C. Koutitas

A breaking wave propagation model based on the Boussinesq equations

Abstract A wave propagation model, based on the Boussinesq type of equations, is proposed, valid both in the shoaling and the breaking region. By introducing a dispersion term to simulate Reynolds stresses (eddy viscosity concept), it is possible to simulate wave deformation in the surf zone, i.e. the turbulence dissipation conditions during breaking. The eddy viscosity coefficient is calculated from the numerical integration of the turbulent transport equation and the mixing length hypothesis. The production term in the proposed k-model is the dissipation of wave energy due to breaking, which is approximated by that of a propagating bore. The model is tested against well confirmed experimental data.

Modeling the impact of climate change on sediment transport and morphology in coupled watershed-coast systems：A case study using an integrated approach

Abstract Climate change is an issue of major concern nowadays. Its impact on the natural and human environment is studied intensively, as the expected shift in climate will be significant in the next few decades. Recent experience shows that the effects will be critical in coastal areas, resulting in erosion and inundation phenomena worldwide. In addition to that, coastal areas are subject to “pressures” from upstream watersheds in terms of water quality and sediment transport. The present paper studies the impact of climate change on sediment transport and morphology in the aforementioned coupled system. The study regards a sandy coast and its upstream watershed in Chalkidiki, North Greece; it is based on: (a) an integrated approach for the quantitative correlation of the two through numerical modeling, developed by the authors, and (b) a calibrated application of the relevant models Soil and Water Assessment Tool (SWAT) and PELNCON-M, applied to the watershed and the coastal zone, respectively. The examined climate change scenarios focus on a shift of the rainfall distribution towards fewer and more extreme rainfall events, and an increased frequency of occurrence of extreme wave events. Results indicate the significance of climatic pressures in wide-scale sediment dynamics, and are deemed to provide a useful perspective for researchers and policy planners involved in the study of coastal morphology evolution in a changing climate.

A comparative study of three mathematical models for wind-generated circulation in coastal areas

The development of a quasi depth-varying mathematical model for wind-generated circulation in coastal areas, expressed in terms of the depth-averaged horizontal velocity components and free surface elevation, but taking into account the velocity variation along the depth (most important in the case of wind-generated currents), is developed as a middle stage between the classical depth-averaged (long waves) model traditionally used, and the modern depth-varying (3-D) model. The proposed model is solved numerically together with the two other models. A comparison of the results for a specific case of wind-generated circulation in a typical semi-enclosed coastal basin is presented to assess the benefits and qualifications of the proposed model. It appears that the description of the free surface elevation and velocity field is very close to that given by the depth-varying model which consumes computer time and memory similar to the classical depth-averaged one.

Numerical solution of the complete equations for nearly horizontal flows

Abstract The complete mathematical model for the non steady hydrodynamic circulation due to wind, waves and density gradients in a coastal area or a lake is solved by the combined application of the weighted residuals—Galerkin method and the finite difference method. The computation of u and v velocity components in x, y, z, t space is achieved through expansion in series of base functions times undetermined coefficients over the depth. The computation of these coefficients giving the vertical variation of the velocity is done by the Galerkin method. The rapid convergence of this procedure permits a quick and economic evaluation of 3-D flows on a 2-D grid (in x, y space). The capabilities of the method were demonstrated by applying it to a tidal flow in an estuary.

Numerical modelling of suspended sediments

Abstract The turbulent advection-diffusion mathematical model in three-dimensional space is solved by a mixed finite element finite difference method. Linear finite elements in the vertical direction and central finite differences in the horizontal directions are used coupled with the Galerkin error minimization procedure. The integration in time is performed in fractional steps (one explicit one implicit) by splitting the differential operator. The method is illustrated by application to the three-dimensional movement of suspended sediment. Its accuracy is checked by comparison to analytical solutions and its efficiency is gauged relative to finite elements and implicit finite difference solutions for two-dimensional suspended sediment transport over a dredged channel.

A microcomputer code for tsunami generation and propagation

A linear long wave mathematical model is formulated and processed to a compact one equation form, for the description of the generation and propogation of seismic origin water waves. The predictive ability and specific properties of the model are tested by means of a number of numerical experiments and comparison to existing analytical or numerical solutions. Two applications are finally presented for Greek coastal areas: tsunami generation in the bays of Patra and Corinthos and reproduction of a rencent (1956) tsunami in South Aegean Sea (the second application leads to comparison of computed and observed wave heights).

Finite element-fractional steps solution of the 3-D coastal circulation model

Abstract The mathematical model for the nearly horizontal circulation due to wind, tides and density gradients in 3-D coastal areas is solved by a combined use of the method of finite elements and the integration in fractional steps. The discretisation of the flow domain is achieved through a system of 1-D finite elements over the depth, z , and 2-D finite elements in x − y space. The differential operators of the momentum equations in x and y , are split and integrated separately in z and x − y dimensions. The method is an extension of a previously presented approach combining finite differences and expansion in series. The application refers to the wind induced circulation in the 3-D coastal basin of Thessaloniki Bay.

Optimum design of the entrance of a fishpond laterally to the main stream of an open channel

In this study, the optimum design of the entrance of a fishpond laterally to the main flow of an open channel was investigated numerically and experimentally. The flow characteristic measurements were realized with the PIV (particle image velocimetry) method. The mathematical simulations were based on the development of a two dimensional -mean in depthhydrodynamic model and a quasi three dimensional sediment transport model which includes processes of advection, diffusion, and settling of conservative suspended matter. The study was completed with the comparison of the final results of the mathematical models with the findings of the physical model revealing the hydrodynamic interaction and coupling between the main flow of the channel and the lateral reservoir—fishpond and leading to the optimum technical design of the system.

Oil Spill Modeling Aiming at the Protection of Ports and Coastal Areas

Oil spill models are used worldwide to provide preventive measures in assessing risks of actual and potential damage to natural resources from spills, and also in assisting coastal facilities and local authorities in their strategic development of oil spill mitigation planning and response. Numerous oil spill simulation models exist in the bibliography. They vary in complexity, applicability to location and ease of use. A synoptic presentation of the types of oil slick models internationally applied in operational mode is done, focusing on the model developed by Aristotle University of Thessaloniki. The current study elaborates on that oil slick numerical model which simulates the transport and weathering (due to a number of physicochemical processes evolving with time) of an oil spill that accidentally occurred in a coastal area, coupled with a 3D hydrodynamic model. The model is applied in a semi-confined water body, namely Thermaikos Gulf, in N. Greece, which contains the Port of Thessaloniki, a potential source of accidentally spilled oil. Findings of the present study highlight the existing experience on the subject and denote the applicability of such models in either tracing the source of a spill or in predicting its path and spread, thus proving their value in real-time crisis management.

Structure and properties of the attractor of a marine dynamical system

We investigate the properties of a marine dynamical system by means of time series of the sea-level height at four locations in the Saronicos Gulf in the Aegean Sea, Greece. In order to characterize the dynamics, we estimate the dimension of the underlying system attractor, and we compute its Lyapunov exponents. Dimension estimates indicate that the dynamics can be explained by a low-dimensional deterministic dynamical system. Lyapunov exponent estimates further substantiate the above conclusion, while at the same time, indicate that the dynamical system is a rather nonuniform chaotic one.