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The flood hunter

Efforts to improve the accuracy of flood modelling are enabling planners and emergency services to target communities at greatest risk. NCE reports.

A freezing winter weekend in 1953 lives long in the memories of residents on the east coast of England. The sea surged over defences, killing 307 people and forcing 30,000 from their homes. Caused by a major depression and driving winds, the surge affected 1,600km of the coastline, opening an estimated 2,000 breaches in natural and artificial defences, overwhelming 650km2 of land and damaging 24,000 properties. The Big Flood is one of the worst natural disasters in Britain’s history.

Paradoxically, today the risk of catastrophic flooding remains significant as the higher, stronger defences built after the 1953 event gave people new confidence to live in low lying coastal areas. In the last 50 years the coastal population has boomed.

Meanwhile, some defences are at or close to the end of their 50-year life expectancy. Consequently the number of people and the value of property susceptible to a major storm surge event are now greater.

Disaster safeguard

Mott MacDonald is at the forefront of efforts to safeguard against another disaster. Modelling flood risk is fundamental to the planning and execution of emergency responses. The Environment Agency’s existing modelling flood forecasting system incorporates forecast data on sea levels, wind speed and direction.

“We know where the effects of tides, wind and waves will be most severe”

Sun Yan Evans, Mott MacDonald

However, it does not take into account the type and condition of sea defences - beaches and dunes, sea walls, salt marsh, earth embankments and rock revetments - making it difficult to pinpoint exactly where the risk of breaches or overtopping and resultant flooding is greatest. This has given rise to false alarms, disrupting people and businesses and stretching emergency services.

To combat this Motts has developed a new modelling system that provides this vital detail and displays it graphically, showing the level of risk faced by different coastal communities. “It significantly improves the agency’s flood preparedness,” says Mott MacDonald technical director Sun Yan Evans.

Initially the Environment Agency is using the system along a 220km length of the east coast between South Ferriby on the Humber Estuary and Hunstanton on the Wash. It trained the Environment Agency’s flood warning officers in autumn 2011.

“Knowing what the defensive line is made of dramatically improves one’s ability to anticipate where overtopping or breaches will occur,” Sun Yan explains. Met Office weather and sea level forecasts are fed into the model in real time.

Algorithms

“We know where the effects of tides, wind and waves will be most severe,” she says. Algorithms combine this data with the information on coastal defences to calculate the probability and severity of flooding.

A mapping library highlights the location and elevation of communities and assets vulnerable to flooding. Flood risk to strategically vital infrastructure such as power substations, water and wastewater treatment works, telecommunications centres and principal transport corridors has been identified. Institutions including schools, hospitals and prisons, where extra time is needed to evacuate occupants, have also been highlighted.

Armed with this information, the Environment Agency and emergency services are better able to anticipate the impacts of flooding and focus warning and evacuation activities on those most in danger while avoiding disruption of people nearby who are not at immediate risk.

The system is also a powerful tool for raising awareness of flood risk. “We use the system to generate time lapse animations showing how flood events would unfold,” says Sun Yan.

“Visualising water flowing up streets and into buildings brings home the potential severity of flood risk for the public and everyone involved in planning for and responding to floods.”

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