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Restoring the Slugain Burn

Cairngorms National Park Authority cbec eco-engineering UK Ltd March 2023

cbec eco-engineering UK Ltd The Green House, Beechwood Business Park North, Inverness, IV2 3BL

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Client: Cairngorms National Park Authority

Project Name: Restoration of the Slugain Burn

Project Reference: 2150308

Document Type: Draft Report

Project Manager: Kate Comins

Author: Dr. Lynsey MacLeary, Kate Comins

Technical Reviewer: Dr. Hamish Moir

Revision History

Version	Issued To	Date of Issue
1.0	Dr. Sally Mackenzie, Cairngorms National Park Authority	26^th January 2023
2.0	Dr. Sally Mackenzie, Cairngorms National Park Authority	9^th March 2023

Services provided pursuant to this agreement are intended solely for the use and benefit of Cairngorms National Park Authority. No other person or entity shall be entitled to rely on the services, opinions, recommendations, plans or specifications provided pursuant to this agreement without the express written consent of cbec eco- engineering UK Ltd., The Green House, Beechwood Park North, Inverness, IV2 3BL.

TABLE OF CONTENTS

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INTRODUCTION 2 1.1 Approach 2 1.2 Site Location 3
DATA REVIEW 5 2.1 Previous Study 5 2.2 Topography and Land Use 6 2.3 Geology and Soils 7 2.4 Historic channel adjustment 7 2.5 Ecology 8 2.6 Hydrology/Flood risk 10 2.7 WFD Classification 10 2.8 Bridge 11 2.9 Land Ownership 11
FIELD SURVEYS 12 3.1 High-Level Topographic survey 12 3.2 Geomorphic Assessment 15 3.2.1. Methodology 15 3.2.2. Assessment of Fluvial Form and Process 16
OPTIONS DEVELOPMENT 25 4.1 Options Appraisal 25
CONCLUSIONS AND NEXT STEPS 39
REFERENCES 42

LIST OF FIGURES

Figure 1.1. Slugain Burn – site location. 4 Figure 2.1. Slugain Burn – historical channel alignment as indicated on mapping published in 1902. .9 Figure 3.1. Topographic survey points 13 Figure 3.2. Existing conditions DEM 14 Figure 3.3. Reach types and morphological pressures 21 Figure 3.4. Sediment dynamics and large wood 22 Figure 3.5. Areas covered during the reconnaissance-level survey 23 Figure 4.1. Option 2 overview map 31 Figure 4.2. Option 3 overview map 34 Figure 4.3. Option 4 overview map 37

LIST OF TABLES

Table 3.1. Summary of fluvial form and process: Reach 1 17 Table 3.2. Summary of fluvial form and process: Reach 2 19 Table 3.3. Photos illustrating character of mainstem River Dulnain and backwater and drainage channels 24 Table 4.1. Options appraisal matrix – Slugain Burn 38

LIST OF APPENDICES

Appendix A: High Level Cost Estimates

INTRODUCTION

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The lower reaches of the Slugain Burn, (a tributary of the River Dulnain) in the Spey catchment, near Carrbridge in the Scottish Highlands, have been artificially constrained through historical straightening, with embankments significantly constraining the channel on both sides. This has impacted natural fluvial processes, which in turn has altered the physical habitat of these sections of river. These changes have limited the ecological diversity of the burn and resulted in both the disconnection of the burn from its adjacent floodplain and the periodic deposition of large volumes of sediment within the channel. Localised flooding has increased in recent years (in particular, it is understood that the burn has breached its eastern bank twice since 2019), resulting in costly repairs and maintenance and negatively impacting access for the estate and the local community.

cbec has been commissioned by the Cairngorms National Park Authority (CNPA) to undertake a feasibility study on the lower stretches of the Slugain Burn where it flows into and over the Dulnain floodplain. This report describes the development and appraisal of a range of sustainable restoration options to address the heavily degraded lower reaches of the Slugain Burn.

