Oluwole P. Akadiri
School of Engineering and the Built Environment (SEBE), University of Wolverhampton, Wulfruna Street, WV1 1SBWolverhampton, UK
One of Britain's principal geographical features is its extensive coastline (12429 km) created by a multitude of land and ocean processes, subjected to anthropogenic influences and characterized by a variety of formations and coastal environments. Greater coastal flood risk due to increased storminess and accelerated sea level rise is one of the most serious impacts of climate change on the coast. Flooding can cause serious damage to buildings, resulting in the need for extensive repairs to be undertaken. The amount of damage depends on the various factors such as the cause, depth and duration of the flood. Repairs to flooded buildings should seek to increase the flood resilience and thereby ensure that in any future flood the amount of damage is minimised. This paper sets out the type of damage caused to different building elements, the assessment of future flood risk and repair options. The options considered in this paper include the use of flood protection products and the use of more flood-resilient materials and repairs methods with a review of the cost- benefit of the latter depending on the type, severity and frequency of flooding.Repairs are in accordance with the level of risk involved and the costs involved. Improving the flood resilience of buildings should result in reduced insurance costs.
Keywords: Climatic protection, Insurance, Housing, Preventive maintenance
Recent natural and human-induced events have highlighted the fragility and vulnerability of the built environment to disasters. Flooding is a major problem for many people in the United Kingdom, posing a risk to health, safety and wellbeing, resulting in widespread damage to property. The worldwide increase in frequency of extreme weather events has been reflected in the UK where, since 1998, there have been several severe flood events. The flood event of Easter 1998, coming as it did after a relatively dry period, sparked a renewed interest in the management of flood risk (Bye and Horner, 1998) which was reinforced by further flooding in 1999 and the widespread 2000 floods (EA, 2001, Clark et al., 2002, Lamond, et al., 2007).
Forecasts of increased flooding due to climate change (Office of Science and Technology, 2003, Evans et al., 2004) and subsequent flood events in Boscastle in 2004, Carlisle and North Yorkshire in 2005 and the most recent summer flooding of 2007 have brought to light how susceptible communities in the UK are to flooding, now and increasingly so in the future. There is need therefore for government, financial institutions, insurers, building industry and the public to improve the local flood protection of buildings in flood risk zones.
Building for resilience against floodwater has become increasinglyimportant given the high demand for new houses in the UK andthe need to build in flood-prone areas (Escarameia et al., 2007). Although it is unlikely that the effects of flooding can be eliminated completely, many practical steps can be taken to reduce the cost of flood damage to buildings by improving flood resilience. This paper looks at the impact of flood on buildings and the approaches that can be taken to make a property more resistant to flood damage. This includes the use of flood protection products and considers the benefits of changing the specification for repair so that, rather than using the same materials and techniques again, a more flood resistant alternative is adopted. While there may be a higher initial cost, for properties that are subject to repeated flooding the future cost of repair after further floods may be reduced with a consequent significant potential saving over an extended period of time.
2. Understanding floods
By definition, a flood generally involves the inundation or overflow of water over land that is not normally submerged (Ward, 1978; Smith and Ward, 1998; Samwinga et al,2004). Flooding occurs as a result of one or more events such as rainfall filling rivers, streams and ditches; coastal storms resulting in overtopping and breaching of coastal flood defences; blocked or overloaded drainage ditches, drains and sewers; heavy rain resulting in run-off flowing overland; or rain soaking into the ground and raising ground water levels (DTLR, 2002). Although flooding is commonly associated with rivers or the coast, localised flooding that does not normally lead to property being flooded above ground level, also occurs due to broken water mains (DTLR, 2002). Human activity is generally blamed for increasing the risk of flooding from rivers and streams in many areas. In particular, development may have reduced the natural capacity of floodplains and increased the rate of surface water run-off.
