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SEDIMENT LOADS AND SOURCES IN THE BUSH
CATCHMENT: A MOVE TOWARDS INFORMED
MANAGEMENT STRATEGIES
D Evans 1 * and C Gibson 2
1
Aquatics Section, Department of Agriculture and Environmental Sciences,
Queen’s University Belfast, Newforge Lane, Belfast, BT9 5PX, UK; 2 Agricultural and
Environmental Science Division, Department of Agriculture and Rural Development,
Newforge Lane, Belfast, BT9 5PX, UK; *Current presenting/correspondence author
address: Dr D J Evans, Atkins Limited, Chadwick House, Birchwood Science Park,
Warrington, Cheshire, WA3 6AE, UK, E-mail: dan.evans@atkinsglobal.com
SUMMARY
A decline in the survival of salmon from ova to smolt had been reported in the River
Bush, Northern Ireland, due partly to habitat degradation. A monitoring programme
was initiated at four study sites on the river in order to promote improved sediment
management at the catchment scale. Bed and suspended sediment loads were
quantified leading to the identification of specific grain types and transport events
contributing to the sedimentation of salmon spawning redds. Temporal and spatial
variations were recorded in fine sediments transported through the channel in
suspension (median value range 26.4–448 kg per week) and along the bed (median
value range 0.214–9.55 kg per week). The resulting data indicated that lower parts
of the river suffered form a relatively high sediment load. This poster also reports
on supplementary work aimed at classifying the sources of these sediments in the
catchment. A combination of visual observations, GIS erosion potential maps and
bank erosion monitoring were used to assess large-scale sediment processes and
guide more detailed study. For instance, bank erosion was highest in regions of
the catchment with the least cohesive bank materials during high flow conditions
(e.g. mean of 38.1 mm per storm at Magherahoney). These data targeted specific
sites where sediment fingerprinting techniques were applied to elucidate the link
between soil erosion in the Bush catchment and downstream sediment delivery
to the four instream study sites. Livestock poaching and peak flows exacerbated
damage to banks at a localised scale and led to selective patches of bare land being
susceptible to further erosion, augmenting the sediment load (approximately 2%
of the suspended sediment load and 60% of the bed load). Drainage maintenance
work (60% and 30%, respectively), forest clearfell (1% and 2%, respectively) and
ploughed land (37% and 8%, respectively) were also shown to influence the quantity
of sediment transported through the study channels. Information generated from
the sediment source ascription process was transferred into a scientifically justified
plan aimed at reducing fine sediment transport in the Bush catchment. Nine key
actions were suggested in order to improve habitat quality in the River Bush. These
included wetland restoration, prohibiting drainage maintenance work, strategies to
control conifer plantation felling, reducing bare ground, livestock access restrictions,
construction site management, systematic macrophyte clearance, employment of
a river warden and systematic dissemination of project recommendations to the
general public to generate community involvement. Above all, this work has showed
that effective catchment management has to be steered by detailed sediment budget
information.
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