PPS 15: Planning and Flood Risk
Annex B: The Impact of Development on Flood Risk
B9 The nature of a flood event is shaped primarily by a combination of the effects of human activity and natural physical conditions.
B10 New development, whether building works, engineering operations or changes in land use, can increase flood risk. Most simply, siting a building within an area likely to flood will increase the direct risks to that building. The greater the quantum of construction, the greater the consequences of a flood event are likely to be. While the direct consequences of flooding for a building are generally readily identifiable, less obvious but equally significant indirect impacts can be caused elsewhere.
B11 New development can increase indirect flood risk in locations beyond the development site simply by increasing the amount of run-off from the developed area. This may occur where permeable surface areas are reduced by construction work leading to increases in the volume and speed of water transported through a catchment.
B12 Where development, for example, a poorly located bridge across a river, restricts the capacity of the drainage system to convey water through an area, this will increase the flood risk adjacent to and upstream of that development. This will create the potential for flooding to occur more often and to greater depths. Development that impairs the conveying capacity of the flood plain can also exacerbate risk by encouraging the gathering of water borne debris that may further restrict floodwater flows.
B13 Buildings and the construction of other hard surfaces also have the potential to increase downstream flood risk by decreasing flood plain storage and increasing the scale or speed of run-off.
B14 Land raising (sometimes called infilling), either to facilitate a development or as an operation in its own right, can also cause flooding where it interferes with existing drainage systems under normal conditions or areas that store or convey water during flood events.
B15 Overland flow is a frequent cause of localised flooding in Northern Ireland (see Annex A). Where no development exists its consequences are not normally a significant source of risk. However, where overland flow is present, development that does not address it may increase potential for flood risk elsewhere by increasing the speed or the volume of flow. Flood risk will also be increased if new buildings or works are located where they may themselves be prone to inundation.
B16 In addition to potential impacts upon public safety, human health and social and economic wellbeing, increased flood risk may also have adverse environmental impacts.
B17 River and coastal flood plains are valuable ecological resources which provide habitat for a wide range of plants and animals, many of which are unique. A number of the priority habitats identified in the Northern Ireland Biodiversity Strategy are associated with floodplains. In addition, flood plains are often important landscape assets and the location of features of the archaeological and built heritage. Flood events can damage ecosystems, habitats, heritage and landscape assets, and development can exacerbate such damage. The natural regulation of floodwater limits ecological damage caused by flooding, while pondage areas provide for the trapping and deposition of sediments and recycling of the nutrients from run-off.
B18 Human activity can therefore have a significant impact in increasing flood risk and new development that is constructed without regard to flood risk may serve only to endanger life, increase property and environmental damage and require wasteful expenditure on remedial works.