2.1 Flood Risk in the UK
Flood risk for a property is generally understood as a combination of the likelihood of a flood occurring and the consequences of the flood in terms of damage caused or impact (DTLR, 2002). In England and Wales, the areas at risk from flooding have been mapped into what is known as the Indicative Floodplain Maps (IFM) which are available on the Environment Agency website. Several criticisms have been advanced against the IFM such as the accuracy of the maps since they do not take into account the effect of flood defences, local topography, and small flood risk areas such as those at risk of flooding due to urban drainage. Estimates suggest that in England and Wales over £220 billion worth of property is potentially at risk of flooding. In particular, the coastal zone, which is occupied by 10 million people (within 10km of the coast), and which accounts for 40% of manufacturing industry, 40% of tourism expenditure and contains a concentration of energy installations, is at risk from both fluvial and coastal flooding. However, the maps are useful in providing a general overview of the risk of flooding.
Over 5% of the people in England live lower than 5 metres above sea level, including large parts of major cities such as York and London(Samwinga. It has also been suggested that about 7% of the country is likely to flood at least once every 100 years from rivers. In addition, approximately 30% of the coastline is developed and around 1.5% of the country is at risk from coastal flooding (DTLR, 2002). The risk levels highlighted in Table 1 suggest that flooding is a potentially costly problem in England and Wales. et al, 2004)
Table 1 - Flood Risk in England and Wales
Number of Domestic properties at Risk of flooding
People living in properties at Risk of flooding
Number of Industrial/Commercial properties at Risk
Capital Value of Property in vulnerable areas
Capital Value of Agricultural Land in vulnerable areas
Total annual average damage at present level of protection (flood defences)
Source: DEFRA (2004)
Most areas at risk from river flooding are protected using man-made flood defences, which reduce the likelihood of flooding. However, these defences can be breached or overtopped due to more extreme events, as they are designed to withstand specific flood heights.
2.2 Factors Affecting Flood Risk
When attempting to quantify how much rainfall is needed to produce a major flood, one has to account for the fact that the amount of rainfall transformed to runoff depends to a large degree on three factors: soil type, the underlying geology and existing soil moisture conditions controlled by the rainfall intensity of preceding storms (AIR, 2008). But an additional factor—and one often neglected even by hydrologists when estimating the risk of flood—is relative rainfall intensity compared to the average precipitation received by a river basin over a year. It is well known among geomorphologists that natural streams adjust their shape and conveyance to average annual precipitation. Very roughly, this means that a river with a 1000 km2 upstream drainage area that receives, say, 2000 mm of rainfall a year will have twice the conveyance (and cross-sectional dimensions) of a reach with the same contributing area and slope, but receiving only 1000 mm of rainfall a year. Of course, this connection between the channel conveyance capacity and the precipitation regime is made more complex by the soil type and underlying geology. Again roughly speaking, these two factors control how much of the rainfall goes directly into the river, potentially producing flood, and how much penetrates down to groundwater and later drains to the river network gradually and benignly over long periods of time.
To illustrate this, the maps in Figure 1 show the annual average precipitation over England and Wales (left), and the Standard Percentage Runoff, or the proportion of rainfall transformed into surface runoff (right). One can infer from the maps that the streams in the mountainous areas of Wales and western England, where basins receive more precipitation and are generally less permeable (i.e. higher Standard Percentage Runoff) will be wider and deeper compared to the streams in eastern England. Consequently, a single storm producing rainfall of 100 mm may produce negligible flooding in northern Wales but could be devastating to eastern England, depending on the existing soil moisture conditions.
Figure 1. Average annual precipitation (left) and normal percentage runoff (right). Source: AIR, 2008
In the context of the above, the runoff volume from a single storm as a percentage of the total annual runoff conveyed at each location along the river network can be used to estimate the risk of flooding along a river network. In Figure 2, and using actual precipitation data for July 18 to July 23, ratio for rivers in Wales and England was determined.
