Mary Dhonau, chairman of the Property Care Association’s Flood Protection Group, explains the two main types of mitigation measure available for properties at risk of floodwater ingress.
Five million people live with the threat of being flooded from rivers and the sea, with 200,000 of those considered to be at very high risk. As yet there is no definitive number for those who are also at risk of flooding from flash floods caused by surface water exceeding the capacity of urban drainage systems. The two main types of mitigation measure available are explained below:
1. Flood resistance
This refers to products and specialist building materials designed to prevent floodwater from entering the property.
Kitemarked versions of many devices are available, having undergone rigorous testing procedures. Examples of resistance products include permanently fitted devices such as flood-proof doors and windows (which seal upon locking); self-closing airbricks; and anti-backflow valves, which can be fitted to sewage pipes and washing machine outlets. There are also products such as brickwork sealants and mortar sealants on the market (though these are not to be confused with ‘damp-proof ’ products which do a very different job). Measures that can be deployed temporarily are also available, many of which also bear the kitemark. These include a wide variety of door boards (including extra wide types for patio doors and garage door protection); airbrick covers; and free-standing barriers capable of keeping floodwater at some distance from the property. Traditional sandbags are not recommended, however, being both cumbersome and largely ineffective (unless used ‘army- style’, in huge quantities and incorporating an impermeable membrane). Modern absorbent bags that don’t contain sand are available, which are not only easier to store and deploy, but also waterproof and, in some cases, capable of being cleaned and re-used. The Property Care Association (PCA) has a list of competent and accredited flood protection manufacturers and fitters who are members of the flood protection group within the association. The PCA are able to offer one stop shop advice on flood protection related queries.
There are limits to this approach, however: where flooding is likely to affect a building over an extended time period; or if the flood depth is likely to exceed 900mm, then water should be permitted to enter, in order to equalise the pressure on both sides of the walls and avert serious structural damage. In such cases, an alternative approach is needed.
2. Flood resilience
Resilience involves adapting a property to minimise the damage that floodwater can do. Following a flood, a clean-up will be needed but not major drying or refurbishment, while the build- ing’s structural integrity is maintained.
Examples include moving electrical sockets, boilers and service meters above the maximum expected flood level; installing tiled floors over a concrete floor (rather than carpets and/or laminate flooring) or using wood such as oak, which is more resilient to floodwater. Modern fitted kitchens are typically constructed from chipboard/MDF panels which disintegrate after inundation, but these can be replaced with stainless steel, plastic carcasses, or solid wood units, while white goods can be raised on suitable height plinths. Ordinary plaster can be replaced with lime-based plaster or cement render. Research has shown that, where measures such as these are undertaken as part of a post-flood restoration programme, they can be highly cost-effective: many PCA members have the necessary skills to be able to undertake resilient measures to a property.
“While resilient repairs were found to be more expensive than traditional methods (average 34 per cent higher) they were found to significantly reduce the repair costs assuming a subsequent flood were to take place. Resilient flood mitigation measures will help in limiting the cost of repairs up to as much as 73 per cent for properties with a 20 per cent annual chance of flooding the up-front investment would be recovered following a single subsequent flood event.”
Joseph, Proverbs, Lamond and Wassell, (2011) An analysis of the costs of resilient reinstatement of flood affected properties: A case study of the 2009 flood event in Cockermouth, Structural Survey, Vol. 29 Iss: 4, pp.279-293
Finally, in some locations a number of property-owners may be able to take joint action to protect their homes and busi- nesses with more wide-ranging measures.
Examples would include permanently installed demountable barriers; automatically activated flood gates; and free-standing barriers of various types (again, kitemarked versions are available). Small scale telemetry systems can also be designed to provide direct warnings to properties not covered by the Floodline Warnings Direct scheme.