International conference ICAWA 2016 : extended book of abstract : the AWA project : ecosystem approach to the management of fisheries and the marine environment in West African waters

This project is an important step to not only improving our understanding of coastal dynamics in vulnerable and non-documented tropical environments, but more generally providing fundamental insight in the functioning of coastal zones. It enable to validate 3D wave-current models and 2D morphological models in challenging conditions. COASTVAR has three key scientific objectives: 1) Bridging the knowledge gap in event-scale beach evolution and more specifically on hydro-sedimentary mechanisms linking the upper beachand surf-zone dynamics. This includes the study of extreme events and crucial but mostly unknown beach recovery. 2) Identifying the hydrodynamic processes involved in cross-shore exchanges between nearshore and shelf zones and the role of transient currents. There is still no consensus on the origin and effect of these currents, on their 2D or 3D dynamics and their relation to the stratification of shelf waters. 3) Understanding the sources of coastal variability at various scales, in particular the link between short-term (event scale) and long-term evolution. This includes determining the long-term impact of individual events (i.e., resilience?) and the coastal response to oceanic forcing with seasonal modulation of wave conditions or regional climatic modes

Application of remote sensing methods to monitor coastal zones

In this Special Issue “Application of Remote Sensing Methods to Monitor Coastal Zones” nine original research papers were published, with topics covering a wide range of ranging of remote sensing applications including coastal topography, bathymetry, land cover, and nearshore hydrodynamics [...

Coastal coverage of ESA' Sentinel 2 mission

Climatologic and anthropogenic pressures in coastal areas affect the coastal zone at different scales. With the development of new missions in open-access, satellites now represent an attractive solution for a broad public to capture local-scale coastal impacts at large scales. Here, the capability of the Sentinel 2 constellation to cover coastal areas and measure coastal processes -physical and biological. We show that Sentinel 2 enables high-frequency measurements across the globe. Cloud coverage at higher latitudes is overcome by decrease revisit time-intervals. Only around the equator, the longest revisit intervals and high cloud cover probability limits coastal measurements there. Sentinel 2 based methods are capable of estimating Digital Elevation Models for mid- to high-latitude coastal zones and sporadic spots for lower latitudes where 2 orbit swaths overlap. For the majority of the world's coastal bathymetries can be obtained with the Sentinel 2 imagery surpassing the depth of closure (beyond this offshore limit sediment transport is limited). Only in sheltered areas, wave-based bathymetry inversion is limited but at these areas inversion through colouring (light penetration) prevails. This works shows that Sentinel 2 enables coastal monitoring as never before, large spatial scale with revisits of a few days at most of the world

Video-based depth inversion techniques, a method comparison with synthetic cases

Applications of (video-based) depth inversion in the near-shore coastal environment are growing in numbers. Video-based capabilities in nearshore monitoring are improving and coastal monitoring programs are expanding due to greater availability and reduced costs. Video-derived beach (state) indicators such as beach width, bar position and wave and current parameters are supplemented by accurate depth estimations through inversion. Video-based depth inversion knows two main approaches, a spectral and temporal method of celerity estimation. The two methods have so far never been compared as video-systems are often tailored for the chosen celerity estimation method. Here, a spectral and temporal method are compared using controlled synthetic datasets obtained using the SERRE1D Boussinesq model to estimate celerity and invert depth. The assessment is carried out on a set of wave boundary conditions with over linear and barred bottom profile. Both methods invert depth with a similar accuracy for the most realistic JONSWAP cases. An evident correlation is found between wave skewness, non-linearity and depth estimation error linked to the limits of the linear dispersion relation. A residual 'sensing' error is linked to method-based parameters and a changing wave shape as incident waves propagate inshore. The inversion-error can be reduced significantly including a wave height dependent non-linear correction. Importantly, a method-based error is introduced for the temporal method to increase the suitability for data assimilation. Likewise, the spectral method has its own existing depth-error estimation to feed into the Kalman Filter. However, these method-based error estimates show very weakly or no relation to the observed error between estimated cross-shore profile and bottom profile used for the model input

Coupling terrestrial LiDAR and video imagery to perform 3D intertidal beach topography

