Hocine Oumeraci

CLASSIFICATION OF WAVE LOADING ON MONOLITHIC COASTAL STRUCTURES

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

Overtopping Flow Parameters on the inner slope of Seadikes

Wave overtopping is one of the most important parameters concerning the design of seadikes. In the past the design was based on average overtopping rates. Nevertheless, average overtopping rates are not suitable for design of seadikes because they represent the direct loading on the dike surface. Direct loads on the dike surface are overtopping velocities and layer thicknesses. These parameters are responsible for erosion and infiltration o f overtopping water into the c lay coverage of seadikes (Fig. 1). A theoretical model was developed b ased on the two d imensional equations of motion to p redict overtopping velocities and layer thicknesses. This model was calibrated and verified by small and large scale model tests and is valid for each dike section (seaward slope, dike crest, landward slope).

Prediction of wave pressures on smooth impermeable seawalls

Abstract Simple prediction methods are proposed to estimate the wave induced pressures on smooth impermeable seawalls. Based on the physics of the wave structure interaction, the sloped seawall is divided into a total of five zones (zones 1, 2 and 3 during run-up (corresponding pressures are called as positive pressures) and zones 4 and 5 during run-down (corresponding pressures are called negative pressures)) ( Fig. 1 Download high-res image (121KB) Download full-size image Fig. 1 . Definition sketch to explain the predicted wave pressure on smooth impermeable seawalls. ). Zone 1 (0 z d − H i /2), where the wave pressure is governed by the partial reflection and phase shift; Zone 2 ( d − H i /2 z d ), where the effect of wave breaking and turbulence is significant; Zone 3 ( d z z d ), where the wave pressure is caused by the run-down effect and Zone 5 (0 z d -Run down), where the negative wave pressures are due to partial reflection and phase shift effects. Here d is the water depth at the toe of the seawall, H i is the incident wave height and z is the vertical elevation with toe of the seawall as origin and z is positive upward. For wave pressure prediction in zones 1 and 5, the empirical formula proposed by Ahrens et al. (1993) to estimate the wave reflection and Sutherland and Donoghues recommendations (1998) for the estimation of phase shift of the waves caused by the sloped structures are used. Multiple regression analysis is carried out on the measured pressure data and empirical formulas are proposed for zones 2, 3 and 4. The recommendations of Van der Meer and Breteler (1990) and Schuttrumpf et al. (1994) for the prediction of wave run-down are used for pressure prediction at zone 4. Comparison of the proposed prediction formulas with the experimental results reveal that the prediction methods are good enough for practical purposes. The present study also shows a strong relation between wave reflection, wave run-up, wave run-down and phase shift of waves on wave pressures on the seawalls.

The SWAN model used to study wave evolution in a flume

The SWAN numerical model is used to model the evolution of JONSWAP wave spectra and hence the significant wave height of waves in a tank. Comparison with experiment has shown that modelling triad interactions in the numerical model leads to too low predictions of spectra and significant wave height and should therefore be excluded. The modelling of the breaking constant was also investigated, by looking at the use of a constant breaking constant, Nelson formula, and Goda formula (added into SWAN for this study). Using a constant value of 0.78 within SWAN gave the best comparison between theory and experiment.

Hydraulic Performance of Artificial Reefs: Global and Local Description

Artificial reefs are increasingly used as an active shore protection measure. Nevertheless, the physical processes occuring at these structures are still not well understood. Existing design formulae mostly take into account only the change in wave height over the reef (transmission coefficient). The energy transfer within the wave spectrum is not considered. Physical model tests on artificial reefs have been performed at Leichtweis Institute for Hydraulic Engineering, focussing on the energy transfer and on local effects occuring at the reef. Among others new formulae are proposed to predict both height and period of the transmitted and reflected wave. With respect to the local effects, new breaking criteria and breaker classifications are proposed, together with a description of the generated vortices and their effects on the hydraulic performance of the reef.

