Open water bodies - mesotrophic and eutrophic standing waters in the UK Biodiversity Action Plan – usually occur where gravel pits are situated in river floodplains. Many are important for a range of wetland wildlife. Clay and some chalk extraction can also create open water habitat. Opencast coal sites are suitable where the overburden includes considerable clay measures.
General design of large open waterbodies
There are several general design principles for large open waterbodies:
- Avoid use of fertile topsoil during habitat creation as this can be detrimental.
- Create as much shallow water as possible (< 1.5 m depth), especially away from future overhanging trees.
- Maximise edge habitat by creating scalloped or sinuous margins.
- Create low wet islands, which ideally can be immersed to manage vegetation.
- Provide conservation features in shelter from the wind to reduce wave erosion.
- Create together with other wetland habitats - ditches, reedbeds, marsh and wet grassland - for optimum biodiversity benefit.
- Create gently sloping to flat beaches of bare sediment - important for predatory invertebrates like ground beetles and wolf spiders.
Landform and island design
- Initial land-forming is vital:
- Reduce human and predator disturbance, by including earth mounds, screens, and moats.
- Factor in the potential effects of wave and wind erosion. Note that limited wave erosion may help to form and maintain beaches.
- Unstable sandy and silt margins support rich invertebrate fauna.
- Tree plantings add shelter, reduce erosion and add riparian habitat, but can enclose a site reducing its value for waterbirds. Plan with care to provide a balance.
Off shore islands provide disturbance-free nesting for ducks, waders and terns. They will usually require annual management to maintain open character; this should be considered during design. Islands should be located in sheltered areas – this is normally the south-west ‘corner’. They should be low in profile, with shallow gradients into the water and extensive shallows, and they should have small pools of shallow open water within them.
Naturally vegetated, clay-capped islands attract redshanks and lapwings. Gravel will attract ringed plovers and common terns. Steep, naturally eroding sunny faces are important for invertebrates and as nest sites for kingfishers and sand martins.
After working, do not bulldoze de-watered pits into flat basins prior to flooding. Keep de-watering trenches and piles of clay. These provide a valuable diverse topography as resource for wetland species to colonise the lake.
Silt lagoons have the potential to develop excellent wildlife habitats. The pumped silt forms deltas extending into the lagoon that consolidate and settle over time. By manipulating discharge pipes and channels, optimum levels and landforms are created for a variety of habitats.
Silts deposited around the margins of pits, using a mobile, floating discharge head, create shallow margins to benefit waterbirds. The damp sand is also important for invertebrate (esp. beetles) and plant communities. Colonising vegetation is partly suppressed by fluctuating water levels and wind and wave erosion. However, natural succession will gradually take over without intervention. These areas could be left to develop a reed fringe.
The wet silt in active silt lagoons is ideal for colonising willows which will quickly succeed to dense scrub. This can become excellent, if small, areas of wet woodland are retained where water levels are high.
Creating shallow water and edge habitat
Shallow water holds most wildlife, therefore it should be maximised. Exact knowledge of eventual water levels and regimes, and supply of fill material (topsoil, overburden and silt) is needed, and should take into account the extent of summer drawdown.
Edge habitat should be gently sloping (1:20) and less than one metre deep. Ideally create an edge with micro-topographical variations for a fringing strip with pools and shallow-flooded grassland.
- Topsoil is generally high in nutrients and should not be spread in shallow waters. Encourage natural colonisation instead of planting of aquatic vegetation.
- Diverse, valuable aquatic vegetation communities, including e.g. Stoneworts, are more likely to develop on nutrient-poor substrates. Elsewhere, the nutrient flush can eliminate important early successsional stages.
- Bare, unshaded shores are important for plants and invertebrates. Use washery silt to create shallows where marsh vegetation can develop
Where large volumes of inert fill are available, shallow water can be created by back-filling. Sufficient fill is only usually available where heavy industry, power generation facilities or development (e.g. house building) occurs. The quality of fill material needs to be thoroughly checked.
Inert fill can be used to enhance existing open water by creating more shallow areas, as well as small, low islands. Water level control allows ideal management of created islands; ideally these can be flooded in winter to at least 450mm to control terrestrial plants, leaving exposed muddy margins for migrating waders.
Ponds and scrapes
Ponds are immensely important for aquatic flora and fauna, supporting 65% of all the UK’s wetland plants and animals. New ponds can, within five years, be colonised by many wetland plants and invertebrates. There is value in creating ponds anywhere water can be retained, and where possible, they should be created in clusters of different depths and shape.
Good semi-natural pond habitats can be created easily by following the key principles of pond design:
- Avoid/ minimise exposure to water pollution. Create new ponds where their catchments can be managed non-intensively.
- Dig trial holes before the pond is created to determine water levels and substrate.
- Avoid linking the pond to enriched or polluted inflowing streams and ditches.
- Preferably, create new ponds close to existing wetland areas but not in existing habitat.
- Pond mosaics within wetland complexes are preferable to single isolated waterbodies.
- Include some very shallow temporary pools with a depth of no more than 20cm, as well as semi-permanent and permanent ponds.
- Do not connect shallow and deep water pools – to avoid predators from one affecting species in the other.
- Vary the size of waterbodies as much as possible.
- Maximise edge habitats and the extent of the drawdown zone.
- Generally the edges of a pond should be shallow: e.g. 2cm / metre (1:50).
- Create ‘hummocks and hollows’ in marginal areas where water levels fluctuate to maximise the hydrological diversity of this rich area.
- Islands add edge, and where the margins are shaded, trampled or grazed, they add different habitats for invertebrates and plants.
- Planting up is rarely necessary as colonisation is usually rapid, especially when other wetlands are close.
- Pond creation is often best considered as a two-phase process, with fine-tuning of the structure of the drawdown zone and margins one to two years after the first construction phase.
- Pond management may be needed during early colonising stages to ensure that one or two plant species do not dominate the new site.
The water supply should be good quality. Ponds fed by poor quality surface water may need buffering. Buffer zones should be:
- > 20 m wide to avoid spray drift from arable farming. In early colonising phase the high proportion of annual plants and seedlings are vulnerable
- Positioned to intercept flows from higher ground
- Checked to ensure they are not compromised by surface any temporary surface flows.
Annually flooded pools are one of the most stable freshwater environments, but are exceptionally scarce and support rare species – these can be built into areas where seasonal flooding will occur
Seasonal pools are exceptionally important for aquatic plants - 25% of the wetland plants protected under the Wildlife and Countryside Act 1981 are restricted to this habitat - and invertebrates. Natterjack toads also depend on temporary pools.
Early phases of colonisation are important for pioneer species that are absent in later stages. Planting limit these species, and could spread non-native species as fragments among other vegetation. It is therefore best to establish vegetation through natural colonisation.
Generally, the best long-term management is to allow gradual, natural succession, whilst excavating new ponds as old ones become overgrown. When left alone ponds will fill with sediment, become shallower and have changing communities. This process needs to occur as a number of specialist invertebrates thrive at different stages of succession.
Where individual species of plants dominate, removing a proportion may promote species diversity. Any alien species, e.g. New Zealand swamp stonecrop or water fern, should be completely removed as a priority.
Relevant case study
The information set out within this advisory sheet in no way constitutes legal or regulatory advice and is based on circumstances and facts as they existed at the time Nature After Minerals compiled this document. Should there be a change in circumstances or facts, then this may adversely affect any recommendations, opinions or findings contained within this document