as they are able to fix atmospheric nitrogen (see ref. 11. (Appendix 11 - Selected references and further reading)) Return to top of page
Table 7.1 Plant nutrients in common manures and organic fertilisers. Figures represent percentage dry weight of each major nutrient (Nitrogen (N), Phosphorus (P), and Potassium (K)). (Adapted from Ref. 11.(Appendix 11 - Selected references and further reading))
It is much easier to introduce nutrients into a wetland than to remove them, and any attempts to do so should, therefore, not be undertaken lightly (see 5.8.2 (Chapter 5 - Water: chemistry and quality)). Before all else, an assessment should be made of the current nutrient status and surveys carried out to determine whether any interesting species are present that might be adversely affected by increased levels (see 3.3 (Chapter 3 - Deciding what to do)).
An increase in nutrients is best achieved by introducing a suitable source of organic matter such as well rotted manure (see above). Chemical fertilisers tend to be relatively soluble and cause a dramatic, but brief, flush of nutrients that will favour algae. Additionally, they are easily washed out of a system. Nutrients are often bound up in wetland sediments owing to prevailing anoxic conditions; these can be released by exposing the bed to the air (see 6.5 (Chapter 6 - Water: the physical environment)).
Smooth substrates such as bedrock and compacted clay may benefit from being covered with a sandier or more soil-like material in order to provide a suitable medium for rooting plants and aquatic invertebrates. In catering for invertebrates a covering deposit need not be very thick; most chironomid larvae, for example, live in the upper 100 mm of substrate. Straw is a suitable surfacing material where the aim is to increase invertebrate populations (see 7.3.1 above). The depth of substrate required by rooting plants varies according to the species being encouraged. In general, emergent plants are deeper rooted than submerged plants; however, a soil capping of 300mm depth will cater for most species. At Amwell Gravel Pits (see Feature 16.1 (Chapter 16 - Reclaiming industrial land)) the gravel washings have been used to provide areas of finer substrate. Recent work has found that by simply breaking up compacted clays using a rotavator, densities of chironomid larvae and other benthic invertebrates can be increased.
At a coarser level, various materials can be placed on the bed of a waterbody to create an artificial reef. Such reefs can provide attachment points for invertebrates and algae and sheltered water and refuges for fishes and other free-swimming creatures. Artificial reefs can consist of all sorts of materials; the more complex the structure the better. Bundles of twigs and branches have been used successfully for this purpose, as have a variety of manmade, non-toxic, waste materials and structures. If the materials used are likely to decompose they should be replaced on a regular basis in order to sustain the aquatic community. Hardwoods, such as oak and elm, would be suitable for constructing longer-lasting, purpose-built reefs with a more natural texture. The construction and location of reefs should be considered in the light of other site uses; reefs could, for instance, interfere with the use of boats or fishing lines.
Many industrial wetlands suffer from very silty bed where it may be helpful to create areas of more stable substrates. One potential solution is to occasionally drain the wetlands thus enabling both organic and mineral
components to dry out and oxidise. Another solution is to add areas of coarser, less mobile materials that project
above the bed of silt. In order to encourage benthic invertebrates, piles of rubble and stones (<100 mm diameter) can be introduced; the interstices offer suitable, stable niches.
It is not unusual for more than one of these problems to be exhibited by a particular substrate. Smooth, hard substrates, for instance, are often lacking in organic matter. The chosen solution may, therefore, have to incorporate a variety of ameliorative materials.
Some schemes may require very specific substrates for specific purposes. Gravel (5-20 mm in diameter) is often used to surface banks and islands chosen to attract nesting Common Tern and Ringed and Little Ringed Plover or to provide loafing areas for waterbirds. If gravel does not occur naturally on site, other local materials may offer similar qualities. It may, for example, be more cost-effective to use builders' rubble or stone chippings as a substitute substrate, topped by a small amount of imported gravel to improve the visual appearance. All materials used for this purpose need to be free of soil or dust in order to limit the potential for plant root development; such features need to be kept free of vegetation. A minimum depth of 150 mm of material should be spread over a synthetic liner, such as heavy duty, black Polythene or a tightly woven geotextile fabric, in order to prevent plants rooting in the underlying soil. The liner can be omitted where the layer of clean gravel or stone is more than 300 mm deep. Large stones or 'pebble-dash' concrete should be placed along the water's edge in order to prevent wave erosion (see 6.8.2 (Chapter 6 - Water: the physical environment)).
