Bioengineering in Delphi to rehabilitate eroding riverbank and enhance salmonid habitat

The Project

Due to a combination of environmental factors, a riverbank along the Glenummera River channel, approximately 1km upstream of where it enters Doo Lough, was severely eroded. Without correct intervention, the integrity of the nearby public road and an area of blanket bog subject to restoration efforts was under threat. As the river channel split into two sections due to severe erosion, 150m of high quality salmonid spawning and nursery habitat was lost in medium and low flow conditions. Active erosion of banks contributed towards higher than normal silt loading within the Glenummera River and Doo Lough. A combination of a series of instream check weirs that were built upstream of the site in question, in conjunction with increased high flow rate events caused by a reduction of the carrying capacity of the surrounding blanket bog from land drainage, put the receiving environment under pressure due to increased tractive force within the channel. A similar scenario is playing out across Ireland in these typical high-energy upland rivers, particularly where the surrounding blanket bog is degraded from drainage and subsequent peat extraction.

The Rivus Solution

Delphi Lodge acquired funding from Inland Fisheries Ireland through the Salmon and Sea Trout Rehabilitation, Conservation and Protection Fund and employed the services of Rivus to investigate the site in question. As this site is within the Mweelrea/Sheefry/Erriff Complex SAC in Co. Mayo, this necessitated an assessment of possible impacts to the qualifying interests and integrity of the designated site.

Site investigation surveys concluded the riverbank in question required stabilistaion through the use of bioengineering (soft engineering) techniques. A stone toe, no higher than a Q80 flow elevation, in conjunction with reprofiling of banks to an angle of repose no steeper than 1:3, a tiered system of revetment using geocoir anti-erosion mattress and willow spiling where appropriate was used. Once works were complete, the immediate works area was seeded with deep rooting riparian grass species. Flows to the dried out 150m section of channel adjacent to the works area were restored by carefully placing naturally deposited large rock armour within the point where the Glenummera River cut through an embankment.

The Result

This project was carried out in August 2019. Since then, it has flourished and successfully stabilised 150m of riverbank, resulting in the rehabilitation of critically important and sensitive salmonid juvenile habitat adjacent and downstream of the site. Careful attention to detail was paid throughout the construction period to ensure a seamless transition between the existing, natural bank and new revetment. All efforts were made to retain as much of the local riparian vegetation as possible in order to facilitate the merging and evolution of the new structure into its surrounding environment.