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Investigating the impacts of the regression of Posidonia oceanica on hydrodynamics and sediment transport in Giens Gulf
Abstract
Posidonia oceanica plays a significant role in the stabilization and protection of the coast in Gulf of Giens. Unfortunately, its distribution has been declining remarkably due to both anthropogenic interventions and natural factors. The present study focuses on the numerical simulation of the presence of Posidonia as well as the influence of its disappearance on hydrodynamics and sediment transport along Alamanarre beach. The model results indicate that the regression of Posidonia leads to significant changes of the hydrodynamic parameters, sedimentation rates, and causes severe erosion along Almanarre beach. In the annual wind conditions, the mean current speed is increased by 50.38%–80.65%, the mean significant wave height increased by 14.03%–16.58%, and the total load of sediment increased by 252%–426%, in absence of Posidonia. Regarding the seasonal wind variation, the disappearance of Posidonia induces an increase of the mean current speed about 52.86%–58.34%, the mean significant wave height by 8.23%–11.27%, and sediment transport rate by 358.99%–394.53%. Under the impact of extreme events with the disappearance of Posidonia, the mean current speed, the mean significant wave height, and the sediment transport rate increase by 12.53%–58.48%, 23.91%–34.67%, and 65.15%–154.62%, respectively.
... In the numerical model, the bed resistance is defined by the Manning number. It is variable due to water depth [28,29]. The calibration process defines the final Manning number in the domain based on drag coefficient and water depth [29]. ...
... It is variable due to water depth [28,29]. The calibration process defines the final Manning number in the domain based on drag coefficient and water depth [29]. Horizontal eddy viscosity is defined by the Smagorinsky model with a constant coefficient of 0.28. ...
... The wave model was coupled with the hydrodynamic model and used the observation wind data in Hon Ngu Island station for the offshore boundary condition. The Mike 21SW is calibrated by adjusting the Nikuradse roughness length k s which is a function of the median grain size D 50 in the study domain [29]. ...
Two-dimensional models of large spatial domain including Cua Lo and Cua Hoi estuaries in Nghe An province, Vietnam, were established, calibrated, and verified with the observed data of tidal level, wave height, wave period, wave direction, and suspended sediment concentration. The model was then applied to investigate the hydrodynamics, cohesive sediment transport, and the morphodynamics feedbacks between two estuaries. Results reveal opposite patterns of nearshore currents affected by monsoons, which flow from the north to the south during the northeast (NE) monsoon and from the south to the north during the southeast (SE) monsoon. The spectral wave model results indicate that wave climate is the main control of the sediment transport in the study area. In the NE monsoon, sediment from Cua Lo port transported to the south generates the sand bar in the northern bank of the Cua Hoi estuary, while sediment from Cua Hoi cannot be carried to the Cua Lo estuary due to the presence of Hon Ngu Island and Lan Chau headland. As a result, the longshore sediment transport from the Cua Hoi estuary to the Cua Lo estuary is reduced and interrupted. The growth and degradation of the sand bars at the Cua Hoi estuary have a great influence on the stability of the navigation channel to Ben Thuy port as well as flood drainage of Lam River.
... With the same technique, Than [2] estimated that the average annual rate of shoreline erosion is (-0.01 to -0.63) ± (0.27 to 1.82) m/year in the northern part and the shoreline accretion is (0.02 to 2.01) ± (0.14 to 5.1) m/year in the central and southern parts over the period from 1920 to 2012. By using numerical simulation, [3] indicated that the disappearance of Posidonia in Giens Gulf would cause a significant increase of current speed, significant wave height and sediment transport rate. In the other hand, by 2100, the global SLR due to climate change, forecasted to be around 44 cm, may have serious impacts on coastal regions [4]. ...
... Particularly, the HD module is used to compute surface elevation and currents which are induced by waves, whilst the SW module is used for transformation of wind-generated waves and swell from offshore and description of the detailed near-shore wave field. The ST module is used to estimate the sediment transport field and the morphological evolution [3]. The computational domain, about 9 km 2 , has only one western opensea boundary and a shoreline-land boundary enclosing remainders ( Figure 2). ...
... The model calibration was done by adjusting some main input parameters, viz. Manning's number, M, Nikuradse's roughness height, k s , and the median grain size, D 50 [3]. The simulated wave and current results are compared with the data measured at Almanarre beach from 31 st October to 12 th November 2000. ...
Rising sea level along with the occurrence of greater and more frequent storms would cause not only coastal flooding, but also beach erosion and shoreline retreat problems. The Almanarre beach along the western Giens tombolo is socio-economically and heavily vulnerable to accelerated sea level rise due to its high touristic value and low-lying topography. Therefore, it is necessary to quantify the impacts of sea level rise (SLR) on the morphodynamics in this area, e.g. to evaluate the relationship between the beach erosion and SLR. Coupled hydrodynamic and sediment transport numerical models are used to investigate the changes in current, wave and sediment dynamics when sea level rises. A total of 16 scenarios with and without SLR are simulated. The effectiveness of the coupled model is assessed by comparison of simulated values with available field measurements. The results presented in this work should be useful in the investigation of other coastal regions.
... As both wave set-up and sediment transport are dependent on wave height, this leads to a reduction of the flooding and erosive potential of waves in this area and, accordingly, contributes to coastal protection. The obtained results, based on numerical modeling are in agreement with observations in flume tests [49,50,61,85], field experiments [98,99] or other numerical studies [117][118][119]. ...
... erosive potential of waves in this area and, accordingly, contributes to coastal protection. The obtained results, based on numerical modeling are in agreement with observations in flume tests [49,50,61,85], field experiments [98,99] or other numerical studies [117][118][119]. ...
In this paper, the effectiveness of transplanted (either created or restored) seagrass meadows as a coastal protection measure is assessed through a five-step methodology. The analysis is focused on a stretch of the Catalan coast (NW Mediterranean) which is a fetch-limited environment. The results show that even considering conservative values for the meadow parameters (plant diameter, meadow density and canopy height), significant reductions of the annual average wave heights reaching the beach may be obtained, reducing flooding and erosion risks. Therefore, the investment in the conservation and restauration of seagrass meadows for protecting coastal areas from erosion and flooding is a measure that must be considered, due to the multiple benefits that they provide including ecosystem services. In addition, the proposed methodology may be a useful tool for coastal managers to help them in the design of seagrass meadows for coastal protection.
... In order to assess the performance of the numerical models, calibration was conducted with the data for waves, currents, and water elevation from offshore and nearshore stations at the Ba Lang beaches from December 20 to December 24, 2016. As the main parameter of the HD model, Manning's number was calibrated, and the Nikuradse roughness height in the SW model was selected for calibration (Vu et al., 2017). Fig. 5 compares the simulated and observed wave data at nearshore and offshore stations. ...
The Ba Lang sand beaches, located north of the Nha Trang Bay in Central Vietnam, are famous tourist attractions. However, they are experiencing shoreline and coastal erosion retreat, which is attributed to natural causes (such as tropical depressions, storms, and monsoons) as well as human impacts (such as hydropower generation, sand dredging, and coastal works). According to the forecast of the Vietnam Ministry of Natural Resources and Environment, global climate change will cause the sea level to rise by 74 cm along the coast from the Dai Lanh Cape to the Ke Ga Cape (including the Ba Lang beaches) by the end of this century in the representative concentration pathway (RCP) 8.5 scenario. Sea level rise (SLR) due to global climate change is expected to aggravate the coastal erosion and shoreline retreat problems. In this study, coupled numerical models with the spectral wave module (MIKE 21 SW), hydrodynamic module (MIKE 21 HD), and sand transport module (MIKE 21 ST) in the MIKE 21 package were used to simulate waves, current fields, and sediment dynamics along the Ba Lang beaches considering the impact of SLR. These models were calibrated with the field data measured in December 2016. The results showed that SLR caused the wave height to increase and reduced the current speed and total sediment load in monsoon conditions. The increase in wave height was even intensified under the joint impact of SLR and extreme events.