The overall aim of the feasibility study is to develop nature-based solutions to deliver morphological improvements, restoring physical (geomorphic) and ecological processes within the lower reaches of the Slugain Burn. Where possible, restoration options have been developed to improve access in the area for the estate and recreational users by addressing the current unsustainable management of the sediment that periodically blocks the bridge within the restoration site. In developing the restoration options, the project also aims to raise awareness within the local community of the many benefits of river restoration, such as a greater level of harmony between land use and natural river processes, an increase in habitat and associated biodiversity and a river corridor that is more adaptable to the effects of climate change.

The work forms part of Heritage Horizons: Cairngorms 2030. Funded by the National Heritage Lottery Fund, the project aims to make the Cairngorms National Park an exemplar of people and nature thriving together in a rapidly changing world by 2030. It is understood that this feasibility study forms part of the overarching development phase of the project. A second phase for detailed design and construction is anticipated to start in Autumn 2023.

1.1 APPROACH

Options to improve the degraded nature of the lower reaches of the Slugain Burn have been developed and assessed by applying a robust options identification process and evaluating the resulting options within the context of the wider catchment as well as the section of the burn to be restored. Data from previous work undertaken by cbec in the catchment, desk-based assessments (including ecology information provided by the project group) and information from a field-based fluvial audit and topographic survey were used to develop an understanding of the current physical condition and constraints of the site. This allowed for an informed assessment of potentially feasible restoration options. An on-site meeting was undertaken prior to the field surveys taking place, with all project partners present including landowners, regulatory authorities and land managers. This meant that key requirements and constraints could be fed into the options development process at an early stage. McGowan Environmental Engineering Ltd also attended this meeting to ensure issues such as site access and buildability were accounted for at an early stage in the options development process.

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To develop sustainable, long-term restoration solutions for the site, the project team adopted a ‘process-based’ approach, allowing nature-based options to be developed within the context of the physical process régime of the wider catchment. Particular focus was given to options that: minimised future maintenance costs and requirements; encouraged a longer, more sinuous course for the burn, increasing the diversity of habitats available; and reduced downstream flood risk. The core principle underpinning this approach is that addressing the processes of water and sediment supply, transport and storage at the largest possible spatial scale (i.e. catchment scale) will permit the river to recover naturally towards a dynamically stable morphology that is self-sustaining and requires minimal post- implementation management intervention over the long term. Since physical form and processes provide the template for many critical ecological functions (and their associated biota), it is reasonable to assume that restoring physical form and process should provide medium-to-longer term benefits to the currently degraded aquatic and riparian biodiversity of the Slugain Burn.

What is the ‘process-based’ approach?

We base our approach on the philosophy of ‘process-based restoration’. The underlying concept of the theory is that consideration of the natural geomorphic processes acting at the site will permit the development of a restoration strategy that is appropriate to imposed physical conditions and, where appropriate, permit recovery of the river to a more diverse and self- sustaining condition. In this way, the river itself will subsequently do the work of maintaining a ‘natural’ and dynamic environment with minimal requirement for subsequent intrusive interventions. It is important to note that the application of this type of ‘natural’ or nature-based approach to river works (i.e. working with natural river processes) is regarded favourably by the regulator in terms of licensing.

Following the development of an initial list of options for the site, feedback was sought from the project group and was used to refine and finalise the preferred list of options for the site.

1.2 SITE LOCATION

The Slugain Burn rises from a bealach between the hills of Garbh-mheall Mor and Carn Sleamhuinn (NGR NH 842 165) and extends a total of 4 km from its headwaters to the confluence with the River Dulnain at OS NGR NH 854 202, ~5.5 km to the west of the A9 road bridge as it passes close to the village of Carrbridge. The specific study site considered here encompasses the confluence of the Slugain Burn with the Dulnain and ~300 m of the Slugain Burn upstream of the confluence. At its confluence with the Dulnain, the Slugain Burn has a catchment area of 5.4 km².

