Planning Committee meeting - Item 5: appendix 3 - 14 November 2025

Cairngorms Nation­al Park Author­ity Item 5 Appendix 3 Ugh­dar­ras Pàirc Nàiseanta a’ Mhon­aidh Ruaidh 14 Novem­ber 2025 Page 1 of 4

Agenda item 5

Appendix 3

2025/0104/DET

Tromie diver­sion pro­ject summary

RSPB SCOT­LAND Cairngorms Con­nect RSPB Insh Marshes Nation­al Nature Reserve — River Tromie Realign­ment: Non-Tech­nic­al Pro­ject Sum­mary. This sum­mary provides a brief over­view of the cur­rent issue being addressed, the solu­tion pro­posed, the options that have been con­sidered and the bene­fits that are expec­ted as a res­ult. This realign­ment is designed to recon­nect the river with the wider flood­plain and to reduce the risk of the river break­ing to the right across improved agri­cul­tur­al land, threat­en­ing neigh­bour­ing prop­erty and access infrastructure.

Intro­duc­tion:

Insh Marshes is one of the largest and most nat­ur­ally func­tion­ing flood plain sys­tems in the UK. It is inter­na­tion­ally import­ant for wild­life and is highly des­ig­nated (SSSI, SAC, SPA and Ram­sar site) for the birds, mam­mals, fish, inver­teb­rates, plants, wood­land and wet­land hab­it­ats found there, attract­ing over 15,000 vis­it­ors to the area annu­ally. The River Tromie flows across Insh Marshes to join the River Spey between Inver­tromie farm and the Dell of Killiehuntly.

The Marshes have a long his­tory of agri­cul­tur­al use as shown in the Nation­al Lib­rary of Scot­land side-by-side map extract below, show­ing Gen­er­al Roy’s Map on the left from circa 1750 and the first edi­tion OS map from the late 19^th Cen­tury on the right. The Roy map shows the more nat­ur­al courses of the Tromie and the Spey pri­or to the peri­od of land improve­ment, with a longer, more mean­der­ing course. In the 1^st Edi­tion OS map, the embank­ments have been con­struc­ted, and the course of the rivers has been straightened to improve con­di­tions for hay cut­ting and grazing.

The issue:

Over time, the river nat­ur­ally trans­ports sands, gravels and cobbles down­stream when the energy in the water­course is high, e.g. dur­ing spates. In a nat­ur­al burn or river sys­tem, this mater­i­al is then depos­ited in lower lying/​low energy areas, spread­ing out across flood­plains to form allu­vi­al fans and nat­ur­al levees. On the Tromie, the river embank­ments have reduced the con­nectiv­ity with the flood plain and sed­i­ment is now depos­ited with­in the chan­nel on the flat­ter sec­tions. This has raised the bed of the Tromie in sev­er­al loc­a­tions to a point where the river is break­ing out of the main chan­nel through the right bank and is hav­ing a sig­ni­fic­ant effect on improved agri­cul­tur­al ground on the Dell of Kil­liehuntly. Approx­im­ately 12ha of land is cur­rently affected by a con­stant over­land flow, redu­cing the area avail­able to the ten­ant Farm­er who now uses the field for sil­age production.

If the right bank fails com­pletely and all of the river water flows across the Dell of Kil­liehuntly fields, then a lar­ger area of improved ground will be per­man­ently affected, neigh­bour­ing ground at Old Milton and below Insh vil­lage will be at great­er risk of reg­u­lar flood­ing, and exist­ing cul­verts on access tracks and at Coull would be at risk, being too small to accom­mod­ate a large increase in flow and are likely to be over­whelmed. Nat­ur­al levees and man-made embank­ments pre­vent the Tromie from rejoin­ing the Spey until it reaches Loch Insh. A breach to the right would also interupt the sal­mon pas­sage and reduce the area of trans­ition mire hab­it­at, cre­at­ing more stand­ing water across the fens which res­ults in a less diverse swamp habitat.

