UBathy (v2.0): a software to obtain the bathymetry from video imagery

UBathy is an open source software developed for bathymetry estimation from video images. The proposed scheme is based on extracting the wave modes from videos of the nearshore surface wave propagation. These videos can be formed either from raw camera images, which must have been previously calibrated, or from georeferenced planviews. For each wave mode extracted from the videos, the wave frequency and the spatially dependent wavenumbers are obtained. The frequencies and wavenumbers from different videos are used to estimate the bathymetry by adjusting the dispersion relationship for linear surface water waves. The bathymetry at different times can further be weighted and aggregated using the Kalman filter. The new software is suitable for Argus-type video monitoring stations and for moving cameras mounted on drones or satellites, and it is meant for users familiar with coastal image processing and suitable for non-experienced users. The software and an application example are available on the GitHub platform.Postprint (author's final draft

Characteristics and dynamics of crescentic bar events at Castelldefels beach

Crescentic sand bars have been studied intensively in the last decades, resulting in good knowledge of some of their characteristics, but the processes behind their formation and destruction are not yet clear. This study aims to increase our understanding of the dynamics of crescentic bars at an open, dissipative Mediterranean beach (Castelldefels, 20 km southwest of Barcelona). Their dynamics have been analysed using a 4.25-year dataset of video images. The crescentic bar events have been identified using visual analysis, including the formation and destruction moments. The results show that crescentic bars hardly occurred when the sandbar was located close to the beach, whilst they developed often when the sandbar was further offshore. Wave conditions during crescentic bar formation were low- to intermediate-energy waves with both oblique and shore-normal angles of incidence. Sandbar straightening was preferably observed for oblique waves (of both intermediate and high energy). The alongshore wavelength and cross- shore amplitude of the crescentic bars have been also quantified, giving some 245 m and 10 m on average, respectively.Peer ReviewedPostprint (published version

UBathy: a new approach for bathymetric inversion from video imagery

A new approach to infer the bathymetry from coastal video monitoring systems is presented. The methodology uses principal component analysis of the Hilbert transform of video images to obtain the components of the wave propagation field and their corresponding frequency and wavenumber. Incident and reflected constituents and subharmonics components are also found. Local water depth is then successfully estimated through wave dispersion relationship. The method is first applied to monochromatic and polychromatic synthetic wave trains propagated using linear wave theory over an alongshore uniform bathymetry in order to analyze the influence of different parameters on the results. To assess the ability of the approach to infer the bathymetry under more realistic conditions and to explore the influence of other parameters, nonlinear wave propagation is also performed using a fully nonlinear Boussinesq-type model over a complex bathymetry. In the synthetic cases, the relative root mean square error obtained in bathymetry recovery (for water depths 0.75m¿h¿8.0m) ranges from ~1% to ~3% for infinitesimal-amplitude wave cases (monochromatic or polychromatic) to ~15% in the most complex case (nonlinear polychromatic waves). Finally, the new methodology is satisfactorily validated through a real field site video.Postprint (published version

Narrow banded wave propagation from very deep waters to the shore

32 pages, 13 figures, 5 tablesA fully nonlinear Boussinessq-type model with several free coefficients is considered as a departure point. The model is monolayer and low order so as to simplify numerical solvability. The coefficients of the model are here considered functions of the local water depth. In doing so, we allow to improve the dispersive and shoaling properties for narrow banded wave trains in very deep waters. In particular, for monochromatic waves the dispersion and shoaling errors are bounded by ~ 2.8% up to kh = 100, being k the wave number and h the water depth. The proposed model is fully nonlinear in weakly dispersive conditions, so that nonlinear wave decomposition in shallower waters is well reproduced. The model equations are numerically solved using a fourth order scheme and tested against analytical solutions and experimental dataAuthors would like to thank support from MICINN through Project 445 CGL2011-22964. G. Simarro and R. Minguez are supported by the Spanish government through the “Ramón y Cajal” programPeer reviewe

Camera calibration for coastal monitoring using available snapshot images

Joint intrinsic and extrinsic calibration from a single snapshot is a common requirement in coastal monitoring practice. This work analyzes the influence of different aspects, such as the distribution of Ground Control Points (GCPs) or the image obliquity, on the quality of the calibration for two different mathematical models (one being a simplification of the other). The performance of the two models is assessed using extensive laboratory data (i.e., snapshots of a grid). While both models are able to properly adjust the GCPs, the simpler model gives a better overall performance when the GCPs are not well distributed over the image. Furthermore, the simpler model allows for better recovery of the camera position and orientation.Postprint (author's final draft