A field-based fluvial audit and topographic survey were conducted throughout the study site and an extended section of the Slugain Burn, along the mainstem Dulnain immediately downstream of the confluence with the Slugain Burn and throughout the wider floodplain. The extended surveys were undertaken to gain information on processes and features outside of the restoration site that may be impacting on it or being impacted by it and to ensure consideration of processes at work within the wider system.

An overview of the study site is provided in Figure 1.1.

SLUGAIN BURN — LOCATION MAP

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Figure 1.1. Slugain Burn — site location.

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DATA REVIEW

The specific character of a section of river is influenced by both catchment- and reach-scale processes. Accordingly, it is important that any local channel management decisions are made with a full understanding of the wider catchment. This desk-based data review considers topography, land use, geology, soils and hydrology (including flooding) as a foundation for the subsequent field-based geomorphic survey (i.e. fluvial audit) and to develop suitable restoration options. The assessment also considers any existing data relating to the sites, including a review of historical mapping for the reach of interest.

2.1 PREVIOUS STUDY

cbec previously undertook a study, in 2013, to develop potential options for the restoration/management of the Slugain Burn¹ in the reach of interest. The overarching aim of this previous study was to provide options for the sustainable restoration of physical and ecological processes within the study site, to offer benefits including: a greater level of harmony between land use and natural river processes; an increase in habitat and associated diversity; a river corridor that is more adaptable to the effects of climate change; and an improved understanding within the local and wider community of these benefits and their importance to society.

cbec’s report describes the Slugain as being relatively confined in its upland section and significantly realigned/confined where it flows across the Dulnain floodplain. The site has experienced considerable management over at least the last 135 years, including drainage improvements for agriculture, particularly in relation to the now-derelict farm ~200 m west of the Slugain channel. The low-clearance bridge that the Slugain flows under upstream of its confluence with the Dulnain results in a backwater effect during high flows. This has induced significant deposition/aggradation near the bridge; periodic dredging is known to have been undertaken to remove this material, which has been piled on the adjacent channel banks. The channel is described as exhibiting a transitional step- pool/plane bed morphology in the upper reaches and a forced plane bed reach downstream. Prior to management of the channel and the adjacent floodplain, it is considered that the Slugain Burn would likely have had a ‘wandering’ or ‘braided’ character, with large active gravel bars resulting in a divided channel morphology. cbec’s report notes that the watercourse flows over a characteristic alluvial fan feature that it has created over the last 10,000 to 15,000 years, with the channel periodically switching laterally across the feature in response to sediment deposition patterns. However, recent human intervention is considered to have impacted these natural processes significantly; in particular, canalisation of the channel has been maintained through periodic dredging focused on the section immediately downstream of the road bridge, in an attempt to address the systematic aggradation that had resulted in the bed of the Slugain being elevated above the adjacent floodplain at the time of the cbec study. This canalisation and embanking has artificially increased forces on the channel bed during high flow events, thus impacting the sediment transport régime, increasing channel confinement (i.e. disconnecting the channel further from its floodplain) and reducing in-channel morphology and habitat diversity.

¹ ‘Dulnain Tributaries Restoration Project Part 2: Allt an t‑Slugain Dhuibh’, cbec report for the Cairngorms National Part Authority, January 2013.

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The report concludes that the Slugain Burn has potential for dynamic behaviour, implying that dynamic river processes can be reinvigorated by the removal or relaxation of artificial constraints to dynamic process. On this basis, three potential restoration options were presented:

Full restoration: removal of all existing constraints (i.e. embankment, existing bridge, some of road) affecting the current channel downstream of the point at which it enters the Dulnain floodplain, constructing an initial new channel just to the east of the existing alignment and allowing the river to develop a more natural equilibrium morphology over time;
Medium restoration: retention of the existing channel upstream of the bridge and development of a ‘transitional’ section downstream of the bridge to train the channel into a zone further downstream in which lateral constraints would be removed and more natural process and form reinvigorated.
Low restoration: widening of the active channel corridor by setting back the existing embankments on one side or both sides of the current channel, constructing a new initial channel through the widened corridor and allowing the channel to evolve over time within the established river corridor.