... Vastila and Jarvela (2017) presented a model for flow and sediment transport and showed that when the vegetation cover density increased, the flow velocity and sediment transport rate decreased. Tuan Vu et al. (2017) studied the numerical simulation of the presence of Posidonia on hydrodynamic behavior and sediment transport rate in Almanarre beach. The results of their developed model indicated that the presence of Posidonia resulted in significant changes in the hydrodynamic parameters of sediment and increased coastal erosion in Almanarre beach. ...
Natural climate changes like global warming and changes in the sea natural phenomena have led to numerous challenges, including changed regional flow patterns and, therefore, coastal erosion and sedimentation. Sediment transport management is an essential factor in coastal management and ecosystem conservation plans. Using forest cover is an environmentally shore-oriented strategy, which can reduce wave-induced sediment transport through the flow blockage and turbulence changes. In this study, the effect of forest cover configuration and density on sediment trapping efficiency was evaluated by implementing a model of coastal forest cover. A flume with an installed acoustic Doppler velocimeter and an output valve for the transported sediment was used, and the experiments were performed at different densities ranging from 12 to 273 stem/m2 of forest cover in both staggered and parallel patterns. The results demonstrated a rising trend in the sediment trapping efficiency following the increase in the cover density so that it reached from 20% in the lowest density to 84% in the highest. The staggered pattern was found to perform better than the parallel pattern in increasing sediment trapping efficiency; however, the difference was not significant (averagely 10%). An equation with an acceptable accuracy was also proposed for determining the sediment transport rate affected by the flow hydraulic and forest cover density.
... Coastal vegetation such as Spartina Alterniflora and Seagrass are being increasingly recognized as unique and crucial for coastal protection. It can attenuate wave energy, thus decreasing wave height (Barbier et al., 2008;Lashley et al., 2020;Möller et al., 2014;Tang et al., 2019) as well as wave setup (Dean and Bender, 2006;van Rooijen et al., 2016), and is also capable of mitigating storm surge hazard (Guannel et al., 2015;Narayan et al., 2017), and enhancing the ability to resist coastal erosion and sea level rise (Duarte et al., 2013;Kirwan and Megonigal, 2013;Vu et al., 2017), thus protecting coastal zone. Moreover, coastal vegetation has several ecological functions such as conservation and restoration of the ecological environment and biodiversity. ...
Quantitative investigation of wave attenuation by flexible vegetation is an increasingly prominent area in wave mechanics. Given that the existing XBeach non-hydrostatic wave model can only simulate wave attenuation by rigid vegetation, a mechanical model for simulating submerged flexible vegetation dynamics driven by waves and a coupling simulation method between the flexible vegetation dynamic model and the XBeach non-hydrostatic wave model were proposed. To assess the performance of the proposed coupled model, a wave flume experiment was conducted. This study demonstrates the validity of the new flexible vegetation dynamic model in simulating forces and motions of a single submerged flexible stem. The validation between modeled results and experimental data shown that the coupled model is capable of reproducing the wave attenuation in the presence of flexible vegetation. The above results further indicate that the coupling implementation is valid and reliable. Besides, this study further confirms that flexible vegetation is able to attenuate wave height, and the elasticity modulus of flexible vegetation plays a vital role in wave attenuation when all the other variables are invariant. This study can broaden the application of the XBeach model and provide technical support for the engineering practice of coastal nature-based solutions.
... The CFC in rigid or flexible forms affects the flow dynamic and the sediment transport (Chen et al., 2020;Västilä and Järvelä, 2018;Wang et al., 2015) and consequently reduces the coastal bed erosion (Chen et al., 2012). Results of the experimental studies (Esteban et al., 2020;Fathi-Moghadam et al., 2018;Romdhane et al., 2018;Lou et al., 2018), field studies Fan et al., 2006;Mazda et al., 2006), and numerical studies (Nardin et al., 2018;Tang et al., 2017;Vu et al., 2017;Maza et al., 2015) show that the vegetation cover creates a drag and a resisting force against the flow passage, leading to wave damping and sediment accumulation, trapping, and settling. ...
The sediment transport phenomenon is considered to be an important factor in improving coastline hydraulic performance. This research studied the effects of long waves as simulated solitary waves on the sediment transport to estimate the sediment transport rate in different coastal forest cover (CFC) densities. To examine how trees altered the wave characteristics, artificial trees were used experimentally to simulate the aligned and
staggered configurations in totally 32 cover densities and 3 different wave heights. Results revealed that the CFC could averagely reduce the total sediment transport rate by up to 41.18% (in the densest case by 54% and 45% in staggered and aligned configurations, respectively, compared with the no-CFC case). The optimum density limit
with maximum reduction effect was determined as 40–50% for the staggered and 70–80% for the aligned pattern. The configuration effects on the CFC efficiency were also investigated and showed the superiority of the zigzag configuration over the aligned arrangement. This study also used the laboratory data to develop two equations for calculating the sediment transport rate in the presence of coastal cover.
... Dead P. oceanica leaves -together with large amounts of sand -have therefore been removed from the beaches and transported to a landfill (or sometimes pushed, using mechanical earth movers, towards one end of the beach). As a result, the now unprotected beaches have been steadily eroded, and attempts are madeunsuccessfully -to compensate the erosion by costly sand replenishment (Guala et al., 2006;Conseil Scientifique des Iles de Lerins and CREOCEAN, 2011;Boudouresque et al., 2017a and references therein;Otero et al., 2017;Vu et al., 2017;Martín Prieto et al., 2018;Vu, 2018). In addition, the fate of the dead leaves of the banquette is to return, sooner or later, to the sea; burying it in landfills deprives coastal ecosystems of an important source of carbon and nutrients, with a negative impact on captures of the fisheries El Zreli et al., 2016). ...
The Mediterranean Posidonia oceanica seagrass meadows provide a variety of ecosystem services; one of them is the protection of beaches against erosion, due to the accumulation on beaches of drift dead leaves, which forms what is known as 'banquettes'. In most Mediterranean areas, the P. oceanica banquette is considered as waste that has a negative impact on seaside tourism and is therefore removed by the municipal authorities. In fact, it is not only unproven that the banquette constitutes a nuisance for tourism, but also, in most cases, its removal has not been insisted on by the majority of beach users. On the contrary, its removal turned out to be an economic and ecological disaster. Interestingly, in an area frequented by international tourists, at Zarzis (southern Tunisia, Mediterranean Sea), thanks to a local initiative, the non-removal of the banquette shows that it is compatible with high beach attendance rates, especially when tourists are properly informed about the issue. This is a lesson for local mayors all around the Mediterranean who are being manipulated by tour operators and misleading information. Résumé. Maintien des banquettes de feuilles mortes de Posidonia oceanica sur une plage très fréquentée par les touristes : leçons depuis la Tunisie. Les herbiers à Posidonia oceanica, une plante à fleurs marine endémique de Méditerranée, fournissent à l'homme de nombreux services écosystémiques ; l'un de ces services est la protection des plages contre l'érosion, grâce aux banquettes de feuilles mortes de posidonie. Dans la plus grande partie de littoral méditerranéen, les banquettes de P. oceanica sont considérées comme des déchets et comme une nuisance pour le tourisme balnéaire ; elles sont par conséquent enlevées par les autorités locales (municipalités, maires). En réalité, le fait que les banquettes nuisent au tourisme est loin d'être démontré ; les enquêtes montrent que l'enlèvement des banquettes n'est pas réclamé majoritairement par les usagers des plages, même non informés de leur rôle, et qu'il est largement rejeté par les usagers informés. En fait, l'enlèvement des banquettes représente un désastre non seulement économique mais aussi écologique. Il est significatif de constater que, dans un secteur très fréquenté par les touristes internationaux, à Zarzis (Sud de la Tunisie), le non-enlèvement des banquettes de feuilles mortes, dans le cadre d'une initiative locale, est compatible avec une fréquentation élevée par des touristes bien informés des enjeux. Ceci constitue une leçon pour les maires des côtes méditerranéennes, qui se laissent manipuler par des tour-operateurs et par des informations inexactes.
