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Planning Advice Note PAN 69
Planning and Building Standards Advice on Flooding

OTHER DESIGN ISSUES

LANDRAISING

110. Landraising is sometimes proposed in order to permanently raise a site above the expected flood levels, using inert and compacted infill material. Under SPP 7 it may be appropriate to raise land above the functional flood plain, or elsewhere to address groundwater problems, poor drainage or other local causes of flooding. The following potential side effects of landraising on functional flood plains should therefore be considered:

• Displacing a volume of water can increase flood risk elsewhere. If a similar volume of 'compensation storage' is created on or near the site it may balance the loss of storage from landraising. However, simply "digging a hole" in the flood plain will not provide storage at the same flood level and any proposed scheme will require detailed engineering design including inlet and outlet controls in the form of weirs and gates.
• Raising land on a site may create an "island effect" with surrounding areas inundated during a flood. This is unacceptable as there must be safe road access, normally from two access points for emergency vehicles and evacuation.
• Landraising affects river geomorphology, with further potential impacts, such as erosion on site and changes to erosion and sedimentation elsewhere.
• Other possible environmental impacts for example on landscape and amenity value of the river flood plain.

111. In some circumstances, and subject to environmental constraints indicated above, landraising may be acceptable from a flood risk perspective. For example for small developments at the edge of estuarine flood plains where the river is very wide and small changes in the landscape will not have a detectable impact on flood levels. SPP 7 highlights the possible opportunities for habitat creation. Subject to a detailed engineering assessment, this could include the development of flood compensation in the form of river or coastal wetlands and grazing marshes on separate sites within the same coastal unit or river catchment.

112. Design and engineering for landraising are specialist topics but the issues to consider are likely to include:

• Top soil stripping, storage and reinstatement;
• Depth of infill material;
• Type of material (granular/cohesive, e.g. clay);
• Slope or embankment stability;
• Compaction of infill material;
• Short term and long term settlement;
• Possibility of mobilising contaminants;
• Impact on hydrology within and outside the infilled area.

The land created by landraising will no longer be part of the functional flood plain (see SPP 7 paragraph 19).

LEVEL ACCESS AND RAISING FLOOR LEVELS

113. There has been a requirement in the building regulations since 1985 for all new buildings, other than dwellings, to be accessible to disabled people. In April 2000 this requirement was extended to dwellings, but only where it is reasonably practicable. Level or ramped access therefore is now required to virtually all new buildings. However the requirements for level access can present a potential conflict with measures to prevent property flooding if not considered at an early stage in the design process.

Building Standard 4.1 requires that:

Every building must be designed and constructed in such a way that all occupants and visitors are provided with safe, convenient and unassisted means of access to the building.

114. It is government policy for all buildings to be accessible by disabled people. However it is recognised that this may not be practical for some dwellings. The Technical Handbooks provide guidance on access issues and include an example of what might be termed reasonably practicable in such cases. Steeply sloping sites can create severe problems to accessibility and the provision of long, zig-zag, ramped access is not considered reasonably practicable. While raising the level of a ground floor may help reduce damage to buildings from minor flooding, it may also make the installation of a simple, short ramp more difficult. Designers should be aware that raising ground floor level alone is not considered a valid reason for avoiding provision of ramped access under the 'reasonably practicable' exemption.

115. Where there are conflicts between the need for level access and the flood risk, this should be discussed with planning and building regulations officers at an early stage. It may be reasonable for only a proportion of dwellings in a development to have access for disabled people, perhaps by landraising in a few localised areas while raising ground floor levels in the other dwellings and providing stepped entrance.

116. Construction of level or ramped access to buildings other than dwellings is generally easier to provide as sites are often larger thus allowing greater flexibility in site modelling and/or localised ground raising. Numerous buildings with multiple entrance points, however require more careful consideration. In both cases, it is vital that this issue is considered as early as is practicable in the design process as it can depend greatly on the considered use of both site layout and levels.

117. There is no reason why a building designed with level or ramped access should be any more susceptible to flooding than one with stepped access. However, careful consideration should be given to appropriate detailing of damp-proofing, weather-proofing and drainage, particularly on and around the accessible entrance and to the landing or platt immediately adjacent. Guidance on level thresholds is provided in 'Accessible Thresholds in New Housing' published by DETR.