The 2013 report recognised that there may be considerable barriers to implementation of these options. For example, the removal of the existing infrastructure required for option (1) would likely prove to be a practical constraint in terms of access, while differences in elevation between the channel bed and the adjacent floodplain would make the implementation of option (2) technically challenging and could result in limited restoration of natural process and form. Implementation of option (3) would have low impact on existing infrastructure but would pose some risk of excessive aggradation in future and of failure of the set-back embankments.

Since cbec’s previous study was undertaken, the scope of possible restoration and management has widened considerably, with potential for restoration of natural form and process across a much wider area of the River Dulnain floodplain, providing opportunity for much larger-scale gains in terms of natural geomorphic process, wider biodiversity, flood risk, climate change resilience and access and amenity value, among other factors. Given the potential benefits of larger-scale restoration and management options (e.g. realignment of the Slugain Burn across the wider floodplain, relative to a much shorter realignment following the course of the existing watercourse), the options provided in cbec’s 2013 report have not been reconsidered explicitly here.

2.2 TOPOGRAPHY AND LAND USE

Catchment topography influences how rapidly the system responds to rainfall, affects the energy of the resulting flows and controls the sediment transport régime within the system. Land use and land cover patterns within a catchment control the influx of water, sediment and large wood to the system.

The burn is a small, upland waterbody that rises near Garbh-mheall Mor and Carn Sleamhuinn at approximately 500 mAOD. The confluence of the burn with the River Dulnain, ~4 km downstream of its headwaters, lies at an altitude of approximately 310 mAOD. The Slugain Burn catchment upstream of its confluence with the Dulnain can be considered an upland catchment. Land use is dominated by moorland in the upper catchment and by forestry and pastoral farming in the lower catchment. The relative lack of woodland in the upper catchment likely results in a limited supply of large wood to the Slugain Burn itself; however, large wood supply to the River Dulnain is likely to be greater, including

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from a strip of forestry along the southern bank of the Dulnain upstream of the Slugain Burn confluence.

2.3 GEOLOGY AND SOILS

Bedrock and superficial/drift geology (predominantly of glacial origin) and soil cover are important considerations in the development of management options because these factors exercise fundamental controls on sediment availability and the response of the fluvial system to rainfall.

Based on consultation of the British Geological Survey’s Geology of Britain viewer², the catchment of the Slugain Burn is underlain primarily by the Monadhliath Pluton (Phase 1 and 2), which extends beneath the upstream section of the River Dulnain. The bedrock underlying the lower reaches surrounding the confluence of the Slugain Burn with the River Dulnain and the mainstem Dulnain downstream of the confluence comprises psammites of the Dava Subgroup.

The superficial geology of the catchment comprises, in the upper reaches, the Ardverikie Till Formation, a poorly sorted sediment of stony, sandy clay. Towards the confluence and the mainstem River Dulnain, alluvial deposits underlie the river corridor. Upstream of the confluence on the mainstem River Dulnain, river terrace deposits and glaciofluvial sheet deposits are present. These widespread sediments provide a source of material to be reworked by the River Dulnain.

Based on the Scottish Government’s ‘Scotland’s soils’ map³, the Slugain Burn catchment is covered predominantly by humus-iron podzols; however, mineral alluvial soils underlie the straightened section of the channel that forms the reach of interest and the confluence with the River Dulnain. The extent of the alluvial soils, often associated with high productivity, corresponds well to the areas of fertile, grassland used for agriculture within the River Dulnain valley.

2.4 HISTORIC CHANNEL ADJUSTMENT

Analysis of historical datasets (such as old maps, photos and aerial imagery) adds valuable context to the data collected during field surveys. Such analysis allows evaluation of historic changes in channel planform along the river as the basis for assessing (a) the degree of dynamic behaviour resulting from natural fluvial processes (i.e. as opposed to human activity) and (b) the ‘reference state’ of the river system. A review of the National Library for Scotland’s historical map archive and available aerial imagery was undertaken to provide historical context, including historical channel adjustment and identification of management practices that may have influenced the supply, transport and storage of water and sediment throughout the catchment.

