River Dee project summary

Date	6th Novem­ber 2023	Issued ver­sion 1.0
Author	Kate Com­ins, cbec eco-engin­eer­ing UK Ltd	[Sig­na­ture]
Review­er	Alis­on Wilson, cbec eco-engin­eer­ing UK Ltd	[Sig­na­ture]
To	Cairngorms Nation­al Park Authority
Pro­ject	Dee Val­ley Restoration
Sub­ject	Non Tech­nic­al Pro­ject Summary

Flood embank­ments, arti­fi­cial drain­age and bank pro­tec­tion on the River Dee, dat­ing from the 18^th and 19^th cen­tury, have dis­con­nec­ted the river from its nat­ur­al flood­plain and impacted in-chan­nel phys­ic­al pro­cesses and wider flood­plain hab­it­at diversity. cbec was com­mis­sioned by the Cairngorms Nation­al Park Author­ity to pro­duce detailed designs that would reju­ven­ate ~135 hec­tares of the Dee val­ley by restor­ing nat­ur­al river pro­cesses, re-estab­lish­ing the con­nec­tion between the chan­nel and flood­plain and improv­ing the qual­ity of chan­nel and flood­plain hab­it­at at the land­scape scale. Designs have been developed con­sid­er­ing exist­ing hab­it­at, land use/​management, loc­al her­it­age assets and infra­struc­ture in order to achieve the max­im­um pos­sible bene­fits for phys­ic­al river char­ac­ter­ist­ics, wild­life, biod­iversity, flood risk and cli­mate change resi­li­ence with­in the exist­ing site constraints.

Fol­low­ing a com­pre­hens­ive desk- and field-based assess­ment of the res­tor­a­tion site and the wider catch­ment, two design options were eval­u­ated: par­tial embank­ment remov­al, and full embank­ment remov­al. Sub­sequent high-level com­puter mod­el­ling of the pro­posed options and stake­hold­er engage­ment helped to fur­ther refine the proposals.

The final design option includes remov­al of the major­ity of the embank­ment totalling ~2,200m (only short sec­tions that are dir­ectly impacted by util­it­ies are retained), remov­al of all hard bank pro­tec­tion along the left bank of the river, wet­land cre­ation, tree plant­ing along the riverb­ank and flood­plain areas and addi­tion of large wood to improve hab­it­at with­in the channel.

Ana­lys­is of the volumes of mater­i­al to be excav­ated, was under­taken for the pre­ferred option and estim­ated this to be 14,000m³. Dis­cus­sions with a spe­cial­ist con­tract­or con­firmed that it would not be eco­nom­ic­ally viable, prac­tic­al or sus­tain­able to remove this volume of mater­i­al from the site. It was agreed that, instead, this should remain on site, using the area of low ground imme­di­ately behind the embank­ment to dis­pose of some of the mater­i­al as well as infilling oth­er low-lying areas on the flood­plain and cre­at­ing areas of high­er ground to improve hab­it­at diversity.

Detailed com­puter mod­el­ling of the pre­ferred design demon­strated that, for smal­ler more fre­quent flood events, the design sig­ni­fic­antly improves inund­a­tion of the flood­plain near the Quoi­ch Water and increases water depths in the north­ern flood­plain chan­nels and ditches, rechar­ging wet­land areas by retain­ing water post-flood in the wet­land scrapes and ponds. The design also improves the con­nec­tion between the river and flood­plain for the less fre­quent and big­ger flood events, although the dif­fer­ences between exist­ing and design con­di­tions are less pro­nounced for these big­ger flood events. Poten­tial risks to prop­er­ties and access tracks res­ult­ing from the pro­posed design were con­sidered as part of the mod­el­ling exer­cise. Allan­more is the only prop­erty shown to come close to the pre­dicted flood extents (i.e. the area of ground flooded), oth­er than farm build­ings at Allanaquoi­ch, which are all unaf­fected by the design. Flood out­line map­ping shows that under the pro­posed design, the Allan­more prop­erty remains just out­side the flood extents at the biggest flood event mod­elled and that there is no sig­ni­fic­ant dif­fer­ence to the access­ib­il­ity of the track West of Allan­more as a res­ult of the design for all flood events modelled.

The Flood Risk Assess­ment has demon­strated that the design will res­ult in a gen­er­al increase in max­im­um flood depths with­in the flood­plain but no increase in flood extents. Addi­tion­ally, the design is not pre­dicted to have any impact on flood­ing down­stream of the site, oth­er than a poten­tial slight reduc­tion in flow energy for the smal­ler, more fre­quent floods.

The pro­posed design (i.e. remov­al of the major­ity of the embank­ment and all of the bank pro­tec­tion in place on the left bank of the river), offers a num­ber of bene­fits. This includes improv­ing the con­nec­tion between the chan­nel and flood­plain, and nat­ur­al move­ment of the chan­nel, pro­mot­ing the recov­ery of nat­ur­al river pro­cesses and phys­ic­al char­ac­ter­ist­ics, while the inclu­sion of large wood and wet­land and wood­land hab­it­at with­in the design is inten­ded to provide wider bene­fits for hab­it­at and cli­mate change resi­li­ence. Whilst recog­nising site con­straints and ensur­ing no increase in flood risk, the pro­posed design fea­tures are inten­ded to pro­mote land­scape-scale reju­ven­a­tion of the Dee val­ley. Ulti­mately, this will increase the rivers capa­city to sup­port a healthy and diverse eco­sys­tem includ­ing for key spe­cies such as Fresh­wa­ter Pearl Mus­sels and Atlantic Sal­mon and for the des­ig­nated hab­it­ats with­in the res­tor­a­tion site and sur­round­ing area.