... The results of his study showed that the southern part of Gapeau river mouth, Ceinturon beach and Pesquiers beach experienced the retreat with an average change rate of − 0.68 m/yr, −0.35 m/yr, and −0.1 m/yr, respectively; whereas the remainders along the eastern tombolo were accreted with an average change rate of +0.26 m/yr between 1972 and 2003. Recently, Vu [9] reported that the regression of Posidonia might increase in wave height, current speed, and shoreline change rate along the coast of Giens tombolo. ...
Giens double tombolo linking Giens island to the mainland is a unique geomorphological formation in the world. However, its existence has been threatened by coastal erosion, especially in the eastern part of this tombolo. The investigation of historical shoreline changes along the eastern Giens tombolo were carried out applying the integration of satellite remote sensing and geographic information system (GIS) techniques. Additionally, the combination of the Digital Shoreline Analysis System (DSAS) and linear regression method was used to predict the location of future shorelines. The results obtained from the analysis of shoreline position showed that the average annual change rate along the eastern Giens tombolo varied around +0.18 m/yr during the duration from 1973 to 2015, revealing a general progradation trend. Even though accretion is dominant, there are some local areas undergoing severe erosion. The most severely vulnerable areas were Les Cabanes du Gapeau, the south of Ceinturon, Pesquiers, and the north of La Capte with the maximum change rates of −1.05 m/yr, −0.77 m/yr, −0.44 m/yr, and −0.29 m/yr, respectively. The change analysis of shorelines in 2020 and 2050 also reveals these severely eroded areas. On the other hand, this work demonstrates that both natural factors and human activities are the main causes of the shoreline changes in the eastern Giens tombolo.
... In recent years, several studies have evaluated the effect of the Posidonia oceanica on the inshore hydrodynamics of the area, addressing the wave attenuation on plant density and on submergence. Posidonia oceanica meadows reduce the inshore wave climate by means of the leaves and the leaf systems and retain the fraction of sediments covered by the vegetation [108][109][110]. ...
Sea hazards are increasingly threatening worldwide coastal areas, which are among the most strategic resources of the Earth in supporting human population, economy and the environment. These hazards enhance erosion processes and flooding events, producing severe socio-economic impacts and posing a challenge to ocean engineers and stakeholders in finding the optimal strategy to protect both the coastal communities and the health of the environment. The impact of coastal hazards is actually worsened not only by an enhancing rate of relative sea level rise and storminess driven by climate changes, but also by increasing urban pressure related to the development of the sea economy. With regard to larger environmental awareness and climate change adaptation needs, the present study focuses on a stepwise approach that supports the actions for coastal protection at Calabaia Beach, which is located in the Marine Experimental Station of Capo Tirone (Cosenza, Italy). These actions first aim to protect humans and coastal assets, then to restore the environment and the local habitat, overcoming the need for the emergency interventions carried out in the last decades and pointing out that healthy ecosystems are more productive and support a sustainable marine economy (“Blue Growth”).
... Calibration and validation of the numerical model focused on the water levels, wave and current characteristics. The Nikuradse roughness height and Manning's number are used as the main calibration parameters for numerical models (Vu et al., 2017). ...
Nha Trang bay, located in the south Central part of Vietnam, have been widely considered as one of 29 most beautiful bays in the world. It is a center of tourism and services, which plays an important role in economic development of Khanh Hoa province. However, its coast is facing severe erosion problems threatening the existence of sandy beaches as well as many seaside properties. Especially, the Ba Lang beach is very vulnerable. In order to halt coastal erosion and stabilize the shoreline, submerged breakwaters (SBWs) are introduced to apply for this area. SBWs are being interested in beach protection, mainly due to less loss of beach amenity and their low aesthetic impact, which are important for maintaining the tourist value of sandy beaches. Nevertheless, SBWs have rarely been used for beach coastal protection in Vietnam; the morphodynamical response to these structures is not clearly understood at present. Therefore, the main aim of this paper is to investigate the effects of SBWs in hydrodynamics and sediment transport by using the numerical models of MIKE21, including hydrodynamic (HD), spectral wave (SW), and sand transport (ST) models. The results demonstrate that SBWs can be an effective measure to protect Ba Lang beach to a certain degree in both ordinary and strong wave conditions.
... However, Posidonia Oceanica commonly present at a depth that deeper than the breaking zones in Hyères bay, while the remainders are only covered by sand, gravel or rock without seagrass. Therefore, the estimation of the drag coefficient and the roughness height is partitioned into two different areas, seagrass and no seagrass (Vu, Lacroix, & Nguyen, 2017). For the seagrass areas, the vegetation's drag expression form of Nepf (1999), which was developed based on laboratory experiments with steady flows through rigid grass mimics, was adopted and used to simulate the existence of Posidonia. ...
The Ceinturon beach in the eastern Giens tombolo has been eroded since 1970s. Beach nourishment has already been applied to recover it. In this paper, the effects of beach nourishment on hydrodynamics and sediment transport of Ceinturon beach are investigated by using the numerical models. These numerical models were calibrated with field data. All calibrations and comparisons show good agreement between the measured results. The results demonstrate that beach nourishment has the considerable impacts on hydrodynamics and sediment transport of Ceinturon beach, but it is not a rational and primary tool for coast stabilization and protection.
... Previous studies highlighted the use of biomass into earthen materials, achieving some improved results in terms of comfort conditions [3][4][5]. This study evaluates the use of Posidonia Oceanica fibres as an additive for adobe bricks.. Posidonia Oceanica, a seagrass widely distributed along coastlines of the Mediterranean sea, leaves residues that reaching the coasts represents a significant economic, and hygienic problem in all coastal zones [6][7][8][9]. Hence, beached leaves of Posidonia Oceanica were proposed as a renewable and low cost material for the production of value-added products [6,10,11]. ...
The main aim of the present study is to measure the capability the "Posidonia Oceanica" as fibre reinforcement for adobe bricks. This sea-plant is distributed in the Mediterranean coast, considered as a solid marine by-product. To analyse the performance, prismatic earthen specimens with seagrass fibres were compared with the most traditional additives for this purpose; straw fibres. Both fibres were included with different lengths and quantities. Previously, in order to understand their behaviour, the fibres themselves were evaluated. Tensile strength and water absorption tests were performed. Meanwhile bricks themselves were subjected to mechanical tests. Concerning the biomass fibres, results show that straw fibres present higher tensile resistance than the seagrass leafs while they are more fragile to breakage than seagrass; on the other hand, they have higher water absorption than seagrass. Mechanical results show variations depending on their fibre content and length. Nevertheless, bricks with long seagrass fibres present a characteristic good behaviour in terms of flexural and compressive strength. Generally, adobe bricks with both types of fibres achieve similar mechanical properties, generating favourable results in terms of comparison between them.
... The consistent buffering effect of seagrass beds against the tsunami contrasts with the impact of coral reefs, which were often seen to accentuate flooding (Cochard et al., 2008). The modelling of Vu et al. (2017) suggests that for decadal to centennial storms off the French coast in the Mediterranean, the drag exerted by Posidonia oceanica meadows decreased the significant wave height by up to 25 per cent and rates of net sediment transport by up to 60 per cent. This is consistent with the observed attenuation effect of Posidonia oceanica meadows on currents and waves and the promotion of sediment stability during a storm off the Spanish coast (Granata et al., 2001). ...
