Publications in Journals

Past and future marsh adaptation: lessons learned from the Ria Formosa lagoon

Carrasco A.R., Kombiadou K., Amado M., Matias A. (2021).
Science of the Total Environment, 148082

Abstract

It is rational to assume that salt marshes in regions where sediment loads are high should remain stable or prograde when facing a range of sea-level scenarios, whereas those in sediment-poor systems may erode or drown. Despite extensive theoretical and laboratory studies, additional marsh ‘persistence’ indicators under human pressures and accelerated sea-level rise rates are still needed. This study investigates the recent lateral and volumetric changes occurring in the lagoon marshes of the Ria Formosa lagoon (south Portugal), under human pressures and sea-level rise. Our analysis assesses the past (1947-2014) geomorphological evolution of marshes based on aerial imagery analysis and estimates its potential future adjustment to sea-level rise (~100 years) based on SLAMM (landscape-based model) simulated land cover changes. We highlight the influence of both stressors on marsh ecosystems and examine how their interactions can contribute to understanding sea-level rise impacts and ecological resilience of lagoon marshes. Salt marshes in the Ria Formosa have slowly expanded over the last 70 years (~0.2 mm/yr), with local erosion in front of tidal inlets and along the main navigable channels, associated with inlet migration and dredging activities. Past evolution shows that the ecosystem was able to maintain its functions and cope with sea-level rise. However, future marsh trajectories under a high sea-level rise rate suggest unbalanced vertical marsh accretion and progressive migration of the tidal flat (and water bodies) towards the salt marsh area. The model results show evidence of non-linearity in marsh response to high sea-level rise rates, which could indicate the presence of a system tipping-point and potential positive (disturbance-reinforcing) feedbacks within the system, with significant implications to marsh resilience.

Simulating Destructive and Constructive Morphodynamic Processes in Steep Beaches

Kombiadou K., Costas S., Roelvink D. (2021).
Journal of Marine Science and Engineering, 9(1):86

Abstract

Short-term beach morphodynamics are typically modelled solely through storm-induced erosion, disregarding post-storm recovery. Yet, the full cycle of beach profile response is critical to simulating and understanding morphodynamics over longer temporal scales. The XBeach model is calibrated using topographic profiles from a reflective beach (Faro Beach, in S. Portugal) during and after the incidence of a fierce storm (Emma) that impacted the area in early 2018. Recovery in all three profiles showed rapid steepening of the beachface and significant recovery of eroded volumes (68–92%) within 45 days after the storm, while berm heights reached 4.5–5 m. Two calibration parameters were used (facua and bermslope), considering two sets of values, one for erosive (Hm0 ≥ 3 m) and one for accretive (Hm0 < 3 m) conditions. A correction of the runup height underestimation by the model in surfbeat mode was necessary to reproduce the measured berm elevation and morphology during recovery. Simulated profiles effectively capture storm erosion, but also berm growth and gradual recovery of the profiles, showing good skill in all three profiles and recovery phases. These experiments will be the basis to formulate event-scale simulations using schematized wave forcing that will allow to calibrate the model for longer-term changes.

Exploring foredune growth capacity in a coarse sandy beach

Costas, S., Bom de Sousa, L., Kombiadou, K., Ferreira, Ó., Plomaritis, T.A. (2020).
Geomorphology, 107435, DOI: 10.1016/j.geomorph.2020.107435

Abstract

Dunes are key elements of coastal landscapes in almost every latitude. They host high levels of biodiversity and provide important benefits to society; e.g. protection against floods and erosion, or recreation. Coastal dune growth is constrained by intrinsic factors, which are critical when managing dune systems or choosing coastal dune restoration as an alternative green solution for coastal protection. Here, the evolution of a beach-dune system, characterized by a reflective coarse sandy beach and low dunes, is explored to identify the favourable and optimal conditions for dune growth in these settings. Dune growth capacity is evaluated by analysing the topographical changes observed along a coastal dune over two different temporal scales (interannual and event scale) and comparing the observations with theoretical approximations of sediment transport potentials. Observations and predictions over interannual scale document that (1) temporal variability in external conditions (wind regime) and spatial variability of estimated wind fetch length, alone, fail to explain alongshore dune growth patterns and (2) optimal conditions for dune growth occur when storms (strong winds) impact the study area, jointly with low runup levels, at zones of shoreline progradation and absence of direct human influence. Conversely, lowest values of dune accumulation are associated with areas where shoreline retreat was documented. Observations from event timescales suggest that sediment transport potential can be reached over zones with no significant signs of beach erosion, if runup levels remain low and the event duration is shorter than the time scale of sand surface depletion within the upper beach.

