Journal of Coastal Research

SI 64

pg - pg

ICS2011 (Proceedings)

Poland

ISSN 0749-0208

A system of models and data base for short term beach processes – ECORS simulator T. Garlan†, J.M. Souffez‡, R. Mauget‡, J.P. Mazé∞ and L. Leballeur∞ † SHOM Research Dept 29200 Brest France [email protected]

‡ Capgemini, 35000 Rennes, France [email protected] [email protected]

∞Actimar 29200 Brest, France [email protected] [email protected]

ABSTRACT Garlan, T., Souffez, J.M., Mauget, R., Mazé, J.P., and Leballeur, L., 2011. A system of models and data base for short term beach processes – ECORS simulator. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), pg – pg. Szczecin, Poland, ISSN 0749-0208 The ECORS project, led by the French Hydrographic Office, aims to demonstrate short-term predictions of the morphodynamics of sandy coasts. These predictions are needed for the understanding of hydrodynamic and morphodynamic processes and for naval amphibious operations. From 2006 to 2012, the ECORS program includes three main tasks: theoretical research, field and laboratory experiments and numerical modeling. Such a wide scope was made possible by the additional contributions from many research institutes and local authorities. This paper presents the conceptual model of the ECORS coastal forecasting simulator. This one includes models and a set of Data Bases of tidal parameters, bathymetrical soundings, sedimentary and hydrographic data. The models are divided in two levels. The first one is composed of simple models to obtain a rapid characterization of some beach parameters and of the 1D beach profile. The second level is a system of models to calculate waves, currents and morphological evolutions. ADDITIONAL INDEX WORDS: Short-term predictions, Sandy coasts, Numerical Models, Morphodynamics

INTRODUCTION ECORS is a project led by the French Hydrographic Office (SHOM), for short-term morphodynamics of sandy beaches, with a general goal of improving the description of the nearshore environments for naval amphibious operations. This project, funded by the DGA (General Direction for Ordnance), is realized by the SHOM in collaboration with numerous university research laboratories affiliated with the French national institute for earth sciences and astronomy (CNRS/INSU), foreign laboratories and companies. The project thus encompasses incremental improvements to the well established forecasting capability of sea states, and demonstrations of new capacities, namely short-term predictions of the morphodynamic of sandy coasts. The understanding of hydrodynamic and morphodynamic processes that will be generated by the project have many other applications, including coastal management and enhancement of numerical models. Over the years 2006 to 2011, the ECORS program includes three main tasks: preliminary studies and research, field experiments and the development of a system for predicting hydrodynamical and morphological evolution of sand beaches. Theoretical research has been developed in sixty laboratories in the framework of four ECORS sub-projects which are led by the Universities of Bordeaux, Caen, Perpignan and Brest. The project includes both field and laboratory experiments and their inter comparison to very different types of numerical models. Such a wide scope was made possible by the additional contributions from many research institutes and local authorities. The exceptional high wave conditions, during the main morphodynamic field experiment (Senechal et al. 2008), has been a welcome energetic complement to existing data sets (Thornton et al., 1996; Ruessink et al., 1998). The experiment was designed

to nurture both process-based studies and numerical modeling simulations of the entire littoral system. Based on these research and campaigns, a coastal forecasting simulator is now developed and the present paper describe it structure. After recalling the results of studies based on publications from the project and from the Truc Vert Beach ECORS Experiment, we present the characteristics of the ECORS models system.

ECORS RESEARCH ACTIVITIES The ECORS research part involved sedimentologists and oceanographers from sixty research laboratories affiliated to INSU. These studies concern the definition of suitable methods to study rapid changes of morphology, the analysis of the impact of hydrodynamic factors and wind on the sediment transport, and the classification and magnitude of beach morphology parameters. The studies cover waves and currents effects, morphologic changes from detailed bottom evolution investigated at the time scale of the wave morphology changes to high resolution optical remote sensing, interactions between water and sediments, sediment properties, etc. Considering the variations of sandy beach morphologies, several experimental beaches have been selected for studying megatidal to microtidal environments, with or without sand bars in the lower intertidal and/or in the subtidal domain. These beaches are all along the French coasts from North Sea, English Channel, Atlantic Ocean to Mediterranean Sea. The three main ECORS sub-projects are: Plamar on megatidal environment led by F. Levoy of the University of Caen, Microlit on microtidal environment led by R. Certain of the University of Perpignan, and Modlit on modeling led by P. Bonneton of the University of Bordeaux 1.

