The seasonal pattern of sediment dynamics on an inner shelf characterized by the presence of sediment delivered by a small, mountainous river (with a “flash-flood” regime) was investigated. Near-bottom suspended sediment fluxes across the shelf (i.e. 20, 30 and 40 m water depth) were estimated using observations from three benthic tripods deployed from September 2007 to June 2008. Near-bottom sediment resuspension was controlled by wave-induced currents and river-born sediment availability, whereas the shelf currents played a secondary role. Fourteen sediment transport events were identified (eight in autumn, two in winter and four in spring), with transport rates according to storm intensity and sediment availability. These few energetic events induced a large percentage of the cumulative sediment transport near the bottom. However, the lack of proportionality between suspended sediment transport rates and the combined wave-current bottom shear stress in some events highlights the importance of the sequence of events in sediment dynamics. Since wave activity, hydrography and river discharges display a strong seasonal pattern in the NW Mediterranean, the resulting sediment dynamics across the shelf also correspond to a seasonal cycle. This seasonal variability leads to a temporal evolution of the bottom grain size (coarser in winter) and the near-bottom sediment transport rates (higher in spring and autumn) which is consistent with the seasonal pattern of the hydrodynamic events and the river discharge load.
Flow intensification episodes lasting more than 12 h are observed occasionally at different locations along the Northwestern Mediterranean coast. In the last years, these pulses have hindered ship operations outside the Barcelona harbour, thus attracting the attention of the port authorities. In this paper, the strongest intensification events in the Barcelona coast area are quantified and characterized in order to identify the mechanisms which generate them. For this, current, sea level and meteorological measured and modelled data, at local and regional scale, are analysed. The results show that the flow accelerations are due to the combination of a narrow coastal shelf and the prevalence of a strong and sustained wind from the NE to SE. The synoptic atmospheric conditions that lead to this meteorological scenario are described. For one of the events, the presence and contribution to the current fluctuations of a coastal trapped wave, likely generated at the Eastern edge of the Gulf of Lions shelf, and other factors such as a freshwater discharge are also identified and discussed.
Many inner continental shelves are characterized by the presence of large rhythmic bedforms, such as shoreface-connected ridges and the more offshore located sand ridges, which have heights of several meters and are spaced several kilometers apart. This study focuses on explaining the observed orientation difference between shoreface-connected sand ridges and the more offshore located ridges. For this, an existing idealized morphodynamic model is used, but modified such that sea level rise simultaneously induces a steepening of the inner shelf and a retreating shoreface. Different settings (rate of sea level rise; landward depth of the inner shelf) are systematically explored. For each setting, the gross characteristics of ridges (growth rate, height, migration, orientation) during their initial formation and long-term evolution are quantified. Model results show that a rising sea level and associated shoreface retreat and shelf steepening lead to new ridges in the shallow area of the inner shelf, which remain active in time (i.e. ongoing growth and downstream migration in time). Old ridges that were already formed in the antecedent area of the shelf and which in the course of time experience deeper water become less active with the rising sea level. In the case that migration of the offshore parts of the ridges vanishes, these parts change orientation to become more shore-parallel compared with the active onshore parts of these ridges. In the case of small landward depths of the inner shelf and a decreasing rate of sea level rise, the active onshore parts migrate too fast, thereby causing the drowned offshore parts to detach and to become inactive. The characteristics of modeled shore-oblique shoreface-connected and more parallel offshore located ridges agree with those of observed sand ridges.
The high-resolution and coupled forecasting of wind, waves and currents, in restricted coastal domains, offer a number of important challenges; these limit the quality of predictions, in the present state-of-theart. This paper presents the main results obtained for such coastal domains, with reference to a variety of modelling suites and observing networks for: a) Liverpool Bay; b) German Bight; c) Gulf of Venice; and d) the Catalan coast. All of these areas are restricted domains, where boundary effects play a significant role in the resulting inner dynamics. This contribution addresses also the themes of the other papers in this Special Issue, ranging from observations to simulations. Emphasis is placed upon the physics controlling such restricted areas. The text deals also with the transfer to end-users and other interested parties, since the requirements on resolution, accuracy and robustness must be linked to their applications. Finally, some remarks are included on the way forward for coastal oceanography and the synergetic combination of in-situ and remote measurements, with high-resolution 3D simulations.
