Theoharris Koftis

2D-V hydrodynamics of wave–floating breakwater interaction

Wave interactions with a fixed floating breakwater (FB) are investigated both numerically and experimentally. Laboratory experiments of large scale have been performed in the CIEM flume of the Catalonia University of Technology, Barcelona and measurements are compared with numerical results obtained with the use of the COBRAS model. The latter solves the two-dimensional, unsteady Reynolds averaged Navier–Stokes (RANS) equations in the vertical plane (2D-V). The k–ε turbulence model is also used for the computation of the Reynolds stresses as well as the Volume Of Fluid method for “tracking” the variation of the free surface. The effects of relative draught dr/D (dr=structure draught, D = water depth) and the FB configuration (breakwater without and with an attached seaward plate with the same dr/D) on the hydrodynamic characteristics (transmission, velocity, vorticity, turbulence) are investigated. Experimental water surface elevation, velocities at selected locations and pressure distribution around the structure are compared satisfactorily with computed results for dr/D equal to 0.325 and 0.2 and the two FB configurations. Detailed computed velocities, vortices and turbulence kinetic energy in the vicinity of the structure indicate the effects of dr/D and FB configuration on the flow pattern and the turbulence structure at the two sides of the structure.

Modeling of Nonlinear Wave Attenuation due to Vegetation

ABSTRACT Karambas, T.; Koftis, T., and Prinos, P., 2016. Modeling of nonlinear wave attenuation due to vegetation. In the present work, a nonlinear wave propagation model is developed and is applied for the simulation of the wave dissipation over vegetation. The free-surface flow over the vegetation is simulated using a Boussinesq model, while the flow within the canopy is simulated by solving simultaneously a canopy flow model. The parameters of the canopy flow model are related to the geometric characteristics of the vegetation, while the drag coefficient is taken from existing formulas found in literature and is related to the Reynolds number. The coupling between the Boussinesq and the canopy flow model is simulated by adding two extra terms, due to vegetation, in the continuity and momentum equations of the Boussinesq model. The numerical results are found to be in good agreement with several experimental measurements found in the relevant literature. The advantage of the proposed methodology is based on a nonlinear Boussinesq-type wave model, with wide range of applications for both engineering and scientific purposes, and the use of a canopy flow model with no calibration needed for the model coefficients. Moreover, a simple formula is extracted from the results for the estimation of the wave damping coefficient depending on the meadow and wave parameters.

Innovative Engineering Solutions and Best Practices to Mitigate Coastal Risk

Engineering solutions are widely used for the mitigation of flood and erosion risks and have new challenges because of the expected effects induced by climate change in particular sea level rise and increase of storminess. This chapter describes both active methods of mitigation based on the reduction of the incident wave energy, such as the use of wave energy converters, floating breakwaters and artificial reefs, and passive methods, consisting of increase in overtopping resistance of dikes, improvement of resilience of breakwaters against failures, and the use of beach nourishment as well as tailored dredging operations.Existing coastal management and defense approaches are not well suited to meet the challenges of climate change and related uncertanities. Professionals in this field need a more dynamic, systematic and multidisciplinary approach. Written by an international group of experts, Coastal Risk Management in a Changing Climate provides innovative, multidisciplinary best practices for mitigating the effects of climate change on coastal structures. Based on the Theseus program, the book includes eight study sites across Europe, with specific attention to the most vulnerable coastal environments such as deltas, estuaries and wetlands, where many large cities and industrial areas are located. * Integrated risk assessment tools for considering the effects of climate change and related uncertainties* Presents latest insights on coastal engineering defenses* Provides integrated guidelines for setting up optimal mitigation measures* Provides directly applicable tools for the design of mitigation measures* Highlights socio-economic perspectives in coastal mitigation

Chapter 3 – Innovative Engineering Solutions and Best Practices to Mitigate Coastal Risk