Beaches along the eastern branch of the Giens double tombolo are subject to coastal erosion. Prediction of the behavior of the beach profile configuration in response to natural and anthropogenic changes using the concept of equilibrium beach profile (EBP) could be useful in finding the most suitable measure to address the erosion problem. Field investigation data of 11 beaches along the eastern tombolo were supplied for this study, and a nonlinear fitting technique was applied to estimate the best parameter values of seven empirical formulations of the relevant EBP. All of the observed beach profiles could be described by a single function, but a single EBP was inadequate to represent all of the beach profiles observed. The variation found could be explained in terms of longshore variation of bathymetry, sediment size, and wave parameters. Analysis of the validity of the EBPs revealed that a representative EBP of each beach is governed by different equilibrium parameters.
Posidonia oceanica meadows are declining at alarming rates due to climate change and human activities. Although P. oceanica is considered the most important and well-studied seagrass species of the Mediterranean Sea, to date there has been a limited effort to combine all the spatial information available and provide a complete distribution of meadows across the basin. The aim of this work is to provide a fine-scale assessment of (i) the current and historical known distribution of P. oceanica, (ii) the total area of meadows and (iii) the magnitude of regressive phenomena in the last decades. The outcomes showed the current spatial distribution of P. oceanica, covering a known area of 1,224,707 ha, and highlighted the lack of relevant data in part of the basin (21,471 linear km of coastline). The estimated regression of meadows amounted to 34% in the last 50 years, showing that this generalised phenomenon had to be mainly ascribed to cumulative effects of multiple local stressors. Our results highlighted the importance of enforcing surveys to assess the status and prioritize areas where cost-effective schemes for threats reduction, capable of reversing present patterns of change and ensuring P. oceanica persistence at Mediterranean scale, could be implemented.
EN: The objective of this thesis is the determination of the causes of sea erosion phenomenon, and the production of policy proposals for the protection of the beach.
The study begins with the collection and analysis of all available data, to find evolution rules of tombolo of Giens. Which aims to explain how the hydrodynamic processes and sediment transport occur.
Then MIKE modeling software was applied to confirm the hypothesis and get a better understanding of the dynamics at work in the Western tombolo.
Then, we coupled factors simultaneously: waves, coastal currents and sediment transport for different regimes of wave and wind in MIKE 21.
Finally, the possibility of stabilizing the Western tombolo is discussed to give suggestions on the choice of solutions.
Keywords: hydrodynamics, Almanarre beach, digital, coastal erosion model, tombolo, beach nourishment, breakwaters, evolution, coupled models.
FR: L’objectif de cette thèse est la détermination des causes du phénomène d'érosion marine, et la production de propositions d'orientation pour la protection de la plage de l’Almanarre.
L'étude commence par la collecte et l'analyse de toutes les données disponibles, pour trouver des règles d’évolution du tombolo de Giens.
Puis, le logiciel de modélisation MIKE a été appliqué pour confirmer l'hypothèse et à obtenir une meilleure connaissance de la dynamique à l’œuvre au tombolo Ouest.
Ensuite, nous avons couplé des facteurs simultanément : les houles, les courants côtiers, et les transports des sédiments pour les différents régimes de houle et de vent dans MIKE 21.
Enfin, la possibilité de stabiliser le tombolo Ouest est discutée pour donner des suggestions sur les choix de solutions adaptées.
Mots clés : hydrodynamique, plage de l’Almanarre, modèle numérique, érosion côtière, tombolo, rechargement des plages, brise-lames, évolution, modèles couplés.
http://www.theses.fr/s110957
The western Tombolo of the Giens peninsula in southern France, known as Almanarre beach, is subject to coastal erosion. We are trying to use computer simulation in order to propose solutions to stop this erosion. Our aim was first to determine the main factors for this erosion and successfully apply a coupled hydro-sedimentological numerical model based on observations and measurements that have been performed on the site for decades. We have gathered all available information and data about waves, winds, currents, tides, bathymetry, coastal line, and sediments concerning the site. These have been divided into two sets: one devoted to calibrating a numerical model using Mike 21 software, the other to serve as a reference in order to numerically compare the present situation to what it could be if we implemented different types of underwater constructions. This paper presents the first part of the study: selecting and melting different sources into a coherent data basis, identifying the main erosion factors, and calibrating the coupled software model against the selected reference period. Our results bring calibration of the numerical model with good fitting coefficients. They also show that the winter SouthWestern storm events conjugated to depressive weather conditions constitute a major factor of erosion, mainly due to wave impact in the northern part of the Almanarre beach. Together, current and wind impact is shown negligible.
Seagrass meadows are considered to be among the most important marine ecosystems, with regard to both ecology and biodiversity and for the services they provide. Seven species occur in the Mediterranean Sea: Posidonia oceanica (the most common in the open sea), Cymodocea nodosa (particularly common in the eastern basin), Ruppia cirrhosa, R. maritima, Zostera marina and Zostera noltii (mainly in estuaries and brackish lagoons), and Halophila stipulacea (introduced from the Red Sea). Seagrass regression may be due to natural processes and/or natural or anthropogenic disturbances and stress. It can also be due to long-term climate trends, e.g., the post-Last Glacial Maximum rise in sea-level, the Little Ice Age (LIA) cooling and the post-LIA warming, resulting in possible misinterpretation. Human-induced losses of P. oceanica have been mainly related to coastal development, pollution, trawling, fish farming, moorings, dredging, dumping and introduced species. All other seagrasses have also undergone more or less dramatic regression events. In fact, accurate data are generally of very local value and they are lacking for most of the Mediterranean Sea. In the absence of a reliable baseline, some widely cited cases of regression are questionable. Relatively healthy P. oceanica meadows, whose limits have changed little since the 1950s, may thrive in highly anthropized areas. In addition, the decline of one species can benefit another, so that the overall seagrass balance may remain unchanged (e.g., Cymodocea replacing Posidonia). However, to conclude that everything is for the best would be erroneous. First, the lack of data supporting the general regression hypothesis does not invalidate the hypothesis. Indisputably dramatic seagrass losses have been documented (e.g., P. oceanica and Z. marina). Second, the Posidonia regression is irreversible at human scales, while other seagrasses can rapidly recover, and the expansion of some seagrasses (e.g., Cymodocea) cannot counterbalance, in terms of ecosystem services, the decline of the P. oceanica meadows. Third, human pressure (demography, tourism, etc.) on Mediterranean seagrass ecosystems is destined to strongly increase in the coming decades. Finally, the rise in sea-level due to global climate change will automatically induce a withdrawal of the lower limit of seagrass meadows whenever the limit is beyond the compensation depth. So the regression trend observed in Mediterranean seagrasses, even if it proves to be currently weaker than postulated, will significantly increase and become a major concern in the future. There is therefore an urgent need for the adoption of a set of efficient indicators and the setting up of a robust comparative baseline in order to draw up an accurate assessment of the losses and, for seagrasses other than Posidonia, possible gains at Mediterranean scale. In addition, seagrasses and seagrass habitats should be granted legal protection and, where such protection already exists, it should be implemented.
A three-dimensional model has been modified to describe the complex interactions between hydrodynamics, sediment dynamics and biological parameters in the presence of Zostera noltei. The model treats seagrass leafs as flexible blades that bend under hydrodynamic forcing and alter the local momentum and turbulence fluxes and, therefore, the benthic shear conditions; these changes cause related changes to the mass balance at the boundary of the bed, in turn affecting the suspended matter in the column and ultimately primary productivity and the growth of the dwarf-grass. Modelling parameters related to the impact of Z. noltei to the local flow and to erosion and deposition rates were calibrated using flume experimental measurements; results from the calibration of the model are presented and discussed. The coupled model is applied in the Arcachon Bay, an area with high environmental significance and large abundance of dwarf-grass meadows. In the present paper, results from preliminary applications of the model are presented and discussed; the effectiveness of the coupled model is assessed comparing modelling results with available field measurements of suspended sediment concentrations and seagrass growth parameters. The model generally reproduces sediment dynamics and dwarf-grass seasonal growth in the domain efficiently. Investigations regarding the effects of the vegetation to the near-bed hydrodynamics and to the sediment suspension in the domain show that dwarf-grass meadows play an important part to velocity attenuation and to sediment stabilisation, with flow and suspended sediment concentrations damping, compared to an unvegetated state, to reach 35–50 and 65 %, respectively, at peak seagrass growth.