The earliest available mapping for the area surrounding the study site dates from around 1747 (Roy Military Survey of Scotland, 1747 – 55). However, the high-level nature of this map means that the Slugain Burn has not been captured. The map does, however, provide insight into the large-scale character of the River Dulnain, which is shown to have a sinuous planform in the mapping. The earliest available historical map depicting the Slugain Burn was published in 18755 and indicates that the present, straightened channel has been in place since at least this time. A drain is shown on this map adjoining the channel on its western bank that, although not marked on modern day mapping, is

² https://www.bgs.ac.uk/map-viewers/geology-of-britain-viewer [Accessed November 2022] ³ https://soils.environment.gov.scot/ [Accessed November 2022] ⁴ https://maps.nls.uk/ [Accessed November 2022] ⁵ OS Six Inch Series: Inverness-shire (Mainland), Sheet XLV, Surveyed: 1867 – 71. Published: 1875.

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visible as a depression on satellite imagery. A sluice is marked upstream of the Slugain/Dulnain confluence and appears to have directed flow from the main river into a pond adjacent to the property at Dalnahaitnach. It is possible this represents a mill offtake, with the aforementioned drain acting to discharge mill water back to the main river via the Slugain Burn. The pond is no longer present in historical mapping published in 1902. Furthermore, the 1875 map shows a road bridge crossing the River Dulnain at Dalnahaitnach; this bridge is no longer present, but appears on mapping until at least the 1950s.

Two secondary channels are present on the Dulnain floodplain to the east of the Slugain, arising approximately from and NGR NH 8576 2040 (northern – ‘Channel A’) and NGR NH 8579 2020 (southern ‘Channel B’); these are indicated in Figure 2.1. These channels join the River Dulnain ~950 m and ~1.5 km, respectively, downstream of the Slugain confluence. Channel A arises within a field ~180 m north of Channel B and exhibits straightened channel sections; Channel B appears similarly straightened. It is considered likely that both channels were originally natural secondary floodplain channels but have subsequently been realigned and deepened to form drains. The historical mapping published in 1902 indicates that the main channel of the River Dulnain previously exhibited a different alignment (Figure 2.1), indicating historical lateral adjustment of the channel. Both Channel A and Channel B are marked on this map, but appear to have been significantly shorter historically. Channel B is now significantly longer due, in part, to the River Dulnain’s northward migration but primarily to the extension of Channel B westwards, parallel to the road. Channel A is shown as a minor floodplain side-channel in the historical mapping. In contrast, the Slugain Burn has remained relatively fixed over the period covered by historical mapping, although the location of its confluence with the River Dulnain has moved in association with lateral adjustment of the mainstem channel planform.

2.5 ECOLOGY

The Slugain Burn itself falls within the River Spey Special Area of Conservation (SAC), specifically designated for Freshwater Pearl Mussel, Atlantic Salmon, Sea Lamprey and Otter. Parts of the site are further designated as a Special Area of Conservation (SAC), Special Protection Area (SPA) and an Important Bird Area, particularly the banks of the River Dulnain and a strip of land to the southern side of the minor access road and vehicle bridge. Remnants of old Caledonian pine forest exist on the banks of the River Dulnain and within the wider site there is open moorland habitat. The site is also located within the Kinveachy Forest, which is designated as a Site of Special Scientific Interest (SSSI) due to notified natural features including native pinewood and breeding birds including capercaillie, Scottish crossbills and crested tits.

These designations will be carefully considered during all aspects of the project to ensure that all work undertaken, as well as the final design proposals, contribute positively to the ecological functioning of the site and the designated habitats and species. Initial discussions with the project group and wider stakeholders have also indicated that areas of the floodplain provide important habitat for waders; this will be explored further during the options appraisal process and incorporated into the options matrix.

