Case study:Upper River Nairn restoration project

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Location: 57° 18' 0.85" N, 4° 16' 53.02" W

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Project overview

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Status	Complete
Project web site
Themes	Environmental flows and water resources, Fisheries, Flood risk management, Habitat and biodiversity, Hydromorphology, Land use management - agriculture, Land use management - forestry, Monitoring
Country	Scotland
Main contact forename	Euan
Main contact surname	Anderson
Main contact user ID	User:Aberarder
Contact organisation	Aberarder Estate
Contact organisation web site
Partner organisations
Parent multi-site project
This is a parent project encompassing the following projects	No

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Project summary

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The site is located on the upper River Nairn, just west of the settlement of Croachy. The study section is 4 km long, extending upstream to where the channel exits from its steep headwater zone. The gradient of the channel generally decreases as it transitions into a floodplain zone, currently managed for agriculture. Under natural conditions, the study site was a highly dynamic alluvial fan with multiple channels. However, because of historical engineering for land management, natural physical processes in this part of the upper Nairn were 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 water body was classified at poor ecological status under EU Water Framework Directive (WFD) classifications.

Following a ‘process-based’ approach, an ‘assisted recovery’ design strategy was adopted, permitting the river to self-adjust towards a quasi-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) and connectivity with wetland/ floodplain environments. Initially a series of conceptual design were developed from which, after stakeholder engagement, a preferred option was identified. A detailed design phase then followed, supported by high-resolution 2D hydrodynamic and morphodynamic (i.e. sediment transport) modelling. An iterative modelling-design process determined the optimal restoration approach for the different river sections.

Three different design strategies were proposed in discrete sections of the site. Due to practical site constraints, the design in the upstream sections of the site could only involve the localised reprofiling of the river corridor (mainly the removal/ re-grading of existing embankments) while, in the downstream section, channel realignment and the reinstatement of ‘online’ wetlands were implemented. Almost 1 km of channel length was realigned into a more sinuous course through a lower elevation area. Importantly, this connected a series of three online wetland areas before tying back into the existing channel course downstream. Furthermore, to mitigate the lack of large wood (LW) processes through the site, riparian planting was conducted in conjunction with the installation of ‘bar apex’ LW structures were installed at appropriate locations through the realigned channel.

Monitoring surveys and results

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Post works monitoring is essential for evaluating the physical and ecological performance of the project. Given the near unique scale of application of the ‘process-based’ restoration approach, the project provided an invaluable opportunity to monitor how the river environment responds to the various components of the implemented works, providing valuable insight and guidance for the application of this type of approach in other areas.

Specific monitoring has included:

• Repeat topographic/ bathymetric surveys (immediately post-construction and subsequent to three high flow events flood events).

• Repeat sediment sampling (pre- and post- implementation)

• Repeat fixed-point photographs

• Repeat spawning surveys/ redd counts (including pre-works and the two spawning seasons post-construction)

• Repeat electro-fishing surveys (in both channel and wetland locations, pre- and post-works)

Immediately after construction, a detailed topographic and sedimentary resurvey were carried out. Further resurveys have been undertaken after significant flow events, determining rapid evolution of the site and in the trajectory intended (and predicted by modelling).

Given fish populations are a good indicator of habitat condition, repeat red counts and electro-fishing surveys have also been undertaken. Latest survey results show significant increases in spawning and juvenile salmon and trout compared to pre-restoration conditions, providing an ‘ecological validation’ the works implemented.

Lessons learnt

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Given the explicit development of an integrated steering group and the associated project management approach, challenges during the project were minimised. Also, experience from previous construction projects by the design/ build team meant that many potential issues were avoided. There was careful planning of the construction phase of the project such that the most intrusive works (i.e. construction of the realigned channel and large wood implementation) were conducted ‘offline’, minimising the risk of high flow events impacting the works and fine sediment pollution. Such ‘assisted recovery’ designs explicitly require a degree of ‘field fitting’, allowing for unexpected issues encountered to be managed through appropriate short notice design variations. This was the case in the furthest downstream section of the site where, due to wet ground, it was not safe for construction machinery to excavate the initially proposed realigned channel in this area. It was therefore decided that the proposed design be modified such that the third (furthest downstream) wetland feature would be instated through this ~200 m long section, subsequently proving to be an appropriate revision. Such a short notice adjustment was only possible due to the close and established working relationship between the designer, regulator (i.e. SEPA) and contractor.

Image gallery

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Aberarder New Lower Channel

Aberarder Upper Wetland

Aberarder

Wood structure

Catchment and subcatchment Select a catchment/subcatchment Site Edit site Name WFD water body codes WFD (national) typology WFD water body name Pre-project morphology Reference morphology Desired post project morphology Heavily modified water body National/international site designation Local/regional site designations Protected species present Invasive species present Species of interest Dominant hydrology Dominant substrate River corridor land use Average bankfull channel width category Average bankfull channel width (m) Average bankfull channel depth category Average bankfull channel depth (m) Mean discharge category Mean annual discharge (m3/s) Average channel gradient category Average channel gradient Average unit stream power (W/m2) Project background Edit project background Reach length directly affected (m) 24002,400 m <br />2.4 km <br />240,000 cm <br /> Project started 2014/11/20 Works started 2017/08/01 Works completed 2017/09/30 Project completed 2018/04/01 Total cost category Total cost (k€) Benefit to cost ratio Funding sources Scottish Water Environment Fund (WEF) Cost for project phases Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname Investigation and design cbec eco-engineering UK Ltd Hamish Moir Stakeholder engagement and communication Works and works supervision Salix - McGowan Post-project management and maintenance Aberarder Estate Euan Anderson Monitoring cbec eco-engineering UK Ltd Hamish Moir Reasons for river restoration Edit reasons for restoration Mitigation of a pressure River engineering relating to agriculture Hydromorphology Biology Physico-chemical Other reasons for the project To restore a heavily modified section of the Upper River Nairn back to a more natural state whilst also reducing the risk of flooding to protect agricultural interests Measures Edit Measures Structural measures Bank/bed modifications Channel realignment, embankment removal, channel reprofiling, installation of large wood structures Floodplain / River corridor Channel realignment, channel reprofiling, reinstatement of three wetlands Planform / Channel pattern Changing from a canalised straightened channel to a meandering/ wondering channel Other Non-structural measures Management interventions Riperian tree planting, fencing Social measures (incl. engagement) Regulatory bodies incl with SEPA and SNH Other Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Biological quality elements Edit biological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Physico-chemical quality elements Edit Physico-chemical quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Any other monitoring, e.g. social, economic Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Monitoring documents Upload monitoring documents Additional documents and videos Upload additional documents Additional links and references Edit links and references Link Description Supplementary Information Edit Supplementary Information