River Rhymney

There are a series of weirs at Cwmlas, Llanbradach  on the Rhymney made out of steel piles. It is difficult for migratory fish and trout  to get over them and move upstream to spawn. The Trust was requested to look at them and work in conjunction with Environment Agency Wales to come up with a solution.  The area was on the River Rhymney   Grid. Ref.  ST 1545 9166 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.

This barrier to fish migration was identified by Environment Agency Wales  walk-over surveys and viewed by SEWRT representatives in 2009.  A barrier is classed as a weir or fall that fish can not jump over. This barrier consisted of a series of 3 steel pile weirs erected perpendicular to each river bank and driven into the river bed across the full width of the river, approx. 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, by re-creating a series of 2 stepped pools downstream of the lower steel pile weir and effectively reducing the height necessary for fish to pass over this weir.

The above slide show shows the work that has been done in the following order.  The first two pictures are the weir before the work started note in the right hand corner of the second picture the strong flow of water.  In the next two pictures the weir is shown when the work was completed.  Note the height of the water has increased making the weir easier for fish to jump over again note the flow in the right hand side. The next four slides show the work in progress and the work completed.

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 actually seen jumping over the lower steel pile weir the same day.

 

Two pictures showing the blockstone being delivered.

 

The effectiveness of this easement will be monitored by requesting reports of migrating fish passing over this barrier and by analyzing the results of future EAW electro-fishing surveys, upstream of this location.

Funding for this project was provided by Environment Agency Wales  Sustainable Fisheries Programme.