The River Nairn at Aberarder, Highlands, Scotland

Under natural conditions, the study site would have been a highly dynamic alluvial fan, possibly with multiple channels. However, because of the historical engineering undertaken for land management purposes, natural physical processes in this part of the upper Nairn were considered to be significantly altered. The channel had been straightened and constrained between embankments, becoming ‘perched’ above its floodplain due to aggradation of its bed. The river corridor lacked the physical features and associated habitats that would naturally occur, mainly through the lack of morphological variability but also limited tree cover and large wood material in the active channel. As a result, the waterbody was given less than good ecological status under EU Water Framework Directive classifications.

Following a ‘process-based’ approach, an ‘assisted recovery’ general design strategy was adopted that permitted the river to adjust itself towards a stable ‘dynamic equilibrium’ morphological condition. The aim was to reinstate a greater potential for the development of lateral channel variability through the evolution of alluvial bar forms and associated bank erosion processes. A high-resolution topographic survey was conducted, which fed into 2D hydrodynamic and morphodynamic (i.e., sediment transport) modelling phases of the work to determine the optimal design for the different river sections. A series of conceptual visualisations was produced, followed by detailed AutoCAD Civil 3D engineering design drawings and an associated method statement to meet the Scottish Environment Protection Agency’s licensing requirements. cbec supported the Aberarder Estate stakeholder engagement and consultation work throughout.

Within the general ‘assisted recovery’ restoration approach, two different design strategies were proposed in discrete sections of the site. Owing to practical site constraints, the design in the upstream sections of the site could involve only the localised reprofiling of the river corridor (mainly the removal or re-grading of existing embankments) while, in the downstream section, a new realigned course was constructed. This moved the channel into a more sinuous course through a lower elevation area and through a series of three online two pond or wetland areas before tying back into the existing channel course downstream. ‘Bar apex’ large wood structures were installed at appropriate locations through the realigned channel (in channel margins). The wood structures incorporated mature trees with root ball and branches still attached. With sufficiently sized trees (minimum 500 mm diameter trunks), no engineered measures (i.e. cables or piling) were required to stabilise the wood structures. Such trees were sourced locally and the structures were ‘field fitted’, matching specific trees to appropriate locations within the realigned channel. cbec’s principal geomorphologist was on-site throughout construction, which was successfully completed in October 2017.

cbec continues to monitor progress at the site. Subsequent physical and ecological monitoring of the site has demonstrated that morphology has evolved in the manner intended, with Atlantic salmon and trout responding positively to the new channel configuration (evidenced through redd and juvenile surveys conduced in 2017 and 2018).

The project was nominated for the 2019 UK River Prize, details of which can be found here.