River Clydach – Tunnel at Ynysybwl

The objective was to create an easement for migratory fish to pass through the Tunnel at Ynysybwl on the River Clydach.   If you look at the picture of the tunnel below you can see it has a strong centre flow. Our aim was to deepen this by placing a timber baulk on the right had side that would concentrate the flow. Subsequently we placed one on the left hand side as well increasing and deepening the flow. This would allow migratory fish to access an additional 5KM upstream of Ynysybwl and reach the spawning gravels in the headwaters and tributaries of the River Clydach. This barrier to fish migration was identified by NRW representatives during a walk over survey.

When working on the lower entrance to the tunnel trout were seen living in it indicating that there must be resting areas in the rocky bottom. The  tunnel, 80 meters (approx.) in length x 2 meters wide x 2.5 meters high (approx.)  and having a flat inclined bed (approx. 10 degrees) with some deeper water channels. Due to the age of the tunnel it was not felt safe to work in it. The picture below shows the bottom entrance to the tunnel.

Using the two pieces of timber also raised the level of water in the tunnel keeping it deeper for fish to swim through. The wooden sleeper was secured to the bed of the tunnel using stainless steel bolts (x2) and chemically anchored into the bed of the tunnel. The deflection/concentration of the river flow out of the tunnel was evident immediately, as was the increased depth of water inside the tunnel, upstream of the baffle.