R. Balaji

Tsunami Wave Interaction with Data Buoys

To plan for proper mitigation measures, one should have an advanced knowledge of the phenomenon of tsunami propagation from the deep ocean to coastal waters. There are a few methods to predict tsunamis in the ocean waters; one method is the effective use of data buoy measurements. Although data buoys have been used along the Indian waters there has been a tremendous growth in the number of buoy deployment recently. Under the National Data Buoy Programme (NDBP) of India, the 2.2 m diameter discus data buoys were deployed along the east and west coasts of India for measuring meteorological and ocean parameters. It would be advantageous if these buoys could be efficiently used to measure rare events such as tsunamis. Understanding the dynamic behavior of the buoy is of prime importance if a tsunami warning system is to be successful. This may be accomplished through experimental or numerical studies. A comprehensive experimental study has been conducted to understand the dynamic behavior of a wave rider buoy exposed to a variety of waves. It is common that tsunami waves are represented in terms of shallow water waves, namely solitary and cnoidal waves. Hence, in the present study, the discus type data buoy is scale modeled and tested under the action of solitary and cnoidal waves in the laboratory. The time histories of wave elevations, as well as heave and pitch motions of the buoy model, were analyzed through a spectral approach as well as through wavelet transformations. The wavelet approach gives more detailed insight into the spectral characteristics of the buoy motion in the time scale. The harmonic analyses were performed for the cnoidal wave elevations and subsequent motion characteristics that give an insight into the energy variations. The details of the model, instrumentation, testing conditions and the results are presented in this paper.

Laboratory Simulation and Analysis of Wave Groups

The coastal and offshore structures are some times exposed to group of waves with successive higher wave elevations exceeding the significant wave height, which is considered to be vulnerable for the stability of the structures. Hence, the knowledge on the existence and frequency of occurrence of ocean wave groups at a particular region of interest is important for the design of the ocean structures. In the present study, the wave groups were simulated theoretically and the same was generated in the laboratory wave flume. The measured wave elevations were analysed through statistical, spectral and wavelet approaches to detect the existence of the groupiness.Copyright

Simple Approaches to Oil Spill Detection Using Sentinel Application Platform (SNAP)-Ocean Application Tools and Texture Analysis: A Comparative Study

Effective and efficient monitoring of oil spills that originate from ships, offshore platforms and any accidents are of immense importance from the viewpoint of public safety and environmental protection. Detection of spilled oil is also essential to estimate the potential spread and drift from the source to the nearby coastal areas. In this regard, utilization of SAR data for the detection and monitoring of oil spills has received considerable attention in recent times, due to their wide area coverage, day-night and all-weather capabilities. In this paper, two oil spills incidents along the coast of Mumbai, India are investigated; (1) The 2010 oil spill that occurred after the MV MSC Chitra and MV Khalijia-3 collided and (2) the oil spill caused due to sinking of MV RAK carrier in 2011. Two simple and relatively quick approaches for oil spill detection have been applied to VV polarized Radarsat-2 imagery of the incidents and a comparison is made of the results obtained. The first approach utilizes the oil spill detection tool of Sentinel Application Platform (SNAP) and the second explores texture analysis using Grey Level co-occurrence matrix (GLCM). The results of the study show that texture analysis proves to be an efficient method for oil spill detection as compared to the SNAP oil spill detection tool. Nevertheless, both the proposed methodologies are useful for detection of oil spills and for subsequent utilization of the results, timely and cost effectively, for the calibration and validation of numerical models that predict oil spill dispersion trajectories.

Inter-comparison of hydrodynamic models of Gulf of Khambhat

The complex seabed contours and geometry of basin lead to a unique tidal pattern along Indias Gulf of Khambhat (GoK). Estimation of tidal levels and associated currents in this basin is challenging due to the amplification of tides and intense current magnitudes. In this study, the finite difference based numerical model, Delft3D-FLOW has been set up. Spatially varying bottom roughness parameters have been used to estimate the tidal levels and currents for the entire basin and the results are compared with an earlier work of Sathish Kumar and Balaji (2014 and 2015) based on an alternative finite element based numerical model, Telemac2D. An inter-comparison of model results shows good agreement with each other and with the field measurements. The tidal levels are observed to vary from 2m to more than 10m in the narrowing portion of the gulf while the maximum current velocity is seen as 3m/s.