The accuracy of wave models in semi-enclosed-basins and orography-controlled wind conditions, especially during fetch-limited storm events, is known to be limited. Wind wave forecasting in the NW Mediterranean Sea is particularly demanding due to the characteristic sharp gradients of the wind and wave conditions. In this work we focus on the commonly observed underestimation of wave parameters even when the wind field is “correct” or overestimated. This is a small step to analyse such a discrepancy, where wind overestimation has been commonly used to get the “right” wave predictions for the “wrong” reason. Here we selected a suitable combination of nested meteorological and wave models to focus on the physics (in parameterized terms) of meso-scale wave generation in restricted domains. First, to better capture the typical sharp gradients in wind and wave fields under those conditions, the spatial resolution of the atmospheric model was progressively increased during a characteristic storm event from 18 km to 4 km; the corresponding frequency of the wind input was increased from 6 to 1 h. Second, the calculated rate of wave growth in the numerical model (i.e. the balance between the input term and the whitecapping dissipation) was analysed and tuned to match the observed local rate of wave growth. The rate of non-dimensional growth in the region of study, which was calculated using measurements along the fetch, turned out to be faster than simulated with the initial model settings and faster than reported in previous studies. Adjusting the wave growth rate in the model to the observations improved the estimated wave height by about 18% and the wave period by about 4%. Decreasing the grid size of the numerical models from 12 km to 4 km improved the timing of the wave peaks but not the maximum values of the storm. Increasing the frequency of the wind input (from 6 to 3 h) improved the estimation of the maximum wave height values (peaks) of the storm by about 13%. Summarizing, the results of this work showed that using high resolution and physically adjusted parameterizations in complex regions with strong wind and wave gradients such as the study area, it is possible to significantly reduce the under-estimation of wave parameters and to locally improve wave growth forecasting.
This study has been motivated by the limited accuracy of wave models under short-duration, fetch-limited conditions. This applies particularly to the wave period, in semi-enclosed domains with highly variable wind patterns as along the Catalan coast. The wave model SWAN version 40.91A is used here in three nested grids covering all the North-western Mediterranean Sea with a grid resolution from 9 to 1 km, forced with high resolution wind patterns from BSC (Barcelona Supercomputing Center) for two study periods, the winter 2010 and the spring 2011. The results are validated in eight locations with different types of instrumentations. In order to improve the results, a modification of the whitecapping term parameters is performed. Also the appropriate frequency integral range used to calculate the integral wave parameters is tested to be sure to compare the simulation results and the measurements for the same frequency interval. The results obtained show a clear improvement of the mean wave period and the peak period for the study area, decreasing considerably the negative bias observed previously, while almost no change is observed in wave height due to the proposed modifications. These results can be generalized to the Spanish Mediterranean coast and may be applicable to study areas with similar characteristics as the ones presented here: semi-enclosed domains with fetch-limited conditions and young sea waves.
The dispersal of freshwater inflow from a flood event is studied using a three dimensional flow model (ROMS). The model domain includes a small part of the Catalan shelf where the combination of local land topography with torrential rainfall can cause high local runoff during a short period but with a large impact on the receiving coastal waters. Both steady, low river discharge, typical of normal (lowdischarge) conditions and a high discharge representative of post-rain conditions are considered. Simulated salinity profiles on the shelf near the river mouth are compared with records from CTD measurements with quite good correspondence. A strong correlation between local wind and plume response was observed. Local winds affect the trajectory of the freshwater plume that enters the Catalan shelf waters. During post-rain conditions, northerly and westerly winds exported the plume further away from the coast, whereas southerly and easterly winds confine the plume closer to the coast. During low discharge conditions, the plume remained closer to the coast due to the weak wind stresses. Results show that freshwater spread, shape and dilution are mainly controlled by local wind forcing at relatively short time scales.
We discuss the performance of two global meteorological models in a difficult enclosed sea area and the possible improvements using two respectively nested high resolution local models. Each of the four sets of wind fields has been used to drive the same wave model. The performances are judged on the base of measured, buoys and satellites, wind and wave data. The analysis shows clearly the general benefits of a higher resolution. However, it also highlights the sensitivity of the nested models to apparently minor changes in the input information from the global models and their consequent possibility of larger errors, particularly in complex meteorological situations.
