Flood mitigation solutions in buildings - Part Two

Barely a year seems to pass nowadays without flooding being a feature of the national news. It is an emotive subject due to the degree of devastation it can cause – not only can severe flooding endanger life, but even at moderate levels can lead to substantial damage and disruption to normal patterns of life.

Water source contamination and inhibited access are significant problems, coupled with the ease with which internal fixtures and decorations can be damaged irreparably. A flooded house can be uninhabitable for months, and subsequent insurance costs can rise considerably as a result. And now the UK government’s plans to overhaul their flood-cover agreement with the insurance industry are coming under scrutiny, over concerns that a significant proportion of the population could find their property not only uninsurable, but also un-mortgageable and, in extreme cases, unsellable. Fortunately there are a number of measures that can be taken to minimise both the risks and impact of flood damage, both for new development and for existing buildings.

Flood avoidance strategies for new buildings (continued)

Building Regulations

 As far as construction regulations in the UK and Republic of Ireland are concerned, the Scottish Building Standards, Technical Handbook 3.3 Flooding and groundwater contains the most stringent requirements, stating that “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 flooding and the accumulation of groundwater”. There then follows extensive guidance on surface flooding, flood risk assessment and resilient construction. Reference is made to the following:

• Scottish Environment Protection Agency (SEPA)
• Scottish Planning Policy 7: Planning and flooding 2003 (SPP 7) – now superseded by the Scottish Planning Policy 2010
• PAN 69 Planning and building standards advice on flooding
• CIRIA document C624 Development and flood risk – guidance for the construction industry 2004

English and Welsh Building Regulations refer to flooding in three separate Approved Documents:

• Approved Document C Site preparation and resistance to contaminants and moisture states that “flood resistance is not currently a requirement ... of the Building Regulations”, thus putting the onus on best practice rather than legislation. However, Sections 3 (Subsoil drainage) and 4 (Floors) contain guidance on mitigation measures that can be adopted.
• Approved Document H Drainage and waste disposal states in the Performance guidance that the Requirement of H1 will be met if the foul water drainage system “does not increase the vulnerability of the building to flooding”. This is then elaborated upon in reference to avoiding flood contamination and damage due to foul drain surcharges. Note also that “all drainage unaffected by surcharge should by-pass the protective measures and discharge by gravity”. Similarly, the Requirement of H2 will be met if “wastewater treatment systems and cesspools are sited and constructed so as not to ... be in an area where there is a risk of flooding”. Perhaps most significant, however, is Requirement H3 Section 2 Drainage of paved areas, which effectively places a requirement for the use SuDS in the form of permeable paving and drainage to other pervious areas such as grassland. Section 3 of Requirement H3 states that design rainfall intensities should be obtained from BS EN 752-4 Drain and sewer systems outside buildings, where low levels of surface flooding could cause flooding of buildings. Finally, Requirement H5 aims to minimise the risk of flooding of foul sewers by demanding separate drainage systems for foul and rain water.
• Although Approved Document J Combustion appliances and fuel storage systems makes no specific mention of flooding, Section 5 Provisions for liquid fuel storage and supply gives guidance on protection measures “where there is a risk of oil pollution”, namely secondary containment in the form of bunds (either integral to the storage tank or provided on site).

The Building Regulations (Northern Ireland ) Technical Booklet C Site preparation and resistance to contaminants and moisture takes a similar stance to the English and Welsh position in Approved Document C, stating that “the Building Regulations do not set any requirements for resistance to flooding ... [but] ... a building ... can be constructed with mitigating measures”.

The Republic of Ireland Building Regulations (at the time of writing) make only passing references to flooding, in Technical Guidance Document H Drainage and waste water disposal.

Flood avoidance strategies

Having established the appropriate Flood Zone in which to site a proposed development, it is recommended that as a first step, the Communities and Local Government publication Improving the flood performance of new buildings (2007) should be consulted. In particular, there is a set of three very useful flowcharts in diagrams 4.2, 4.3 and 4.4 of that document which enable the designer to establish which ‘defensive’ design strategies should be employed. These strategies are covered in detail in the mitigation section for refurbishment and repair below; they are equally applicable to both new build and existing.

At a strategic level, consider alternative locations for the building as described above. If this is not possible, set the ground floor level above the maximum predicted flood height. This is obviously going to be subject to local planning restrictions, and may be a requirement due to a SSFRA.