Barrier island resilience assessment: Applying the ecological principles to geomorphological data

Kombiadou, K., Matias, A., Costas, S., Rita Carrasco, A., Plomaritis, T. A., & Ferreira, Ó. (2020).
CATENA, 194, 104755, DOI: 10.1016/j.catena.2020.104755

Abstract

Applying the ecological resilience principles to barrier island geomorphological evolution requires approaches that perceive and interpret resilience far from predefined barrier characteristics and static views. Accepting that barrier islands, like all natural systems, are dynamic and adaptively changing in response to external disturbances is fundamental to the formulated approach. To this aim, geomorphological units and dimensions were used to describe barrier island stability landscape as an actively shifting ‘topography’, reshaping in response to exogenous events and in relation to intrinsic properties. The structure of the subaerial barrier was characterised using the environmental units of Beach, Dune and Marsh (or BDM), where different combinations of BDM structure define distinct barrier stable states, under a simplified framework that is applicable over a wide range of barrier structures. The methodology is based on reconstructing resilience trajectories of barrier islands through identifying the distinct BDM states and related shifts (thresholds crossed) and assessing resilience dimensions (latitude, resistance and precariousness defined as barrier width and height and proximity to mainland, respectively) that, jointly, define the shape of the stability domain. The approach was applied to the Ria Formosa barrier island system (S. Portugal), using multi-decadal geomorphological data and gradually decreasing spatial discretisation, passing from individual barrier transects to sectors and to entire barriers. The joint evolution of two inland-migrating barriers (Cabanas Island and Cacela Peninsula) was used as an exemplar of adaptive capacity in barrier geomorphic change and, therefore, of resilient response to external pressures. Resilience pathways showed that the Ria Formosa barriers have been resilient over the studied timeframe, with a tendency for maintaining or increasing BDM structural complexity. In general, the stability domain tends to shift from low latitude and high resistance forms (narrow-deep basins of attraction) in the west part of the barrier chain, to higher latitudes and lower resistance ones (wide-shallow basins) towards the east. Precariousness peaks near the edges of the system (low lagoon width) and minimises towards the central part (most detached barriers). Scaling issues regarding smoothing of longshore variability and potential consequences on masking thresholds and critical dimensions are highlighted and discussed, along with the key role of the meaning of specified resilience (of what?) in the assessment. The methodology is a novel approach, easily transferable to different systems and spatiotemporal scales of analysis, representing a step forward in interpreting and assessing barrier island resilience.

Rethinking suspended mussel-farming modelling: Combining hydrodynamic and bio-economic models to support integrated aquaculture management

Konstantinou, Z.I., Kombiadou, K. (2020)
Aquaculture, DOI: 10.1016/j.aquaculture.2020.735179