Shoreface-connected sand ridges occur on many storm-dominated inner shelves. These rhythmic features have an along-shelf spacing of 2-10 km, a height of 1-12 m, they evolve on timescales of centuries and they migrate several meters per year. An idealized model is used to study the impact of sea level rise on the characteristics of the sand ridges during their initial and long-term evolution. Different scenarios (rates of sea level rise, geometry of inner shelf) are examined. Results show that with increasing sea level the height of sand ridges increases and their migration decreases until they eventually drown. This latter occurs when the near-bed wave orbital velocity drops below the critical velocity for erosion of sediment. In contrast, in the absence of sea level rise, the model simulates shoreface-connected sand ridges with constant heights and migration rates. Model results furthermore indicate that sand ridges do not form if the rate of sea level rise is too high, or if the initial depth of the inner shelf is too small. A larger transverse bottom slope enhances growth and height of sand ridges and they drown quicker. When shoreface retreat due to sea level rise is considered, new ridges form in the landward part of the inner shelf, while ridges on the antecedent part of the shelf become less active and ultimately drown. Only if sea level rise is accounted for, merging of ridges is reduced such that multiple ridges occur in the end state, thereby yielding a better agreement with observations. The physical mechanisms responsible for these findings are also explained. (C) 2014 Elsevier Ltd. All rights reserved.
Lavoie, C.; Jimenez, J.A.; Canals, M.; Lastras, G.; de Mol, B.; Amblàs, D.; Liquete, C.; de Batist, M.; Hughes, J. Continental shelf research Vol. 74, p. 94-104 DOI: 10.1016/j.csr.2013.11.021 Data de publicació: 2014-02 Article en revista
We used high-resolution swath-bathymetry data to characterise the morphology of the abandoned subaqueous Sol de Riu delta lobe in the Ebro Delta, Western Mediterranean Sea. This study aims to assess the influence of an abandoned delta lobe on present-day coastal dynamics in a micro-tidal environment. Detailed mapping of the relict Sol de Riu lobe also showed a set of bedforms interpreted as footprints of human activities: seasonal V-shaped depressions on the middle shoreface due to boat anchoring and old trawling marks between 16 and 18 m water depth. Estimations of the mobility of bottom sediment showed that the shallowest shoreface (i.e. less than 7 m depth) is the most dynamic part of the relict lobe, while the middle shoreface experienced significant morphological changes since the lobe was abandoned. The deepest shoreface (i.e. water depth in excess of 15 m), which corresponds to the front of the lobe, is defined by a very small potential for morphological change. Simulations showed that while the relict lobe does not significantly affect the typical short period waves (T-p approximate to 4 s) in the study area, it does interfere with the most energetic wave conditions (T-p >= 7 s) acting as a shoal leading to the concentration of wave energy along the shoreline northwest of the lobe. The consequence of such modification of the high-energy wave propagation pattern by the relict lobe is an alteration of the wave-induced littoral sediment dynamics with respect to a situation without the lobe. (C) 2013 Elsevier Ltd. All rights reserved.
Manca, E.; Caceres, I.; Alsina, J.M.; Stratigaki, V.; Townend, I.; Amos, C. Continental shelf research Vol. 50-51, p. 100-116 DOI: 10.1016/j.csr.2012.10.008 Data de publicació: 2012-12 Article en revista
Nearshore sandbars are often characterized by three-dimensional bed patterns. To analyze the influence of wave direction on the morphological response of a double sandbar system, this paper uses the 2DH nonlinear surf zone model MORFO55. Depending on wave direction, different morphologies emerge as free instabilities. These morphological responses differ in terms of geometry (shape of the alongshore rhythmic patterns) and temporal evolution. Nearly shore-normal waves favor the emergence of crescentic patterns along both the inner and outer bars. These instabilities arise from “bedsurf” coupling, which is the positive feedback between the bed perturbations and the wave-breaking patterns resulting in rip-cell circulations. The system is stable for intermediate wave incidence angles because the longshore current shifts the circulation cells alongshore, inhibiting the growth of initial perturbations. For larger angles, the system is again unstable. Two types of oblique bars emerge. The first type forms between the crest of the inner bar and the shoreline and is oriented down-current. The second type is superimposed on the two bars and is oriented up-current. Although previous studies attribute the formation of oblique bars to the deflection of the longshore current (that is a cross-shore process), we show here that this mechanism contributes to the bar formation but the growth rate is mainly governed by a longshore process. Specifically, it is the positive feedback between the bathymetry and the longshore gradient of the sediment concentration. Finally, interactions between the patterns in the two shore-parallel bars are analyzed. In the observed configurations, the influence is always one way as the inner-bar dynamics never influence those of the outer bar. The outer-bar instabilities cause undulations in the inner bar when the outer-bar instabilities grow faster than those of the inner bar.
