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​​​​​​​Rhymney River Weirs

There are a series of weirs at Cwmlas, Llanbradach on the Rhymney constructed from steel piles. It is difficult for migratory fish and trout  to get over them and move upstream to spawn. The Trust was requested investigate and work in conjunction with Environment Agency Wales to develop a solution. By helping fish over this barrier it would allow them access to an additional 10KM of spawning gravels in the headwaters and tributaries of the River Rhymney.

The weirs were identified as a barrier to fish migration by Environment Agency Wales walk-over surveys and reviewed with SEWRT representatives in 2009.  The cascade consists of 3 steel pile weirs, erected perpendicular to each river bank and driven into the river bed across the full width of the river, approximately 50 metres apart. Previously, two arched blockstone pre-barrage type weirs had been built downstream but they were badly damaged and not functioning as originally planned.  

As a  result  migratory fish were having to jump approx. 1.5 metres to pass through the lower steel pile weir to access their spawning grounds.  The heights of the 2 upstream steel pile weir crests were approx. 0.4 metres above the surface of both plunge pools and were not considered to be barriers for migratory fish. It was decided that rebuilding both the downstream boulder weirs would be the most effective solution, re-creating 2 stepped pools downstream of the lower steel pile weir and effectively reducing the height necessary for fish to pass over this weir.​


​​Environment Agency Wales  expressed some concern over the stability of the river beds in the vicinity of these steel pile weirs, and the possible flood risk that existed if the rebuilding work was not carried out to the highest possible standards.  As a result, the Agency's  Operations Team, who are highly experienced in this work, were engaged and work began on Thursday 21st October 2010 and was completed as per plan on Tuesday 26th October 2010. On completion of the reconstruction work, migrating fish and trout were seen jumping over the lower steel pile weir the same day!

​Project Grid Ref.  ST 1545 9166