Aquatic plants convert mean,kinetic energy into turbulent kinetic energy at the scale of the plant stems and branches. This energy transfer, linked to wake generation, affects vegetative drag and turbulence intensity. Drawing on this physical link, a model is developed to describe the drag, turbulence and diffusion for flow through emergent vegetation which for the first time captures the relevant underlying physics, and covers the natural range of vegetation density and stem Reynolds’ numbers. The model,is supported by laboratory and field observations. In addition, this work extends the cylinder-based model for vegetative resistance by including the dependence of the drag coefficient, CD, on the stem population density, and introduces the importance of mechanical diffusion in vegetated flows.
Aquatic vegetation controls the mean and turbulent flow structure in channels and coastal regions and thus impacts the fate and transport of sediment and contaminants. Experiments in an open-channel flume with model vegetation were used to better understand how vegetation impacts flow. In particular, this study describes the transition between submerged and emergent regimes based on three aspects of canopy flow: mean momentum, turbulence, and exchange dynamics. The observations suggest that flow within an aquatic canopy may be divided into two regions. In the upper canopy, called the ``vertical exchange zone'', vertical turbulent exchange with the overlying water is dynamically significant to the momentum balance and turbulence; and turbulence produced by mean shear at the top of the canopy is important. The lower canopy is called the ``longitudinal exchange zone'' because it communicates with surrounding water predominantly through longitudinal advection. In this region turbulence is generated locally by the canopy elements, and the momentum budget is a simple balance of vegetative drag and pressure gradient. In emergent canopies, only a longitudinal exchange zone is present. When the canopy becomes submerged, a vertical exchange zone appears at the top of the canopy and deepens into the canopy as the depth of submergence increases.
The distribution of suspended sediment in the combined wave-current motion is theoretically predicted in the case of a plane bed. This will be the case if the wave-induced motion close to the bed is sufficiently strong. The theory is able to predict the average value of the concentration as well as the instantaneous values at a given distance from the bed. The theory is compared with laboratory and field measurements.
Mediterranean nearshore sandy and rocky bottoms are colonized by the endemic reef-building seagrass Posidonia oceanica. This species loses leaves in autumn that form large litter patches in the surf zone and huge litter banks on adjacent beaches, resulting in wedge and layered structures of few centimetres to several meters in thick (banquettes). Some authors pointed the importance of those banquettes for the protection of sandy beaches because they dissipate wave energy. By contrast other authors state that this effect is almost negligible. This work deals with the role of Posidonia oceanica accumulations in Mediterranean beach morphodynamics. By means of coastal video-monitoring and wave records we assess the marine conditions related to the formation and destruction of banquettes and evaluate their role in the protection of two sandy beaches (Cala Millor and Son Bou, Balearic Islands). Results indicate that banquettes are common beach features at the study sites although they are not persistent and experience complex construction and destruction dynamics throughout the year. Two different types of banquette construction can be differentiated over the year: one related to the reworking of older seagrass beach cast by alongshore currents and a second as a response to the incorporation of new volumes of dead leaves after energetic winter storms (Hs similar to 2 to 3 m). In both cases, seagrass cast accumulations are continuously built up and destroyed and rarely persist before the arrival of new sea storms. Therefore, at least for semi-enclosed sandy beaches, the protection role of seagrass banquettes should be reconsidered.
The paper presents a simple mathematical model for sediment transport in straight alluvial channels. The model, which is based on physical ideas related to those introduced by Bagnold (1954), was originally developed in two steps, the first describing the bed load transport (Engelund 1975) and the second account- ing for the suspended load (Fredsae and Engelund 1976). The model is assumed to have two advantages as compared with empirical models, first it is based on a description of physical processes, secondly it gives some information about the quantity and size of the sand particles in suspension and the bed particles.
Successful management and restoration of coastal vegetation requires a quantitative process-based understanding of thresholds hampering (re-)establishment of pioneer vegetation. We expect scouring to be important in explaining the disappearance of seedlings and/or small propagules of intertidal plant species, and therefore quantify the dependence of scouring depend on plant traits (flexibility, size) and physical forcing by current velocity. Flume studies with unidirectional flow revealed that scouring around seedlings increased exponentially with current velocity and according to a power relationship with plant size. Basal stem diameter rather than shoot length was found to be the factor controlling scouring volume. Flexible shoots caused far less scouring than stiff shoots, provided that the bending occurred near the sediment surface as was the case for Zostera, and not on top of a solid tussock base as we observed for Puccinellia. Therefore, shoot stiffness is likely to strongly affect the chances for initial establishment in hydrodynamically exposed areas. Plant traits such as shoot stiffness are subject to a trade-off between advantages and disadvantages, the outcome of which depends on the physical settings.
We demonstrate the utility of using the equivalent bottom roughness for calculating the friction factor and the drag coefficient of a seagrass meadow for conditions in which the meadow height is small compared to the water depth. Wave attenuation induced by the seagrass Posidonia oceanica is evaluated using field data from bottom mounted acoustic doppler velocimeters (ADVs). Using the data from one storm event, the equivalent bottom roughness is calculated for the meadow as ks 0.40 m. This equivalent roughness is used to predict the wave friction factor fw, the drag coefficient on the plant, CD, and ultimately the wave attenuation for other storms. Root mean squared wave height (Hrms) is reduced by around 50% for incident waves of 1.1 m propagating over 1000 m of a meadow of Posidonia oceanica with shoot density of 600 shoots m-2.
It is widely accepted that light availability sets the lower limit of seagrass bathymetric distribution, while the upper limit depends on the level of disturbance by currents and waves. The establishment of light requirements for seagrass growth has been a major focus of research in marine ecology, and different quantitative models provide predictions for seagrass lower depth limits. In contrast, the influence of energy levels on the establishment, growth, and maintenance of seagrasses has received less attention, and to date there are no quantitative models predicting the evolution of seagrasses as a function of hydrodynamics at a large scale level. Hence, it is not possible to predict either the upper depth limit of the distribution of seagrasses or the effects that different energy regimes will have on these limits. The aim of this work is to provide a comprehensible methodology for obtaining quantitative knowledge and predictive capacity for estimating the upper depth limit of seagrasses as a response to wave energy dissipated on the seafloor. The methodology has been applied using wave data from 1958 to 2001 in order to obtain the mean wave climate in deep water seaward from an open sandy beach in the Balearic Islands, western Mediterranean Sea where the seagrass Posidonia oceanica forms an extensive meadow. Mean wave conditions were propagated to the shore using a two-dimensional parabolic model over the detailed bathymetry. The resulting hydrodynamics were correlated with bottom type and the distribution of P. oceanica. Results showed a predicted near-bottom orbital velocity of between 38 and 42 cm s-1 as a determinant of the upper depth limit of P. oceanica. This work shows the importance of interdisciplinary effort in ecological modeling and, in particular, the need for hydrodynamical studies to elucidate the distribution of seagrasses in shallow depths. Moreover, the use of predictive models would permit evaluation of the effects of coastal activities (construction of ports, artificial reefs, beach nutrient-input, dredging) on benthic ecosystems.
