Whitminster Lock and Bridge

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Key Dimensions
Existing Structure:	Width at water	5.1m min	Width of roadway	3.65m
	Headroom	small	Depth bottom stop planks	1.8m
			Depth top cill (assuming 10cm rise)	1.55m
Proposed Replacement:	Width at water	m	Width of roadway
	Headroom	m
	Draft	1.5m
Owners	British Waterways/Mrs Teasdale

Background Information

Whitminster Lock originally dropped the Stroudwater Canal by approx 1.9m but this was reduced to as little as 10cm when the G&S Canal was built; the remaining difference being taken up by the new Junction Lock. There is no evidence that any changes were made to the structure at that time apart from building up the tail walls to accommodate the much higher water level. The lower gates remained at their original height with the risk that silt accumulation might have caused difficulties.

In the 1960s, the lock was infilled and severed from the upstream part of the Stroudwater Canal when Lockham aqueduct was removed and the canal and river channels merged.

There is a low level fixed span bridge at the tail of the lock carrying a farm crossing and footpath.

Replacement Structure

W.S.Atkins in their "The Stroudwater Canal Engineering Feasibility Study of Restoration", of April 1991, identified that a restored Whitminster Lock would have to posses additional capabilities the the original. In particular, it had to be possible to lower the level of the River Frome below that of the G&S Canal in the event of a pollution incident on the M5 in order to safeguard Bristol's domestic water supply which is derived from the G&S Canal.

W.S.Atkin's solution was to rebuild the lock with sector gates at an estimated cost of in excess of £300,000. The Cotswold Canals Trust realised that the same function could be achieved by simply replacing one set of mitred gates with a guillotine gate, which could cope with differential water pressure on either side, with a consequential reduction in cost of about £200,000. The airdraft provided under the fully raised gate needs to be decided in the light of the desire to provide a higher air draft to Fromebridge.

The lock also has to be incorporated into the flood banks of the river such that the southern bank has to stay on the south side of the lock and then effectively cross at the guillotine gate to rejoin the existing flood bank between the lock and the river.

The CCT started work on Whitminster Lock in 1995 with the full excavation of the lock chamber after long negotiations with the NRA. Repairs were carried out to the structure and stop plank grooves were provided at the tail of the lock on a new concrete cill which reduced the draft at this point to about 1.8m.  Repairs were made to the lock chamber brickwork which appears to be a constant 1m (approx) in thickness rather than increasing in thickness with depth. New upper gates, which have gate paddles, were installed but work ceased due to the landowner withdrawing permission whilst attempts were made to gain planning approval for a nearby proposed marina and housing development. The CCT has funding available for the guillotine gate once the land for the lock and link to the river has been secured.

The depth over the top cill is currently 1.45m at the G&S level, historic records indicate that the upper water level was 12cm above this giving 1.5m.

It was intended to replace the bridge at the tail of the lock with a new high level fixed structure. There were some issues with the NRA (now EA) about the approach ramps being on the flood plain. A bigger issue may be the desire to facilitate larger airdraft craft at least as far as Fromebridge as the difficulty of providing a fixed structure increases significantly as the desired air draft increases. In this location, as a lift or swing bridge may be viable or it may even be possible (but unlikely) to negotiate away the need for a bridge completely. When considering this bridge, the structural strength of the tail walls of this lock need to be considered and it has been suggested that a swing or lift bridge across the lock chamber itself might be the best solution as it can be left open for much of the time.

The CCT commissioned design plans for the lock and connecting channel from M.D.Hughes and these can be made available and BW have more recently produced a full design for this site with mitred gates at both ends. Pollution could be prevented from entering the G&S Canal by closing the stop gates at Saul.

 





 New top gates being installed in Whitminster  Lock in 1996 following the restoration of the    brickwork by the Cotswold Canals Trust. The  original ground paddles at the head of the lock  have been decommissioned in favour of gate  paddles which will always discharge underwater.

















The route of the connection channel between Whitminster Lock and the Frome. Note that the flood bank will need to move from the north (left) side of the lock to the south (right) side.











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