Stream enhancements at Munreery Culvert

As part of our commission on the A6 Dungiven to Drumahoe dualling scheme in Northern Ireland, ROD, in conjunction with fisheries consultants Paul Johnston & Associates, undertook the upgrade of Munreery culvert, a large box culvert with a flat concrete base.

As our understanding of hydrological and hydraulic processes developed over the 20th century, sizing proposed river crossings to enable them to convey extreme flows became commonplace. While this focus on conveyance was done with the best of intentions, it led to degradation of the channel morphology, negatively impacting biodiversity.

Key ecosystem services and habitat types cannot return to the urban catchments without river restoration measures being undertaken within the main river channel.

Typical measures include:

• De-culverting of watercourses

• Introduction of large woody debris

• Establishment of in-stream vegetation

• Creating new meanders in impounded river channels

• Reconnecting remnant meanders

• Current deflectors

• Narrowing channel with aquatic ledges

• Creating a sinuous low-flow channel in an over-widened channel

• Creation of on-line bays

• Fixing whole trees into the riverbank for flow diversity

• Gravel reworking to restore a low-flow channel

• Weir removal

• Review of/reduction in maintenance

While the impact of these measures on current channel morphology and maintenance practices may vary, they will assist us in meeting our obligations under the EU Water Framework Directive.

Munreery culvert is an example of how common hydromorphological pressures can negatively impact watercourses. In low flow conditions, the depth of flow in the culvert was so low (<4cm) that it prevented the migration of fish, including salmon and trout. Conversely, in high flow conditions, the high velocities at the outlet resulted in erosion of the downstream channel.

ROD undertook a hydraulic assessment of the culvert and obtained schedule 6 approval for enhancement measures that allowed for the migration of sensitive fish species and stabilised the downstream channel. This was achieved without adversely affecting flood risk in the vicinity of the culvert.

Hydraulic assessment