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Guillou N.,Laboratoire Of Genie Cotier Et Environnement Lgce | Thiebot J.,Normandie University
Energy | Year: 2016

Numerical assessments of environmental disturbances induced by a tidal farm project rely usually on local modifications of the friction coefficient over the area covered by a proposed array. Nevertheless, no study has investigated the sensitivity of predictions to surrounding seabed friction. The present investigation focuses on impacts of roughness parameterisation of rock outcrops, a typical seabed of tidal stream sites. A high-resolution depth-averaged circulation model is implemented in the Fromveur Strait off western Brittany, a region with strong potential for array development, integrating the heterogeneity of sediment bottom types. Rock roughness strongly influences initial predictions of tidal current and kinetic energy in the Strait with variations of available power up to 30 %. Tidal energy extraction induces noticeable reductions of tidal currents and bottom shear stresses up to 15 km from the array considered till surrounding sandbanks. Rock roughness impacts farm-induced modifications of tidal currents, bottom shear stresses and stream powers till north-eastern and southward edges of the Strait with major absolute differences identified in its central part. Surrounding sandbanks are finally suggested to variations of shear stresses from 9 to 17 % over the Bank of the Four with possible implications on local sediment deposition. © 2016 Elsevier Ltd Source

Guillou N.,Laboratoire Of Genie Cotier Et Environnement Lgce
Ocean Engineering | Year: 2015

Abstract The present study investigates the uncertainties in wave power characterisation associated with the computational-methods accuracy of two state of the art phase-averaged numerical models: SWAN and TOMAWAC. The target area is the Sea of Iroise (western Brittany) considered as one of the most energetic regions along the French coasts with a mean offshore energy flux around 50kWm-1. Model predictions based on the same settings are first evaluated against available measurements of significant wave height and peak period at nine wave buoys. Wave power predictions are then compared during a eight-year period (2004-2011). Patterns of wave energy flux present in both cases the same spatial and seasonal distributions identifying coastal hot spots off the isles of Ushant and Sein, the bay of Audierne, the headland of Penmarc'h and the Crozon peninsula. In offshore waters, SWAN provides however wave power estimates by 15% lower than TOMAWAC predictions. A part of these differences is attributed to the computational methods for extracting the wave energy flux from model predictions as SWAN method neglects additional terms in the output evaluation of the total wave potential. Differences are reduced to 5-10% in shallow waters where uncertainties in wave power characterisation appear mainly associated with the inter-annual and inter-seasonal variabilities of the wave climate. © 2015 Elsevier Ltd. Source

Guillou N.,Laboratoire Of Genie Cotier Et Environnement Lgce
Ocean Engineering | Year: 2014

The energy dissipation by bottom friction of wind-generated surface-gravity waves is evaluated in relation to the seabed roughness magnitude in the English Channel (western Europe). The investigation is based on the phase-averaged wave model SWAN (Simulating WAves Nearshore) modified to account for a new parameterisation of the wind-drag coefficient at high wind speeds. Two formulations of the bottom-drag coefficient are evaluated: (1) the default constant empirical values derived from the JONSWAP experiment and (2) the eddy-viscosity model of Madsen et al. (1988) integrating the hydrodynamic conditions and the bottom roughness length scale considered successively constant and parametrised according to the grain size of bed sediments. Model performances are evaluated by comparing predictions with available measurements of the significant wave height and the peak period at (1) three offshore lightships and (2) two nearshore wave buoys off Le Havre and Cherbourg harbours. The heterogeneous bottom roughness length scale associated with the grain size of seabed sediments improves globally numerical estimates. Mappings of coastal regions influenced by bottom friction are produced exhibiting significant energy dissipation in areas of pebbles and gravels of the Normano-Breton Gulf and the surroundings of the Isle of Wight exposed to the incoming waves from the North-Atlantic ocean. © 2014 Elsevier Ltd. Source

Guillou N.,Laboratoire Of Genie Cotier Et Environnement Lgce | Chapalain G.,Laboratoire Of Genie Cotier Et Environnement Lgce
Renewable Energy | Year: 2015

An unstructured version of SWAN (Simulating WAves Nearshore) is implemented in the Sea of Iroise (western Europe) to assess the wave energy resource at high spatial resolutions in coastal areas. Numerical results are compared with available measurements of the significant wave height and the peak period at nine locations including (1) long-term offshore observations and (2) medium to short-term data acquired during field campaigns. A medium-term evaluation of the wave energy resource for a eight-years period (2004-2011) is performed exhibiting major nearshore energetic patterns off the isles of Ushant and Sein and in the coastal areas of the bay of Audierne, the Crozon Peninsula and the northern coastline. The variability of wave power production is estimated revealing in accordance with numerical modelling conducted over the European shelf seas significant inter-seasonal and inter-annual evolutions of the resource in the Sea of Iroise. These changes appear particularly noticeable during the winter period with opposite situations in the distribution of monthly average wave energy flux. In the perspective of the implementation of wave energy converters devices, the present assessment is finally exploited to investigate the local distributions of wave energy flux against periods and directions in areas of maximum mean wave power. © 2015 Elsevier Ltd. Source

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