Numerical modelling of wave overtopping-induced erosion of grassed inner sea-dike slopes

Recent field wave overtopping experiments on grassed sea-dikes indicate that a moderate-quality grass cover can surprisingly resist severe overtopping rates. This high erosion-resistant capacity against wave overtopping of grass covers is owing to the root reinforcement of grass in the subsoil. Inspired by this finding, the present study makes first attempt to develop a numerical model of wave overtopping-induced erosion of the inner slope of grassed sea-dikes, considering the effect of the root reinforcement. The critical velocity for grass erosion is described as a function of the root cohesion and thus decreases with the erosion depth. This depth-dependent strength of grass allows for a better description of the nature of grass erosion, especially eligible for modelling erosion initiated from a naturally or artificially weak spot in a grass slope. Wave overtopping on grass slopes exhibits high turbulence with entrained air bubbles, for which the bed shear stress determined according to the conventional approach for ordinary open-channel flows is underestimated. To resolve this, it is assumed that the structure of wave overtopping resembles that in a bubbly turbulent wall jet, so that the bed shear stress can be determined in connection with the degree of flow turbulence. Model validation against the data from field overtopping experiments conducted in the Netherlands shows that main features of grass erosion on landward dike slopes are successfully simulated. Numerical experiments on various aspects of grass erosion are carried out which give some new insights into the nature of wave overtopping-induced grass erosion. Also, if the process of breach initiation of a grassed dike can be divided into subprocesses, erosion of the grass turf and erosion of the bare clay cover, an attempt is successfully made to roughly anticipate the breach initiation time of grassed sea-dikes composed of a sand core and a grassed clay cover.

Numerical Modelling of Fixed Oscillating Water Column Wave Energy Conversion Devices: Toward Geometry Hydraulic Optimization

The Oscillating Water Column (OWC) is one of the simplest and most studied concepts for wave energy conversion. The commercial scale diffusion of the OWC technology is, however, strongly dependent upon the device optimization. Research at a fundamental level is therefore still required. Analytical, numerical and experimental models are necessary tools for advancing in the knowledge of the system and thus promoting its passage at the commercial level. In this work, a simplified frequency domain rigid piston model has been applied to preliminary select expected ranges of air pressures and air velocities for the instrumental set up of laboratory experiments. The set up of a Computational Fluid Dynamic (CFD) model implemented in the open source OpenFOAM®1 environment is then presented. The multiphase model solves incompressible 3D Navier-Stokes equations, using Large Eddy Simulation (LES) for turbulence modelling, and adopts a Volume of Fluid method (VOF) to track the air-water interface. A preliminary validation of the model with physical tests data is conducted. The numerical approach seems to be promising for an accurate simulation of the OWC device energy conversion process. Hence, the validated model can be a useful research tool for different problems, particularly for systematic parameter studies to extend the range of conditions tested in the laboratory.Copyright

WAVE PRESSURE DISTRIBUTION ON PERMEABLE VERTICAL WALLS

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

Spatial Modeling of Tangible and Intangible Losses in Integrated Coastal Flood Risk Analysis

This paper describes an integrated spatial modeling concept for flood losses which has been developed within the joint research project “XtremRisK”. For the final step of an integrated coastal flood risk analysis based on the “risk source-pathway-receptor” approach, the “Cellbased Risk Assessment” (CRA) concept is implemented for the spatial modeling of both tangible and intangible flood losses and their aggregation into the so-called “integrated risk”. Finally, all results are utilized for the hazard and risk mapping, which serve as a basis for the decision making on risk management strategies. The different steps of the CRA concept and its applicability for different types of spatial input data are shown in the paper. Furthermore, advantages and limitations of this spatial modeling concept for integrated flood risk analysis are discussed. The practical implementation of the approach is described for the study area Hamburg-Wilhelmsburg (Germany) and the related categories of flood losses. The results show that the newly developed CRA concept is a suitable spatial modeling framework with respect to the comprehensive requirements in this integrated coastal flood risk analysis.

Experimental study on tsunami attenuation by mangrove forest

Laboratory experiments on the effectiveness of mangroves to reduce tsunami energy were performed. A complex tree structure of Rhizophora sp. was parameterized using the stiff structure assumption (root system and trunk) for different submerged root volume ratios and frontal tree areas. The hydraulic resistance of the prototype and the parameterized models under steady flow conditions was compared and the most appropriate parameterized model in terms of both equivalent flow resistance and practical feasibility was selected for further investigation. The damping performance of the mangrove forest was determined from laboratory tests performed synchronously in a twin-wave flumes (with and without the forest model in 1 and 2 m-wide wave flumes, respectively) for varying incident height of solitary wave, water depth and forest width. The role of the different types of wave evolution modes on wave damping is discussed based on the measurements of the forces exerted on the single tree models along the entire forest width. A new approach for the wave transmission coefficient, which is based on the ratio of the forces exerted on the trees placed in the last and first forest row, is proposed. In the paper, the most important results of the tree parameterization procedure and the wave flume experiments are discussed.