Restoration of a gauging weir to aid fish passage

• We examine hydraulic conditions for the restoration of a gauging weir to aid fish passage.
• We use one-dimensional hydraulic model to represent weir geometry and river channel.
• The weir does not fulfil the fishery requirements in the original condition.
• A rock ramp fish passage is assessed to aid fish passage.

This paper presents a case study of restoration of a flow gauging station namely Pontneddfechan located in South West Wales, UK. The weir was found to be affected by significant deterioration due to prolonged use and also created obstacles to fish migrating upstream. The main objective of this paper is to examine the hydraulic conditions of the weir considering the opportunities for fish passage while still retaining the accuracy of flow measurement. A one-dimensional hydrodynamic model called ISIS was used to estimate hydraulic parameters under different flow conditions. Results indicated that the weir did not fulfil the fishery requirements in the original condition. The stilling basin was modified as a way of improving the fish pass. While the modified weir was favourable for strong swimmers like salmon and trout, it was not adequate for weaker swimmers such as coarse fish. A rock ramp structure has been designed and assessed to aid fish passage across the weir.