3.3 Flooding and groundwater
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Flooding can be diverse, often site specific and brought about by a range of factors including heavy rain, raised groundwater levels, increased rain water run-off and blocked or surcharged drainage systems. It is also generally recognised that climate change may play a major role in increasing the risk of flooding in the future, for example, local pluvial (rainfall) flooding from more frequent short intense rain storms.
The effects of flooding on a building can include significant damage to materials, services and structure. Contamination could result where waste water drainage is present in the floodwater. Where there is a risk that flooding can affect a building it is important that any proposed construction is designed to be more resistant or resilient.
Pressure for land development may mean that development may be given planning approval on land subject to some risk of flooding. Where development is to take place on land assessed by the planning authority as having a flood risk, advice should be sought from sources such as the local planning authority, the Scottish Environment Protection Agency (SEPA) and those responsible for coastal defences. Further guidance may be obtained from the ‘Scottish Planning Policy SPP.
The Scottish Environment Protection Agency (SEPA) provides flood risk information on their indicative river and coastal interactive flood maps on their website. http://www.sepa.org.uk/.
When near surface level groundwater is present on a building site there is the potential for construction activity to affect it or for the groundwater to pose a hazard to any new buildings. To reduce the risk to buildings from groundwater, subsoil drainage of a site may be necessary to protect against penetration of groundwater through a building and damage to the building fabric. Any existing drains that will be affected by the construction of a building should also continue to function properly and guidance is provided under Standard 3.5.
Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).
New building sites should be initially assessed to establish the existing groundwater level and any fluctuation to the established level brought about by seasonal effect, new construction, excavations or other related activities.
Ground below and immediately adjoining a dwelling that is liable to accumulate groundwater, at a level that could affect the structure or penetrate the building, requires subsoil drainage or other dewatering treatment to be provided to mitigate against the harmful effects of such water.
The drainage of groundwater may also be necessary for the following reasons:
to increase the stability of the ground
to avoid surface flooding
to alleviate subsoil water pressures likely to cause dampness to below-ground accommodation
to assist in preventing damage to foundations of buildings
to prevent frost heave of subsoil that could cause fractures to structures such as concrete slabs.
The selection of an appropriate drainage layout will depend on the nature of the subsoil and the topography of the ground. Field drains, where provided, should be laid in accordance with the recommendations in clause 10 of BS 8301: 1985.
Surface water run-off to adjacent sites - with the removal of topsoil from a development site, developers should be aware of the dangers from possible surface water run-off from their building site to other properties. It is good practice to have procedures in place to overcome this occurrence. Depending on conditions, the formation of channels or small dams to divert the run-off or, where conditions are particularly serious, the installation of field drains or rubble drains may overcome the problem.
Any identified site specific risk of flooding to a building or its occupants should be assessed to allow sustainable design mitigation. Building site flood risk assessments should be an integral part of the design and construction process with the appraisal also considering the effects that the development may have on adjoining ground.
‘Planning and Building Standards Advice on Flooding’ (PAN 69) sets out flood risk and probability assessment procedures including the need for drainage assessments to demonstrate a neutral or better effect on sites where flooding is an issue. For site specific flood risk assessments the CIRIA document ‘Development and Flood Risk – guidance for the construction industry’ (C624) 2004 provides detailed guidance on carrying out flood risk assessment and suggests design considerations for developers.
3.3.3 Resilient construction in flood risk areas
Where it is intended to develop in areas that may be at some identified risk of flooding, buildings should be designed and constructed to offer a level of flood resistance and resilience that can reduce the flood impact on structure and materials.
The May 2007 CIRIA document ‘Improving the Flood Performance of New Buildings – Flood Resilient Construction’ is a national document that provides design guidance on suitable materials and construction details for use in low or residual flood risk developments.
‘The Design Guidance on Flood Damage to Dwellings, 1996’. This document describes the likely effects of flooding on materials and elements of the building and assesses various forms of construction and measures to reduce the risk of flood damage in dwellings.