Abstract

Suspended mussel-farming using cultivation socks, either in long-line or traditional pole cultivation structures, is a widely spread activity around the world. The sustainability and efficiency of the activity depends on a number of interrelated parameters, as is the natural availability of food, the physical and ecological characteristics of the coastal or marine area and the farmers' choices. The natural parameter which influences the activity the most and which can be a major controlling parameter for integrated management is adequate and uninhibited water circulation. The individual characteristics of the farming structures, as well as their spatial placing, can influence circulation, thus affecting food distribution, growth rates, productivity and profits of the mussel-farming activity. To assess this influence in an integrated manner, it is necessary to work in different levels of analysis and transdisciplinary domains, utilising multiple simulation models and taking into consideration the particular characteristics of the activity in different areas. This paper aims to demonstrate such an integrated management approach through: i) the high resolution assessment of the effect of farming structures on circulation, using a novel parameterisation of the cultivation socks as porous media; ii) the calibration of a traditional drag coefficient based on the results of the high resolution experiments, allowing a lower resolution analysis, necessary to implement assessments in cultivation area level and iii) the analysis and evaluation of alternative spatial planning configurations, both regarding hydrodynamics and bio-economic characteristics of the mussel-farming activity. The case study, used to demonstrate the methodology, was Thermaikos Gulf, in Northern Greece, the area producing more than 80% of the national mussel production and which has been facing critical management problems for more than 20 years. The integrated modelling approach resulted in the successful use of porosity for high resolution modelling of suspended mussel-farming in socks and in the multi-level understanding of how the operational characteristics of the activity can influence its sustainability. Additionally, the results shed light in some of the most important problems of the Greek mussel-farming sector. Overall the approach demonstrates that integrated, social-ecological management of productive activities, like aquaculture, require the combination of multiple and transdisciplinary levels of analysis, the development of tailor-made modelling approaches and some outside-the-box thinking, to overcome difficulties related to the availability of information.

Coastal ridge constructive processes at a multi‐decadal scale in Barreta Island (Southern Portugal)

Herrero, X., Costas, S., Kombiadou, K. (2020)
Earth Processes and Landforms, 45(2), pp.411-423

Abstract

Multiple ridges across prograding coasts may display variable geometries, commonly expressed through varying elevations. Changes in ridge elevation have been traditionally related to the occurrence of fluctuating progradation rates, which might, in turn, be driven by shifting environmental conditions.

Here, we explore the geometry and growth mechanisms of multiple ridges, generated at Barreta Island (Ria Formosa, Southern Portugal), as a consequence of the rapid progradation of the island over the last 70 years, following the artificial fixation of the downdrift Faro‐Olhão inlet with jetties in 1955. The variability in the morphology of these features was analysed in combination with available wind and wave data, in order to better distinguish growth mechanisms and understand the main parameters determining the final geometry of the observed ridges.

The results suggest that (1) most of the identified ridges fall in the beach ridge classification, as they have been mostly built by marine processes, and (2) the parameters derived from, or closely related to wave climate variability (e.g. progradation rates, storm occurrence) can jointly explain most of the observed morphological changes, while aeolian processes played a secondary role. Indeed, ridge geometry appears mainly controlled by progradation rates, with higher ridges associated with lower progradation rates. Progradation rate, in turn, is mostly related to longshore wave power, storminess, and the occurrence storm groups. Yet, the final configuration of ridges can also be affected by runup levels and onshore winds. Therefore, establishing the relation between ridge geometry and wave climate is not a straightforward task, because of the complex processes and interactions that control coastal morphodynamics.

Bridging the gap between resilience and geomorphology of complex coastal systems

Kombiadou, K., Costas, S., Carrasco, A.R., Plomaritis, T.A., Ferreira, Ó., Matias, A. (2019)
Earth-Science Reviews, 198, 102934

Abstract

Resilience has been used over a wide range of scientific fields and often ambiguously, causing confusion over terminology and concepts and giving rise to distinct interpretations and misconceptions, even within the same scientific discipline. Starting by providing clarifications and definitions of the main terminology and key principles of ecological resilience theory, we pass on to expressing them through geomorphic dimensions of barrier islands. Three distinct environments (beach, dune, marsh) are proposed as the panarchical levels of analysis, along with potential feedbacks between them and geomorphic dimensions that can express the changes of the stability landscape. Morphological changes induced by storms and subsequent recovery are transferred to stability landscapes, over a range of storm impacts and recovery. We postulate that post-perturbation recovery should not be restricted to regaining pre-disturbance barrier dimensions, but should be viewed in terms of reorganisation and adaptation, accounting for maintaining the existence of functions, or the ability of the system to regain them. The proposed scheme and dimensions are tested using geomorphological data from barrier response to distinct disturbances, over different temporal scales that range from event to multi-decadal ones. The case of a barrier island migrating landwards is conceptualised in terms of alternative states and thresholds arising during the process and related phases and changes to the adaptive cycle. The methodology and approach presented is a step towards more holistic views of geomorphic systems' resilience that we hope will contribute to furthering interdisciplinary understanding and cooperation in the area of sustainability and resilience of natural systems.