⁶ ⁶ OS Six Inch Series: Inverness-shire, Mainland XLV, Surveyed: 1900, Published: 1902.

SLUGAIN FEASIBILITY — HISTORICAL CHANNEL ALIGNMENT

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Figure 2.1. Slugain Burn – historical channel alignment as indicated on mapping published in 1902.

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2.6 HYDROLOGY/FLOOD RISK

The hydrology of a catchment controls the movement of water through the system and affects the rate and magnitude of any changes in water level and extent. Although the Slugain Burn is ungauged, the River Dulnain benefits from a SEPA level gauge located approximately 1.5 km downstream of the site at Inverlaidnan Bridge (OS NGR NH 8655 2116). The gauge has been operational since February 2017 and the maximum level recorded at the stie was 2.322 m, recorded on 7^th August 2019.

The Slugain Burn lies within the Findhorn, Nairn and Speyside Local Plan District and is not located within a Potentially Vulnerable Area. SEPA’s Flood Maps indicate that the floodplain of both the Slugain Burn and River Dulnain in the vicinity of the restoration site are inundated during high- likelihood flood events (i.e. the 1:10 year flood). The area with greatest risk of inundation lies across agricultural land just to the east of the straightened channel section. In particular, SEPA’s mapping indicates that, during flood events, flood waters arising from the Slugain Burn upstream of the access road flow north and east across the floodplain, forming secondary (high-flow) channels that connect to Channel A and, in particular, Channel B shown in Figure 2.1. The site is at minimal risk of surface water flooding.

Reports from the Project Group suggest that localised flooding has become more frequent in recent years, with the Slugain Burn breaching its eastern bank twice since 2019. Flooding upstream of the current bridge (NH 85552 20054) may be exacerbated by its low clearance, causing water to back up during high flows. Further flooding has been reported due to backing up around the bridge at Inverlaidnan (NH 86539 21162).

SEPA’s online Natural Flood Management (NFM) opportunity maps represent a high-level tool for identifying opportunities for the implementation of NFM, including the designation of areas with potential for sediment management. Although these maps do not classify the sediment régime within the fluvial audit reach, they do provide background to the wider River Dulnain environment. They indicate that the confluence of the Slugain Burn with the Dulnain corresponds to a boundary between primarily ‘moderate deposition’ upstream and ‘moderate erosion’ downstream, suggesting that the Slugain Burn flows into a channel that is erosion-dominated, potentially with accumulation of alluvial sediment on the River Dulnain upstream of the confluence. These classifications should be considered indicative only and a detailed assessment of sediment dynamics within the Slugain Burn and in adjacent sections of the mainstem River Dulnain will be provided by the fluvial audit. No opportunities for runoff reduction have been highlighted within the study site, although the opportunity maps do indicate medium to high potential for floodplain storage on a section of the eastern bank of the studied reach; these opportunity areas indicate locations in which SEPA’s high-level screening has identified the potential for storage of flood waters on the floodplain and the attenuation of flooding based on the natural features of the landscape.

2.7 WFD CLASSIFICATION

Information on Water Framework Directive (WFD) status has been obtained from SEPA’s Water Classification Hub. The Slugain Burn is a non-main river and, as such, is not classified under the WFD. The burn does, however, converge with the River Dulnain within the lower reaches of the site. The River Dulnain (WFD Waterbody ID: 23106) is situated within the wider Spey catchment and is approximately 26.5 km long. During the most recent assessment (2020) the waterbody was classified as having ‘Good’ status. Both the ecological classification and hydromorphological designation are also

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‘Good’. No existing pressures have been noted by SEPA as part of the WFD classification for the waterbody.

2.8 BRIDGE

Just downstream of the point at which the Slugain Burn meets the floodplain of the Dulnain valley, it flows under a low-clearance road bridge, a feature that appears to result in a backwater effect during high flows. This feature is inducing sediment deposition/aggradation of the bed in the vicinity of the crossing, requiring periodic removal of material that has been stored on the immediate channel banks. Given the significant impact of this bridge on natural river processes, a critical component of this project involves working with Moxon Architects and Highland Council to design a new bridge to replace the current bridge.

