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No piddling matter

Construction of an aquatic bypass is saving one west Dorset village from future flooding.Andrew Mylius discovers how.

The River Piddle lends its name to a string of villages as it trickles its way through West Dorset. On its upper reaches, north of Dorchester, the Piddle passes through Piddletrenthide, whose houses and main road hug the river's modest banks. Most of the year there is just enough water flow to keep resident ducks happy.

But heavy rainfall running off surrounding hills can swell the normally placid river, doubling its flow from 3m 3/sec to about 6m 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. A rapid succession of floods through the last decade has brought misery to 50-odd homeowners, and climate change forecasts suggest that increased rainfall intensity will raise peak river flows, inching flood water over the thresholds of other, hitherto dry, houses.

West Dorset District Council (WDDC) came under pressure from Piddletrenthide inhabitants to relieve them from regular wetting in the early 1990s, and appointed local consultant Ian Howick & Partners to draw up options. The firm proposed upgrading the river channel to improve flood storage, says the council's principal engineer Nick Browning.

'But that scheme met environmental objections, ' he explains. 'The Piddle is a perched chalk stream. Dredging and widening would have broken the river's impervious bed, allowing water to drain into chalk strata below - there was a chance that in summer the river would have dried up altogether.' River widening also demanded demolition of several small bridges, the abutments of which would have presented constrictions.

'And the owners of potential flood storage sites upstream of the village weren't happy about losing their land. Frankly neither were we. The sites proposed for storage were in very pretty, environmentally valuable valleys, ' says Browning But before any alternative designs could evolve, changes to the allocation of funding for flood relief projects put Piddletrenthide's scheme on the shelf, however. Severe floods in 1996 and 2001, combined with the impending transfer of responsibility for 'critical ordinary watercourses' from local authorities to the Environment Agency (due this year) put the village's flood problem back on the live projects list, however.

This time WDDC tackled the design itself using a computer generated model of the village and river catchment. The solution was to build an aquatic bypass that would 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 this it would have involved 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 102m 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. The work is being undertaken by a joint venture of Mowlem Civil Engineering and Mowlem Johnston Microtunnelling, under a £1.4M construction contract.

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

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