Impacts of human interventions on the evolution of the Ria Formosa barrier island system (S. Portugal)

Kombiadou, K., Matias, A., Ferreira, Ó., Carrasco, A.R., Costas, S., Plomaritis, T. (2019)
Geomorphology, 343, 129-144

Abstract

Human interventions on sandy barriers disturb natural barrier dynamics, to the extent of having become key forces in modifying geomorphological evolution. This work identifies natural and human-induced drivers and analyses their importance to the multi-decadal evolution of the Ria Formosa barrier island system, in South Portugal. Aerial photographs from the last six decades and historical maps are used to assess changes in cross-shore rates, morphological characteristics (barrier and dune widths, inlet morphology and migration) and barrier areas, through systematic methods that can be easily transferred to other barrier systems. Interventions, and especially hard engineering ones (jetties, inlet stabilisations), affected barrier evolution trends. Shore-perpendicular works increased shoreline progradation updrift and initiated coastal retreat downdrift, with strongest erosive impacts along the edges of the system. Inlet stabilisations changed tidal inlet hydrodynamics and initiated ebb-shoal attachment to the barriers on either side of a non-migrating inlet that experienced loss of tidal prism. This shoal attachment was the main factor for the increase in total barrier area of Ria Formosa during the 60 years of analysis. Barrier growth after 2005 was slower, which could indicate that the system is reaching morphodynamic stability.

Climate change effects on the marine characteristics of the Aegean and Ionian Seas

Makris, C., Galiatsatou, P., Tolika, K., Anagnostopoulou, C., Kombiadou, K., Prinos, P., … Rusu, E. (2016)
Ocean Dynamics, Vol. 66(12), pp. 1603–1635

Abstract

This paper addresses the effects of estimated climate change on the sea-surface dynamics of the Aegean and Ionian Seas (AIS). The main aim is the identification of climate change impacts on the severity and frequency of extreme storm surges and waves in areas of the AIS prone to flooding. An attempt is made to define design levels for future research on coastal protection in Greece. Extreme value analysis is implemented through a nonstationary generalized extreme value distribution function, incorporating time harmonics in its parameters, by means of statistically defined criteria. A 50-year time span analysis is adopted and changes of means and extremes are determined. A Regional Climate Model (RegCM3) is implemented with dynamical downscaling, forced by ECHAM5 fields under 20C3M historical data for the twentieth century and the SRES-A1B scenario for the twenty-first century. Storm surge and wave models (GreCSSM and SWAN, respectively) are used for marine climate simulations. Comparisons of model results with reanalysis and field data of atmospheric and hydrodynamic characteristics, respectively, are in good agreement. Our findings indicate that the dynamically downscaled RegCM3 simulation adequately reproduces the present general circulation patterns over the Mediterranean and Greece. Future changes in sea level pressure and mean wind fields are estimated to be small, yet significant for marine extremes. In general, we estimate a projected intensification of severe wave and storm surge events during the first half of the twenty-first century and a subsequent storminess attenuation leading to the resettlement of milder extreme marine events with increased prediction uncertainty in the second half of the twenty-first century.

