Search

As the tidal energy industry grows, developers are interested in the influence that large scale arrays might have on their surrounding environment including hydrodynamic and sediment. Understanding of local and regional sediment dynamics is an important element for environment impact and site assessments, as changes in sediment processes could alter bed ecosystems. In Australia, the Banks Strait located in the Northeast of Tasmania has been identified as a promising site for tidal energy. Very large sand waves have been observed near the area suitable for tidal turbines installation and they may interfere with human activities. Building from previous work, a 2D regional scale model investigated the influence of different tidal farms on the morphology of Banks Strait and especially the sand waves. Results showed that tidal farm of more than 49 MW rated power will alter the circulation of residual currents in Banks Strait leading to significant changes in the sediment dynamics, and farms less than 24.5 MW will have negligible influence on the migration of sand waves. • Morphological impact assessment of Banks Strait with tidal farms. • Hydrodynamic and morphodynamic models including wave forcing are used due to the wave-current interaction in Banks Strait. • Tidal farms less than 24.5 MW (rated power) have negligible influence on the migration of sand waves. [ABSTRACT FROM AUTHOR]

Sediment transport plays a crucial role in coastal ecosystems and is one of the least known parameters in high energy sites. The recent development of tidal energy projects challenges the scientific community to better understand this natural phenomenon and the interactions with tidal turbines. Using MIKE21/3 software from DHI, a benchmark for sediment transport model was developed with simulations with and without a tidal farm in idealised two and three-dimensional tidal channels. Results reveal that a 2D approach is sufficient for regional scale morphological assessments, however 3D models allow for a closer examination of influences around the tidal farm. Differences in calculating sediment transport rates based on approaches after Engelund & Fredsøe and Van Rijn formulae illustrate the degree of uncertainty in modelling sediment transport rates. Results for the Engelund & Fredsøe models also show that they are sensitive to mesh resolution in equilibrium conditions and are more stable in non-equilibrium conditions for the bed level change rates. Tests have also demonstrated that models using Engelund & Fredsøe formulae are more sensitive to physical characteristics than models based on Van Rijn, accentuating the necessity to evaluate sediment transport formulae with field data before making a choice of model. • Benchmark outlined for tidal energy site sediment transport modelling. • 2D approach is sufficient for regional scale morphological assessment. • Difference in bed level change predictions of 1.4 found between Engelund & Fredsøe and Van Rijn models. [ABSTRACT FROM AUTHOR]

Estuaries that feature large tidal ranges (macrotidal) have recently received attention for their potential to generate energy by tidal stream turbines. As estuaries are delicate ecosystems and often have large human populations residing on their shores, determining where to place the turbines and how they will impact the hydrodynamics and sediment transport of these systems is vital to their successful implementation. The aim of this study is to provide a framework for assessing an estuary for turbine placement, including determining how the turbines will impact the environment (focusing on the physics), and examining other items that could peripherally affect the tidal turbine farm location or impact. This work was carried out using the Gironde Estuary as a case study and included a combination of in-situ data collection and numerical modeling. In terms of the turbine farm impact on the hydrodynamics and Suspended Sediment Concentration (SSC), results showed that the inclusion of a farm reduced currents up to 10% and decreased SSC up to 15 mg/l. The decrease in SSC was due to the attenuated current velocities decreasing friction velocity and thus near bottom generated turbulence that suspends bottom sediment. Over time, the decreased SSC will lead to sediment accumulation and new bottom features. • Framework established for assessing an estuary for turbine placement and environmental impacts. • Turbines decreased current velocities and suspended sediment concentration in the near and far field from turbine locations. • Decreased suspended sediment concentration due to decreased friction velocity and near bottom turbulence production. [ABSTRACT FROM AUTHOR]

Highlights • A maximum overlap of 5% of the smallest particle reproduces macroscopic flow. • Monopile filling is controlled by the distance between the residuals and the hole. • Tidal current direction has a negligible effect when this distance remains below 20 m. • Tripod filling depends on the seabed morphology and the hole dispositions. • Impact of ambient sediments and roughness is coupled with seabed and tide direction. Abstract Tidal energy is a promising way to reduce the carbon fossil energy. Installing tidal converters remains difficult particularly due to the bore hole filling by drill residuals and ambient sediments. To fix this issue, we perform a coupling between a coastal circulation model and a discrete element model, with an application to Alderney Race, and considering spherical particles. The coupled model is firstly described, validated and then used to investigate the parameters controlling the filling volume of monopile and tripod technologies. The results are analysed for different disposition of residuals and initial current direction and intensity. We show that: the distance between the bore hole centre and the residuals is the key parameter controlling the filling by drill residuals; the current direction plays a negligible role in monopile while this distance remains smaller than 20 m; the filling of tripod is strongly influenced by current effects and seabed morphology. Impacts of bed roughness (modelled by steady spherical particles inlaying in the seabed) and ambient sediments are quantified and discussed. Interactions between moving particles and bottom roughness lead to a slight increase of the filling while the impact of ambient sediments strongly depends on seabed morphology and current effects. [ABSTRACT FROM AUTHOR]

