Slu­gain Burn River Res­tor­a­tion Non-Tech­nic­al Sum­mary Report

Draft

June 2025

Pre­pared for: The High­land Council

www​.jba​con​sult​ing​.com

JBA con­sult­ing

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Doc­u­ment Status Issue date 27/06/2025 Issued to The High­land Coun­cil BIM ref­er­ence PQZ-JBA-XX-XX-RP-EN-0001-S3-P01- Non_​Technical_​Summary Revi­sion P01

Pre­pared by Jenny Per­ratt BA, MSc Assist­ant Geomorphologist

Reviewed by Alice Gent BSc MCIWEM C.WEM CEnv Prin­cip­al Geo­mor­pho­lo­gist Steven Thom­son BSc (Hons) MSc CEng MICE, Prin­cip­al Engineer

Author­ised by Douglas Scoular MEng Pro­ject Manager

Car­bon Foot­print The format of this report is optim­ised for read­ing digit­ally in pdf format. Paper con­sump­tion pro­duces sub­stan­tial car­bon emis­sions and oth­er envir­on­ment­al impacts through the extrac­tion, pro­duc­tion and trans­port­a­tion of paper. Print­ing also gen­er­ates emis­sions and impacts from the man­u­fac­ture of print­ers and inks and from the energy used to power a print­er. Please con­sider the envir­on­ment before printing.

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con­sult­ing

JBA con­sult­ing Con­tract JBA Pro­ject Man­ager Douglas Scoular Address Unit 2.1, Quantum Court, Research Aven­ue South, Heri­ot Watt Research Park, Ric­car­ton, Edin­burgh, EH14 4AP JBA Pro­ject Code 2025s0598

This report describes work com­mis­sioned by the High­land Coun­cil, by an instruc­tion dated 11 April 2025. The Client’s rep­res­ent­at­ive for the con­tract was Simon Far­row of the High­land Coun­cil. Jenny Per­ratt and Klaudia Mularska of JBA Con­sult­ing car­ried out this work.

Pur­pose and Dis­claim­er Jeremy Benn Asso­ci­ates Lim­ited (“JBA”) has pre­pared this Report for the sole use of the High­land Coun­cil and its appoin­ted agents in accord­ance with the Agree­ment under which our ser­vices were performed.

JBA has no liab­il­ity for any use that is made of this Report except to the High­land Coun­cil for the pur­poses for which it was ori­gin­ally com­mis­sioned and prepared.

No oth­er war­ranty, expressed or implied, is made as to the pro­fes­sion­al advice included in this Report or any oth­er ser­vices provided by JBA. This Report can­not be relied upon by any oth­er party without the pri­or and express writ­ten agree­ment of JBA.

The con­clu­sions and recom­mend­a­tions con­tained in this Report are based upon inform­a­tion provided by oth­ers and upon the assump­tion that all rel­ev­ant inform­a­tion has been provided by those parties from whom it has been reques­ted and that such inform­a­tion is accur­ate. Inform­a­tion obtained by JBA has not been inde­pend­ently veri­fied by JBA, unless oth­er­wise stated in the Report.

The meth­od­o­logy adop­ted and the sources of inform­a­tion used by JBA in provid­ing its ser­vices are out­lined in this Report. The work described in this Report was under­taken between April and June 2025 and is based on the con­di­tions encountered and the inform­a­tion avail­able dur­ing the said peri­od. The scope of this Report and the ser­vices are accord­ingly fac­tu­ally lim­ited by these circumstances.

Where assess­ments of works or costs iden­ti­fied in this Report are made, such assess­ments are based upon the inform­a­tion avail­able at the time and where appro­pri­ate are sub­ject to fur­ther invest­ig­a­tions or inform­a­tion which may become available.

JBA dis­claims any under­tak­ing or oblig­a­tion to advise any per­son of any change in any mat­ter affect­ing the Report, which may come or be brought to JBA’s atten­tion after the date of the Report.

