Sorn Castle Dam has long been a concern for salmonid migration having an incomplete Larinier fish pass that was virtually impossible for fish to use. There were no ideal alternatives for lower flow conditions leaving fish struggling to get over if they missed the optimal levels. The fish pass was in the process if construction back around 2007 when the contractors went into liquidation leaving the job half done and a problem to migrating fish ever since. It required considerable work to complete and the cost for this was expected to be high.
Yes, salmon made it over the Castle weir in high water but we often saw fish lying on their side and exhausted, particularly as spates subsided. We also saw fish lying in shallow water with their tails exposed as they recovered from their efforts to get over, before trying again. These issues were not something that missed the attention of those willing to take salmon illegally and over the years, considerable numbers of fish have been poached at this dam. This problem has long been an issue raised at the River Ayr DSFB AGM.
Two weeks ago, we managed to agree a way forwards with the estate factor and SEPA and immediately ordered the timber hoping to beat any rain. Thankfully we did and now the heavens can open and allow the river and fish to come up and staff a chance to assess the effectiveness of this work.
We’ve spent the last 4 days designing and building a timber baffle & baulk fish pass to the left side of the dam face where most fish seemed to attempt to get over. We installed untreated Red and English Oak baffles, the best and most enduring timbers for use in water. Two larch baulks were scribed to match the face of the weir and will capture water and create an attraction flow that should allow the fish over the last and steepest part of the slope. We also removed some 30 tonnes of sediment built up behind the dam and used this to fill holes where fish often became trapped and the target for poachers. Some of this material will be redistributed downstream in spates and that is an essential part of managing sediment behind dams in rivers.
While digging gravel and sediment from the river, inevitably fine silt was released and coloured up the water behind the dam however, Sedimats were in place throughout the works and minimised impacts downstream. Most of the fine solids were removed within just 20m. We often see work going on in rivers without any silt protection, yet Sedimats offer a very effective and a relatively cheap solution to contractors. These mats can be reused time and again making them an excellent investment for river engineers.
Bearing in mind there is no water to test the function yet as the river is well below normal level, we have attempted to predict how another few inches of water will affect the flow through the new fish pass and designed it accordingly. Hopefully this will be a big improvement but it may need some fine tuning once we see it as the river rises and in spate. Baffles should create depth and white water and provide fish more protection as they make their way over the dam. Of course nothing will prevent poachers from trying but that is not our responsibility and new HD security cameras are to be added. Most of the undergrowth beside the fish pass will also be removed.
Our thanks to Sorn Castle Estate, Kevin Weir and Factor, Nick Wright for they assistance in bringing this about.