Figure 2. Runoff volume from the July floods as a percentage of the total annual runoff. Highest risks are rivers shown in red. Source: AIR, 2008
The probability of flooding is highest for the river locations shown in red, since the channel capacity at these locations is predicted to be approached or exceeded. It is these rivers—the Severn, Avon and Thames—that were among the most severely affected in July
2.3 Experiencing a flood event and its aftermath
Floods can have devastating effects, especially when they occur without warning. The most visible and obvious impact of floods upon households is the physical damage to the fabric of the building and household contents, which may or may not result in financial loss to the homeowner. The impact of flooding on households is influenced by factors broadly classified into two categories (Green et al., 1983; Samwingaet al, 2004):
i. Flood characteristics - duration, depth, speed of development, whether anticipated or not; weather conditions; contaminants (sewerage, oil, silt, etc.);
ii. Individual's characteristics - age, prior health status, prior stress levels, whether or not evacuated and duration of; event anxiety; aftermath anxiety.
Similarly, Business & Marketing Research (2001) concluded on the basis of their qualitative research, that whether or not a homeowner perceived a flood event as a catastrophe depends largely on three factors, namely:
i. The physical impact of the flood event;
ii. The characteristics of the individual concerned; and
iii. Where the claim sits in relation to other events in the homeowner’s life.
The physical damage caused by flooding has been found to be highly dependant on flood characteristics - the scale and nature of the flood event (Soetanto et al., 2002; Samwingaet al, 2004). Flood damage to domestic property presents unique challenges from a restoration perspective, particularly the nature of 'projects' (flood-damaged domestic properties) and ‘clients’ (insured homeowners) involved; both the flood damaged property and the homeowner have unique characteristics summarised by Samwinga and Proverbs (2003) as being:
i. Recovery and restoration - returning the flood-damaged property to its pre-incident condition;
ii. Flood restoration works, by nature, usually involve processes such as cleaning, drying, 'deodorising', sanitation, etc., which are unique;
iii. The property characteristics such as size, usage and contents;
iv. They involve flood claims, which can be very complex to handle;
v. The parties involved in flood reinstatement projects are typically the homeowner, insurer, contractor(s) (cleaning, drying, and repair), loss adjuster, and sometimes loss assessors, whereas 'ordinary construction projects' typically assemble a team consisting of the client, designer, consultants and contractors;
vi. The 'clients' (insured homeowners) undergo a potentially traumatic experience often resulting in anxiety during and after the flood event (Green et al., 1983);
vii. Loss of symbolic objects or irreplaceable assets of sentimental value, underinsurance on buildings and no insurance on contents (resulting in financial loss), may exasperate the trauma experienced by homeowners.
3. What can be done?
Flood defence and management schemes are being improved to ease the situation but these often take time and may not protect all areas prone to flooding. There are measures that can be taken by individual property owners to increase the resistance of their properties to flood damage and it may be the case that the mortgage provider, insurer or local authority will be helpful in taking these forward. In some cases, these measures could be cost-effective in that they reduce the repair cost of subsequent damage should flooding occur again. Every property is different and has to be considered on its own merits when deciding the most appropriate flood-resisting measures.
The most suitable or cost-effective approach will depend upon the frequency of flooding, its likely depth and duration as well as the nature of the flood whether it be from watercourses, groundwater, burst water main, sewer surcharge or surface run off. Further guidance is available in “Preparing for Floods” published by the Department for Transport, Local Government and the Regions (DTLR, 2002) and in useful leaflets published by CIRIA and the Environment Agency (2003a, b).
3.1. Demountable flood protection products
There are products coming onto the market which provide demountable, temporary protection to prevent water ingress to buildings through openings and vents in the construction. These are installed by the homeowner in the event of a flood warning, sometimes onto a pre-installed frame. A “Kitemark” scheme operated by British Standards Institute (BSI) is in operation based on newly developed “Publicly Available Specifications” (PAS) (British Standards Institution, 2003a, b). The scheme provides assurance to users of flood protection products that these have been independently tested and manufactured in accordance with the specification. Some products are already approved and others are on the way. Further information on this is available from CIRIA and the Environment Agency (2003c).
4. Flood-resilient repairs
More permanent measures can be considered which improve the resistance to flood damage of domestic buildings. These would normally be installed as part of the repair following a flood but could be installed in anticipation of damage, if required. A process of brainstorming was undertaken involving Damage Management Specialists and Cost Consultants in the UK to select a range of measures which could be taken to improve the flood resistance of a building. A number of these were selected and reviewed. Although not necessarily a fully exhaustive list, it is indicative of the most likely and practicable options. These are considered for their practicability, effectiveness in reducing damage caused by flooding of differing depths and durations, and the benefits in reducing future claims. This list of measures will be of useful initial guidance for those considering the repair of flood-damaged houses.