The study aims to investigate the influence of tip clearance on the scour rate of pile-supported horizontal axis tidal current turbine (TCT) and also attempts to correlate time-dependent scour depth of TCT with the tip clearance. A physical model of TCT was placed in a flume for scour test and the scour rate of the fabricated model was investigated. The results suggest that the decrease in tip clearance increases the scour depth. In addition, the shortest tip clearance results in the fastest and most sediment transport. The maximum scour depth reached 18.5% of rotor diameter. Results indicate that regions susceptible to scour typically persist up to 1.0D t downstream and up to 0.5D t to either side of the turbine support centre. The majority of the scour occurred in the first 3.5 h. The maximum scour depth reaches equilibrium after 24 h test. An empirical formula to predict the time-dependent scour depth of pile-supported TCT is proposed. [ABSTRACT FROM AUTHOR]

Large-scale extraction of power from tidal streams within the Pentland Firth is expected to be underway in the near future. The Inner Sound of Stroma in particular has attracted significant commercial interest. To understand potential environmental impacts of the installation of a tidal turbine array a case study based upon the Inner Sound is considered. A numerical computational fluid dynamics model, Fluidity, is used to conduct a series of depth-averaged simulations to investigate velocity and bed shear stress changes due to the presence of idealised tidal turbine arrays. The number of turbines is increased from zero to 400. It is found that arrays in excess of 85 turbines have the potential to affect bed shear stress distributions in such a way that the most favourable sites for sediment accumulation migrate from the edges of the Inner Sound towards its centre. Deposits of fine gravel and coarse sand are indicated to occur within arrays of greater than 240 turbines with removal of existing deposits in the shallower channel margins also possible. The effects of the turbine array may be seen several kilometres from the site which has implications not only on sediment accumulation, but also on the benthic fauna. [ABSTRACT FROM AUTHOR]

Banks Strait, Tasmania, Australia, has been identified as a potential site for the deployment of tidal turbines. In this study, the characterization of sediment transport and large sand waves for this site is performed. Observations of bed level change collected from surveys in 2018 showed a migration of large sand waves over a period of nine months. Migration rates in an excess of one hundred meters for nine months were found, which are large compared to the rate reported at other coastal sites, by several meters per year. A validated hydrodynamic model is coupled with a morphodynamic model to perform sensitivity tests and identify what parameters influence migration to better understand sediment dynamic in the Banks Strait. Numerical analysis showed a constant shift of the sand waves profile in an eastward direction, consistent with the observations. This migration was strongly linked with tidal asymmetry, with a residual current flowing towards the east. The principal parameters driving the migration of sand waves in the Banks Strait were found to be sediment sorting, bed friction and residual current. This study gives new insights for the seabed of Banks Strait and provides an assessment of the natural variability of sediment for futures tidal farms deployments. [ABSTRACT FROM AUTHOR]

The development of tidal energy in Australia is still a challenge with few studies performed on the characterisation of the resource, due to the difficulty to acquire data and uncertainties about the influence of this anthropogenic activity on the marine environment. Changes in flow could lead to alterations in sediment transport and have further influence on the marine habitat. A case study in a promising area, Banks Strait (Australia), was created using high resolution 2D and 3D models validated against in situ data to investigate changes to hydrodynamic conditions with two scenarios of tidal farms (100 and 300 turbines). Comparison between 2D and 3D is performed to find the best compromise between model accuracy and computational time for preliminary assessment. Changes to current speed and bed shear stress over a 35 day period were found to be localised around the tidal farms and did not extent more than 7 km from the farm (300 turbines) for both 2D and 3D. The results showed that for near field and far field, 2D models are sufficient to give a first approximation of the hydrodynamic influence of tidal farm deployment on its environment. [ABSTRACT FROM AUTHOR]