Usually this term is applied to new "greenfield" developments, largely in urban areas, where it is a Planning requirement that the surface-water run-off from any new buildings and other hard surfaces must not add to the existing downstream flood-risk, after already taking into account how that risk will be increased by climate-change. Where a "brownfield" redevelopment is to take place, it can be an additional requirement to restore the flood-risk to what it was before the original development took place.
A Suds System is the arrangements put into place by a Developer to deliver sustainable drainage. Proposals in this respect will need to be signed-off by the local Planning Department (usually on the advice of the Environment Agency), and implementation of the finished system signed-off by Planning Control Officers
SuDS Systems are usually based upon the two main principles of "Source Control", which means not allowing surface water to leave the piece of land or hard-surface on which it falls or, where that is not possible, "Attenuation" which means temporarily storing water where it falls, then slowly releasing it later when downstream drainage infrastructure can deal with it without posing a flood risk.
Taking the building of a new house as an example, rainfall can be retained on the plot by a combination of collecting some for later re-use (ie RWH), making hard-surfaces porous wherever possible, and setting aside part of the garden to collect water when necessary (ie a water-garden feature). On housing developments, the development as a whole is taken as the source, rather than each individual property.
The need or otherwise to use attenuation to achieve Source Control will depend upon a number of factors, but is a regular requirement. Traditionally this has been achieved installing the necessary calculated additional water-storage capacity that fills during a peak weather event and then slowly empties at a rate with which downstream drainage infrastructure can cope. Modern best practice, however, is to use surface features such as swales (ditches) and/or balancing ponds to store this excess water as they are easier to inspect/service, and add amenity qualities and thus value to the development. A typical new housing development based upon these principles is illustrated below:
The extent to which undeveloped land retains surface-water when it rains will vary with soil-type and topography, but broadly the sequence when rain falls on a flat greenfield meadow is firstly the rain soaks into the ground, then it forms puddles in any undulations, before finally running into water-courses and causing a downstream flood risk. Once the land is developed, any rain falling onto hard surfaces such as roofs, paths, parking areas and roads immediately runs-off, causing a downstream flood risk
Droughts cause the ground to harden, making it similar from run-off point of view to a developed site; this aspect is exacerbated in the open countryside when the terrain slopes. Long dry spells are also often broken by rainfall of tropical intensity which, when combined with hardened sloping terrain, means that drainage infrastructure such as ditches, streams and rivers cannot cope and flooding occurs.
The underlying principle behind flood defences is to improve downstream drainage infrastructure so that it can cope with expected rainfall, in effect improving their ability to transport surface water out to sea without causing flooding. Without massive future investment in new infrastructure storage capacity (ie reservoirs), this water is no longer available during subsequent long dry spells unless expensively desalinated.
The agricultural industry provides a good example of how this can be done. The graphic below shows that throughout large parts of England south of the Humber estuary, the supply of water for agricultural purposes is already under great stress due to climate-change, exacerbated by exhaustion of historic ground sources, and loss of extraction rights as river-levels too fall during the summer months. To maintain the value of their land, therefore, farmers are increasingly needing to invest in farm-level reservoirs in which to store the still ample winter rainfall for later summer use. Typically, such reservoirs will be full by mid-winter, after which they will have no impact upon later downstream flood risks unless it becomes national policy to fund additional attenuation capacity as part of national flood defence strategy.
The same principles that apply to the open countryside, also apply to urban landscapes, driven partially by SuDS, but also by the need for the water utility companies to sign-off all new developments from the perspectives of their ability to supply them with water. Tackling SuDS design on new developments from the perspective of also minimising the need for mains water therefore makes good commercial sense, particularly when it unlocks a Planning Consent that might not otherwise be granted. The Welsh Government "SuDS Standard" therefore already reflects this by making the preferred method of managing surface water, its harvesting for subsequent non-potable use - thereby reducing household mains water consumption by around 50%.
When building new homes, developers take into account from the outset the mandatory requirement for SuDS and therefore include associated costs in their affordability calculations. Having done so, Developers tend to view RWH as an optional issue, which involves additional unrecoverable costs, from which only the eventual homeowner will benefit. The UKRMA view is that this separation of costs/benefits is un-helpful, and can be resolved by taking an integrated approach to SuDS/RWH systems as required by the Welsh Government's SuDS Standard. Similar considerations do not generally apply to commercial projects where the developer and the end-user are often the same organisation.
SuDS are covered by the "2010 Flood and Water Management Act", which focuses on water-quality and flood issues, rather than droughts. The Act envisages that SuDS on significant developments will need to be adopted (in a similar way to the adoption of roads and sewerage infrastructure) to ensure the long-term maintenance and effectiveness of the SuDS. To-date, the national formula for setting-up Agencies to undertake adoption and fulfill associated responsibilities has yet to be found. Meanwhile, Developers have to convince Planners that they are leaving robust inspection and maintenance arrangements in place, usually in the form of a "Resident's Management Company". Either way, ie Adoption or Management Company, it is in the financial interests of all concerned that SuDS Systems are easy to inspect and maintain. This is achieved through integrated architecture along the lines below, particularly if the principle is accepted that the RWH element will have its own maintenance regime (as water supplies in-part depend upon it), and the the SuDS element therefore starts at the point where the RWH element overflows into the SuDS element.