4 minute read
River Nairn
Aberarder, Scotland, United Kingdom
Restoring a wandering gravel-bed river using online wetlands. Beginning as a steep mountain stream in the Highlands, the River Nairn historically wandered through a wide valley of wetlands and wooded floodplain. But between 1750 and 1860, the river was straightened and embanked, most likely to improve the valley for agriculture, leaving the channel perched above its floodplain. The gradual in-filling of the channel with gravel had increased flood risk to surrounding property, and the river corridor lacked naturally occurring physical features. In 2013, the landowner and regulator discussed two solutions to address the flood risk of the river: the traditional approach of regular dredging or a process-based restoration approach. They chose the latter for the project, which involved embankment removal, channel realignment, and restoration of wetland areas (creating online wetlands by reconnecting them to the river channel) to facilitate sediment storage without compromising flood risk. The project team also placed large wood in the channel to facilitate natural physical processes such as the development of riverine habitats. Construction began in 2017 and finished in just four weeks. Within two weeks of completion, sea trout (Salmo trutta) were already using typical spawning locations, with Atlantic salmon (Salmo salar) following a few weeks later.
Article cover: The alternating gravel-bar features and large wood structures are evident in the restored channel, which now meanders through low-lying wetlands. (Photo by Scottish Environment Protection Agency)
Producing Efficiencies
The project began with a river reconnaissance survey to identify the main processes operating within the wider river and the reach of interest. The team then used a two-dimensional hydrodynamic and morphodynamic model to design the restored watercourse, a type of modeling never before used in Scotland for this type of project. An iterative modeling-design process determined the optimal restoration approach for the different river sections. Postconstruction monitoring showed that the channel evolved as predicted by the model, validating this approach for future projects.
Using Natural Processes
The project team constructed a simple trapezoidal channel with a gravel bed, anticipating that highenergy flows would quickly and naturally create a dynamic pool-and-riffle morphology characterized by alternating gravel bar features. The day after completion, a high-flow event occurred that adjusted the channel geometry into the predicted dynamic morphology. Reconnecting online wetland floodplain areas helped trap fine sediments and provided extensive low-velocity habitat. Strategically placed large wood structures also forced the physical evolution and associated ecological improvements. The team’s work resulted in the project winning the 2019 UK River Prize for “Restoring Natural Processes.”
Broadening Benefits
The project site provides important habitat for Atlantic salmon (Salmo salar), a species in decline. The project also improved overall water quality by taking advantage of the reconnected wetlands’ ability to regulate fine sediment transport, eliminating the need for (and cost of) regular dredging. And together with the lowered channel, meandering curves, and reestablishment of native woodland, these wetlands slow flows, increase water storage times, and prevent excess runoff. The channel evolution has performed as predicted by the team’s modeling, proving the efficacy of this holistic approach to habitat restoration and flood risk management.
Promoting Collaboration
To restore the reach within the tight time frame and funding constraints, communication between the landowner, designer, regulatory agency, contractor, and local community was essential. From this communication, the project team developed a restoration strategy that balanced the improved physical and ecological functioning of the river with local landowners’ desire to protect agricultural land and access tracks from flooding. In addition, the stakeholder’s sense of ownership helped to secure future monitoring and maintenance of the scheme.