Wave mixing rise inferred from Lyapunov exponent

We study the horizontal surface mixing and the transport induced by waves in a coastal environment. A comparative study is addressed by computing the Lagrangian Coherent Structures, via Finite Size Lyapunov Exponents, that arise in two different numerical settings: with and without wave coupled to currents. In general,we observe that mixing is increased in the area due to waves. Besides, the methodology presented here is tested by deploying a set of eight Lagrangian drifters at different locations. This dynamical approach is shown as a valuable tool to extract information about transport, mixing and residence embedded in the Eulerian time dependent velocity fields obtained from numerical models.uthors would like to thank financial support from Spanish MICINN thought projects CTM2010-16915; CGL2011-22964 and from EU through MED Programme Project TOSCA (G-MED09-425).Peer reviewe

Characterization of coastal upwelling events during the generation of the water column stratification in spring (Vilanova i la Geltrú, NW Mediterranean)

This article presents some preliminary results on the characterization of inputs of offshore waters (upwelling events) in the coast off Vilanova i la Geltrú (NW Mediterranean) in the spring of the years 2010, 2012 and 2014. The upwelling events are studied in terms of meteorological and oceanographic conditionsPostprint (author’s final draft

Water level control on coastal landform distribution and associated processes in a highly fluctuating shallow lake (Gallocanta Lake, NE Spain)

Shallow lakes in semi-arid environments are very sensitive to hydrological alterations associated with climate change. Their shorelines and geometry can change according to water level fluctuations. Gallocanta Lake (NE Spain) is a typical example of such lacustrine conditions because it is exposed to strong winds parallel to its elongation axis and is located in a semi-arid Mediterranean environment. In this work, a high-resolution digital elevation model (DEM) of the area is used to compare the distribution of coastal forms (beaches, barrier islands, deltas, lagoons, etc.) with the frequency at which different water levels are attained. As a result, a clear relationship is obtained between presently active forms and the water levels most frequently reached in the lake. It is deduced that, once formed, the surrounding coastal plains related to these coastal forms control the permanence of water around a given height interval, favouring the development of these morphologies, in a positive feedback mechanism only broken by subsequent climate warming and lake water lowering. The hydrodynamic conditions responsible for activating the coastal forms have been analysed by applying a mathematical model of wind-driven currents in the lake that predicts the present erosional/progradational trends associated with them. The combination of the different results obtained was used to generate a synthetic map of active coastal processes and trends along the lake shore during high water episodes, with two versions according to the two dominant wind scenarios in the region. The distribution of erosion/sedimentation trends along the lake shores has helped to propose the existence of longitudinal littoral cells, each one recording different shoreline trend (retreat, progradation, and stability), depending on the prevailing wind scenario. This synthetic scheme can be useful for predicting the eco-morphological trends of the lake shore and adapting the present management practices in this protected area accordingly.Postprint (author's final draft

Riprap Protection of Vertical-Wall and Spill-Through Bridge Abutments

Scour around bridge abutments frequently leads to the collapse of bridge superstructures. Consequently, scour countermeasures are usually designed and installed by river engineers, riprap matting being the most widespread solution. The associated design criteria must attend to two groups of failure mechanisms: i) those associated the abutment body stability, which include particle erosion, translational slide failure, modified slump and slump failure; ii) those associated with the riprap apron stability, including shear, winnowing, edge failure, bed-form undermining and river bed degradation. The work presented in this communication consists on the review of design criteria recently published by the authors to face particle erosion, shear, winnowing and edge failure mechanisms at vertical-wall and spill-trough bridge abutments, under clear water flow conditions. These are the key aspects to be faced as soon as abutments are located in flood plains, their side slopes are not too steep and pore pressure is negligible. The work is based on a large experimental campaign carried out at several facilities and allows the specification of blocs’diameter, plan layout and thickness of riprap mattresses

Observed alongshore sandbar and shoreline variability at an open, fetch-limited beach

andbars often display an alongshore variable morphology (crescentic bars), which is often mimicked at the shoreline (megacusps). The processes resulting in these patterns are not yet clear and particularly the coupling between sandbar and shoreline deserves more attention. This contribution presents 8 years of video observations on the dynamics of crescentic bars and megacusps, including coupling, at the open, fetch-limited beach of Castelldefels (Spanish Mediterranean coast). Wave conditions were propagated from the nearest wave buoy to the study site (10 m depth). Crescentic bars and megacusps were detected visually from video images, after which the barlines and shoreline were extracted to compute characteristics like wavelengths, amplitudes and alongshore migration rates. Finally, the bar-shoreline coupling was evaluated using cross-correlation. Crescentic bars were only observed when the sandbar- shoreline distance was at least 10 m. They developed during lower energetic waves with both oblique and shore-normal angles, whereas straightening occurred primarily during high-energetic obliquely-incident waves. Megacusps only developed in the presence of a crescentic bar and were less pronounced and dynamic compared to crescentic bars. During megacusps presence, a highly-significant coupling was present during 73% of the time but the relative phase varied, partly due to different alongshore migration rates of the two patterns.Peer ReviewedPostprint (published version