NERC Quota PhD studentship proposal 2012/2013
A field study of the effects of offshore tidal power arrays on the transport of suspended
sediment
D.G. Bowers (Bangor University) and G.E.L. Harker (Harmonicquay Ltd.)
The collaborating CASE partner(s) will be Harmonicquay Ltd., and possibly other companies
involved in the Shetland tidal power array project.
BACKGROUND/CONTEXT OF PROJECT
Renewable energy generated by offshore arrays of tidal current turbines will make a growing
contribution to the UK’s energy supply in the coming decades. Although modelling work has been
carried out to assess the likely environmental impact of these arrays, very little field work has been
possible up to now. However, during the next year, the world’s first community-owned tidal turbine
will be installed near the Shetland Isles. The PhD project proposed here will take advantage of links
between SOS and companies involved in the scheme to carry out field measurements of the impact of
tidal energy extraction on suspended sediment transport processes.
By design, tidal current turbines remove energy from the tidal flow. This will have two effects on the
suspended sediment load downstream of the turbine. Because there is less energy there will be less
sediment in suspension. However, the change in turbulence in the wake of the turbine will also affect
flocculation processes in the suspended sediment. For example, a turbulent wake will tend to break
up flocs, creating smaller particles with lower settling speeds, which are therefore easier to maintain
in suspension (the figure below illustrates this process of changing particle size with changing in
current speed in the Menai Strait). The consequence is that there are two opposing processes
affecting sediment dynamics in the wake of a tidal turbine: the net effect will depend on the relative
importance of these processes. The question then arises: what is the net effect? In this project we
will therefore make measurements of suspended load, particle size and settling speeds in the wake of
a turbine and in the ambient water to shed light on these processes and answer this question.
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time (days)
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current speed (m/s)
median particle size
(microns)
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Time series of
current speed (dark
diamonds) and
median particle size
(open squares)
showing the effect of
changes in
turbulence on
flocculated particles
AIMS AND OBJECTIVES (max 200 words)
The aim of this project is to determine, by measurements in the field, the effect of a tidal current
turbine on the concentration, size and settling speed of particles and the consequences of this for
sediment transport.
Our objectives are to:
1) Carry out in situ measurements of current speed, suspended load, particle size distributions and
particle settling speeds at a grid of stations around a tidal current turbine.
2) Make measurements over tidal cycles at a fixed location to measure temporal changes and estimate
sediment fluxes.
3) Use the results of the field work to develop process models of the effect of tidal energy extraction
on particle suspension and flocculation.
4) Use the insights gained from the objectives above to draw general conclusions about the effect of
tidal current arrays on suspended sediment transport.
MATERIALS/METHODS/TRAINING ELEMENT
The project will begin with a literature review of the effects of tidal turbines on flow and of flow on
suspended sediment properties. The student will carry out fieldwork in collaboration with our
industrial partners and will gain experience of acoustic Doppler measurements of currents, LISST
measurements of particle size and settling tube experiments. It is anticipated that the fieldwork will
be conducted early in the project: almost certainly in the first year. The student will be expected to
synthesise the data collected to better understand the processes affecting suspended sediment load in
the turbulent wake of a tidal turbine.
COSTS
Fieldwork will be carried out in collaboration with the commercial partners. We will make use of
their boat operating in the area to make measurements and we anticipate field work costs to the
studentship will therefore be minimal. Instruments are available from the pool at Bangor. Some travel
money will be needed but this will be covered by the allowance given to NERC students.