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PIDDLE FLOW TAMED

FLOOD RELIEF - Construction of an aquatic bypass means flooding should become a thing of the past for one village in south west England. Andrew Mylius reports.

The River Piddle lends its apt name to a string of villages as it trickles its way through West Dorset.

On its upper reaches, north of Dorchester, it passes through Piddletrenthide, where houses and the main road hug the river's modest banks. Most of the year there is just enough water flow to keep the ducks happy.

But when heavy rainfall runs off surrounding hills, the normally placid flow doubles from 3m 3/sec.

The Piddle is prone to flash flooding as well as more gradual and sustained floods, which rise up to submerge the flat valley bottom.

Over the last decade a rapid succession of them has brought misery to 50-odd homeowners, and climate change forecasts suggest increased rainfall intensity will raise peak river flows, threatening more houses.

In the early 1990s West Dorset District Council (WDDC), under pressure from Piddletrenthide inhabitants to relieve them from these regular wettings, appointed local consultant Ian Howick & Partners to draw up options.

The council's principal engineer Nick Browning says the fi m proposed upgrading the river channel to improve flood storage.

'But that scheme met environmental objections, ' he says. 'The Piddle is a perched chalk stream.

Dredging and widening would have broken the river's impervious bed, allowing water to drain into the chalk below - there was a chance that in summer the river would have dried up altogether.' River widening would have also demanded demolition of several small bridges.

'The owners of potential flood storage sites upstream of the village weren't happy about losing their land. Frankly neither were we, ' Browning adds. 'The sites proposed for storage were in very pretty, environmentally valuable valleys.' Before other designs evolved, changes to the allocation of funding for UK flood relief projects put Piddletrenthide's scheme on the shelf.

Severe floods in 1996 and 2001, combined with the impending transfer of responsibility for 'critical ordinary watercourses' from local authorities to the Environment Agency got the village's flood problem back on the live projects list.

This time WDDC tackled the design itself using a computer generated model of the village and river catchment. Its solution was to build an aquatic bypass to carry excess flows for a one in 50 year storm around the village rather than through it. But installing a culvert alongside the river was out of the question, says Browning, as it would have involved fairly high risk tunnelling under houses.

The £1.7M bypass alignment chosen will take high flows off the Piddle via a 12m long weir box into a 200m long, 2.4m wide by 1.4m high concrete box culvert, installed in a cut and cover trench.

This travels at right angles to the river, joining the head of a 520m long, 1.6m internal diameter jacked pipe. At the downstream end, this connects to a 20m long culvert, from which water will spill down a gabion wall and across a water meadow to rejoin the Piddle.

A joint venture of Mowlem Civil Engineering and Mowlem Johnston Microtunnelling (MJM) is carrying out the work under a £1.4M construction contract.

The 520m pipejacked tunnel section on the scheme has just been completed, using a remotely operated Herrenknecht TBM 22m below ground level.

MJM major projects manager Andy Crawford says although the firm's tender price was higher than its competitors', it was chosen because its unmanned machine eliminated the risks of working in tunnels.

Route geology is mainly chalk.

'It's sound, stable, and good for boring, ' Crawford says. 'We hit a few flint cobbles, maximum size 600mm diameter, soon after starting, but the TBM coped with them no problem. The rest was plain sailing.' Spoil was removed as a slurry.

The TBM's ideal working conditions are in wet ground - up to 30m head of water - in which slurry is created by water ingress at the face. At Piddletrenthide the contractor was forced to bowser in water to keep spoil fluid.

Site workers treated arisings with screens, and cyclonic and centrifugal processes to separate solids from water.

Jacking started from a thrust pit at the scheme's downstream end, with 800t hydraulic rams driving 2.5m lengths of pipe and the TBM forward.

As a precaution against excessive friction MJM created four 'interjacking' points, but only needed to use two.

The annulus between concrete pipe and TBM overream was fi ed by remotely injecting bentonite grout.

The scheme is due for completion in mid-April, but can already handle excess river flows, says Mowlem Civil Engineering site manager Martin Crawford.

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