Storm surges in the Mediterranean Sea: variability and trends under future climatic conditions

Androulidakis Y.S., Kombiadou K.D., Makris C.V., Baltikas V.N., Krestenitis Y.N. (2015)
Dynamics of Atmospheres and Oceans, Vol. 71, pp. 56-82

Abstract

The trends of storm surge extremes in the Mediterranean Sea for a period of 150 years (1951–2100) are explored, using a high-resolution storm surge model. Numerical simulations are forced by the output of regional climate simulations with RegCM3, which uses IPCC’s historical data on greenhouse gasses emissions for the (past) period 1951–2000, and IPCC’s A1B climate scenario for the (future) period 2001–2100. Comparisons between observations and modeling results show good agreement and confirm the ability of our model to estimate the response of the sea surface to future climatic conditions. We investigate the future trends, the variability and frequency of local extremes and the main forcing mechanisms that can induce strong surges in the Mediterranean region. Our results support that there is a general decreasing trend in storminess under the considered climate scenario, mostly related to the frequency of local peaks and the duration and spatial coverage of the storm surges. The northward shift in the location of storm tracks is a possible reason for this storminess attenuation, especially over areas where the main driving factor of extreme events is the inverted barometer effect. However, the magnitudes of sea surface elevation extremes may increase in several Mediterranean sub-regions, i.e., Southern Adriatic, Balearic and Tyrrhenian Seas, during the 21st century. There are clear distinctions in the contributions of winds and pressure fields to the sea level height for various regions of the Mediterranean Sea, as well as on the seasonal variability of extreme values; the Aegean and Adriatic Seas are characteristic examples, where high surges are predicted to be mainly induced by low pressure systems and favorable winds, respectively.

Effective mussel-farming governance in Greece: Testing the guidelines through models, to evaluate sustainable management alternatives

Konstantinou Z.I., Kombiadou K. and Krestenitis (2015)
Ocean & Coastal Management, Vol.118B, pp. 247-258

Abstract

The last twenty five years multiple management issues concerning the suspended farming of filter feeding bivalves in Greece have emerged from the deficient implementation of different legislation regimes, that, on their own account, have been proven insufficient. Specifically, regarding the planning and management of cultivation areas, multiple alternatives have been proposed through the years, with some still being under consideration. Yet most of them were selected based on purely spatial criteria and none has been thoroughly investigated, regarding the system's carrying capacity. As the current governance structure, in combination with the previous and current legal framework, seems unable to solve the existing problems, a question arises: are the “legal”solutions the optimum –or even among the best-regarding the environmental and socio-economic sustainability of the coastal aquaculture in the areas under consideration? In this paper we attempt a discussion between science and management, regarding the 3 main mussel-farming areas of Thermaikos gulf, Greece: Chalastra, Imathia and Pieria. The tools that were designed in the context of integration between science and policy are used in order to investigate the appropriate structure and flexibility of the legal framework controlling the activity and the governance level in which this control should take place. Previous and ongoing research results from combined management models, implemented or developed, for the mussel-farming areas of Thermaikos gulf are the starting point of this discussion. The goal is to detect “weak spots” of the current legal framework and to investigate if and how scientific management tools can assist in the development of more flexible, yet more effective environmental legislation.

Modelling the effects of Zostera noltei meadows on sediment dynamics: application to the Arcachon lagoon

Kombiadou K., Ganthy F., Verney R., Plus M. and Sottolichio A. (2014)
Ocean Dynamics, Vol. 64(10), pp. 1499–1516

Abstract

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.

Interannual Variability of the Physical Characteristics of North Thermaikos Gulf (NW Aegean Sea)

Krestenitis Y.N., Kombiadou K.D. and Androulidakis Y.S. (2012)
Journal of Marine Systems, Vol. 96–97, pp. 132–151

Abstract

Thermaikos Gulf is a marine ecosystem of major importance, not only environmental, but also due to the various socioeconomic activities associated with the area. The physical characteristics of the gulf’s waters were studied, analyzing in-situ measurements of oceanographic parameters, collected during 5 oceanographic surveys from 1994 to 2007, on the same grid of 26 sampling stations. Aim of this paper is the detection and description of the main changes (seasonal and interannual) in the water masses’ characteristics that are related to the seawater quality of the North Thermaikos. The connection between the main forcing factors and the major circulation patterns is also under investigation. The interannual analysis of the collected data revealed the existence of strong seasonal fluctuations that present significant deviations from a mean seasonal pattern in specific periods. A general decreasing trend of the salinities of the domain is observed during the study period. At the same time, a strong relation between open Aegean Sea waters and riverine freshwater fluxes is identified, factors that significantly influence stratification, circulation and renewal of the gulf. Based on the thermohaline properties, two dense water formation events (February 2000 and 2005), not previously reported, are detected and analyzed for the first time.

