by Anthony Lymath
The issue of flooding is not a new one. Whilst overheating has become a relatively commonplace talking point in recent years, widespread areas of the UK have suffered from intermittent floods, whether due to swollen rivers, excess rainwater surface run-off, or tidal surges, as far back as records exist. Key events still remain in the public consciousness, such as the 1953 North Sea flood, York’s worst recorded floods in 2000, the Boscastle flood of 2004, Carlisle in 2005, Tewkesbury in 2007 (when floodwaters entered the abbey for the first time in 247 years), and the widespread flooding of 2012 – not to mention the February 2014 inundation of the Somerset Levels and other areas.
Unlike overheating, flooding is such 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. With the debate surrounding global warming, therefore, it is perhaps not surprising that:“An increase in the risk of flooding is widely reported as one of the most likely impacts of climate change across the UK (Evans et al., Future Flooding: Scientific Summary Vol 1 2004; Werritty with Chatterton, Future Flooding Scotland 2004; CCRA, Assessment for the Devolved Administrations 2010). This is a direct result of the physical principle that a warmer atmosphere holds higher amounts of water vapour and UK regional climate models (UKCP09) predict increased winter rainfall (especially in the north and west) and more intense, highly localized summer rainfall (especially in the south and east). These predictions also accord with recent changes in rainfall over the period 1961–2006 (Jenkins et al., 2008)”1
Flooding is currently seen as the biggest UK climate change risk at present, both immediately and in the short term2. In the UK, the Flood and Water Management Act 20103 deals with flood and coastal erosion risk management, whilst the Flood Risk Regulations 20094 implement the requirements of the 2007/60/EC European Floods Directive5. Flooding can occur from three primary sources:
- Coastal (sea) flooding
- Fluvial (river) flooding
- Pluvial (rain) flooding (e.g. surface water run-off).
Coastal flooding is primarily due to tidal variations in particular at spring tides, and these can be exacerbated by stormy weather conditions. In such circumstances it is coastal areas, obviously, which are greatest affected. Rising sea levels pose a risk to existing defences, such as the Thames Flood Barrier which was designed in the 1970s to a predicted annual sea level rise of 1.8mm per year, which has increased to 3.1mm per year currently6. In early 2013, the ‘ice2sea’ project warned that there is a less than 1:20 risk that global sea levels could rise by as much as 0.84m by 2100, due to a combination of melting ice sheets and the expansion of warmer oceans7. Fluvial flooding occurs due to periods of excessive rainfall and the subsequent drainage into the river system which makes its way downstream, swelling river channels. Whereas parts of the country have flood defences in place for such events, including barrages, dikes and relief ponds (‘flood plains’), these are not 100% effective in protecting all property and infrastructure under extreme conditions.
Pluvial flooding is generally due to of excess rainfall falling onto ground which cannot absorb it, whether it is saturated, frozen or covered in an impermeable surface. Whereas the first two matters are largely natural and hence unchangeable, the third 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 load8,9. 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 201110.
Floodwater is invariably contaminated in some form and these contaminants can cause further damage to buildings and services, besides posing a threat to public health.
Floodwater is invariably contaminated in some form and these contaminants can cause further damage to buildings and services, besides posing a threat to public health. Contaminants can include sewage, hydrocarbons, silt, salt and other biological or chemical substances, depending on the location of the flood and any unsecured hazards that may be present in the vicinity. Hydrocarbons are perhaps most commonly present in this manner in the form of petroleum, although asphalt and wood preservatives such as creosote are other sources for hydrocarbon pollution.
Apart from the obvious routes, water can enter buildings through many places. The wall and floor materials themselves can be permeable; concealed voids such as wall cavities and party walls; at junctions between, for example, suspended timber floors and walls; air bricks and other ventilators; inadequate or broken seals around window and door frames (including thresholds); weepholes in facing brickwork; services entry points; cracks in mortar or render; subfloor voids; inadequate or defective damp-proof membranes or tanking; and through sanitary or washing appliances via drainage backflow.