However, for flood proofing measures that reduce repair costs in the long-term, each building must be treated individually and there are no hard and fast rules that can be applied to all. It should be noted that despite concerns about rotting wood, it can be surprisingly resilient and there may only be cause for concern in cases where there is long-term, regularly repeated, water immersion. Further, there is a waste issue related to disposing of treated timber, as it is harder to dispose of than untreated and should be taken to landfill and not burned.
The resilient repairs considered include general items, treatment of floors and walls and interiors, as follows:
- Survey a property for advice on maintenance and repairs that may improve the water resistance when the recommended works are carried out. Water penetrates through the construction of a wall but also through cracks, defects, service penetrations and other openings above and below the dpc.
- Keep a record of measures taken to improve the building. This allows future owners and surveyors to understand the flood resistant measures that have been installed and prevents measures from being taken twice and money wasted. It can also give an indication of where complementary work may be taken.
- Move services meters to at least one metre above the floor level and place them in plastic housings.
- Move electrics to at least 1m above the floor level with cables dropping from first floor level distribution down to power outlets at high level on the wall.
- Put one-way valves into drainage pipes. This prevents contaminated floodwater entering houses through pipes.
- Mount boilers onto the wall above the level that floodwater is likely to reach.
- Install a drainage channel across the driveway to intercept water flowing towards the property.
- Replace sand – cement screeds on solid concrete floor slabs. Where screeds are damaged in floods, resistance to future damage may be improved by replacement with a denser proprietary concrete screed.
- Replace the floor, including joists, with treated timber to make it water resistant/repellent. The timber is less likely to absorb water, enabling the floor to dry out more quickly and be more resistant to rot or distortion.
- Replace the floor timber, including joists, with more robust timber/treated timber which is hardened. This timber is more resistant to becoming saturated with water, enabling it to dry out more quickly and be less likely to rot or distort.
- Replace timber wall plates and joists on sleeper walls with corrosion resistant steel alternatives. The steel joists are surmounted with treated timber boards.
- Install a damp proof material around the ends of the floor joists where built into the walls, turning up the wall and with the timber on top. This will protect the joist ends from persistent dampness and consequent rot. To achieve this will also require the replacement of the joists and floor and item 10 will also be necessary.
- Replace oak floorboards with treated wooden board. Oak boards are difficult to dry out and expensive to replace.
- Remove ash-bedding from underneath quarry tiles in Victorian houses. Ash bedding retains moisture and impedes drying out.
- Replace chipboard flooring with treated timber floorboards. Chipboard has to be replaced if there is any chance of contamination. Other treated floorboards are more resilient to flood.
- Raise the floor level above the most likely flood level. In general, this is applicable only when floodwaters do not rise much above the existing floor level and where the ceiling height of the property can accommodate it. Raising floors may require resetting doors and windows to higher cill levels and this will be an additional cost. Associated works relating to items 4, 10, 23, 26 and 27 will be needed to complete the effectiveness of this option.
- Replace timber floor with solid concrete and provide a tiled finish with falls to allow draining to a sump and pump.
- Clear and repair air bricks/vents to suspended timber ground floors. This improves the under floor air flow and aids the drying out process making it less likely that building components will get damaged by long-term water logging. However, this may make water ingress easier.
- Install air bricks above the expected flood level and duct down to the floor void. Floodwater can travel easily through airbricks into buildings. Raising the vent above floodwater level may reduce the flow of water into a property, particularly to the sub-floor in the event of shallow flooding.
- Install a chemical DPC below joist level. This enables the structure below to be treated in a different way from that above. This helps to minimise the amount of dampness that gets above the DPC, potentially reducing the damage to the property and the amount of repair work that must be done above this level. To achieve this will also require the replacement of the joists and floor and floor timber will also be necessary.