Cer­tain state­ments made in the Report that are not his­tor­ic­al facts may con­sti­tute estim­ates, pro­jec­tions or oth­er for­ward-look­ing state­ments and even though they are based

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JBA con­sult­ing on reas­on­able assump­tions as of the date of the Report, such for­ward-look­ing state­ments by their nature involve risks and uncer­tain­ties that could cause actu­al res­ults to dif­fer mater­i­ally from the res­ults pre­dicted. JBA spe­cific­ally does not guar­an­tee or war­rant any estim­ates or pro­jec­tions con­tained in this Report.

Unless oth­er­wise stated in this Report, the assess­ments made assume that the sites and facil­it­ies will con­tin­ue to be used for their cur­rent pur­pose without sig­ni­fic­ant changes.

Where field invest­ig­a­tions are car­ried out, these have been restric­ted to a level of detail required to meet the stated object­ives of the ser­vices. The res­ults of any meas­ure­ments taken may vary spa­tially or with time and fur­ther con­firm­at­ory meas­ure­ments should be made after any sig­ni­fic­ant delay in issu­ing this Report.

Copy­right Jeremy Benn Asso­ci­ates Lim­ited 2025

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Con­tents Exec­ut­ive Sum­mary vii 1 Intro­duc­tion 1 1.1 Pro­ject over­view 1 2 Back­ground 3 2.1 Pro­ject area 3 2.2 Work com­pleted to date 5 3 Design Prin­ciples 5 3.1 Design over­view 5 3.2 Pro­posed Vehicu­lar Cross­ing Con­struc­tion 6 3.3 Rationale for design change from feas­ib­il­ity study 6 3.4 Hydraul­ic Mod­el­ling 7 4 Con­clu­sion 7

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List of Fig­ures Fig­ure 2 – 1 Pro­ject area 4

Abbre­vi­ations OS NGR Ord­nance Sur­vey Nation­al Grid Ref­er­ence WFD Water Frame­work Dir­ect­ive SAC Spe­cial Area of Con­ser­va­tion SPA Spe­cial Pro­tec­tion Area SSSI Site of Spe­cial Sci­entif­ic Interest

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JBA con­sult­ing Exec­ut­ive Sum­mary This report provides a non-tech­nic­al sum­mary for the out­line design of the Slu­gain Burn res­tor­a­tion pro­ject, com­mis­sioned by The High­land Coun­cil. The pro­ject aims to realign 1km of the Slu­gain Burn, a trib­u­tary of the River Dul­nain, along a more nat­ur­al mor­pho­logy and replace a vehicu­lar bridge over the Slu­gain Burn with a new cross­ing that has the capa­city to con­vey flood flows and sed­i­ment load in order to main­tain access dur­ing storm events and reduce main­ten­ance. The pro­posed bridge will also deliv­er addi­tion­al bene­fits includ­ing improved access for the loc­al com­munity, biod­iversity and hab­it­at qual­ity whilst restor­ing nat­ur­al river and flood­plain processes.

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1 Intro­duc­tion 1.1 Pro­ject over­view 1.1.1 Over­all vis­ion for the design JBA con­sult­ing Water flow along the Slu­gain Burn is con­strained by embank­ments, his­tor­ic straight­en­ing of the chan­nel, and a vehicu­lar bridge. This has led to increas­ingly fre­quent flood­ing across the flood­plain, extens­ive main­ten­ance require­ments, and poor eco­lo­gic­al and geo­mor­pho­lo­gic­al health of the water­course and ripari­an cor­ridor. To reduce flood risk and deliv­er addi­tion­al bene­fits, the High­land Coun­cil pro­pose to realign the burn along a more nat­ur­al, sinu­ous, and longer mor­pho­logy which in addi­tion will replace the exist­ing vehicle bridge which has reached the end of its design life.

JBA Con­sult­ing were com­mis­sioned by the High­land Coun­cil to devel­op an out­line design for this work, build­ing upon a pre­vi­ous feas­ib­il­ity study and options assess­ment. The aim of the design is to reduce flood risk, restore nat­ur­al pro­cesses, alle­vi­ate flood­plain dis­con­nec­tion, and reduce main­ten­ance require­ments on the Slu­gain Burn and Dal­na­hait­nach floodplain.

This report provides a non-tech­nic­al sum­mary of the out­line design which has been developed for the works proposed.