A nonlinear morphodynamic model is analysed to gain fundamental knowledge about the initial growth and long-term behaviour
of observed shoreface-connected sand ridges. Themodel describes quasi-steady, depth-averaged flow on a storm-dominated inner shelf with an erodible bottom and a transverse slope. Both bed load and suspended load sediment transport are incorporated. The formulations are linear with respect to the current and account for depth dependent stirring of sediment by waves as well as for the effect of local bed slopes.
A linear stability analysis has already revealed the initial growth of bed forms that resemble observed shoreface-connected
ridges. Here, a nonlinear analysis is carried out to study the long-term dynamics of these bed forms. The method is based on an expansion of the flow and the bottom perturbations in a truncated series of eigenfunctions of the linear problemfor a coastal stretch with a fixed longshore length. The result is a set of nonlinear algebraic equations, describing the flow over the topography, and differential equations for the bottom amplitudes.
Results indicate finite-amplitude behaviour in the mode amplitudes. The long-term bottom pattern shows the observed asymmetries of the ridges with steep bottom gradients on the downstream side. The migration speed of this finite-amplitude perturbation appears to be unaffected by nonlinear effects. Extrapolation of the results to Long Island
shelf yields bed forms with a characteristic height of about 4 m in a saturation time of B850 yr; which are consistent with observations.
An idealized morphodynamic model is used to gain further understanding about the formation and characteristics of shoreface-connected sand ridges and tidal sand banks on the continental shelf. The model consists of the 2D shallow water equations, supplemented with a sediment transport formulation and describes the initial feedback between currents and small amplitude bed forms. The behaviour of bed forms during both storm and fair weather conditions is analyzed. This is relevant in case of coastal seas characterized by tidal motion, where the latter causes continuous transport of sediment as bed load.
The new aspects of this work are the incorporation of both steady and tidal currents (represented by an M2 and M4 component) in the external forcing, in combination with dominant suspended sediment transport during storms. The results indicate that the dynamics during storms and fair weather strongly differ, causing different types of bed forms to develop. Shoreface-connected sand ridges mainly form during storm conditions, whereas if fair weather conditions prevail the more offshore located tidal sand banks develop. Including the M4 tide changes the properties of the bed forms, such as growth rates and migration speeds, due to tidal asymmetry. Finally a probabilistic formulation of the storm and fair weather realization of the model is used to find conditions for which both types of large-scale bed forms occur simultaneously. These conditions turn out to be a low storm fraction and the presence strong tidal currents in combination with strong steady currents during storms.
Maidana, M.; Naudin, J.; Espino, M.; Garcia, M.; Sanchez-Arcilla, A. Continental shelf research Vol. 22, num. 2, p. 229-245 DOI: 10.1016/S0278-4343(01)00055-3 Data de publicació: 2002-01 Article en revista
A steady-state quasi-3D finite element model is forced with in situ measured wind conditions in order to obtain an estimate of the mean circulation off the Ebro delta and to assess its consequences upon water fluxes. The model is spectral in the vertical direction and is run with a reduced number of vertical degrees of freedom (modes). The boundary conditions prescribed on the ocean contours account for the existence of a slope jet. The hypothesis is that the direct wind forcing on the upper layer and the shelf mesoscale circulation are the main mechanisms driving the flow in the area and that it should be possible to reproduce the main current features even in the vicinity of the Ebro river mouth with this relatively simple model setup, as the usual river freshwater discharge rates are rather low. This is clearly different from the case of other region of freshwater influence systems, in which the river plume dynamics and the related density currents play relevant roles. The model results are compared with surface drifter trajectories obtained during field campaigns.
Sierra, J.P.; Sanchez-Arcilla, A.; González, J.; Flos, J.; Movellán, E.; Mösso, C.; Martínez, R.; Rodilla, M.; Falcó, S.; Romero, I. Continental shelf research Vol. 22, num. 2, p. 361-378 Data de publicació: 2002-01 Article en revista