The top-soil covering a site potentially offers a suitable substrate for capping a new wetlands To maintain the
beneficial properties of top-soil it needs to be handled and stored very carefully. Many soil organisms which help to maintain the condition of the soil are destroyed by incorrect stripping, storage and compaction. Earthworms are particularly important; while breaking down and distributing organic matter, they also aid the circulation of gases and water through the soil. Clayey soils need particularly careful handling; they compact very easily, resulting in anoxic conditions and a poor rooting medium for plants. By following a few simple principles, a high quality soil can be maintained.
a) Avoid running vehicles and machinery over soil-covered areas in order to prevent compaction.
b) Identify suitable sites for stockpiling soil prior to commencing work. Look for sites which can be left undisturbed, but which can be easily accessed when the soil is required for re-spreading.
c) Store top-soil and sub-soil separately. Additionally, different soil types should be kept in discrete mounds.
d) Where soil is to be used to restore terrestrial habitats, including wet meadows, it should be stockpiled in mounds as low as practicable. At depths greater than about 300 mm, especially in clay soils, earthworms will die out, together with the soil microfauna and bacteria. Once destroyed, it can take a long time for such soil fauna to re-establish. Ideally, soil mounds should be sown with a grass-clover mix if they are to be left for more than a few weeks, in order to reduce erosion and prevent colonisation by persistent weeds. Top-soil destined to line ponds and lakes needs less critical attention; once saturated much of the fauna associated with a terrestrial soil will die anyway. Where, however, sediments are being transferred from the bed of an existing wetlands they should be kept moist and moved in the shortest time possible.
e) Soil should be re-spread loosely and allowed to settle under its own weight.
f) Compacted soils can be improved by using techniques such as ploughing or sub-soiling which are undertaken by many agricultural contractors. The depth of cultivation should be influenced by the make-up of the substrate.
All but the very smallest wetlands are best dug using heavy machinery such as excavators; not only are they cost-effective but they also offer the most accurate means of obtaining the required gradients in most situations. Some information on earth-moving machinery is given in Appendix 8. Heavy machines do not usually work efficiently in wet conditions, and depending on the substrate, work may have to be suspended if a site becomes waterlogged. It is, therefore, advisable to aim to carry out such works between July and September, the most reliable period for dry conditions. Few heavy plant contractors are likely to have much experience of wetland creation, and some of the concepts used to encourage wildlife may be alien to their way of working. It is important, therefore, to brief the machine operators fully and to provide someone conversant with the objectives of the project to supervise the work. Any mistakes made at this stage can be very costly to rectify.
Grading wetlands for wildlife generally requires much less precision than most engineering projects; unevenness in the bed of a lake, for instance, will often benefit wildlife. (Guidance on the gradients to incorporate into wetlands for wildlife are given in (Chapter 10 - Wetland designs for wildlife).) However, there will be situations where design specifications need to be closely adhered to, such as the construction of wetlands for wastewater treatment, water retaining embankments and water level control mechanisms.
Excavation inevitably leads to the generation of spoil. The ease with which spoil can be 'lost' on site will be influenced largely by the size and topography of the site. It is quite likely that the design of a wetland may in part be dictated by the opportunities for spoil disposal. Exporting spoil off site is advantageous in that it leaves a larger area for wetland creation and results in a more natural landscape; however, it can add very significantly to the earth-moving costs and would normally require a licence from the local waste disposal authority. The next alternative to consider is the opportunity for losing spoil in the construction of useful features; islands and shallows in existing wetlands, screening embankments, raised walkways, foundations for hides, viewing areas and car parks could all be constructed using excavated spoil.
Where spoil cannot be put to a good use, the location and design of mounds should be considered in the light of the following recommendations:
Site as far as possible from the shore of any waterbodies designed to attract waterbirds
Avoid casting significant shade across the wetland
Avoid mounding spoil on top of islands (steep-sided islands are of little value to nesting birds and look unnatural)
Make every effort to blend mounds into the landscape by, for instance, using tree and shrub planting to disguise mounds. See 8.7 (Chapter 8 - Other aspects of wetland design).
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