AECOM was commissioned to carry out an Options Appraisal and Outline Business Case for a Weir Decommissioning Project, to meet various environmental regulations and improve local ecological conditions.
The primary aim was to decommission the weir and restore the river channel to its natural state, enhancing fish passage in compliance with the Eels Regulations, the Salmon and Freshwater Fisheries Act, and the Water Framework Directive.
The project sought to remove physical modifications within the watercourse, improve fish passage, and enhance the overall health of the ecosystem. The initiative was designed to ensure compliance with environmental legislation and achieve a biodiversity net gain, targeting a 20% increase.
Comprehensive services and expertise
AECOM delivered a comprehensive range of services, including ecological and geophysical surveys, hydraulic modelling, geomorphological risk assessment, and desk-based ground investigation. Our expertise also extended to heritage assessment, options appraisal, outline design, buildability and cost estimation, as well as adherence to CDM Safety regulations. Additionally, our team performed assessments under the Water Framework Directive (WFD) and biodiversity net gain assessments to ensure environmental objectives were met.
Significant challenges and risk management
The project presented several significant challenges. The presence of an abstraction site upstream of the weir necessitated careful management of water levels to prevent disruption. Additionally, a defence factory downstream was prone to flooding, which required solutions to avoid exacerbating this issue. The most critical challenge involved a high-pressure gas main running under the channel upstream of the weir, which posed a risk of exposure and potential bursting if bank erosion occurred following the weir's removal.
Strategic solutions and collaborative approach
AECOM addressed these challenges through strategic planning and collaboration. They engaged with the gas pipe owner to mitigate risks associated with bank erosion and infrastructure exposure. Hydraulic modelling and geomorphological assessments were conducted to predict and manage the impacts of weir removal. Early supplier engagement with the contractor allowed for refinement of designs and ensured buildability. Hard bank protection measures were developed to prevent erosion and safeguard the gas main. Furthermore, ecological surveys and tree constraints mapping were performed to ensure no adverse impacts on protected species, while identifying opportunities for positive ecological impacts.
Empowering environmental decision-making
AECOM’s Options Appraisal and Outline Business Case provided the client with a strategic plan to achieve environmental objectives, addressing challenges through detailed assessments and proposed solutions. This thorough planning equips the client for decision-making, paving the way for successful weir decommissioning and river restoration.
While this example is early in its project life cycle, AECOM has a solid track record in weir removal, demonstrated by projects like the Dowie's Mill Weir Fish Pass in Edinburgh, Crossmill Weir Removal in Barrhead, and Limefield Falls Fish Pass in West Calder.