Ponds, Pools and Lochans

7.2 Ecological design principles to maximise the nature conservation value of constructed retention ponds and wetlands

As in all pond design, there are three key factors 12 which influence the quality of SUDS ponds for nature conservation purposes:

  • water quality
  • proximity to existing wetlands and freshwater habitats and
  • physical shape and structure.

Clearly, since most SUDS schemes are primarily created to deal with contaminated surface water runoff, maintaining good water quality in SUDS ponds is a key issue. It is not, however, an insoluble problem. Much can be done to ensure that at least parts of a waterbody or series of waterbodies have good water quality. In addition, good design and location can be used to partly mitigate water quality impacts.

SUDS ponds and wetlands incorporating the full range of new pond design techniques have considerable biodiversity potential, including the ability to provide habitats for species of high nature conservation importance (e.g. Biodiversity Action Plan species). Experience indicates that relatively small refinements to traditional pond designs, based on new understanding of pond ecosystems, can lead to big improvements in the ecological quality of ponds.

A list of 20 suggestions for improving the ecological design of ponds incorporated into SUDS schemes are given in Box 10. The main principles behind these suggestions are discussed in more detail below.

Box 10. Twenty ways to maximise the nature conservation value of SUDS ponds
This box describes ways to maximise the nature conservation value of new ponds in Sustainable Urban Drainage Systems (SUDS). Clearly SUDS schemes vary considerably in terms of their functions and constraints and not all of the features will be viable in all schemes. However, include as many as possible.
  • Maximise water quality reaching pond basins by fully implementing SUDS treatment sequences to prevent or ameliorate the export of pollutants into pond basins.
  • Where possible locate SUDS basins in, or adjacent to, non-intensively managed landscapes where natural sources of native species are likely to be good.
  • In particular, locate water treatment ponds near to (but not directly connected to) other wetland areas e.g. natural ponds, lakes and river floodplains. Plants and animals from these environments will be able to colonise the new ponds, and potentially recolonise after pollutant influx events.
  • Create habitat mosaics with sub-basins of permanent, temporary and semi-permanent ponds; vary these in size (from 1 ha down to 1m 2) and depth (1m down to 5 cm).
  • Ensure that some ponds, or parts of basins, are not exposed to the main pollutant burden allowing many more sensitive animals and plants to exploit some parts of the site.
  • Create small pools around the margins of larger ponds which are fed by clean surface runoff from non- intensively managed grassland, scrub or woodland on the basin sides.
  • Create shallow grassy ponds along swales and floodways, particularly towards their cleanest ends - pools just 1 or 2 metres across and only 10 cm deep will be valuable for wildlife.
  • Maximise the area of shallow and seasonally inundated ground dominated by emergent plants - these are generally more tolerant of pollutants than submerged aquatic plants. To do this, create very low slopes at the waters edge (e.g. 1:50) and try to avoid fixing pond levels at a predetermined height.
  • Create undulating hummocky margins in shallow water; these mimic the natural physical diversity of seminatural habitats.
  • Avoid smoothly finished surfaces as traditionally used in ditch, drain and river engineering; although giving an impression of tidiness, they provide less physical habitat diversity for plants and animals.
  • Plant trees scrub and wet woodland around ponds: these provide a valuable habitat for amphibians, a food source for invertebrates and tannins from decaying bark which will help to suppress algal blooms.
  • Encourage development of open, lightly shaded and densely shaded areas or pools; this will add to the diversity of habitats available.
  • Encourage or install dead wood in ponds (anchor securely where necessary). Dead wood provides firm substrates for pond animals and can provide egg laying sites for dragonflies and other animals.
  • Encourage the development of mosaics of marginal plants (rather than single species stands) to maximise habitat structural diversity.
  • Avoid planting-up ponds (other than the plants needed for the water treatment function of the pond or the creation of safety barriers). This will allow native plants more opportunity to colonise.
  • Dont plant non-native water plants, trees, shrubs or grass mixes; take special care to avoid invasive alien plants such as Crassula helmsii by dealing with nurseries that only deal in native stock.
  • If planting is essential, stick to native plants of local origin. Include species which are wildlife friendly e.g. grasses such as Glyceria fluitans (floating sweet-grass) and Agrostis stolonifera (creeping bent).
  • Check planting schemes 1 and 2 years after establishment to ensure that specifications have been carried out and undertake immediate remedial action if invasive alien species are found.
  • Consider whether grazing livestock can be given access to ponds; grazing has been shown to be a viable and effective way of managing some SUDS schemes in agreement with conservation organisations or farmers.
  • Wherever possible include a brief post-implementation stage about 1 year after SUDS creation. Use this to (i) undertake fine-tuning of the pond design and (ii) capitalise on new opportunities that have arisen (e.g. pooling of natural areas of standing waters or natural seepage areas etc.). Fine tuning of this sort costs very little but will often greatly increase the biodiversity value of a SUDS scheme.

12. See Section 5 for further information on these factors

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