The Solu­tion:

The pro­posed new chan­nel will encour­age the river to move to the left, onto the unim­proved pas­ture and wet­land com­munit­ies on the lower Inver­tromie Farm field. Approx­im­ately 3ha of this land will be affected, although 2ha of this area is already covered by mire and swamp hab­it­ats that have developed over the last 10 – 15 years.

It is import­ant to note that this is not a hard engin­eered realign­ment of the river into a new con­strained route, but is the form­a­tion of an under­size chan­nel which will take the major­ity of the stand­ard sum­mer flow. In high­er flows, the river will flow freely down both the new and the cur­rent chan­nels, but will over­top the banks of the new chan­nel more reg­u­larly, recon­nect­ing with the flood plain eas­ily, slow­ing the waters pas­sage over the ground and depos­it­ing sed­i­ment on the wet­land area. This is an approach that encour­ages nat­ur­al pro­cesses, is more sus­tain­able as it does not require main­ten­ance and pre­serves the con­nec­tion to the Spey for the pas­sage and spawn­ing of sal­mon and trout. The inter­ac­tion of the Tromie and the Spey makes the last 350m of the new chan­nel to the con­flu­ence a low-energy envir­on­ment, which also lim­its erosion.

The man­age­ment of live­stock on the Reserve in response to changes in river levels is already a well-estab­lished prac­tice, with sea­son­al­ity of graz­ing redu­cing risk and stock

being moved to high­er and drier ground if flood warn­ings are issued. The new chan­nel will be more dynam­ic than an embanked chan­nel, so per­man­ent fen­cing will be less suited to this situ­ation and that can be replaced with satel­lite con­trolled No-Fence style col­lars or tem­por­ary elec­tric fencing.

Options Con­sidered:

His­tor­ic­ally, the build-up of sed­i­ment would have been dug out peri­od­ic­ally to keep the river con­strained with­in the embank­ments, pro­tect­ing the fields from sum­mer floods. The neg­at­ive impacts of repeated dredging are well under­stood now, with it res­ult­ing in hab­it­at loss for fish and inver­teb­rates, heightened down­stream flood risk, increased tur­bid­ity and is only a tem­por­ary solu­tion. The fre­quency of spate events is fore­cast to increase with cli­mate change, mean­ing that the trans­port­a­tion and depos­ition of sed­i­ment will become more reg­u­lar. Dredging may be appro­pri­ate in some spe­cif­ic cases, e.g. close prox­im­ity to infra­struc­ture, but is not a sus­tain­able or desir­able option to under­take in this case.

The Con­sult­ant, Dynam­ic Rivers, mod­elled 7 dif­fer­ent options employ­ing lim­ited inter­ven­tions to man­age the river on the left and right sides, but all of these options had sig­ni­fic­ant neg­at­ive impacts on one or more of the neigh­bours, agri­cul­tur­al land or des­ig­nated hab­it­ats or species.

Fur­ther com­puter mod­el­ling of the pro­pos­al has shown no neg­at­ive impact on des­ig­nated hab­it­ats and spe­cies, it has no effect on the flood­ing régime of the Spey and does not increase the flood risk to the Inver­tromie Steadings.

Bene­fits:

The pro­pos­al will:

• Help to reduce the impact of cur­rent flood­ing on 12ha of improved agri­cul­tur­al land on the Dell of Killiehuntly.
• Help to reduce the risk of the Tromie break­ing through the right bank, which would affect neigh­bours land and access infra­struc­ture. This would also have a neg­at­ive effect on fish pas­sage and large areas of rare wet­land hab­it­ats and spe­cies on the Dell, Insh and Coull fens.
• Cre­ate a more sus­tain­able and diverse river sys­tem which is allowed to devel­op through nat­ur­al processes.
• Retain fish pas­sage to the Spey and have a net gain on the rare trans­ition mire habitat.
• It will not increase flood risk loc­ally, or impact on archae­olo­gic­al heritage.

Steve Blow: Septem­ber 2025.