Fine sediment transport model for river influenced microtidal shelf seas with application to the Thermaikos Gulf (NW. Aegean Sea)

Kombiadou K.D. and Krestenitis Y.N. (2012)
Continental Shelf Research, Vol. 36, pp. 41-62

Abstract

Modelling the transport of fine sediments in the marine environment is a highly complex task with a broad field of application. Various processes take place and interact with one another to determine the movement of fine sedimentary particles in the aquatic domain. Amongst these processes stratification significantly affects the vertical propagation of a sedimentary plume, especially in areas of freshwater influence, with the possibility of matter entrapment and accumulation along the interface; this possibility is higher the finer the sedimentary matter is. The proposed modelling approach describes the physical processes that take place in the water column (advection, dispersion, coagulation, settling, effects of stratification), at the boundary of the seabed (benthic shear) and the mass-exchanges between the two (deposition, consolidation, resuspension, erosion). The Fine Sediment Transport Model was applied in the gulf of Thermaikos (NW Aegean Sea), an area of significant freshwater and riverine sediment inflow, considering the major rivers of the domain as sources of particulate matter. The results were, generally, in agreement with available in-situ measurements of suspended particulate matter. In cases of significant deviations between simulation and measurement, the source of the error did not lie within the modelling approach, but was due to the considered input parameters. The investigation revealed the existence of transport and sedimentation patterns related to the riverine source of the material and the individual parts of the gulf. The sedimentation rates were estimated at 18, 7 and 3 mm/y locally at the outflows of Aliakmon, Axios and Pinios respectively, while the areas of high sediment accumulation coincided with locations where the benthic material ranges from very fine to fine silt.

Simulating the fate of mechanically eroded masses in the Thermaikos Gulf

Kombiadou K.D. and Krestenitis Y.N. (2011)
Continental Shelf Research, Vol 31(7-8), pp.817-831

Abstract

Benthic trawlers, through the scraping of the upper layers of the seabed sediment, are responsible for major alterations to the distribution of suspended matter in coastal areas. A major trawling site in Greece is that of the gulf of Thermaikos, located in the NW Aegean Sea (Eastern Mediterranean). In an attempt to assess, for the first time, the quantities of the mobilized sedimentary masses and the properties of their motion in the aquatic domain of the gulf of Thermaikos, a mathematical model has been formulated. The modeling approach includes the determination of the mechanical erosion rate, the production of turbulence due to the flow through the mesh of the net and the physical processes that control the propagation of the generated sedimentary plume in the marine environment. In the absence of information regarding the haul paths in the gulf, their determination was made stochastically, based on the existing legislative restrictions and trawlers’ fishing practices. The trawling-activated masses thereby defined for the duration of the trawling period in the gulf were very high, indeed several times greater than the related contribution from the major rivers of the area, emphasizing the significance of the activity to the sedimentary input of the local marine environment. The total annual erosion by trawls in the gulf was calculated at 3.38·106t/y. Τhe mean mechanical erosion rate per trawled surface (parameter free from the bias raised by the stochastic simulation of the trawl trajectories) was estimated at 430gr/m2, values directly comparable to ones defined for similar coastal areas. Results from a simulation of the 2001-02 trawling period were well correlated with in-situ measurements for the same period, while the investigation of the matter’s suspension level and times revealed that the matter exhibits low residence times in the water column and at the same time moves in the vicinity of the bed, contributing to the formation and distribution of benthic nepheloid layers in the gulf.

