Dudbridge bypass cleared
Dudbridge bypass cleared
by: Buffs
This week’s relenting of the rain storms and flood surges along the canal gave Western Depot volunteers the chance to inspect and clear the bypass at Dudbridge. Regular readers will recall that since the middle of the autumn we have been regular visitors to the bypass grille to clear it of significant volumes of debris which has been driven into the canal from the Slad and Painswick streams. The huge down flows have not just driven debris into our water but have added large quantities of silt to the canal bottom making the lock gates difficult to open and depositing several inches on the bottom of Dudbridge lower lock where we are trying to reinstate the lock wall. The extreme flows sank Annette II and filled her with upwards of 3 tonnes of silt. The continual rainfall delayed her recovery from the water.
The grille at Dudbridge was reported to be clear by our lengthsman. Armed with grapples ropes and kebs, two volunteers set off to finish the removal of accumulated debris around the eel pass from the hydro inlet. There are spectacular photos of this obstruction and its effect on the water flow in earlier posts in this blog. It was a relief to see that the bypass was quiet and the blockage had partly self-cleared but a substantial debris dam still remained.
The flow of water along the bypass was reduced to a trickle by careful adjustment of the flow through the lock. While this was happening attempts were made to remove floating debris from the canal using the grapple. A road sign was very visible and first to come out. A large branch was partially visible through the water and several casts of the grapple brought out some small branches and fronds. The main structure of the branch remained visible through the water but resisted all attempts to bring it to the surface. It will be left for the dredger to remove it from the mud. In the meantime more silt will be attracted to it making it more of a task to remove it.
On to the clearing activity. The dam by the hydro entrance was quickly cleared. The inevitable cones were first to make it to the bank followed by a large pallet made very heavy by the water it had absorbed whilst in the canal. The rest of the dam was small twigs and leaves forced into a solid mass by the press of the flow onto the larger obstructions. Once cleared of the overburden it was clear that any eel mat which was in the trough had been torn out by the extreme turbulence around and over the dam.
Next onto the fish and eel pass which runs under the bank against the playing field. There were two significant debris dams to clear and the head weir needed clearing. The head weir where the water enters the passes was fouled by the expected leaf and stick dam but also by an considerable silt bank which had built up there. The flow at this point creates a back eddy which slows the water causing it to drop some of the entrained silt. This bank extended about 18 inches beyond the mouth of the passes. Clearing the debris and reducing the silt bank reopened the normal water path into the passes. The two large dams were cleared from the fish pass and flow returned to a semblance of normal. It was clear that the convoluted path design was also acting as a major silt trap. This essential feature encourages fish to travel upstream by breaking the otherwise strong flow downstream and providing resting places. The excess silt was encouraged to move downstream by the careful application of the mark one wellie enclosed foot of the volunteers to the silt piles. By this action more debris and small sticks were removed to the main bypass.
The eel mat will require a further clearing later in the spring to remove the heavy silting in the knap of the mat. There will also be further work to reinstate the grille and the matting around the mouth of the culvert.
During the latter part of the task we were unexpectedly joined by Jasper and her crew of two. They were giving Jasper her first outing since her holiday by the river at Lockham. The crew tried to lift the branch from directly above but were unable to move it in the mud. They did however find the log which had been lashed to the bank. This had been a target for the morning but remained frustratingly hidden from the towpath so our chainsaw man Mike left unfulfilled and joined the block moving party at Ryeford. The log was of an impressive size and was taken in tow to Strachan’s Close slip for recovery and disposal there.
Thanks to Frank, Chris, Ian, Mike L and Jasper’s crew for the work to complete the clearance.