This paper reviews recent work on flow and transport in channels with submerged vegetation, including discussions of turbulence structure, mean velocity profiles, and dispersion. For submerged canopies of sufficient density, the dominant characteristic of the flow is the generation of a shear-layer at the top of the canopy. The shear-layer generates coherent vortices by Kelvin-Helmholtz (KH) instability. These vortices control the vertical exchange of mass and momentum, influencing both the mean velocity profile, as well as the turbulent diffusivity. For flexible canopies, the passage of the KH vortices generates a progressive wave along the canopy interface, termed monami. The KH vortices formed at the top of the canopy penetrate a distance δ
e into the canopy. This penetration scale segregates the canopy into an upper layer of rapid transport and a lower layer of slow transport. Flushing of the upper canopy is enhanced by the energetic shear-scale vortices. In the lower layer turbulence is limited to length-scales set by the stem geometry, and the resulting transport is significantly slower than that of the upper layer.
We present an overview of a large collaborative field campaign, in which we collected a long-term (months) high-resolution (4Hz measurement frequency) hydrodynamic data set for several locations at the mudflat–salt marsh ecosystem and linked this to data on sediment transport and to a biological description of the organisms on the mudflat and the marsh. In this paper, part of this database has been used to identify general relationships that can be used for making hydrodynamic characterisations of mudflat–salt marsh ecosystems. We observed a clear linear relation between tidal amplitude and the maximum current velocity, both at the mudflat as well as within the marsh vegetation. Velocities in the vegetation were however a magnitude lower than those on the mudflat. This relationship offers promising possibilities for making hydrodynamic habitat characterisations and for validating hydrodynamic models.
The effects of seagrass bed geometry on wave attenuation and suspended sediment transport were investigated using a modified
Nearshore Community Model (NearCoM). The model was enhanced to account for cohesive sediment erosion and deposition, sediment
transport, combined wave and current shear stresses, and seagrass effects on drag. Expressions for seagrass drag as a function
of seagrass shoot density and canopy height were derived from published flume studies of model vegetation. The predicted reduction
of volume flux for steady flow through a bed agreed reasonably well with a separate flume study. Predicted wave attenuation
qualitatively captured seasonal patterns observed in the field: wave attenuation peaked during the flowering season and decreased
as shoot density and canopy height decreased. Model scenarios with idealized bathymetries demonstrated that, when wave orbital
velocities and the seagrass canopy interact, increasing seagrass bed width in the direction of wave propagation results in
higher wave attenuation, and increasing incoming wave height results in higher relative wave attenuation. The model also predicted
lower skin friction, reduced erosion rates, and higher bottom sediment accumulation within and behind the bed. Reduced erosion
rates within seagrass beds have been reported, but reductions in stress behind the bed require further studies for verification.
Model results suggest that the mechanism of sediment trapping by seagrass beds is more complex than reduced erosion rates
alone; it also requires suspended sediment sources outside of the bed and horizontal transport into the bed.
Deterministic and probabilistic Profile models have been compared with hydrodynamic and morphodynamic data of laboratory and field experiments on the time scale of storms and seasons. The large-scale laboratory experiment is a pure 2D case and offers and ideal test case for cross-shore Profile models, as disturbing alongshore non-uniformities are absent. The field experiments are performed at the Egmond site (The Netherlands) during the EU-COAST3D project and represent storm time scale (Oct.–Nov. 1998) as well as seasonal time scale conditions (May 1998–Sep. 1999). The objective of the paper is to present information of coastal processes on these time scales and to assess the predictive capabilities Coastal Process-based Profile models with respect to hydrodynamics and morphodynamics at sandy beaches on the time scales of storms and seasons. Profile models can quite accurately (errors smaller than 10%) represent the cross-shore significant wave height distribution in the surf zone, if the wave breaking model is properly calibrated. The wave breaking coefficient should be a function of local wave steepness and bottom slope for most accurate results. Profile models can reasonably represent the cross-shore and longshore currents (undertow) in a pure 2D case and in 3D field conditions. Profile models including cross-shore mixing effects and breaker delay effects do not produce better predictions of the longshore and cross-shore current velocities. Profile models using default settings can quite reasonably simulate the behaviour of the outer and inner bars on the storm time scale; the behaviour of the beach cannot be modelled with sufficient accuracy on the storm time scale. Profile models can reasonably simulate the post-storm onshore bar migration, provided that the near-bed orbital velocities and wave asymmetry-related sand transport are represented in a sufficiently accurate way (using non-linear wave theories). Profile models cannot simulate the beach recovery processes on the post-storm time scale, because these essentially 3D processes are not sufficiently known to be included in the models.Profile models using default settings cannot simulate the behaviour of the outer and inner bars and the beach on the seasonal time scale; the behaviour of the outer bar on the seasonal time scale can only be represented properly after tuning using measured bed profiles. The simulation of the inner bar and beach morphology on the seasonal time scale could not be improved by tuning.
Underwater observations of eelgrass (Zostera marina L.) beds at the mouth of the Jordan River, Maine, USA, indicated that the eelgrass blades gently undulated with low-amplitude movements under low current speeds. When the above-canopy speeds exceeded 10 cm s−1, dramatic large-amplitude waving of many blades in synchrony occurred. The eelgrass waving caused wide variability in horizontal water current speeds measured above the canopy. During three summers (1986, 1989, 1990), the blade tips (distal 30 cm) had an average of about 3 × more recently settled (plantigrade) blue mussels (Mytilus edulis) compared to regions lower (30–90 cm) on the blade. Because waving of seagrass blades results in the blade tips moving through much more of the water column than lower regions of the blade and in enhanced turbulent mixing above the plant canopy, we hypothesize that such movements increase the likelihood of blade encounter with mussel larvae, and explain enhanced mussel abundances on blade tips. We further hypothesize that the enhanced mixing may direct larvae into seagrass beds generally. Large-amplitude, synchronous waving of terrestrial grasses has been termed ‘honami,’ (Japanese: ho = cereal; nami = wave) and has been shown to dramatically alter aerodynamical conditions within and above the grass canopy. We suggest that ‘monami’ (mo = aquatic plant) is important in coastal hydrodynamics and has major implications for larval settlement and recruitment.
The role of Posidonia oceanica in promoting sediment stability and accretion was studied in a 15 m deep meadow at Fanals Point (NW Mediterranean, Spain) by comparing particle deposition within the meadow and adjacent bare sediment. Small sediment traps were used to measure deposition within and above the meadow and over bare sand. A model, based on measurements of particle deposition at increasing distances from the bottom, was used to partition the total depositional flux between primary (sediment particles deposited for the first time at the measuring site) and resuspended deposition (sediment particles that have been previously deposited at the measuring site). Measurements were conducted monthly over a year to establish the magnitude and seasonality of deposition, and to form a balance of particle transport at the annual time scale. Significant differences in total deposition were found over time, ranging from 1·5 to 500 g DW m−2 d−1, including those between bare and vegetated sediments. The effect of P. oceanica in increasing primary deposition at an annual scale was modest, however, P. oceanica significantly buffered sediment resuspension, which was reduced more than three fold compared to the unvegetated bottom. The annual flux of deposition was dominated by settling of resuspended materials, which represented 85% of the total flux within the meadow, but 95% of the total deposition on bare sand. Thus, seagrass meadows reduce resuspension in the NW Mediterranean littoral, thereby contributing to increased sediment retention and, therefore, reducing erosion in the coastal zone.
