Location: Northern Ireland.
Project: Silt Plume Dispersion & Water Quality Assessment relating to operation of floating suction dredging barges.
Planned Workings: Suction Dredging.
Mitigation: In development.
Approximate Consented Reserve: TBC.
Project Areas: EIA.
Commercial extraction of sand from the bed of Lough Neagh is undertaken using suction dredging barges operated by several companies, collectively known as the Lough Neagh Sand Traders (LNSTs).
The barges are loaded through a suction hose, its trailing drag-head recovering sand in slurry form from the Lough bed at depths of between c.5 metres (m) & c13.m.
The LNSTs’ total annual saleable sand production sourced from Lough Neagh is c.1.7 million tonnes (mt), implying between c.4,800 and c.4,900 individual suction loading operations per annum.
The material raised on to the barges also contains finer sized particles; comprising silts, clays and organic debris. Whilst sand is retained aboard the bay of the barge during suction loading, a proportion of the finer dredged particles are returned back to the Lough; being held in suspension within water draining under gravity via discharge ports in the barge’s gunwales.
Particles entrained within water draining back to the Lough and particles mobilised from the Lough bed by the agitating action of the dredger’s suction drag-head, together form a temporary “plume” within the water column immediately alongside and below the barge.
Field study was undertaken to collect data to allow:
- Identification of the three dimensional extents of the suspended particle plume associated with dredging;
- Characterisation of the three-dimensional spatial distribution of suspended particle concentrations;
- Assessment of the suspended particle loadings generated in the wake of loaded barges returning to berth;
- Examination of the particle size distribution of plume material, and;
- Understanding of the dissolved concentrations for basic water quality parameters within and surrounding the plumes associated with dredging and within the wake of loaded barges returning to berth.
The exercise involved the collection of 124-no. time-stamped water samples from varying depth horizons surrounding 2-no. NEG suction dredgers (the NEG Bayshore and the NEG Norman vessels), both during active suction loading and upon the return navigation routes from their respective dredging grounds.
GPS based position-logging equipment was used to track the positions of the dredgers and shadowing sampling boat, facilitating accurate identification of the position from which water samples were taken.
The results of investigations are currently being considered as part of a multi-disciplinary Environmental Impact Assessment of the sand dredging and related activities.