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Sediment characteristics The two main elements that should be known to understand the characteristics and the life of a beach are the current velocity near the bottom and the granularity of sediment set in motion. These two elements are complex. The speed of bottom currents is affected by the hydrodynamics of waves and the swash which depend on weather, tide and the initial morphology. Sediments, often described by a simple medium grain, are frequently heterogeneous with variability in the three dimensions. The vertical layering of fine sediments overlying coarse sands has been observed during the ECORS experiment (Garlan and Ardhuin, 2009; Garlan and Bonneton, 2010). This fact seems to be common but not noticeable at first glance of a beach observation. Thus beaches with the same average grain may have very different sedimentary characteristics and must have different trends. An experimental simulation of sediment grain size analysis of shoreface nourishments, under storm events, has been done (Grasso et al (in press)).

Morphological changings High resolution satellite Remote sensing and an unmanned photogrammetric helicopter were used during ECORS experiment. A comparison of Spot and Formosat-2 shows that Spot scene gives an interesting complement to the numerical terrain model, in particular where topographic measurements are too scarce (Dehouck et al, 2009). These data gives very interesting results on the sea-sand limit in large nearly flat surfaces. A higher topographic resolution was given by a second approach with measurements by an unmanned photogrammetric helicopter which allows to constructed Digital Elevation Models and orthorectified images with a spatial resolution better than 5 cm (Delacourt et al 2009). Video images were used to study high frequency observations of double-sandbar dynamics in meso-macrotidal settings during storm conditions. One of the results presented by Almar (2010) show a simultaneous straightening of the outer-bar and an increase in the alongshore non-uniformities in the inner bar during the largest wave event which may point to inner-outer bar interactions during extreme wave conditions. These studies give also very interesting results on wave and current celerity measurements (Almar et al, 2008).

Modeling Most of the ECORS modeling studies are done in the subproject Modlit, led by Philippe Bonneton. It focuses on the numerical and physical modeling and is operated by 23 searchers from ten laboratories from France, The Netherlands and Chile. The studies concern adjustments of parameters used in models and impact of beach dynamics. Between numerical models and field measurement, the laboratory experiments are used to test detailed hypothesis under different controlled conditions. Some laboratories involved in this sub-project have developed experiments based on real beach parameters to reconstruct bedforms and to observe their movements. A lot of papers have been already published on these studies which concerned physical modelling of intermediate cross-shore beach morphology (Grasso et al, 2009, Grasso et al 2010), the nearbed flow - sediment processes under irregular breaking waves (Chassagneux and Hurther, 2009), coupling mechanisms in double sandbar systems (Castelle at al 2010a, 2010b). This last study presents tank flume experiments conducted over a mobile PVC sediment bed reproducing the cross shore variation of sediment transport regimes typically seen along sandy beach profiles and look at the

distinction between turbulence induced by wave breaking (bore turbulence) and bed friction effects. The mechanisms which control the generation of wave-induced mean current vorticity due to dissipating waves in the surf zone are investigate (Bonneton et al., 2010). The predictive capability of the shock-wave approach will be compared with ECORS laboratory (Castelle et al., 2009) and field (Bruneau et al., 2009) experiments. The numerical simulation of such topographically controlled macro vortices requires the use of high order robust well balanced schemes which is currently under investigation. The last experiment in this domain has been undertaken in the SOGREAH (Grenoble France) multidirectional 30*30 m square wave basin to compare field observations with laboratory results. This large scale laboratory experiment led by H. Michallet (University of Grenoble/LEGI) has been undertaken over a moveable bed, during a 5-week period at the end of 2008. First results from this experiment on rip current have been presented by Castelle et al. (2009) and by Michallet et al (2010). Research has already been made on parameterizations for wave models, with a proven enhanced accuracy compared to preexisting models (Ardhuin et al. 2008). Likewise, a similar effort is pursued to translate the more accurate offshore wave conditions into better coastal and surf zone wave and current fields, and to turn this hydrodynamic forcing into accurate sediment fluxes and morphological response. Wave-breaking model for Boussinesqtype equations which take into account roller effects in the mass conservation equation (Cienfuegos et al, 2010), Green-Naghdi modelling of wave transformation, breaking and runup, using a high order finite-volume finite-difference scheme (Tissier et al, 2010) and the modeling of vortex ripple morphodynamics (Marieu et al, 2008) have been developed by universities of Bordeaux, Grenoble, Santiago de Chile and the SHOM.

THE TRUC VERT BEACH EXPERIMENT The ECORS Truc Vert international field campaign, led by F. Ardhuin (SHOM) and N. Sénéchal (Univ. Bordeaux), took place in March and April 2008 in the South West of France, on the Aquitanian Coast. This beach is located about 20 km to the north of the Arcachon Lagoon entrance and is far enough from this entrance to be wave-dominated and far enough from anthropic impacts. More than a hundred of persons, from sixteen research groups, from 6 countries took part to this campaign. They have participated to the ECORS experiment to improve the knowledge on the short-term response of a high-energy natural beach, particularly for storm wave conditions with a common goal, the understanding of hydrodynamics and morphodynamics of sandy beaches. All was done to measure the group of processes implemented in this kind of environment from the micro elements, like turbulence and sand grain dynamics, to the macro elements like wave, sandbar and topographic movements. This field experiment, one of the largest to take place in Europe, provided a unique data set to explore surf zone and swash processes and to improve numerical models in presence of very energetic events (5 storms with Hs > 5m) during low tides and high tides, on mesomegatidal multi-barred sandy beaches. During this 5-week campaign, highly accurate topographic and bathymetric data acquisition was required with satellite imagery, unmanned air vehicle, video imagery, jet-ski, boats and topographic surveys to assess the morphology and the evolution of both the outer and bars. An analysis of the different steps of computation (frequency of survey, grid of computation, interpolation methods), have been made to define the accuracy of the data collected to merge the different data and to make an accurate sea-earth numerical model (Parisot et al., 2009).

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A persistent slow shoreward migration, with no significant alongshore movements, of the sandbar was observed during the experiment. This onshore migration during the high wave event can be explained by down-state adjustment of the inner bar resulting from a 6 meters storm event which had occurred two weeks before the field campaign (Bruneau et al, 2009). Results assume a strong temporal to spatial variability, related to wave forcing on either beach morphologies. The small cross-shore migration rates point to the protecting role of the outer-bar on the inner-bar and the beach. These observations highlight the complexity of the short-term dynamics of double-sandbar systems and the role of the inner-outer bar interactions during storm events that may result in outer-bar straightening and inner-bar nonuniformities (Almar et al, 2010). The sediments have shown a small evolution of surface granularity but in fact these sediments are coated at the end of the tide and the real evolution concern the thickness of the surface fine sediment and the granularity of coarse under layer (Garlan and Bonneton, 2010). The analysis of bottom evolution in the mid intertidal zone with an Acoustic Doppler Velocity Profiler (ADVP) able to measure simultaneously the local velocity field, bed elevation and thickness of the sheet flow layer and a set of optical fibers and pressure sensors aligned vertically throughout the sediment/water interface, able to detect the states of the medium in front of each sensor: low or highly concentrated flow, unstable or stable bed (Berni et al, 2009). The correlation between thickness of the sheet flow and excess pore pressure indicates that in most cases, the excess pore pressure is negative under the wave crest and becomes positive under the trough, promoting the development of the sheet flow. Consequently, exfiltration from the porous bed would be an important mechanism for sheet flow initiation during backrush (Berni et al, 2009). At the same scale, the same time and nearly the same place, Arnaud et al (2009) have measured local electrical resistivity along five vertical poles half buried into the sediment. The apparatus is able to characterize in situ, over several tidal cycles on a beach submitted to energetic waves, the time evolution of local bed level changes, with an acquisition frequency of 10 Hz. The bed level detection has been applied on wave by wave frequency time series over 10 tidal cycles. Real time bed level monitoring has been realized in the surf zone allowing the measurement of 1cm.min-1 erosion and deposition rate. A succession of sea bed erosion, creation of moving sand ridge, their disappearance and the accretion of the seabed have been observed. During a high tide these movement could exceed 10 cm and the resulting elevation could be positive, equal or negative at the end of the high tide (Arnaud, 2010). These results must be added to those of Russel et al, (2009) and Blenkinsopp et al, (2009) which measured over a tide period a swash zone enrichment exceeding 900 kg per meter width. An ultrasonic bed-level technique was used to measure shoreline erosion and accretion. The swash instrument rigs included arrays of mini electromagnetic current meters to measure vertical profiles of cross-shore and longshore current velocities, and the suspended sand concentration profiles (Russell et al, 2009). The ultrasonic sensors were deployed in a linear array across the high-tide shoreline and measured bed-level changes on a swash-by-swash basis with an accuracy of 1mm. During an accretionary tide the sand berm built up, close to the high-tide swash limit, accreting around 0.07 m, with the sand berm extending 10 m cross-shore. The swash-by-swash accretion events were slightly larger and slightly more numerous than the erosive events, resulting in overall accretion and berm formation (Russell et al, 2009). A key-point of the understanding of sandy beaches dynamics is the knowledge of the interactions between surface and

groundwater hydrodynamics, in particular between swash and beach watertable, and their impact on accretion and erosion processes above the still water level. A dedicated cross-shore line of 18 relative pressure sensors has been deployed from the berm to the intertidal lowest point, combining buried and free sensors. David de Drézigué et al (2009) show that the amplitude of propagating waves inside the beach clearly depends on the frequency, i.e. tides propagate more easily than gravity waves. These observations confirm existing results describing the sandy beach as a low-pass filter and a damping medium. Comparison between rising and falling tide shows that the beach fills up faster than it drains. The presence of a hump at rising tide can be deduced from the pressure measurements. This hump is due to swash infiltration processes and certainly extends under the upper part of the swash zone (David de Drézigué et al, 2009). With the analysis of ADVP profile Mignot et al (2009) look at cross shore and vertical flow velocity components over a distance of 33cm above the bed. For a moderate wave climate, 5cm high ripples were seen to migrate across the acoustic beam in the shoaling zone. Like Arnaud (2010), they observe that the flow field in the near bed region is strongly affected by ripples. The characteristic pattern of the orbital stress field differs strongly for both faces of the ripple. Finally, the transverse vorticity field also reveals different behaviors at each face of the ripple. The analysis of the whole set of data reveals the consistency of this orbital stress and vorticity organization, with more intense values encountered near the ripple crest (Mignot et al, 2009). Breaking waves over high-energy meso-macrotidal doublebarred beach induce rip currents. One of the reasons of the difficulties to model these one is the lack of intensive, high-spatial resolution, flow field measurements in the rip channel vicinity. During the ECORS intensive field measurements, an intertidal inner-bar rip channel was instrumented with fixed eulerian current meters. In addition a Horizontal ADCP (HADCP) was implemented in the vicinity of the rip current to measure alongshore non-uniformities and to validate model in this environment (Castelle et al, 2009). A coupled wave-induced circulation model was applied to a 5-day period and compared with field data. Results show that the HADCP provides unique information on the shear in the vicinity of the rip neck, which is particularly useful for model calibration (Castelle et al, 2009). To improve the knowledge of this kind of complex environment, Bruneau et al (2009) focused on very low frequency motions of a rip current system over a well-developed bar-rip morphology. Using both a drifter experiment and virtual drifter modeling, their study aims at analyzing the rip current pulsations and the drifter retention in the surf zone. The main results show the oscillating behavior of the rip currents, in particular within the rip neck where the very low frequency pulsations are intense (reaching 1m/s on time scales of 10 to 30 minutes). In addition, most of the drifters are retained within the surf zone (about 80%), with the other 20% exiting the surf zone. These results are reproduced with a numerical model, which shows that shear instabilities of the rip current can be the cause of such retention/expulsion proportions (Bruneau et al, 2009a). The Truc Vert Beach Experiment is a great source of data which gave and will give many results. Papers continue to be published, and new research and PhD are under process.

THE ECORS COASTAL FORECASTING SIMULATOR Principles

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The third part of the ECORS project is under process with the development of a morphodynamic beach simulator, which contain software and a system of models of equilibrium beach profile, and of models to calculate tide, waves, current, and changing of beach morphology. These models are connected to hydrographic and sedimentological data bases, to characterize nearshore environments and their short time evolution. It is divided into two main parts, a first one with simplified local beach models and a second one with 1D cross shore and 2D models. The morphodynamic is impacted by local characteristics of waves and sediment but also by human activities (dams, dredging, ...), by proximal river sediment, wind and runoff. The sediments contain in their composition (grain size, grading, density) the specific regional footprint and thus the translation of the history of the local environment. Hydrodynamic is the forcing factor that shaped the beach. So, for the forecast over a period of several weeks to several months, human actions can indeed be neglected, and the complexity of these environments can be limited to consideration of sediment and hydrodynamics. The demonstrator, without allowing tracking the beach changes at a very high resolution, intended to provide a visualization of the knowledge updated by the recent dynamics. It is composed of three parts. a: The simulation environment that integrates different sequences of models themselves containing the various models as a model database; b: The data space, comprising the Database ECORS but also workspaces, which centralize all data necessary for the operation of the simulator; c: The graphical interface, which is the component with which users will interact with the simulator. These three components interact with each other to fulfill the desired functionality. Thus, the simulation environment feeding data into the data space, but it also generates results. The graphical interface allows to format data from the data space. It also allows the constitution of the chosen modeling sequence.

The ECORS simulator first level The first level of the simulator includes simple models, ie they do not use a surface approach of the study area, and are only based on some parameters that allow to quickly assess the general information of the domain. For example, at this stage the fall velocity is calculated of sediment from the formulations of Stokes

and of Soulsby based on grain size data (size and density of sediment supplied by the operator or retrieved from the ECORS Data Base. After evaluating the dimensionless Dean number, characterizing the type of reflective or dissipative beach, according to Wright and Short 1984, recently improved by Grasso et al. (2009), the characteristic equilibrium beach profile is calculated using data from sedimentary and wave climatology database from statistical data or from seasonal characteristics of the study period. For instance, Bernabeu and Dean equilibrium beach profiles are integrated in the simulator, as well as, the alternative model Bernabeu (2003) model which is built for environments where the tide has a significant influence. In this latter case the model is coupled to the tidal model MARMONDE order to provide an indication of tidal level. This tidal model developed by SHOM reconstitutes the variations of the free surface from tidal harmonic constants. These data are used as reference water level for the calculation of the RTR parameter (Relative Tidal Range), for wave models and current models boundary conditions (Masselink and Short, 1993). Other parameters are calculated at this level. The model of Short and Aagaard (1993) provide an indication of the number of bars that can be formed on the study area. Another useful information is the knowledge of the offshore hydrodynamic boundary. This information is obtained by setting a limit based on the medium wave action, according to the formulation of Cowell (1999). The type of wave breaking (Iribarren number) is obtained from the slope of the beach and offshore wave characteristics extracted from the historical database or field measurements.

The ECORS simulator second level Models of this level require to have digital elevation models (DEM) which are based on depth data from different sources (soundings, mulibeam echosounder data, charts, grided data), with heterogeneous quality and density of data. These DEM provide the basic data of hydrodynamic and morphodynamic models but they are also used to display data and results. There is two kinds of DEM in the simulator, first the offshore one defines large scale conditions at a low resolution (some hundreds of meters to few kilometers), and yields wave open boundary conditions to the second model, and secondly, the nearshore model, embedded in

Figure 1. The conceptual model of the ECORS simulator second level Journal of Coastal Research, Special Issue 64, 2011

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the offshore model, which covers a small zone and presents a high resolution (from ten to twenty five meters). The offshore DEM does not evolve during the simulation and it results feed the coastal wave model. Instead, the high-resolution DEM is used to support the coastal wave, currents and morphodynamic models. Each nearshore numerical result is updated from the morphological changes calculated at each iteration of modeling. These DEM are integrated into ECORS database to archive the models configuration and to replay scenario with the same morphology. For offshore wave model, we used the existing SWAN wave models forced by WAVEWATCH3 wave climatology (Ardhuin et al, 2009). SWAN is also used for the nested nearshore wave model. The other models come from French research Institutes: IFREMER with MARS, University of Bordeaux with SEDIMORPH and 1DBEACH, or from the scientific community: XBEACH, SHORECIRC and WENO HH. The conceptual model of this system is schematized in the figure 1 and will be described in more details during the Symposium. This figure depicts the principle of the sequence implementation of models. It is characterized by the use of nested offshore and nearshore wave models in order to better represent wave propagation from large scale to the beach. Thus, the results of the coarse model are used as boundary conditions of the nearshore wave model. These results contribute to the pattern of currents and to give informations to calculate the evolution of morphology. This sequence of models is successively repeated by updating the conditions of stirring nearshore boundary. In the case of a sea without tides, SHORECIRC current model can be taken into account. The sea level is then maintained throughout the modeling as a constant. In the other cases, MARS, XBEACH and 1D BEACH could be used, and tidal information can be injected from MARMONDE tidal predictions. Then the simulator involves different models in terms of complexity, but also of local characteristics. It should be possible to use it in environments with or without tidal energy on normal and extreme conditions. On the figure 1, the numbers 1 is always SWAN. The sequence 2-3-4 could be either 1D BEACH (for 1D cross-shore modeling), or XBEACH, or SWAN-MARS-SEDIMORPH, or REF/DIF SSHORECIRC-WENO HH.

CONCLUSION Beach morphology evolves with the hydrodynamic conditions which re-mobilize sediment thus this one can move massively over short periods. Such results mentioned in previous chapters are changing the understanding of the phenomena involved to specify the hydrodynamic and sedimentary processes set in. The knowledge of physical processes affecting the beach morphology and gradually refined through the development of acquisition systems better and better adapted to extreme conditions of surf zones and their implementation during measurement campaigns in many hydrodynamic constraints. In the field of morphodynamic beach modeling, enormous progress has been made in particular by improving the wave patterns and characterization of hydrodynamic processes. These advances are gradually included in the models. One field has hardly changed over recent years, this is the digital representation of the sediment which is still oversimplified in models of morphodynamic beach. Between these real conditions and what can be assessed by the numerical models, there is a very important gap that will have to be further reduced by advanced models. These advances will be based on beach and flume experiments, to reproduce the physics of natural phenomena with simplified features and best constraints.

The ECORS simulator is a first step in assembling different models with sets of data directly measured at sea or from database reflecting the state of the knowledge. The system at present in test on different environments will be used to validate this concept for different applications. According to the results obtained with this system we should be able to establish the characteristics of an operational system for urgent needs such as landing operations for the evacuation of persons in distress or to the voluntary grounding of vessels in difficulty.

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ACKNOWLEDGEMENT The authors wish to thank all the teams involved in the ECORS Project. In particular, they are grateful to N. Sénéchal (U. Bordeaux 1), to F. Ardhuin (SHOM), and to P. Bonneton (U. Bordeaux 1), F. Levoy (U. Caen), and R. Certain (U. Perpignan) which manage the most important ECORS sub-projects. We also wish to thank all the searchers, students and technicians implied in the ECORS Project. We are grateful for the financial support provided by the DGA to this project.

Journal of Coastal Research, Special Issue 64, 2011