Retention of particles in seagrass canopies is usually attributed to only the indirect, attenuating effects canopies have on flow, turbulence and wave action, promoting sedimentation and reducing resuspension within seagrass meadows. Yet recent evidence suggests that seagrasses are also able to affect particle flux directly through loss of momentum and increased path length derived from collisions with leaves and binding of particles. We evaluated the role of Posidonia oceanica on flow and associated particle trapping through flume experiments. Our results confirm the existence of 2 dynamically different environments, viz. (1) the below-canopy habitat, with low shear stress and reduced turbulence, and (2) the canopy-water interface region, characterized by high shear stress and turbulence intensity, where vertical transport of momentum is enhanced. At relatively low free stream velocities (i.e. 0.05 and 0.10 m s-1) sediment concentration decreased much faster when a Posidonia meadow was present in the flume, indicating major particle trapping in the seagrass canopy. Fluxes to the sediment (as shown by large negative peaks in Reynolds stress inside the Posidonia meadow) indicated 2 to 6 times more sediment transport to the bottom when a meadow is present. However, calculations based on the experimental results point to loss rates an order of magnitude larger in a Posidonia meadow. We hypothesize that direct effects of particle collisions with leaves are responsible for this discrepancy, and we explore possible interactions with a simple model. Using only collisions as a loss factor, the model predicts that the probability a particle is lost from the flow upon a collision is 2 to 3%. Previously observed leaf density and flow velocity effects on particle loss rates were explained by the model. Fitting the model to our experimentally obtained particle disappearance rates in vegetation indicated that around 27% of particle momentum is lost upon each collision with a leaf. We hypothesize that physical filtration by sediment collisions with plant structures plays a role in particle removal in aquatic systems.
A quasi-theoretical analysis has been given for the flow retardance in vegetated channels. Based on this, a logarithmic flow formula has been derived for flow in these channels. The analysis has been illustrated with flow data obtained from channels in which natural vegetation was present and from a laboratory flume having flexible artificial roughness elements fixed to the bottom.
This book addresses both fundamental and applied aspects of ocean waves
including the use of wave observations made from satellites. More
specifically it describes the WAM model, its scientific basis, its
actual implementation, and its many applications. The three sections of
the volume describe the basic statistical theory and the relevant
physical processes; the numerical model and its global and regional
applications; and satellite observations, their interpretation and use
in data assimilation.
An experimental study, conducted in the large wave flume of CIEM in Barcelona, is presented to evaluate the effects of Posidonia oceanica meadows on the wave height damping and on the wave induced velocities. The experiments were performed for irregular waves from intermediate to shallow waters with the dispersion parameter h/λ ranging from 0.09 to 0.29. Various configurations of the artificial P. oceanica meadow were tested for two stem density patterns (360 and 180 stems/m2) and for plant's height ranging from 1/3 to 1/2 of the water depth.The results for wave height attenuation are in good agreement with the analytical expressions found in literature, based on the assumption that the energy loss over the vegetated field is due to the drag forces. Based on this hypothesis, an empirical relationship for the drag coefficient related to the Reynolds number, Re, is proposed. The Reynolds number, calculated using the artificial P. oceanica leaf width as the length scale and the maximum orbital velocity over the meadow edge as the characteristic velocity scale, ranges from 1000 to 3500 and the drag coefficient Cd ranges from 0.75 to 2.0.The calculated wave heights, using the analytical expression from literature and the proposed relationship for the estimation of Cd, are in satisfactory agreement with those measured. Wave orbital velocities are shown to be significantly attenuated inside the meadow and just above the flume bed as indicated by the calculation of an attenuation parameter. Near the meadow edge, energy transfer is found in spectral wave velocities from the longer to the shorter wave period components. From the analysis it is shown that the submerged vegetation attenuates mostly longer waves.
The hydrodynamics of simulated patches of the Mediterranean seagrass Posidonia oceanica were studied in laboratory flume experiments in which the height of the pronated canopy was always greater than half the total water depth. The effects of variations in speed (from 0.08 m s 21 to 0.24 m s 21 ) and patch configuration on the hydrodynamics were investigated. Significant speeds penetrated the patches to approximately half their height. Reducing speed did not change the flow patterns observed, except to weaken and blur them. Flow encountering a single patch formed a turbulent wake at the height of the top of the canopy. Within this wake, the vertical shear stress decreased monotonically downstream, but the Reynolds stress increased initially and then decayed. When a second patch was positioned within the region where Reynolds stress increased (referred to as the ''06 patch''), the wake center penetrated it, causing average turbulent velocities with horizontal components 3.3 times higher and vertical components 4.2 times higher than in the upstream patch. When this patch was positioned where the Reynolds stress decayed (referred to as the ''14 patch''), the wake center rose above it. Nevertheless, the turbulence in the 14 patch had horizontal components 12% higher and vertical components 22% higher on average than in the 06 patch because its upstream end was closer to the Reynolds stress maximum. Thus the ratio of the patch separation to the length of wake in which the Reynolds stress increased was identified as central to quantifying the turbulence within the downstream patch. The increased turbulence is likely to be important in determining sedimentary and ecological patch characteristics by increasing retention of particulates in suspension and thus reducing depositional rates of, for example, larvae, nutrients, and dead organic matter.
The distribution of sea level in the Mediterranean Sea is recovered for the period 1945–2000 by using a reduced space optimal interpolation analysis. The method involves estimating empirical orthogonal functions from satellite altimeter data spanning the period 1993–2005 that are then combined with tide gauge data to recover sea level fields over the period 1945–2000. The reconstruction technique is discussed and its robustness is checked through different tests. For the altimetric period (1993–2000) the prediction skill is quantified over the whole domain by comparing the reconstructed fields with satellite altimeter observations. For past times the skill can only be tested locally, by validating the reconstruction against independent tide gauge records. The reconstructed distribution of sea level trends for the period 1945–2000 shows a positive peak in the Ionian Sea (up to 1.5 mm yr−1) and a negative peak of −0.5 mm yr−1 in a small area to the south-east of Crete. Positive trends are found nearly everywhere, being larger in the western Mediterranean (between 0.5 and 1 mm yr−1) than in the eastern Mediterranean (between 0 and 0.5 mm yr−1). The estimated rate of mean sea level rise for the period 1945–2000 is 0.7±0.2 mm yr−1, i.e. about a half of the rate estimated for global mean sea level. These overall results do not appear to be very sensitive to the distribution of tide gauges. The poorest results are obtained in open-sea regions with intense mesoscale variability not correlated with any tide gauge station, such as the Algerian Basin.
The ability of turbulence models, based on two equation closure schemes (the k-ε and the k-ω formulations) to compute the mean flow and turbulence structure in open channels with rigid, nonemergent vegetation is analyzed. The procedure, developed by Raupach and Shaw (1982), for atmospheric flows over plant canopies is used to transform the 3D problem into a more tractable 1D framework by averaging the conservation laws over space and time. With this methodology, form/drag related terms arise as a consequence of the averaging procedure, and do not need to be introduced artificially in the governing equations. This approach resolves the apparent ambiguity in previously reported values of the drag-related weighting coefficients in the equations for the turbulent kinetic energy and dissipation rates. The working hypothesis for the numerical models is that the flux gradient approximation applies to spatial/temporal averaged conservation laws, so that the eddy viscosity concept can be used. Numerical results are compared against experimental observations, including mean velocities, turbulence intensities, Reynolds stresses, and different terms in the turbulent kinetic energy budget. The models are used to further estimate vegetation-induced flow resistance. In agreement with field observations, Manning's coefficient is almost uniform for some critical plant density and then increases linearly.