Install SuDS – including hydrobrakes, dry swales (which fill to act as balancing ponds during a deluge, releasing the water slowly into the surface water drainage system usually in conjunction with a hydrobrake); green roofs and rainwater harvesting. Reference should be made to CIRIA publication C697 The SuDS manual (2007).

Pluvial flooding is exacerbated by impermeable surface materials, and is becoming of increasing concern to the government, particularly with regard to the increasing popularity in paving-over of residential front gardens to provide additional car parking spaces. Where this is carried out with impermeable hard paving materials, run-off which would otherwise have percolated into the ground is being directed instead into the surface water drainage system, increasing the load. Permitted development for householder applications now only allows permeable paving materials to be used, or for the run-off to be directed to a permeable area such as a planting bed or lawn. It is estimated that the proportion of urban domestic gardens that have been paved-over, increased from 28% in 2001 to 48% in 2011.

Specify permeable surfaces if possible, to minimise reduction of surface water run-off and hence pressure on the drainage system. ‘Soft’ landscaping materials such as vegetation are the most effective; also gravels and permeable hard pavings such as tarmacadam and proprietary permeable brick paving units are useful where planting is not suitable. Site car parking areas and planting at the lowest levels on the site, to act as sacrificial flood retention areas.

The issue of insurance should also be considered. At the time of writing, the UK Government (in conjunction with the insurance companies) is completely revising the scope and structure of insurance provision against flood damage to buildings. The previous scheme had been set up as a temporary measure and now it is due to be replaced in the summer of 2015 with a new scheme, known as Flood Re. Under the not-for-profit scheme, annual insurance premiums would be capped for high-risk residential properties by imposing a £10.50 levy per household across the country; pay-outs would come from a central fund. One of the key tenets of the scheme is to support the NPPF’s discouragement of new construction on high-flood risk land, by excluding any new property built on Flood Zone 3 land from being covered. It is proving to be contentious, however, as in its present form a swathe of property including recently-constructed dwellings (post-January 1st 2009) and business premises – which includes residential landlords – looks set to be excluded. According to the British Property Federation, some 840,000 properties could be at risk of flooding (including 70,000 deemed high-risk), and a question mark hangs over whether the definition of business premises extends to businesses run from the home. The Government has just U-turned on the exemption of the most expensive houses (Council Tax band H, or about 3,800 properties) from the scheme, which had been excluded on the grounds that owners could be seen as being more able to pay for their own repairs; but not so far on the other exemptions. In total, the proposed exclusions could affect some 9,000 households (either newly built or high-value). Should any doubts exist about the availability and validity of flood-risk insurance for buildings, then consultation with the Association of British Insurers (ABI) or the Department for Environment, Food and Rural Affairs (DEFRA) is advised.

With all of the debate currently surrounding Flood Re, developers would be wise to exercise prudence in avoiding new building on high-risk land for the foreseeable future, no matter how attractive the proposition may seem.

Flood mitigation measures for refurbishment and repair

In the unfortunate case that a building owner/occupier should find their property to become flooded, the first issue likely to face them is where to turn for help. Most householders are unlikely to have the time or knowledge to deal with damage and repairs, which for a typical house could easily run in to the tens of thousands of pounds (in fact the current estimate is £12-19,000 per dwelling). It is worth noting that companies exist nowadays, who specialise in both mitigation and repair services.

Flood mitigation measures can fall into one of three strategies (as identified in the Communities and Local Government publication Improving the flood performance of new buildings [2007]), depending on the depth of predicted flooding: these are termed Water Exclusion, Mitigation and Water Entry. The following diagram – adapted from that document – illustrates the rationale behind each of these strategies:

The ‘Water exclusion strategy’ focuses on either avoidance or prevention of flooding, and is suitable for a design flood depth of up to 0.3m. This is intended to give the building occupants more time to relocate vulnerable contents to higher levels, and should not be considered to be effective for more than a relatively short duration flood event.

Flood avoidance strategies such as those discussed above, or resistance measures (covered in part three) are both appropriate for this category, which aims to keep the property interior dry during low-level and short-term flood events.

Parts in this series

This article is Part Two of a series - 'Flood mitigation solutions in buildings'. Links to other parts in the series are provided below: