On the Sediment Dynamics in a Tidally Energetic Channel : The 9 Inner Sound , Northern Scotland .

Sediment banks within a fast-flowing tidal channel, the Inner Sound in the 22 Pentland Firth, are mapped using multi-frequency side-scan sonar. This novel technique 23 provides a new tool for seabed sediment and benthic habitat mapping. The sonar data are 24 supplemented by sediment grab and ROV videos. The combined data provide detailed 25 maps of persistent sand and shell banks present in the Sound despite the high energy 26 environment. Acoustic Doppler Current Profiler data and numerical model predictions are 27 used to understand the hydrodynamics of the system. By combining the hydrodynamics 28 and sediment distribution data, we explain the sediment dynamics in the area. Sediment 29 particle shape and density, coupled with persistent features of the hydrodynamics, are the 30 key factors in the distribution of sediment within the channel. Implications for tidal energy 31 development planned for the Sound are discussed. 32


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Over the past decade, the strong tidal flows of the Pentland Firth have been a focus of attention for 37 tidal energy developers, with a number of sites identified and leases granted for tidal energy extraction 38 using tidal turbines. Tidal energy potential for the whole Firth has been estimated to be in the range of 39 1 -18 GW [1], though Adcock et al. [2], suggest that a more realistic figure for the maximum  To date, one of the potential impacts of tidal energy that has received less attention is sediment 67 transport and the effect of energy extraction on the movement of sediment through these tidally 68 energetic channels. An early study by Neill et al. [6], demonstrated that installations of tidal energy 69 convertors in the Bristol Channel had the potential to influence large scale sediment dynamics and bed 70 level changes, and that these impacts were most pronounced in regions where tidal asymmetry was 71 stronger. In a further study, Neill et al. [7], found that tidal turbines could also impact sand bank  Firth as comprising predominantly of exposed bedrock, but localised sediment deposits are known to 126 exist around Stroma. We conducted acoustic surveys using side-scan sonar, including a novel 127 prototype tri-frequency ("colour") digital sonar, in a focused area within the sound to map the 128 sediment banks to the south of Stroma. The acoustic data were ground-truthed using sediment grab and 129 a remotely-operated-vehicle (ROV) data from earlier surveys.

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The survey area was approximately 900 metres by 1200 metres in size. Depths within the study 131 area ranged between 10 and 39 metres, with a mean depth of 28 metres (depth derived from multi-  A prototype multi-frequency side-scan sonar system developed by Kongsberg GeoAcoustics Ltd.

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[17], was pole mounted to 'Aurora' approximately 1 meter below the hull. The sonar pings at three 164 frequencies 114, 256 and 410 kHz. The sonar is a basic range-only system and provides no 165 bathymetric data along trace. The Kongsberg GeoAcoustics Ltd. propriety software 'GeoTexture' was 166 used to process the sonar data. GPS navigation and heading data were provided by a Hemisphere

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Vector V110 unit mounted directly above the side scan pole mount.

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The sonar acoustic data acquired at three ping frequencies constitutes acoustic colour data. These          (Table 1), a fundamental 481 requirement since currents in the Firth are largely driven by sea level pressure gradients [15]. We  April 2009. The tidal currents were corrected for temporal distortion using a method 495 described in [12], resulting in currents estimated at mid-track time. Study area marked 496 with black rectangle.

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Comparison with moored ADCP data also showed similarity in the magnitudes and phases of 498 velocity for the principal tidal constituents ( Table 2). The M 2 tidal constituent dominated both the 499 regional tides and particularly the tidal currents in the Inner Sound, being 2.5 times stronger than the 500 next largest constituent S 2 (Table 2).   known that other such banks exist in the local region ([11], [12], [13]), an example of which is the 571 Sandy Riddle, located approximately 13 km to the east composed almost exclusively of carbonate shell 572 fragments [12]. Based on the ROV evidence, sample grab and surface sonar reflectivity of the bank it 573 is expected the bulk composition of the southern wedge shaped bank is also carbonate shell fragments. (2) 580 where R is the submerged specific gravity, g is the acceleration due to gravity, v is the kinematic 581 viscosity, D is the grain diameter in m, and C 1 is a constant with a theoretical value of 18 and 582 represents the smoothness of the grains. C 2 is the constant asymptotic value of the drag coefficient.

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The drag coefficient is 1 for a spherical grain, natural mineralogical sediment have a value ranging 584 from 0.8 to 1.15 dependent on shape (Table 3).  The important aspect to all such equations is the behaviour of a particle in a fluid in relation to the 589 particles density and shape and most such equations are based on a near spherical particle such as 590 natural mineralogical sand. Shell is not spherical or a near-spherical particle with near-spherical 591 particles showing preferential transport in comparison [36]. Broken shell consists of a large diameter 592 grain which is very thin in cross section [18]. This type of particle will have a larger drag coefficient 593 when settling due to the large cross section area exposed to the upward force of the fluid, or a large lift 594 surface [37]. The opposite is true once the shell material is settled, shell material generally settles on 595 the side with the largest surface area, and the thin cross-section exposed to the current. In this 596 configuration the drag coefficient is now much lower than natural sediment. Studies  . The shell material is provided by organisms growing on the local shelf which is 608 thought to be of low diversity and abundance ( [12], [13]).   blocking, and enhance the current speeds around the sides. It seems plausible, therefore that the shell 680 bank may ultimately be eroded or, more likely, relocated by the changing hydrodynamics.

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Monitoring the evolution of these sediment features following deployment of the turbines may be  modelling work and co-wrote the paper; Lonneke Goddijn-Murphy undertook the underway ADCP survey, 721 analysed the data and edited the manuscript.

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The authors declare no conflict of interest.