They are best used for new developments, such as: where intense development has already occurred and the traditional system is not coping: where there are other reasons why localised flooding, and water pollution are a foreseeable issue: or where one simply wants to use best possible sustainable practices. Sustainable drainage systems may also be used where there is redevelopment or regeneration. It’s best for developers to incorporate drainage thinking into plans early on as there are implications for buying land and deciding on layout. Development may be new or redeveloped areas, individual household, housing estate, roads, offices, factories and so on.
Aims are achieved through: prevention and control at source, on site and at regional level. Techniques commonly employed include: secure substance, storage, safe waste disposal, minimisation of hazardous substance use; green roofs; rainwater harvesting; permeable pavements; infiltration trenches; filter drains; swales; basins; ponds and wetlands. In place of the most rapid water movement, as happens with traditional drainage and due to hard paving, hard roofing and water diversion into pipes...precautionary action is taken, water is absorbed, temporarily stored, allowed to settle, is more gradually released, filtered, subject to natural microbial action amongst stones or in soil, has nutrients removed by algae and plant action, has flow variations smoothed out.
Sustainable drainage systems facilitate appropriate development, allowing development in places already built-up, so relieving pressure to build on green land. They enhance water quality, biodiversity, leisure/recreational value and aesthetic appeal. There will be some costs in maintaining ponds, removing silt from sumps. Green roofing cuts the heat island effect, insulates heat and sound, provides habitats, and helps purify air. Upfront costs, maintenance, repair and fixing costs of green roofs tend to be higher. Sustainable drainage systems follow legal requirements in the EU Water Framework Directive and in Local Development Plans. They can save on construction and maintenance costs such as when swales replace roadside kerbs, often generally simplifying construction. They allow storage and use of water and recharging of groundwater when climate change is bringing more drought risk. Some additional costs may be incurred: in getting to know the legislation/regulations; in education and training in new techniques.
Good guide to sustainable drainage systems here (the source of the image top left), with links to good material on flood resilience and all sorts of other water-related issues.