- Replace mineral insulation within internal partition walls with closed cell insulation. Closed cell insulation is more likely to survive a flood without having to be replaced.
- Replace gypsum plaster with a more water resistant material, such as lime plaster or cement render on walls or silicon/mineral board in place of plasterboard. This reduces the extent to which floodwater will penetrate and significantly increases the probability that a wall will survive a flood without damage. Provide a dado rail at the dividing line between flood resistant treatment and normal construction as an indicator for future repair requirements.
- Fix plasterboards horizontally on timber framed walls rather than vertically. In the case of a flood, this means less plasterboard will have to be replaced when repairing the walls.
- Coat the exterior walls with micro-porous spray coating every 5 years. This may be of some benefit in reducing floodwater ingress through the wall construction although it will not prevent penetration through cracks, joints and openings. It may make properties more difficult to dry out and may create durability failures in the existing materials.
- Re-point brickwork with a mix of 1:2:9 – cement/lime/sand mortar. Mortar often tends not to be very flood-resistant and may disintegrate if immersed in floodwater. This could result in expensive repairs and potential structural damage to the property. The replacement mix would be significantly more likely to survive flood conditions without need for repair.
- Replace doors, windows, skirting boards, architraves, doorframes and window frames with fibreglass (grp), plastic, PVC-U or other similar water resistant alternatives. These do not absorb water or warp and so are more readily functional after a flood.
- Replace door hinges with rising butt hinges. These allow doors to be lifted off and placed in a dry place until the flood subsides.
- Fit kitchen units with extendable plastic or stainless steel feet or support on raised brick or stone plinths to raise the units above the water level for shallow flooding. The units should then not be damaged by a shallow flood of less than 50mm above floor level.
- Replace ovens with raised, built under type. These are more likely to be above the flood line but are lighter to move for deeper floods.
- Move kitchens to first floor rooms. Kitchen equipment can be difficult to remove in a flood and can be expensive to replace after one.
- Move washing machines to first floor rooms. Washing machines are heavy and impractical to move before a flood and are expensive to replace after one.
- Replace chipboard kitchen/bathroom units with plastic or similar units. Chipboard units generally have to be thrown out after a flood, but plastic units may be disinfected and used again.
- Specify the least expensive kitchen possible and to expect to replace it after a flood as an alternative to the previous point.
- Replace ground floor baths with chipboard stiffening panels with cast iron or pressed steel models. Chipboard often distorts in floods and this can result in bathtubs being broken. Baths made from traditional materials are more resilient and are more likely to survive a flood.
5. Which should I choose?
Not all the measures are appropriate for all cases. For shallow flooding, below the damp proof course, some measures are worthwhile, for example, replacing oak floorboards with treated softwood, replacing mineral insulation with closed cell alternatives, removing ash from below quarry tiled floors and replacing chipboard with treated floorboards. However, most other measures cannot be justified because of the large number of repeat floods that would be needed to repay the initial investment. For deeper floods, which rise above floor level in the house, many more of the measures can be justified in that the investment is repaid by savings made over two or three repeat floods. In deciding the most appropriate measures to take each situation must be considered on its own merits and advice should be taken from experienced professionals. It must be remembered that none of the normal methods of domestic house construction are watertight and water will penetrate into the building through the materials themselves and through cracks, joints and openings in the construction. Specific design for water tightness can be achieved but not with the commonly used forms of construction considered here, but rather with specially designed, tanked construction which is rarely found in housing.
There is a lot that can be done by the homeowner to reduce the extent of damage caused by flooding. This may include the use of specially designed products to reduce water ingress at door and window openings or the use of construction materials and location of services which will be less prone to water damage. While it is important that the homeowner be aware of the options available, it must be remembered that each house is different and selection of the most appropriate approach depends on many factors. These include the flood risk, frequency and depth of flooding, type of flood water, construction and condition of the fabric of the building and the cost of the repair compared with the potential saving in the event of subsequent floods. In order to achieve the best solution for the householder specialist advice should be sought from an appropriately experienced surveyor or engineer in consultation with the insurer.
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