Influence of the 18.6-year lunar nodal cycle on the tidal resource of the Alderney Race, France

Resource assessment is an important phase of tidal energy projects. Extensive literature is nowadays available on this topic with most studies focussing on the time-scales of the tide or the spring-neap cycle. The evolution of the resource over longer time scales has yet received very little attention in the scientific literature. In the present contribution, we investigate the effect of the 18.6 year lunar nodal cycle on the depth-averaged current of the Alderney Race (France). Firstly, we identify the major constituents of the study zone with the UTide Matlab functions. Two datasets are used: a 36.5 day time-series of depth-averaged current and elevations acquired in the Alderney Race and a 24 year times-series of tidal heights acquired in a neighbouring harbour. The analysis shows that the tide is mostly controlled by the semi-diurnal constituents M2, S2 and N2. Secondly, we verify the ability of UTide functions to predict the changes in amplitude of the major components over decadal time-scale. Thirdly, we predict the tidal resource of the Alderney Race over the period 2014 – 2034 and analyse the yearly changes within this period. The results show that the change of resource is of the order of +/-10% and that the next decade (2020–2029) will be characterised by a significant reduction of the resource

In situ measurement of hydrosedimentary dynamic in the surf zone : a versatile dedicated mooring

International audienceSandy beaches represent 39% of French coasts. They are submitted to erosion, notably because of the energetic wave conditions observed during extreme events (storms). On the contrary, when moderate wave conditions are observed, the sand tends to be brought back on the beach. Processes controlling erosion are quite well known thanks to numerous studies on the topic. As a consequence, erosion events are quite well predicted by models (VAN RIJN et al., 2011). It is not the case when it comes to accretion; indeed, sand quantities transported onshore tend to be underestimated in predictive models (DALY et al., 2017). In order to improve models, parameters controlling accretion need to be clarified. Onshore transport occurs mainly through bedload in the bottom boundary layer. One challenge still to overcome in coastal dynamic studies is to obtain in situ measurements allowing to observe the whole water column with enough resolution. The new mooring built in the frame of ANR WEST is versatile, easy to deploy in all kinds of areas, solid and allows to insonify the whole water column so as to obtain co-localized current, concentration and granulometry measurements. Thanks to the instrument positions, no acoustic interference is observed. Hydrodynamic parameters can be computed and agree with previous studies. The newly presented mooring is therefore well adapted to surf zone data collection

Assessing the turbulent kinetic energy budget in an energetic tidal flow from measurements of coupled ADCPs

Two coupled four-beam acoustic Doppler current profilers were used to provide simultaneous and independent measurements of the turbulent kinetic energy (TKE) dissipation rate ε and the TKE production rate P over a 36 h long period at a highly energetic tidal energy site in the Alderney Race. The eight-beam arrangement enabled the evaluation of the six components of the Reynolds stress tensor which allows for an improved estimation of the TKE production rate. Depth-time series of ε, P and the Reynolds stresses are provided. The comparison between ε and P was performed by calculating individual ratios of ε corresponding to P. The depth-averaged ratio ε/P averaged over whole flood and ebb tide were found to be 2.2 and 2.8 respectively, indicating that TKE dissipation exceeds TKE production. It is shown that the term of diffusive transport of TKE is significant. As a result, non-local transport is important to the TKE budget and the common assumption of a local balance, i.e. a balance between production and dissipation, is not valid at the measurement site

A comprehensive assessment of turbulence at a tidal-stream energy site influenced by wind-generated ocean waves

Velocity measurements collected by an upward-looking acoustic Doppler current profiler were used to provide the first study of ambient turbulence in Alderney Race. Turbulence metrics were estimated at mid-depth during peak flooding and ebbing tidal conditions. The dissipation rate ε and the integral lengthscale (L) were estimated using two independent methods: the spectral method and the structure function method. The spectral method provided ε and (L) estimates with standard deviations twice lower than that obtained from the structure function method. Removal of wave and Doppler noise-induced bias when estimating the dissipation rate was shown to be a crucial step in turbulence characterization. It allowed for a significant refining in (L) estimates derived from the spectral and structure function methods of 35 and 20 respectively. The integral lengthscale was found to be 2–3 times the local water depth. It is considered that these findings could be valuable for current turbine designers, helping them optimizing their designs as well as improving loading prediction through the lifetime of the machines