OTHER BUILDING DESIGN CONSIDERATIONS

118. The following Building Standards, although not written with flooding in mind, will have a positive influence on how buildings react after flooding has occurred and may therefore be relevant:

Building Standard 3.10 states:

Every building must be designed and constructed in such a way that there will not be a threat to the building or the health of the occupants as a result of moisture from precipitation to the inner face of the building.

Building Standard 3.14 states:

Every building must be designed and constructed in such a way that the air quality inside the building is not a threat to the hygiene or health of the occupants or the condition of the building.

Building Standard 3.15 states:

Every building must be designed and constructed in such a way that there will not be a threat to the building or the health of the occupants as a result of moisture caused by surface or interstitial condensation.

119. In areas at risk of short duration flooding a number of measures can be combined to help to prevent water entering buildings. See for example the Severn Trent Water Hydraulic Toolkit described below.

SUSTAINABLE DRAINAGE AND FLOODING

120. Any development will tend to modify the existing drainage, particularly on greenfield sites. Conventional drainage collects rainwater from roofs and other impermeable surfaces and conveys it to the receiving watercourse through a network of underground pipe-work comprising either the separate or combined sewerage systems. The water flows through the pipes more rapidly than through the natural drainage network, adding to the likelihood of downstream flooding.

121. Sustainable drainage systems attempt to mimic the natural drainage patterns as closely as possible. They aim to manage the surface water runoff as near to the source as possible, and minimise runoff rates and volumes using attenuation and infiltration techniques.

122. Drainage systems are generally designed to cope with heavy to severe rainfall but their design may be exceeded by extreme rainfall events. SuDS, in particular, are less effective during extreme rainfall so alternative drainage may also required to deal with excess water. SPP 7 indicates that SuDS should be designed to deal with a storm inflow very soon after a flood subsides and if this is not possible, they are unlikely to be acceptable. PAN 61 provides further advice.

123. Paragraphs 62 to 76 deal with Drainage Assessment and explain the need to obtain a building warrant prior to constructing any surface water drainage system. For more information see also the CIRIA SUDS Design Manual for Scotland and Northern Ireland, first published 2000. Scottish Water is preparing guidance on specification for publicly vestable SuDS systems as part of 'Sewers for Scotland' 2nd Edition.

SUDS and Groundwater Levels

124. Where there is a likelihood of seasonal variations in groundwater levels, careful consideration should be given to the type of surface water drainage system that could be incorporated effectively. The variations of water table level during extreme conditions (including following long periods of above average rainfall) should be assessed to ensure that the water table would not rise above the base of any SuDS system. It may also be necessary to consider the implications of changes in the local water table or moisture content of the soil for existing buildings in the vicinity.

Design Exceedance

125. Consideration must be given to managing the excess water which will overload the SuDS when the volume of rainfall exceeds the design specification. On site and off site arrangements should be in place to ensure that it is routed to avoid causing flooding of property or access routes.

CULVERTS

126. Culverts carry watercourses below ground and beneath roads, railways, buildings, embankments etc. They are typically constructed of brick, concrete or iron and may be pipes. The policy in SPP 7 is that watercourses should not be culverted unless there is no practical alternative, and existing culverts should be opened wherever appropriate. If a culvert is unavoidable it must be designed to maintain or improve existing flow conditions and aquatic life.

127. Existing culverted watercourses are frequently polluted by misconnected sewers, overflows from blocked sewers or contaminated surface water. They cause flooding if they block internally or if their entrances become blocked. Erosion downstream of a culvert may be caused by increases in water velocity leaving the culvert.

128. GDPO article 15(1)(h) amended 1996 requires that planning authorities must consult SEPA before granting planning permission which involves, among other things , 'the carrying out of works or operations in the bed or on the banks of a river or stream'. Therefore a planning application to construct a culvert will almost certainly require consultation with SEPA. SEPA's policy and role is further explained in SEPA Policy Note No. 26 - Culverting of Watercourses.

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