Development and Flood Risk
The susceptibility of land to flooding is a material planning consideration and NPPF (the Government's planning advice on development and flood risk) requires local planning authorities to apply the precautionary principle to the issue of flood risk.
Within the fluvial flood plain and tidal flood zone the risks of flooding should be minimised by the incorporation of appropriate flood protection measures in the design and construction of buildings.
This could include for example, providing access to upper storeys that have a point of escape, and placing electrical circuitry at a higher level.
The Building Research Establishment Scottish Laboratory's publication 'Design guidance on flood damage to buildings' (1996) provides details of such measures. In England the Environment Agency is promoting further research which should be available by the end of 2002.
Bungalows, caravans and camping and caravan sites in these areas pose an unacceptable risk to their occupants. However, no development will be permitted in areas where the Environment Agency considers flood defences to be inadequate.
Protection of the Fluvial Floodplain
The Rivers Darent and Cray and other main rivers, such as the Stanham have significant areas of associated floodplain. Any flooding that could arise might, in certain situations, be exacerbated by extreme tidal conditions and the need to shut the Dartford Creek Barrier.
The high risk flood zone with a 1% probability of flooding, which equates to an average of once in a hundred years or greater flood risk), is shown on the Proposals Map. However, this map does not take account of recent research in climate change or recent experience. The flood risk maps will be updated regularly by the Environment Agency and their most up to date maps will be when making planning decisions.
In general, the Environment Agency is opposed to development in the flood plain which could impede flood flows, reduce the flood storage capacity of the flood plain or pose an unacceptable risk to life, infrastructure and property.
On the fluvial reaches of the Rivers Darent and Cray and other main rivers it will be necessary to maintain an 8m margin from the top of the bank, or within 8 metres of the landward toe where one exists, clear of obstruction for maintenance purposes.
On the tidal reaches of the River Darent it will be necessary to maintain a 15 metre margin from the top of the bank, or within 15 metres of the landward toe where one exists, clear of obstruction for maintenance purposes.
Applications for development in the flood plain or which would increase flood risk elsewhere should be accompanied by a flood risk assessment.
Where development is permitted that requires flood protection work and those works are not programmed by the responsible agencies, the necessary protection measures will be fully funded by the developer. This will include maintenance costs for 30 years.
This will apply to flood protection works required to protect either the development or other areas at risk as a consequence of development. In most cases the authority will require the measures to be implemented before development proceeds.
Where appropriate a contribution will be required towards the cost of additional emergency planning capital costs resulting from the development, such as suitable warning systems.
Developments at risk of flooding may face difficulties with the cost or availability of insurance and this, in turn, could cause problems for the purchaser. To avoid the risk of blight PPS25 advises developers to seek the views of insurers at an early stage.
In undeveloped or sparsely developed areas and the functional flood plain no development will be acceptable unless it is for a use for which such a location is essential, for instance a boathouse.
Tidal Flood Zone
Land fronting the River Thames is protected by continuous flood defences. However there is a theoretical risk - notionally 0.1% per annum, or once in a thousand years - of these being over-topped in extreme tidal conditions.
There is also a risk of breaching if a floodgate is left open or through over-topping and breaching of the defences in extreme circumstances. Areas behind the defences, particularly low lying areas, would be at risk from flooding.
The Environment Agency therefore resists development immediately behind defences. It is therefore important that all living accommodation, including hotels, hostels and guest houses, is constructed or located above the flood risk level.
Land for residential development must be raised to between 6.5m and 6.62m above Ordnance Datum Newlyn (depending upon location), or alternatively storeys below this level must not be used for residential accommodation (for instance, used for car parking instead).
The Environment Agency prefers that living accommodation is located above flood level and that sleeping accommodation is located above flood risk level. Where land is raised it will be necessary to provide the sufficient and appropriate drainage to prevent on-site flooding.
Unless properly sited and designed, new development, including redevelopment can increase the quantity and rate of surface water run-off, creating the risk of flooding locally and farther down stream and adversely affecting the aquatic environment.
It will therefore be important that new development does not increase run-off over and above the current 'undeveloped' level of run-off from the site, although it would be preferable if development resulted in a reduction in the level of run-off.
Redevelopment should result in a reduction in run-off. Appropriate attenuation measures will be required, such as sensitively designed balancing ponds and sustainable drainage systems. The system should be so designed to ensure that there is sufficient capacity to deal with surface water during periods when outfall to the river is tide locked or there are surge tides.
Wherever possible, attenuation measures should be designed so that they improve the amenity and wildlife interest of developments. The views of the Environment Agency should be sought in such cases.
The scope for incorporating 'soft' sustainable drainage systems in new development should be fully explored. These can be a cost-effective means of reducing the rate of run-off, maintaining water quality, and improving the amenity and wildlife interest of a development.
Such measures can include: -landscaped balancing ponds with reed beds for filtration -filter strips and grass Swales (gently sloping landscaped areas that mimic natural drainage patterns) -porous pavements and filter drains -infiltration devices (such as soakaways and infiltration basins)
Watercourses have the potential to add to the quality of the urban environment and the Council will welcome proposals that enhance these assets. The culverting of watercourses, on the other hand, represents a wasted opportunity: it can create flooding problems, disrupt the river environment, result in a loss of amenity and represent a safety hazard, especially to children.
The Environment Agency now resists culverting proposals and encourages the opening up of culverts wherever possible.
General advice on flooding and flood risk is provided by the Environment Agency: