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Praying for rain

Flooding Somerset Levels

High water flows are helping reduce flood risk on Somerset's River Parrett. Adrian Greeman donned his wellies to find out more.

Medieval monks who fought floods on the Somerset Levels would be amazed at the tools used now.

Instead of faggots of wood and hand dug embankments there are lasers, satellites, relational databases, sonar, and automated excavation. There is even electronic protection against drowning.

All these technologies are being deployed on a £100,000 dredging project just beginning on the River Parrett, one of the biggest running through Somerset's flat and flood prone landscape. They will help both the accuracy of the work and the future understanding of river behaviour.

The river needs particular attention because, unlike most of the drainage from this area, it has no tidal gate. This means it takes the full brunt of the extraordinary 14m plus tides that sweep the Severn Estuary, which has the second highest tidal range in the world.

'The river is tidal for a good distance beyond Bridge water [close to 10km upstream], ' explains Mark Doyle, project manager for the regional Environment Agency office, based in the town.

It also has a large catchment.

Heavy rain means large fluvial flows. 'When the two meet head on it can create surges in the river which overtop the embankments and levees.'

He says that in the past deliberate inundation of low areas known as the 'moors' would absorb enough surplus flow to prevent flooding of the town and local roads.

But recent years have seen flooding grow in scale as land use has changed.

The Parrett's tendency to flood has been compounded by a heavy silt load in its waters.

During the turn of the tide the static water will drop a layer of sediment 'the thickness of a pound coin'. That is 35mm of silt every week, building up in shoulders either side of the river and reducing river capacity.

To retain capacity in the past the Agency has let contracts to remove the build up, first with land based excavators and later using barge mounted units to reshape the shoulders.

Dredging has proved very popular with locals, Doyle says.

They see it as a significant flood protection measure and have demanded more.

So this winter a 2.5km section is being reprofiled. But new technology is being deployed.

The first innovation comes from contractor Land & Water, which has fitted its barge-mounted excavator with an automated digging control system, based on Topcon technology. The firm is using a 22t long reach machine with another crawler-mounted long reach unit for the work on the upper embankments.

Sensors fitted to the excavator's boom sections measure its extensions and angles. An on-board computer converts this data into boom position, which is then displayed on a cab video display.

'Rather than guess the bucket position under water the driver can 'see' it on the cab display, ' says Land & Water services division regional manager Matt Cain.

'But most of the digging is pre-programmed. The driver just punches in what the batter angle and depth is to be and positions the bucket against a starting point on the slope.'

More electronics are deployed to see just what slope profiles are produced. First of all a bathymetric survey is carried out from a boat which passes fairly rapidly up the river.

A hull mounted Submetrix Isis 2000 sonar can perform a 270infinity underwater swathe scan of the river bed. The scan is synchronised with readings from a GPS unit, and the tens of thousands of readings are stored in a database from which a 3-D profile image can be generated later on.

'By sweeping before and after, the quantities moved can be very accurately determined, ' says Doyle. Previously an estimated profile was used, based on a cross section survey done by hand from a boat every 50m.

Not only was the interpolation inaccurate but it took a long time to do - perhaps three days for a 2km length - and calculations might take weeks. It was also hazardous on the choppy tidal surges, which can be like a miniature Severn bore on occasion. The new survey creates figures within a day.

A fast survey suits Land & Water which is able to track its costs and quantities much more easily for the remeasured quantities contract it is working under.

But a boat survey is not enough on its own because it cannot 'see' the section above the water line. For this the Environment Agency is using an aerial LIDAR survey.

'A small plane has a laser scanner on board which is tied to a GPS receiver and also an inertial navigation unit which can compensate for the effects of pitch and turning in the plane, ' says Doyle. A full three dimensional picture of the river banks is built up - a software filter 'removes' the vegetation - and this is combined with the boat readings.

Back on the river there is a second stage to the work.

Silt is not removed but dumped into the centre of the river where it is agitated to put it into suspension in the fast flowing water.

'It is thought that the silt should not leave the system but flow with it, ' says Doyle.

Land & Water is using a Finnish made rotating agitator originally developed for ground mixing improvement. It fits on the excavator arm and is powered by the excavator's hydraulics.

To keep cut material in suspension 'we shall also run a marine plough up and down behind a tug', says Cain.

Moving the excavated material down the river and out to sea requires fairly rapid water flow, so work is being carried out when the Parrett's volume is at its highest. And Land & Water must be one of the only contractors in the country actually hoping for rain.

Keeping the team on board

The tug serves a dual role as aquatic tractor and safety boat - the Parrett is notoriously choppy and changeable. It is on hand to respond if the alarm is raised by Land & Water's self-developed 'man over board' marine safety system.

This is based on water sensitive detectors sewn into the operator's life jackets. They are equipped with a mobile phone chip which can transmit a pre-configured safety text message to several telephone numbers sequentially. It will usually say there is a man in the water and pinpoint his location.

'There is an audible alarm set off in the excavator cab as well, ' says Cain. He hopes it will not be tested in earnest.

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