Engineering solutions are widely used for the mitigation of flood and erosion risks and have new challenges because of the expected effects induced by climate change in particular sea level rise and increase of storminess. This chapter describes both active methods of mitigation based on the reduction of the incident wave energy, such as the use of wave energy converters, floating breakwaters and artificial reefs, and passive methods, consisting of increase in overtopping resistance of dikes, improvement of resilience of breakwaters against failures, and the use of beach nourishment as well as tailored dredging operations.Existing coastal management and defense approaches are not well suited to meet the challenges of climate change and related uncertanities. Professionals in this field need a more dynamic, systematic and multidisciplinary approach. Written by an international group of experts, Coastal Risk Management in a Changing Climate provides innovative, multidisciplinary best practices for mitigating the effects of climate change on coastal structures. Based on the Theseus program, the book includes eight study sites across Europe, with specific attention to the most vulnerable coastal environments such as deltas, estuaries and wetlands, where many large cities and industrial areas are located. * Integrated risk assessment tools for considering the effects of climate change and related uncertainties* Presents latest insights on coastal engineering defenses* Provides integrated guidelines for setting up optimal mitigation measures* Provides directly applicable tools for the design of mitigation measures* Highlights socio-economic perspectives in coastal mitigation

Numerical simulation of turbulent exchange flow in aquatic canopies

ABSTRACT In the present study, lock-exchange flows through rigid emergent vegetation are investigated numerically. The volume-averaged Reynolds-averaged Navier–Stokes equations and the transport equations of the renormalization group k-ε turbulence model, which include additional terms due to vegetation, are solved numerically. The drag coefficient CD is estimated as a function of the local volume-averaged velocity, which varies in space and time. The dynamics of gravity currents in regions covered by vegetation (fully and partly) is examined. The solid volume fraction of vegetation ϕ ranges from 0.03 to 0.35. The numerical model is validated with available experimental data for sparse vegetation. The characteristics of gravity currents in dense vegetation, where there is no sufficient information, are investigated. Ιt is found that the toe velocity in the open area is influenced by the adjacent vegetation for ϕ > 0.104. The currents become drag dominated from the beginning of the exchange flow for ϕ > 0.141.

Reynolds stress modelling of flow in compound channels with vegetated floodplains

Flow in compound channel with vegetated floodplain is complex and efficient modelling should include the effects of vegetation on velocity, secondary flow and shear stress. In the present study, the Volume-averaged Reynolds-averaged Navier–Stokes equations, in conjunction with a Reynolds Stress turbulence model, are solved numerically for a non-symmetrical compound channel of a trapezoidal main channel and a vegetated floodplain. The numerical results agree well with available experimental data, while the model is capable to reproduce the evolution of vortices. The cross-sectional flow characteristics reveal the momentum exchange mechanism between main channel and floodplain due to increased shear stresses and turbulence anisotropy near the vegetation interface. Also, the recently improved analytical Shiono and Knight method [1991. Turbulent open-channel flows with variable depth across the channel. J Fluid Mech. 222:617–646] is applied for the determination of the depth-averaged velocity and shear stress, together with simple Manning calculations.

2D-V HYDRODYNAMICS OF DOUBLE FLOATING BREAKWATERS

Interactions of regular waves with a double, fixed, pontoon type, floating breakwater (FB) are studied numerically with the use of a 2DV Unsteady Reynolds Averaged Navier-Stokes (URANS) model. The study is focused on the effect of the structure width and spacing, both made dimensionless with the wave length (W/L and S/L), on the reflection, transmission and dissipation characteristics. The results of the numerical model are in good agreement with available large scale experimental data. Ct decreases with increasing W/L for all S/L; for a 100% increase of W/L a 62% (maximum) and a 34 % (minimum) decrease of Ct is found for S/L=0.25 and S/L=1.04 respectively . The effect of relative spacing S/L is not monotonic since Ct exhibits several maxima and minima with S/L. Better results are obtained for S/L<0.75 with Ct values less than 0.1.