a b s t r a c t Seagrasses develop extensive or patchy underwater meadows in coastal areas around the world, forming complex, highly productive ecosystems. Seagrass canopies exert strong effects on water flow inside and around them, thereby affecting flow structure, sediment transport and benthic ecology. The influence of Zostera marina canopies on flow velocity, turbulence, hydraulic roughness and sediment movement was evaluated through laboratory experiments in 2 flumes and using live Z. marina and a mobile sand bed. Profiles of instantaneous velocities were measured and sediment movement was identified upstream, within and downstream of patches of different sizes and shoot density and at different free-stream velocities. Flow structure was characterised by time-averaged velocity, turbulence intensity and Turbulent Kinetic Energy (TKE). When velocity data were available above the canopy, they were fitted to the Law of the Wall and shear velocities and roughness lengths were calculated. When a seagrass canopy was present, three layers were distinguishable in the water column: (1) within canopy represented by low velocities and high turbulence; (2) transition zone around the height of the canopy, where velocities increased, turbulence decreased and TKE was high; and (3) above canopy where velocities were equal or higher than free-stream velocities and turbulence and TKE were lower than below. Shoot density and patch-width influenced this partitioning of the flow when the canopy was long enough (based on flume experiments, at least more than 1 m-long). The enhanced TKE observed at the canopy/water interface suggests that large-scale turbulence is generated at the canopy surface. These oscillations, likely to be related to the canopy undulations, are then broken down within the canopy and high-frequency turbulence takes place near the bed. This turbulence 'cascade' through the canopy may have an important impact on biogeochemical processes. The velocity above the canopy generally followed a logarithmic profile. Roughness lengths were higher above the canopy than over bare sand and increased with increasing distance from the leading edge of the canopy; however, they were still small (o 1 cm) compared to other studies in the literature. Within and downstream of the canopy, sediment movement was observed at velocities below the threshold of motion. It was likely caused by the increased turbulence at those positions. This has large implications for sediment transport in coastal zones where seagrass beds develop.
Biologically mediated modifications of the abiotic environment, also called ecosystem engineering, can significantly affect a broad range of ecosystems. Nevertheless, remarkably little work has focused on the costs and benefits that ecosystem engineers obtain from traits that underlie their ecosystem engineering capacity. We addressed this topic by comparing two autogenic engineers, which vary in the degree in which they affect their abiotic environment via their physical structure. That is, we compared two plant species from the intertidal coastal zone (Spartina anglica and Zostera noltii), whose shoots are exposed to similar currents and waves, but differ in the extent that they modify their environment via reduction of hydrodynamic energy. Our results indicate that there can be trade-offs related to the traits that underlies autogenic ecosystem engineering capacity. Dissipation of hydrodynamic forces from waves was roughly a factor of three higher in vegetation with stiff leaves compared to those with flexible leaves. Drag was highest and most sensitive to hydrodynamic forces in stiff vegetation that does not bend with the flow. Thus, shoot stiffness determines both the capacity to reduce hydrodynamic energy (i.e., proxy for ecosystem engineering capacity) and the drag that needs to be resisted (i.e., proxy for associated costs). Our study underlines the importance of insight in the trade-offs involved in ecosystem engineering as a first step toward understanding the adaptive nature of ecosystem engineering.
Rapid warming of the Mediterranean Sea threatens marine biodiversity, particularly key ecosystems already stressed by other impacts such as Posidonia oceanica meadows. A 6-year monitoring of seawater temperature and annual P. oceanica shoot demography at Cabrera Archipelago National Park (Balearic Islands, Western Mediterranean) allowed us to determine if warming influenced shoot mortality and recruitment rates of seagrasses growing in relative pristine environments. The average annual maximum temperature for 2002–2006 was 1 °C above temperatures recorded in 1988–1999 (26.6 °C), two heat waves impacted the region (with seawater warming up to 28.83 °C in 2003 and to 28.54 °C in 2006) and the cumulative temperature anomaly, above the 1988–1999 mean annual maximum temperature, during the growing season (i.e. degree-days) ranged between 0 °C in 2002 and 70 °C in 2003. Median annual P. oceanica shoot mortality rates varied from 0.067 year−1 in 2002 to 0.123 year−1 in 2003, and exceeded recruitment rates in all stations and years except in shallow stations for year 2004. Interannual fluctuations in shoot recruitment were independent of seawater warming (P>0.05). P. oceanica meadows experienced a decline throughout the study period at an average rate of −0.050±0.020 year−1. Interannual variability in P. oceanica shoot mortality was coupled (R2>0.40) to seawater warming variability and increasing water depth: shoot mortality rates increased by 0.022 year−1 (i.e. an additional 2% year−1) for each additional degree of annual maximum temperature and by 0.001 year−1 (i.e. 0.1% year−1) for each accumulated degree water temperature remained above 26.6 °C during the growing season. These results demonstrate that P. oceanica meadows are highly vulnerable to warming, which can induce steep declines in shoot abundance as well indicating that climate change poses a significant threat to this important habitat.
Seagrass beds and the communities they form are well known for their ability to alter their local hydrodynamic environment, reducing current velocities and altering turbulent structure in and around the canopy. Much of the quantitative information that has been published on the interaction of seagrass canopies with flowing water has been derived from laboratory flume studies. The few studies that have been conducted all point to similar patterns of flow alteration around the seagrass canopy. Differences among the results of the study are likely primarily due to different experimental configurations. Some studies have used seagrass beds much narrower than the width of the flume while others have used seagrass beds extending the full width of the flume. The validity of the latter design has often been called into question because of scaling issues. In this study, artificial seagrass was used to examine the effects of bed width in a laboratory flume on the spatial pattern of water velocity and turbulence intensity within the bed. As seagrass bed width was increased, blocking more of the cross-sectional area of the flume, the seagrass became less effective at reducing within-canopy current velocities while over-canopy flow was increased. Narrow patches (0.3 m in a flume, 1.0 m wide) were significantly more effective at reducing current velocity within the canopy than were wider patches, but experienced higher turbulence intensity. Using laboratory findings from experiments to predict field flow conditions when patch geometry differs substantially from that of a flume may either over- or under-estimate flow reduction and turbulence intensity. This is particularly the case within the first meter of horizontal distance as flow enters the canopy. Therefore, flume conditions where the bed width equals the flume width may be more appropriate for mimicking flow interaction with broad and shallow seagrass beds. Use of bed widths narrower than the flume width are likely more accurate for modeling small, developmentally arrested patches, or recently established patches such as those arising from restoration projects.
The radiation stresses in water waves play an important role in a variety of oceanographic phenomena, for example in the change in mean sea level due to storm waves (wave “set-up”); the generation of “surf-beats”; the interaction of waves with steady currents; and the steepening of short gravity waves on the crests of longer waves. In previous papers these effects have been discussed rigorously by detailed perturbation analysis. In the present paper a simplified exposition is given of the radiation stresses and some of their consequencies. Physical reasoning, though less rigorous, is used wherever possible. The influence of capillarity on the radiation stresses is fully described for the first time.
Seagrass beds have traditionally been considered to act as sinks for particles due to the reduction of flow velocities by the plant canopy. Yet, there is a paucity of measurements to confirm this role. In this work we illustrate changes in flow in the presence and absence of Posidonia oceanica using an ADV, and provide direct measures of particle trapping by the use of sediment traps. We also describe a model to estimate sediment resuspension after measuring particle flux at different distances from the bottom. Measurements of particle flux are conducted parallel to the study of structural parameters of the Posidonia meadow potentially involved in both particle trapping and retention. Data obtained on velocity profiles confirm previous findings that seagrass canopies slow down current velocities with intensities proportional to the canopy height of the plants. The projected surface area of the plants (LAI) significantly correlated with the total amount of particles trapped within the Posidonia meadow, thus indicating seagrass canopy slightly increased particle trapping in the absence of resuspension. The trapping capacity of the canopy was not linearly correlated to LAI but significantly decreased at LAI above four, thus suggesting that other factors such as bending of the leaves and particle attachment to the surface may interfere with particle free sinking within the canopy at high projected surface area. The model proposed to estimate resuspension allowed to measure the retention capacity of the P. oceanica meadow, this being up to 15 times higher compared to a barren bottom during situations of high energy (large eddies reaching the bottom). The results obtained provide direct quantitative support to seagrass beds promoting sediment accretion and demonstrate a promising avenue to provide the needed empirical support for the effect of seagrasses on depositional processes.
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