King's Sedgemoor Drain - Construction

Construction

An Act of 1791 authorised improvements to the drainage of the peat lands of King's Sedgemoor. The existing outfalls for the moor were inadequate. Cowhouse Clyse, near Andersea on the River Parrett, was too high to work well, and Bennett Clyse, a little further downstream, was not working at all. The five commissioners who were appointed under the act proposed a new drainage channel from the area to an existing sluice called Dunball Clyse, and asked William White, a surveyor from Wedmore near Wells, to carry out a survey. White found a suitable route, but there were some misgivings, as the area is separated from the coast by a slightly higher ridge of clay, and the scheme involved cutting nearly 2.5 miles (4.0 km) of drain to a depth of 15 feet (4.6 m) through this ridge, in order to provide the necessary gradient for the water to flow. He was supported in this by John Billingsley, one of the commissioners who farmed near Sutton Mallet, but who had written a book on the agriculture of Somerset, and the drainage of the levels. He could see no alternative, and so the advice of the engineer William Jessop was sought. Jessop supported White and Billingsley in his assessment of the options, and so the route was adopted.

The 10.5-mile (16.9 km) channel of the King's Sedgemoor Drain was completed in 1795, running from the River Cary at Henley Corner to Dunball, to divert the River Cary around the south-western edge of the Polden Hills. The engineer for the scheme was Robert Anstice, who lived in Bridgwater and went on to become the first County Surveyor for Somerset. Problems were experienced with new foundations for the sluice at Dunball, which was built on ground which had until 1677 been a meander in the river. White's plans had proposed running the channal further downstream to Nine Streams Reach, but this had been opposed by landowners, and so the clyse was built at Dunball. The unsuitable conditions at Dunball resulted in the clyse being built about 4 feet (1.2 m) too high, and the shallower gradient caused sedimentation in the channels. Earth banks, built from the spoil, caused the underlying peat to rise up in the centre of the drain. A timber floor in the drain was built to restrict this, but the recurrent problem of the peat rising up was not finally solved until the banks were moved further back and in many cases, spread over the adjoining fields.

The scheme included a number of new bridges, with the main ones being at Crandon, Bawdrip, Parchey and Greylake. Penning sluices were fitted to the bridges at Bawdrip and Greylake, to allow the water to be used for irrigation, but in many cases, the bridge openings were too small, with the result that water stacked up behind them in flood conditions. In addition to the main channel, which increased in width from 26.5 to 55 feet (8.1 to 17 m) as it approached Dunball, the "Eighteen Feet Rhyne" was built to drain the moor to the north of Henley Corner, and rhynes at Aller, Chedzoy, Shapwick and Street were also 18 feet (5.5 m) wide. Other rhynes were 15 feet (4.6 m) wide. When the scheme was finished, it had cost over £32,000. £15,000 of the total had been spent on engineering work, and the scheme resulted in improvements to the drainage of an area of 11,000 acres (4,500 ha).