PPS 15: Planning and Flood Risk
FLD 1: Undefended Areas
8.10 Undefended flood plains perform an invaluable function in mitigating the natural fluvial and tidal processes important in the wider flood management system. Increasing the amount of development within them not only places the property constructed and its occupiers at risk, but also impairs the flood plain's natural function and erodes its ecological integrity. It is also important that piecemeal reduction of undefended flood plains is avoided because of the cumulative effects on their storage and conveyancing capacity.
8.11 Built development will therefore only be permitted in undefended areas in exceptional cases. This may include infrastructure works where it is demonstrated that a specific flood plain location is essential for operational reasons and that an alternative lower risk location is not available.
8.12 Replacement of an existing building will generally be acceptable where it is demonstrated that the proposed redevelopment will not materially increase flood risk.
8.13 Where, exceptionally, built development is permitted, flood prevention or mitigation measures will normally be required. Such measures might include proposals to mitigate loss of flood storage capacity, the adoption of design and construction measures to minimise the impact of flooding (such as the introduction of a freeboard for new buildings) or the provision of safe means of evacuation in the event of a flood occurring.
8.14 Land raising, i.e. the raising of ground levels in association with building works or as an operation in its own right is not considered for the purposes of this PPS to be a flood prevention measure. It can adversely affect drainage systems and it removes flood storage capacity and interferes with the conveyance of floodwater. This can create or increase the risk of flooding elsewhere. It can also damage visual amenity and impair the natural functioning of the floodplain with consequent adverse effects on the natural and built heritage. Accordingly, proposals to raise ground levels in undefended flood plains will not be permitted.