An important impact of urban development is the reduction of the permeability of the land surface by replacing free ground (permeable) with impermeable areas. This not only reduces the infiltration capacity of soil, it also decreases evapotranspiration due to the removal of green coverage. Local bodies can integrate sustainable urban drainage systems within their strategy to reduce this environmental impact with other practices.
From a historical point of view, most cities started on the water management path by ensuring water supply. Over time, they incorporated additional measures such as sewerage, drainage and, more recently, pollution abatement and natural resource protection. The ideal end point of the sustainable transition framework is the water-sensitive city – a city where water management is adaptive, multifunctional and aligned with the principles of urban design. The EU Water Framework Directive is in line with this approach and, consequently, an integrated approach is needed within the urban water cycle.
In general, drainage systems in local, regional and national policies should:
- be part of a holistic approach to manage the water environment, which should be based on river basins
- integrate quantitative and qualitative objectives – all discharge activities of urban run-off will need to be managed to avoid damage to the receiving environment (e.g. surface water).
Relevant policy approaches to avoid soil sealing and to improve water drainage are:
- reduction of urban sprawl (e.g. Austria)
- reuse of brownfield land (e.g. Belgium)
- protection of the best agricultural land and landscape fragmentation (e.g. Czech Republic)
- management of flood risks (e.g. Germany)
- water drainage (e.g. Ireland, UK).
Taking an overarching approach can be considered best environmental management practice in land planning. Surface water drainage systems, developed in accordance with a sustainable development policy, managing environmental risks resulting from urban run-off and contributing to environmental enhancement are the best examples of the developed Sustainable Drainage Systems philosophy in the UK. This is an exemplary approach, consisting of a three-way concept (improve water run-off quality, optimise water quantity and maximise amenity and biodiversity), replicating, as closely as possible, the natural drainage of sites before development.
Techniques used in the water drainage management of a new development should be adopted according the following hierarchy:
- Prevention: good design practices and site housekeeping measures prevent run-off and pollution (e.g. rainwater harvesting or dust removal).
- Source control: highly polluted (or large volumes of) run-off should be controlled at their source.
- Site control: water flows on site should be controlled (e.g. piping water flow to infiltration or detention basins).
- Regional control: send excess run-off water to a pond or wetland, shared by several sites.