This study investigates the possibility of combining two shore-based remote sensing techniques, 2D LiDAR and video imagery, to measure 3D intertidal beach topography. The shoreline elevation, measured on a single cross shore section of the beach by a 2D LiDAR, was combined with the video-derived shoreline contour to finally obtain intertidal beach topography. Vertical root-mean-square error between differential GPS and remotely derived intertidal beach profiles had an average value of 0.12 m, with maximum disparities of 0.14 m. Two 3D Digital Elevation Models (DEMs) of the beach produced over two tidal cycles demonstrated the feasibility of shore-based LiDAR-video system of describing in detail the short-term morphological evolution of the intertidal area. Results suggest that the LiDAR-video system can improve the capability of remotely surveying intertidal beach topography providing high quality measurements

Recent shoreline changes in the Volta River delta, West Africa : the roles of natural processes and human impacts

The Volta River delta developed as an asymmetric lobe in a tectonic offset on the coast of Ghana. The delta comprises a large curvilinear spit that widens in its central portion due to the adjunction of successive sandy beach ridges. The appearance of a distinct spit, in lieu of a continuous barrier from the present mouth of the Volta River to the Bight of Benin coast, may be an outgrowth of a natural change in the location of the mouth of the Volta. The spit marks a segmentation of the unique sand drift cell that hitherto prevailed on this bight coast. Spit growth has been accompanied by a wave of erosion over the last century of the immediate downdrift sector of the bight coast, endangering the town of Keta. Erosion since the 1960s may have been aggravated by the construction of the Akosombo hydropower dam. The tip of the spit has recently welded to the shoreline, thus assuring resumption of sand supply from the Volta towards the rest of this formerly sand-starved sector of the bight coast. Blocking of sediment by the Akosombo Dam is, in due course, likely to become the overarching factor in delta shoreline stability

Contrasted influence of climate modes teleconnections to the interannual variability of coastal sea level components-implications for statistical forecasts

Sea level variations at the coast can have drastic environmental and socio-economic impacts in particular in the context of an ever-increasing coastal population and anthropogenic climate change. Regional to global climate variability influences all these factors and exerts a strong control on the coastal sea level over a wide range of time scales. Here, we focus on understanding interannual changes which is paramount to improve interannual forecasting systems as well as to constrain and reduce uncertainties on the secular trend in global mean sea level. We consider the coastal total water level (TWL) as the compound effect of three main components: the wave setup, mean regional sea level anomaly (i.e., steric and ocean circulation influences) and atmospheric surge (i.e., influence of local wind and surface atmospheric pressure). To understand their variability at a global scale, we focus on the effect of four climate modes that affect the major oceanic basins: the El Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO) and the Southern Annual Mode (SAM). The contrasted regional influence of these different climate modes on the interannual variations of TWL components are quantified. Results suggest that even if the regional mean sea level is overall the main contributor to the interannual variations of TWL variations at the coast and mostly related to ENSO, the contributions from wave setup and atmospheric surge are not negligible in particular at high latitudes and mostly related to the NAO in the Northern Atlantic and to the SAM in the Southern Hemisphere. Such influences from the NAO and SAM can be seen far away from their extratropical regions of action due to their atmospheric forcing of ocean waves that can significantly propagate their imprint towards tropical areas. Implications for interannual to decadal forecasts of the coastal TWL and related hazards are discussed in the light of regression statistical models and the climate modes own predictability

Radon-augmented sentinel-2 satellite imagery to derive wave-patterns and regional bathymetry

Climatological changes occur globally but have local impacts. Increased storminess, sea level rise and more powerful waves are expected to batter the coastal zone more often and more intense. To understand climate change impacts, regional bathymetry information is paramount. A major issue is that the bathymetries are often non-existent or if they do exist, outdated. This sparsity can be overcome by space-borne satellite techniques to derive bathymetry. Sentinel-2 optical imagery is collected continuously and has a revisit-time around a few days depending on the orbital-position around the world. In this work, Sentinel-2 imagery derived wave patterns are extracted using a localized radon transform. A discrete fast-Fourier (DFT) procedure per direction in Radon space (sinogram) is then applied to derive wave spectra. Sentinel-2 time-lag between detector bands is employed to compute the spectral wave-phase shift and depth using the gravity wave linear dispersion. With this novel technique, regional bathymetries are derived at the test-site of Capbreton, France with an root mean squared (RMS)-error of 2.58 m and a correlation coefficient of 0.82 when compared to the survey for depths until 30 m. With the proposed method, the 10 m Sentinel-2 resolution is sufficient to adequately estimate bathymetries for a wave period of 6.5 s or greater. For shorter periods, the pixel resolution does not allow to detect a stable celerity. In addition to the wave-signature enhancement, the capability of the Radon Transform to augment Sentinel-2 20 m resolution imagery to 10 m is demonstrated, increasing the number of suitable bands for the depth inversion