Extreme weather

Floods

A flood occurs when the discharge of a river exceeds that capacity of the river channel to contain it. Once bankfull discharge is exceeded, inundation of the river valley flood plain must follow. An extremely flashy storm hydrograph can indicate that a flood is imminent. Flood conditions are often associated with precipitation moving through a drainage basin via the process of overland flow as part of the hydrological cycle. Water flows directly over the land surface as result of one of two rainfall scenarios:

1. soils are saturated, perhaps due to rainfall duration, with a large volume having fallen previously

2. extremely high intensity rainfall outstrips the infiltration capacity of even dry soils

Both conditions result in overland flow, although the latter is more likely to produce the most dramatic effects. Overland flow – and the potential for flooding – is also exacerbated by steep slopes, thin soils, impermeable bedrock (or concrete) and a lack of vegetation which would otherwise intercept some of the rainfall (this can reduce or slow the transmission of water through the system, thereby alleviating flood risk).

Unpredictable flash flooding (associated with the sudden onset of high-intensity rainfall) is less common in the UK than gradual-onset flooding associated with a longer period of steady rainfall, often over several days or weeks.

It has been suggested that the flash flood in Boscastle (Devon) in 2004 may be a harbinger of climate change – and that such extreme flood events may become more frequent in the future.

Is climate change increasing the volume and intensity of rainfall and flooding in the UK?

Flash floods in Cornwall devastated the tourist village of Boscastle during August 2004. The village experienced the heaviest rains in living memory, with 185 mm falling in just five hours. Given that the total annual rainfall for much of southern England is only around 1,000mm, this was an exceptional amount for a single rainfall event. Attention must be paid not just to the total volume of rain but also the intensity with which it fell. the majority falling in the first two hours. Under such conditions, infiltration-excess overland flow was inevitable, with the rate of input of rainwater greatly exceeding the infiltration capacity of the soil. In total, an input of 3 million tonnes of water was added to a tiny drainage basin whose size is just 40 square kilometres.

Viewed in isolation, one very heavy storm is not necessarily a symptom of climate change. However, the Boscastle flood is part of a "bigger picture" that suggests Britain's weather is changing. The summer of 2004 as a whole was the wettest in the UK since 1956. In fact, more than double the normal amount of rain fell in England and Wales during June, July and August 2004. It is also notable that it followed on more or less directly from the widespread European floods of 2002 (when 600,000 people experienced economic losses topping £10 billion and over 100 died).

Could northern Europe be moving towards a more unpredictable and stormy climatic regime, as some meteorologist currently suggest? Warming temperatures associated with climate change certainly have the potential to bring more extreme flood damage. Higher temperatures result in raised evaporation rates over water surfaces, meaning that there is greater moisture available to create rain; and if air masses are warming in response to climate change, then their potential to hold ever-greater levels of moisture is increasing (water vapour content is determined by the amount of heat energy available). The possibility therefore exists that the volume and intensity of frontal rainfall over parts of the UK is set to increase. In addition to getting warmer, we may also get wetter - due to an increase in high-intensity rainfall events and flash flooding.

Student Practice Question:

"Heavy rainfall causes floods" - discuss.

A discussion of this statement should begin by recognising that the word "heavy" has two interpretations: long-duration and high-intensity. The role of climate change as a factor that might be driving an increase in high-intensity events is definitely worth flagging up. However, other factors must also be brought into the discussion - land use changes and physical catchment characteristics such as vegetation and slopes help determine how quickly water moves through the hydrological system and whether heavy rainfall actually prompts a flashy response from a river.