1.1.2 Pro­ject object­ives Based on the exist­ing issues, the primary object­ive of the Slu­gain Burn pro­ject as described in the scope is to restore a poorly func­tion­ing water­course whilst improv­ing access via water­course cross­ing that does not restrict the burn dur­ing floods.

Sec­ond­ary object­ives include the fol­low­ing: • Reduce future main­ten­ance require­ments of the burn • Restore nat­ur­al geo­mor­pho­lo­gic­al pro­cesses of the burn and flood­plain • Increase and improve hab­it­ats in the burn and on the flood­plain • Improve pub­lic access and park­ing near the burn

The design aims to achieve these object­ives by: • Realign­ing the Slu­gain Burn to fol­low a longer and more sinu­ous path • Relo­cat­ing and redesign­ing the vehicle bridge to min­im­ise obstruc­tion to water flow • Infilling the exist­ing chan­nel • Cre­at­ing and encour­aging the growth of hab­it­ats in the water­course and on the flood­plain • Redesign­ing pub­lic access and park­ing • Using min­im­al inter­ven­tion and Nat­ur­al Flood Man­age­ment where feas­ible. PQZ-JBA-XX-XX-RP-EN-0001-S3-P01-Non_­Tech­nic­al_Sum­mary 1

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JBA con­sult­ing The Slu­gain Burn pro­ject is partly fun­ded by the Nation­al Lot­tery Her­it­age Fund (NLHF). The pro­ject is part of the Cairngorms Nation­al Park Authority’s Cairngorms 2030 pro­gramme, which aims to empower com­munit­ies, improve trans­port, restore and enhance land­scapes, and improve health and wellbeing.

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2 Back­ground 2.1 Pro­ject area JBA con­sult­ing Slu­gain Burn is a trib­u­tary of the River Dul­nain, in the Spey catch­ment. The Slu­gain Burn over­all is approx­im­ately 4km long. The reach of the Slu­gain Burn with­in the pro­ject area starts approx­im­ately 100m upstream of a vehicle bridge at Ord­nance Sur­vey Nation­al Grid Ref­er­ence (OS NGR) NH 85603 20002. The pro­ject area extends north­east along the pro­posed relo­cated chan­nel for the burn and fin­ishes approx­im­ately 1.5km to the north­east at the con­flu­ence of the pro­posed chan­nel with the River Dul­nain at OS NGR NH 86526 21077.

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N Inverness Allt an Aon­aich River Dul­nain 0 200 400 600 m CAIRNGORMS NATION­AL PARK Pro­posed vehicle bridge New chan­nel to be excav­ated for Slu­gain Bun Cur­rent chan­nel for Slu­gain Burn Drain­age ditch River Dul­nain Con­tains OS data Crown copy­right and data­base right (2025)

Fig­ure 2 – 1 Pro­ject area PQZ-JBA-XX-XX-RP-EN-0001-S3-P01-Non_­Tech­nic­al_Sum­mary 4 JBA consulting