Mathematical investigation of the applicability of benthic ascidians as biological filters in coastal areas

Kombiadou K.D., Krestenitis Y.N., Antoniadou C. and Chintiroglou C. (2010)
Journal of Marine Environmental Engineering, Vol. 9(2), pp. 85–97

Abstract

The effect of filter-feeding organisms to the rearrangement of particulate matter in the hyperbenthic parts of the water column in marine ecosystems is significant, as their positive contribution in eutrophication control and lucidity improvement has been well established. A suspended sediment three-dimensional transport-dispersion model, based on the tracer method, has been employed for the investigation of the ability of benthic ascidians to remove suspended matter from the water column. The model, apart from the various physical processes that control the transport and alterations to characteristic properties of a sedimentary plume, describes the action of benthic ascidians as mass-loss terms of particles that reenter the flow after filtration by the benthic organisms. Both laboratory and in-situ data have been utilized for the formulation of the model that has been applied in the southeastern part of the Thermaikos Gulf (NW Aegean sea), area in which dense populations of the ascidian Styela plicata occur. Four simulations have been performed to establish the level of particle removal from suspension due to ascidians, each considering a different benthic population. The results of the simulations have indicated high percentages of filtered mass by ascidians and, correspondingly, reduction of the mass in suspension at satisfactory levels, supporting the perspective that benthic ascidians can be applied as biological filters for the improvement of the quality of coastal waters.

Modelling the cohesive sediment transport in the marine environment: the case of Thermaikos Gulf

Y. N. Krestenitis, K. D. Kombiadou and Y. G. Savvidis (2007)
Ocean Science, Vol. 3, pp. 91–104

Abstract

The transport of fine-grained sediments in the marine environment entails risks of pollutant intrusions from substances absorbed onto the cohesive flocks’ surface, gradually released to the aquatic field. These substances include nutrients such as nitrate, phosphate and silicate compounds from drainage from fertilization of adjacent cultivated areas that enter the coastal areas through rivers and streams, or trace metals as remainders from urban and industrial activities. As a consequence, knowledge on the motion and distribution of sediment particles coming from a given pollutant source is expected to provide the 'bulk' information on pollutant distribution, necessary for determining the region of influence of the source and to estimate probable trophic levels of the seawater and potential environmental risks. In that aim a numerical model has been developed to predict the fate of the sediments introduced to the marine environment from different pollution sources, such as river outflows, erosion of the seabed, aeolian transported material and drainage systems.
The proposed three-dimensional mathematical model is based on the particle tracking method, according to which matter concentration is expressed by particles, each representing a particular amount of sedimentary mass, passively advected and dispersed by the currents. The processes affecting characteristics and propagation of sedimentary material in the marine environment, incorporated in the parameterization, apart from advection and dispersion, include cohesive sediment and near-bed processes. The movement of the particles along with variations in sedimentary characteristics and state, carried by each particle as personal information, are traced with time. Specifically, concerning transport processes, the local seawater velocity and the particle’s settling control advection, whereas the random Brownian motion due to turbulence simulates turbulent diffusion. The vertical stratification of the water-column is taken into consideration by appropriate damping of the vertical diffusion term. Variations in cohesive sediment properties during the abidance in the aquatic environment include coagulation and flock break-up processes, quantification of the effects of ambient density to the density of the cohesive aggregate and the associated alterations to the falling speed of the particle. In the vicinity of the seabed particles may deposit and gradually consolidate with time the particles remain settled onto the bed, renter the flow at a later temporal point or may enter the water column for the first time, originating from the erosion of the bed. The occurrence of each of the aforementioned near-bed processes is defined accordingly to the prevailing benthic shear stress conditions. The mathematical model has been applied to the Thermaikos Gulf, an area of high environmental and socioeconomic importance but also a region of significant pollutant forcing from various anthropogenic activities taking place in the adjoining land. Various kinds of outputs can be extracted, such as trajectories of the overall movement of specific particles and related alterations of their characteristics with time, snapshots of the domain with respect to suspended or deposited matter and naturally concentrations of sediments at every required temporal and spatial point. Indicative results from yearly and monthly simulations, using input baroclinic circulation data from the North Aegean Sea model and river discharges are presented and discussed, including outputs from a Typical One-Year Simulation (TOYS), the simulation of the period from September 3rd 2001 to August 31st 2002 (S1A2) and the January 2003 experiment (J03). The description of the processes that have been incorporated in the parameterization covers the most significant factors controlling transport and mixing of fine grained sediments in the marine environment, thus validating the accuracy and completeness of the model. One of the major advantages, apart from the observation of the phenomena in scales smaller than the grid size, hence describing the natural processes more accurately, is the flexibility in accepting various pollutant sources and the applicability to different domains with minor modifications. The model has been incorporated in the MFSTEP project, as part of the developed operational forecasting system for the Mediterranean Sea. The application can be used for the prognosis of the seawater quality for current and for future conditions, enabling employment as part of a near-real time observation system or to formulate decisions for the protection of the seawater environment.