2.9 LAND OWNERSHIP

All land likely to be affected by the present project is owned by Seafield Estate, although the land is currently tenanted to a local farmer. The needs of both the landowner and the tenant will be considered at the options appraisal stage and will feed into the appraisal matrix to determine the preferred option.

FIELD SURVEYS

3.1 HIGH-LEVEL TOPOGRAPHIC SURVEY

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A topographic survey was undertaken to inform options scoping for the restoration/management of the Slugain Burn. A combination of a Trimble RTK GPS and S6 Total Station was used to capture data using a ‘rod-based’ methodology. In areas of the study site where vegetation cover was more prevalent or mature, the Total Station ensured adequate coverage beneath tree canopies.

The main channel of the Slugain Burn was surveyed from upstream of the road bridge at NGR NH 85601 20014, to the confluence with the River Dulnain. This included details of the existing bridge such as the soffit level. Existing road levels were captured from the existing walkers’ car park and along an ~230 m long stretch extending eastwards, including details of adjacent drainage ditches. To inform possible relocation of the walkers’ car park, a coarse-resolution gridded survey of the small, grassy area on the right bank of the Slugain Burn was undertaken.

Initial options for realignment included connecting the Slugain Burn to existing channels or topographic low points within the study site extent. Two main possibilities have been identified, referred to in the historical analysis (Section 2.3) as ‘Channel A’ (northern) and ‘Channel B’ (southern). These channels were captured using channel cross sections spaced approximately 20 m apart, including 5 m of floodplain either side of the main channel. The remaining floodplain area was surveyed in coarse grid format, to allow for the identification of low points at which the realigned channel could be tied in.

The topographic survey points collected are illustrated in Figure 3.1. Following post-processing of these data points, a georeferenced Digital Elevation Model (DEM) was created using Autodesk Civil 3D (Figure 3.2).

SLUGAIN — TOPOGRAPHIC SURVEY

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Figure 3.1. Topographic survey points.

SLUGAIN — DEM SURFACE

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Figure 3.2. Existing conditions DEM.

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3.2 GEOMORPHIC ASSESSMENT

3.2.1. Methodology

A field-based geomorphic assessment of the physical condition of ~1 km of the Slugain Burn (from approximately OS NGR NH 8567 1973 to NH 8545 2024) and the surrounding areas of the mainstem River Dulnain and its floodplain was undertaken to assess the distribution of morphological, sedimentary and ecological factors in combination with human impacts along the length of the studied sections. A ‘fluvial audit’ was undertaken along the Slugain Burn itself, encompassing the historically straightened and embanked section and the sections upstream and downstream. The fluvial audit was undertaken on 15^th and 16^th November 2022; the weather on 15^th November was dominated by blustery showers, while that on 16^th November was generally fair. Water levels at the time of the assessment were at the lower end of the normal range. The fluvial audit procedure is a location- specific inventory of the physical form of the river (i.e. morphology and sedimentology) that creates a template for key habitats and all likely influencing factors, providing an understanding of both form and function; this enhances our understanding of the causes of river degradation and supports the implementation of sustainable measures to address such degradation. The less detailed geomorphic walkover assessment was undertaken to allow assessment of the restoration reach in the context of the wider river system, to help define an appropriate ‘reference state’ for the river and to investigate areas of the floodplain that may be suitable for channel realignment. Information collected included, but was not limited to, the following:

Reach-scale channel morphology (e.g. step pool, plane bed, pool-riffle, wandering). We use a classification system that is a combination of recognised procedures (i.e. Montgomery and Buffington, 1997; Brierley and Fryirs, 2000).
Morphological/habitat units (i.e. pools, riffles, runs). These are specific ‘mesoscale’ features that, together, define reach-scale morphology. Such features can be regarded as the fundamental physical ‘building blocks’ of river channels and are closely related to habitat patterns. Therefore, such data can provide potentially valuable information to support assessments of ecological condition and habitats.
Indicators of the sediment transport régime (e.g. the size, form, texture, dominant particle size and vegetation cover of bar features and bed forms). This information is essential for interpreting physical process within the river and has implications for ecological condition and habitats.
Sediment sources (e.g. from upstream on the main river, tributaries, bank/terrace erosion). These sources have been recorded in terms of severity and extent.
In-channel sediment storage (including alluvial bar features and evidence of bed accumulation). This data also provides an indication of the rate and distribution of sediment supply to downstream areas from within-channel sources. This includes any indicators of sediment transport (e.g. the size, form, texture and vegetation cover of bar features and bed forms).
Large wood. The incidence, location (e.g. mid-channel, bank-side) and extents of large wood within the active channel, including their physical and ecological influence, have been documented.
Vegetation. Both in-channel vegetation (e.g. macrophytes) and riparian/bank-side cover have been recorded, as well as invasive/non-native species.

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River engineering pressures (e.g. weirs, lades, impeded side channels, bank protection, canalisation, embankments, bridge crossings). These features have been characterised in terms of their extents and the severity of their impacts on river process.
Floodplain morphology, including drainage channels/ditches, relict natural secondary channels, wetland areas and swales.
Other indicators of the dynamic physical behaviour of the channel (e.g. abandoned channel courses, historic side channels, age structure of vegetation within the riparian corridor).
Other land use pressures in the areas draining directly into the watercourses surveyed (e.g. urban drainage, livestock poaching, poor forestry drainage, field cultivation close to channel margins).

The collected data were recorded using a mobile GIS platform, Qfield, with integral GPS capability. This allowed accurate determination of the position and extent of important features (e.g. length of bank erosion, areas of sediment stored in active bar features). High-resolution georeferenced photos were also taken throughout the survey reach to capture significant features/structures and illustrate the general character of specific reaches.

3.2.2. Assessment of Fluvial Form and Process

For the purposes of the fluvial audit, the Slugain Burn has been divided into two separate reaches based on differences in boundary conditions. The upper reach is characterised by a high degree of lateral confinement by steep valley slopes, while the lower reach is naturally unconfined but artificially constrained by embankments. The dominant features of each reach are summarised in Table 3.1 and Table 3.2, with maps provided in Figure 3.3 and Figure 3.4. A reconnaissance-level survey/targeted walkover of the mainstem River Dulnain and several drainage and backwater channels was also undertaken to determine the character of the Dulnain and to assess the potential for the drainage channels to be incorporated into any channel realignment options. The general areas covered by this walkover are illustrated in Figure 3.5. The River Dulnain here is a highly dynamic gravel-bed river exhibiting a wandering morphology, with extensive alluvial deposition, particularly in the form of point bars, and erosion along the outsides of meander bends. The channel is often split around active to stabilised gravel islands and there is evidence of lateral migration of the main channel and activation of secondary channels during flood events. Based on field evidence and aerial photos, the backwater channel is likely connected to the Dulnain during high flows. Under normal conditions, the backwater channel has standing to slowly flowing water along much of its length and flows alongside an area of native woodland in its lower sections. The channel is incised in places but otherwise represents an area of existing good habitat; for this reason, and owing to the potential for the Dulnain to avulse into this channel, the backwater channel is not considered further as a potential option for an alternative route for the restored Slugain Burn. In contrast, both Channel A and Channel B are considered to offer good restoration potential. Both are currently straight, incised drainage ditches along much of their length and could be naturalised and tied into a realigned Slugain Burn; both channels already have confluences with the River Dulnain, although the confluence of Channel B with the Dulnain is protected by extensive hard bank protection and flow along this channel is variable. A smaller ditch flows into Channel B in the middle of the large floodplain area; although this ditch hasn’t been considered explicitly in the optioneering process, there would also be potential to connect a realigned

Slugain Burn feasibility study report 2023

Restor­ing the Slu­gain Burn

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Restoring the Slugain Burn