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JBA con­sult­ing 2.2 Work com­pleted to date The High­land Coun­cil pre­vi­ously com­mis­sioned cbec eco-engin­eer­ing UK Ltd to under­take a feas­ib­il­ity study and options apprais­al for the res­tor­a­tion of the Slu­gain Burn, and com­mis­sioned Mox­on archi­tects to devel­op a con­cep­tu­al design of a replace­ment bridge. The High­land Coun­cil have selec­ted an option to be taken for­ward to Out­line and Detailed Design stages. This option would involve the fol­low­ing actions: • Realign a sec­tion of the Slu­gain Burn upstream of the exist­ing vehicle bridge, along a new chan­nel to the east, to even­tu­ally con­verge with an exist­ing drain­age ditch • Relo­cate the vehicle bridge to span the new chan­nel • Nat­ur­al­ise the new chan­nel and the exist­ing ditch that will con­vey the Slu­gain Burn • Remove embank­ments, the exist­ing high­way bridge and infill the exist­ing chan­nel, down­stream of the diver­sion point • Enhance hab­it­ats in the chan­nel and flood­plain JBA have been com­mis­sioned by the High­land Coun­cil to devel­op an out­line design and detailed design for the chosen option. 3 Design Prin­ciples 3.1 Design over­view This pro­ject uses a ​‘pro­cess-based’ approach to under­stand the cur­rent nat­ur­al pro­cesses and con­di­tions of the water­course, in order to devel­op the design of a vehicu­lar cross­ing which does not restrict flood flows whilst pro­du­cing a design that is resi­li­ent to cli­mate change. The design prin­ciples for restor­ing the burn involve mim­ick­ing the upstream approach chan­nel by estab­lish­ing step-pool fea­tures fol­low­ing a sim­il­ar chan­nel gradi­ent. Once with­in the much flat­ter River Dul­nain flood­plain the aspir­a­tion is to cre­ate a ​‘Stage 0’ approach — a concept which encour­ages con­nectiv­ity of flows with the flood­plain, to address the straight­en­ing and over­deep­en­ing of the chan­nel as res­ult of human modi­fic­a­tions to the land­scape. This approach involves min­im­al inter­ven­tion and encour­ages the river to nat­ur­ally evolve itself so that the res­tor­a­tion takes a more ​‘nat­ur­al’ approach for nat­ur­al geo­mor­pho­lo­gic­al and eco­lo­gic­al pro­cesses along the burn. This approach improves hab­it­at diversity in the water­course and on the flood­plains and encour­ages the water­course to restore itself to min­im­ise require­ments for long-term man­age­ment. The design splits the pro­posed course of the realigned Slu­gain Burn into three dis­tinct reaches: • Start­ing at the upstream end, to over­come the steep gradi­ent whilst improv­ing the biod­iversity of the chan­nel the first reach con­sists of a step-pool sys­tem with­in a two-stage chan­nel. The reach will end down­stream of the pro­posed vehicle

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JBA con­sult­ing 3.2 • bridge, where the gradi­ent slack­ens onto the wider River Dul­nain flood­plain. This reach runs along a newly excav­ated chan­nel align­ment. • The second reach con­tin­ues along the newly excav­ated chan­nel Icoated along the south­ern edge of the River Dul­nain Flood­plain. The typo­logy along this reach is a pool-riffle sys­tem. This new chan­nel then con­verges with an exist­ing drain­age ditch so that the burn is steered towards this water­course to dis­charge into the River Dul­nain. • The third reach is along this ditch, where a stage zero res­tor­a­tion will be encour­aged, until the ditch con­verges with the River Dul­nain. Pro­posed Vehicu­lar Cross­ing Con­struc­tion Two main options were con­sidered for the river cross­ing: portal frame bridge and box cul­vert. While a bridge cap­able of span­ning the 1:200yr+cc flood event is a prefer­able option in terms of min­im­ising the envir­on­ment­al impact it is likely to prove cost pro­hib­it­ive. Instead the span of the cross­ing is to be reduced to allow a 1:2year flood event to pass through unim­peded. Storm events lar­ger than this will come into con­tact with the abutments/​wingwalls with a min­im­um free­board of 600mm to the sof­fit. Redu­cing the span allows for the bridge to be replaced with a box cul­vert. A precast pox cul­vert offers sig­ni­fi­ant cost and build­ab­il­ity bene­fits as well as a much lower main­ten­ance bur­den for the asset own­er. For the out­line design a box cul­vert with intern­al dimen­sions of 5.1m by 1.8m was assumed but the final decision will need to be con­firmed fol­low­ing fur­ther detailed hydraul­ic mod­el­ling. The step pool mor­pho­logy will con­tin­ue through the cul­vert as such the cul­vert will fea­ture nat­ur­al­ised bed and banks. There is also a sig­ni­fic­ant site access con­straint in the form of a 3.7m height restric­tion from an arched rail bridge which needs to be care­fully con­sidered. It is pro­posed that the exist­ing rail­way bridge is included as part of the sup­ple­ment­ary topo­graph­ic sur­vey to inform the full extent of access restric­tions and aid in poten­tial con­sulta­tions with precast sup­pli­ers. 3.3 Rationale for design change from feas­ib­il­ity study Fol­low­ing a site walkover and ini­tial hydraul­ic mod­el­ling, the pre­ferred option from the feas­ib­il­ity study com­mis­sioned by the Cli­ent as men­tioned in Sec­tion 2.3 was altered. The pro­posed design adopts the over­all concept from the pre­vi­ous option, how­ever, based on the exist­ing topo­graph­ic sur­vey and res­ults of the baseline hydraul­ic mod­el­ling, slight alter­a­tions were pro­posed and accep­ted by the Cli­ent. The mod­i­fied design alters the align­ment of the new chan­nel to cre­ate a less sharp curve, to reduce the erosion of the bank along the out­er curve of the new chan­nel. The loc­a­tion of the pro­posed river cross­ing was altered to span the new chan­nel align­ment at a per­pen­dic­u­lar angle. Most sig­ni­fic­antly the loc­a­tion of the pro­posed cross­ing was also moved upstream to a sec­tion of the chan­nel with high­er energy to pre­vent depos­ition dir­ectly under the bridge and min­im­ise the need for future maintenance.