Publications in Collective Volumes

Mitigating Flood and Erosion Risk using Sediment Management for a Tourist City: Varna, Bulgaria

Ekaterina Trifonova, Nikolay Valchev, …, Katerina Kombiadou, …, Sergey Kuznetsov and Yana Saprykina (2014)
Zanuttigh B., Nicholls R.J., Vanderlinden J.-P., Thompson R.C. & Burcharth H.F., editors, ‘Coastal Risk Management in a Changing Climate (Chapter 7: Case Studies Worldwide)’ (ISBN: 978-0-12-397310-8, pp. 358-383

Abstract

The practical application of a holistic approach to flood and erosion assessment is shown by a number of case studies worldwide. The description of these sites allows the identification of key drivers of future risk at the coastal areas, weak points of present risk assessment and management, and consequent main challenges to be addressed. In some cases, suitable mitigation options and their benefits are also identified, and example application of methodologies for the selection of the best portfolio to preserve the areas and promote their sustainable development is provided.

The sites here considered are very diverse, comprising a wide variety of physical, social, economic, and ecological conditions. From west to east, the sites are: Cancun, Mexico; Santander Bay, Spain; Teignmouth estuary, United Kingdom; Gironde estuary, France; Scheldt estuary, Belgium and The Netherlands; Elbe estuary, Germany; Cesenatico coastal area, Italy; Hel Peninsula, Poland; Varna coastal area, Bulgaria; and Yangtze estuary, China.

Modelling Cohesive Sediment Dynamics in the Marine Environment

Katerina Kombiadou and Yannis N. Krestenitis (2013)
A.J. Manning, editor, ‘Sediment Transport Processes and Their Modelling Applications’ (ISBN: 978-953-51-1039-2), pp. 213-246

Abstract

One of the most challenging tasks in modelling marine processes is the description of fine sediment dynamics. Cohesive sediments enter the marine environment from physical or anthropogenic sources and affect physical, biochemical and biological processes. Their movement and fate is the result of various physical processes and forces that interact with one another. These processes extend from the scale of the microfloc and the effects of the small-scale forces, like cohesion, to the macroscale of the hydrodynamic field and the effects the local hydrodynamics. The present chapter aims at covering the most significant processes that control the motion and fate of cohesive sediment in microtidal areas, focusing both on the physical and mathematical description of the phenomena.
More specifically, the physical processes that take place in the water column and near the seabed and the corresponding mass-exchanges between them are presented and analyzed. The motion of sediment in suspension is determined by the seawater velocities and local shear, the stratification of the water column and the characteristics of the cohesive flocs that control the processes of advection, dispersion flocculation-deflocculation and settling. Near-bed processes are highly dependant on the prevailing shear stress conditions and the characteristics of the sedimentary matter and control the mass-exchanges between the water column and the seabed; these processes include deposition, self-weight consolidation, resuspension and erosion of the bed. Modelling results from a three-dimensional sediment transport model, formulated based on the aforementioned considerations, are also presented and discussed; the applications were performed in the domain of Thermaikos, a microtidal gulf in the NW Aegean Sea (Eastern Mediterranean), for different (point or distributed) sources of fine particulate matter, including river-borne matter, physically and mechanically (trawling-induced) eroded sediments and aeolian transported dust.