Restoring fish passage in Aughrim with a series of weirs with low flow notches downstream







The Project

Coatesbridge Weir, located on the Aughrim River in Co. Wicklow, was originally built to back up river flows in order to supply Goatsbridge Trout Farm with fresh water. The hydraulic drop on this weir is 2m and presented significant fish passage problems over a range of different flows. These problems were further exacerbated by continuous degradation of the structure after high flow flood events which required regular maintenance using heavy machinery.





The Rivus Solution

Goatsbridge Trout Farm acquired the services of Rivus to conduct a topographical site survey, design a plan to restore fish passage and construct the solution. Based on site conditions and fish passage requirements for all species within the Aughrim River, it was proposed to reduce the height of the original weir to 1.5m and construct a series of weirs at lower elevations downstream with individual hydraulic drops of 0.25m. A low flow notch was incorporated into each weir to ensure fish passage was facilitated at low flow rates between Q95-Q80. At higher flow rates, fish passage is provided across the entire width of the structure.






The Result


Heavy rainfall coincided with the construction period of this project in September 2020. Challenging conditions persisted throughout this phase however continued determination in less-than-ideal water levels resulted in the successful completion of all weirs. The beauty of this project and the chosen design meant that all weirs were constructed using stone already present within the river system onsite, thus lowering construction costs as less stone had to be imported. The implementation of this project has now given the green light to all fish species within the Aughrim River to reach valuable habitat upstream.