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JBA con­sult­ing 3.4 Hydraul­ic Mod­el­ling A hydraul­ic mod­el was cre­ated to mod­el the baseline con­di­tions of the pro­posed course of the burn to under­stand water depths and velo­cit­ies dur­ing the low flow and high flow con­di­tions with­in the design. The out­puts of the baseline mod­el scen­ario were used to inform the out­line design of the pro­posed new chan­nel, par­tic­u­larly in terms of siz­ing the chan­nels and design­ing the step-pools. Mod­el res­ults for water depth and velo­city were used to cal­cu­late the shear stress along the pro­posed chan­nel, to inform the siz­ing of boulders and oth­er sed­i­ment that would be placed in the chan­nel — such as for the steps of the step pools, or for the riffles in the pool-riffle sys­tem. The hydraul­ic mod­el will be refined dur­ing detailed design and will also form the basis of a Flood Risk Assess­ment. 4 Con­clu­sion The out­line design of a realigned Slu­gain Burn and relo­cated vehicu­lar cross­ing seeks to reduce flood risk and deliv­er addi­tion­al bene­fits to geo­mor­pho­logy, eco­logy, and pub­lic access. The design con­veys the burn through a pro­posed new chan­nel and along an exist­ing drain­age ditch before encour­aging the burn to spread across a cor­ridor along the south­ern edge of Dul­nain flood­plain in a stage zero res­tor­a­tion inter­ven­tion. Over­all, the design accom­mod­ates the estab­lish­ment of a more nat­ur­al river mor­pho­logy that ties in more closely to the exist­ing flu­vi­al fea­tures of this area, while still allow­ing for an engin­eered approach to the bridge design to ensure the sta­bil­ity of exist­ing road infrastructure

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USA JBA Risk Man­age­ment Inc Ire­land JBA Con­sult­ing Engin­eers and Sci­ent­ists UK JBA Con­sult­ing JBA Risk Man­age­ment Romania JBA Con­sult SRL CHAS Accred­ited Con­tract­or CHAS​.co​.uk JBA con­sult­ing Offices at Bris­tol Cole­shill Don­caster Dub­lin Edin­burgh Exeter Glas­gow Hay­wards Heath Isle of Man Leeds Lim­er­ick New­castle upon Tyne New­port Peter­bor­ough Ports­mouth Saltaire Skipton Tad­caster Cam­bod­ia Mekong Mod­el­ling Asso­ci­ates Singa­pore JBA Risk Man­age­ment Pte Ltd MEM­BER OF The 5% Club ◆VEST­ING IN A GEN­ER­A­TION APPREN­TICES AND GRADU­ATES Aus­tralia Jeremy Benn Pacific (JBP) Thirsk Walling­ford War­ring­ton Registered Office 1 Broughton Park Old Lane North Broughton SKIPTON North York­shire BD23 3FD United King­dom +44(0)1756 799919 info@​jbaconsulting.​com www​.jba​con​sult​ing​.com Fol­low us: X in Jeremy Benn Asso­ci­ates Lim­ited Registered in Eng­land 3246693 JBA Group Ltd is cer­ti­fied to: ISO 9001:2015 ISO 14001:2015 ISO 27001:2022 ISO 45001:2018 CYBER ESSEN­TIALS IEMA CER­TI­FIED CIEEM REGISTERED ARMED FORCES

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