Boscastle disaster drives flood defence

Following catastrophic flooding in Boscastle, Cornwall, three years ago, a substantial scheme of improvements is being implemented.

It is believed that a consequence of climate change will be more extreme weather events. And the Boscastle flood on August 16, 2004, would certainly fall into that category. Rainfall levels up to 200.4mm in one day were measured close to the Cornish village on the day of the flood - the 11th highest recorded daily rainfall in the UK since widespread records began in the early 1860s.

But the Environment Agency's (EA) Roger Bailey says: "Most of the rain fell within a four-hour period, and the intensity of the rain coupled with Boscastle's steep catchment led to a rapid rise in the River Valency, instigating flows that have been estimated at 130m3/s in this one-in-400-year event." The River Valency caused millions of pounds' worth of damage when it burst its banks. People were rescued by helicopters, homes were flooded and cars swept out to sea when about 2Bl of water washed through the village. The flood defence scheme will cost more than £4.5M.

Major construction work was undertaken by contractor Carillion to repair damaged buildings and revive the local tourist industry in time for Easter 2005. Further measures are currently being undertaken to reduce flood risk. This involves new flood walls and an increase in channel capacity by widening and by excavation to lower the river bed.

The 2004 flood effectively scoured the river bed, removing gravel, rocks, and wildlife. Weirs and pools are therefore being created to provide new habitats. But work was hampered this summer by two further periods of heavy local rainfall, each of which represented a one-in-five-year event. A nationwide study known as the Rapid Response Catchment Project is currently under way to identify areas, such as Boscastle, that represent the greatest risk.

While the EA is responsible for monitoring major river flows in Wales and England, it does not continuously monitor the many hundreds of minor rivers such as the Valency. But, by coincidence, level monitors had been installed in tributaries to the Valency during 2001.

Roger Bailey's team installed the level monitors as a result of concerns that statistical software tools for flood prediction are generally designed for larger rivers and not short, steep rivers and streams. But level monitors do not generally give an accurate indication of flow, so the more sophisticated Argonaut Doppler flow meter has been installed about 2km upstream of Boscastle.

The Argonaut SW (which stands for shallow water) is suitable for monitoring flows in small channels from less than 0.2m up to 5m deep. It offers advanced Doppler performance for sites previously thought impossible to measure. Typically mounted on the bottom of a channel, stream, or pipe, the SW measures water level and 2D vertically integrated velocity. This, the company says, makes it ideal for sites with reversing or stratified flow conditions.

The Argonaut was chosen because it employs an advanced pulsed Doppler technique that integrates accurate velocity measurement from distinct known locations in the water column rather than the traditional lower-cost Doppler method. The latter treats the body of water being measured as a single unit and does not have the ability to identify where the reflected sound has come from.

Cliff Willis, a member of Roger Bailey's hydrometry team, has designed an attachment for the Argonaut that anchors the device securely to the river bed (with a kerbstone) and maintains the sensing elements in the correct position.

One of the 2007 events resulted in a breakage of the Argonaut's cable but Willis was able to retrieve the device and download a complete set of data. Willis manages Argonaut data using a SonTek/YSI

software package known as FlowPack. He says: "The software allows me to generate easy-to-interpret Velocity-Index ratings, and makes report creation simpler and faster. FlowPack stores flow, velocity, and stage measurements in one software program, which is very convenient."

Willis's experience of the Boscastle flood highlights the localised nature of the rainfall. He was sitting in a sunny traffic jam 15 miles away in Wadebridge when he was shocked to hear on the radio that helicopters were rescuing people from a torrential flood in Boscastle.

At present, data from the Boscastle Argonaut is stored within the instrument and collected periodically by laptop. But, with an SDI12 or MODBUS output, it will be simple to connect the instrument to the EA's telemetry system, while retaining the data within the Argonaut for additional security. Unfortunately, it is not currently possible to issue flood warnings for Boscastle based on flow data alone. Bailey explains: "Small, steep catchments such as this can generate sudden increases in flow rates when high levels of rain occur in specific locations and when the ground's ability to absorb the rain is lowered either by a high moisture content or by the intense nature of the rain. It is therefore necessary to gather a complete picture of a catchment so that models are able to make accurate flood predictions."

It is hoped that data derived from such installations will contribute to the ongoing refinement of national software tools for flood prediction.

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