A Case Study on Curtailed Tidal Hydrodynamic Modeling Along UAE Coast

A curtailed numerical model has been developed to assess the tidal hydrodynamics of entrance of a navigational channel in Abu Dhabi coast, United Arab Emirates. The curtailed model is developed using a finite element based numerical scheme, RMA2 (Donnell et al., 2006). The boundary conditions for the model were extracted from a large scale numerical model covering entire Abu Dhabi coast, developed using TELEMAC (Hervouet, 2000) modeling software. The hydrodynamic results of the curtailed model are validated with that of large scale model. The comparisons of water levels and current velocities obtained from the two models are found to be in agreement, demonstrating the efficiency and accuracy of the curtailed numerical model. The features of the tidal current pattern in the vicinity of the entrance of the navigational channel are also discussed. The details of the numerical scheme, model setup and methodology are presented and discussed in this paper.

Identification of breaking events from the responses of a data buoy

Conventional data buoys record wave spectral parameters such as the significant height, peak period, and mean direction. Information about the existence of wave groups and breaking waves is also important for the design of ocean structures. To derive such information from data buoy measurements, understanding of the motion characteristics of data buoys under a variety of wave conditions is essential. In the present study, the motion characteristics of a scale-modelled discus data buoy were tested under the action of deep-water breaking waves of different intensities, covering spilling to steep plunging types. The laboratory generation of breaking waves was based on the constructive wave—wave interaction method. The buoy heave and pitch motions were measured using potentiometers and non-contact motion-capturing cameras. The measured time series of the breaking-wave surface elevation and the resulting buoy motions were analysed by continuous wavelet transformation and phase—time methods. A closer insight into the wavelet phases reveals that the breaking events leave a constant phase variation over a wider range of higher frequencies. Through appropriate filtering of the localized frequencies, the phase—time method could also be adopted to detect breaking events in the time histories of the measured wave elevation and motion response. The details of the model, instrumentation, testing conditions, and analysis are presented and discussed in this paper.

WAVE ATTENUATION CHARACTERISTICS OF CHAMBERED BREAKWATER

ABSTRACT The reflection characteristics and the dynamic pressures exerted on an impermeable wall on the leeward side of permeable wave screens due to action of regular waves were taken up for investigation through theoretical and experimental programs. The two wave attenuating chambers are formed by placing horizontal slotted wave screens consisting of equally spaced circular wave-intercepting elements. The theoretical model is based on the eigenfunction expansion theory for linear waves, which has been successfully applied earlier for evaluating the hydrodynamic performance of vertical slotted screens by Issacson et al. (1998, 1999) and subsequently for pile supported barriers by Laju et al. (2006). The agreement between theoretical and experimental results is found to be good for the tested range of wave conditions. The present theoretical model is also found to be good in predicting the reflection characteristics of multi-chambered breakwaters. The formulation of theory, solution methodology, details of experiments and the results are discussed in this paper.

RESPONSE CHARACTERISTICS OF A DISCUS SHAPED DATA BUOY IN NONLINEAR WAVES

The response characteristics of a discus data buoy under the action of nonlinear waves are investigated through an experimental set-up in a wave tank. The details of the model, instrumentation, testing conditions and the analysis of the results are presented and discussed in this paper. The nonlinear waves were generated in the wave tank such that their characteristics fall under the Cnoidal and Stokes wave theory regions. The measured wave elevation and responses of the buoy model were analyzed qualitatively through phase-portrait and, quantitatively through spectral and harmonic analysis. In addition, the effects of the nonlinear wave height, wave period and Ursell parameter, Ur , on the response characteristics of the buoy model are addressed. Using a linear Transfer Function (TF) obtained from linear waves, an attempt to estimate the nonlinear-wave spectral density from the buoy model dynamics is made and the uncertainties in such predictions are detailed.

EXPERIMENTAL AND NUMERICAL STUDIES ON THE TSUNAMI WAVE CHARACTERISTICS

ABSTRACT The devastating effects of the great Indian ocean tsunami has forced researchers in focusing their attention more vigorously on understanding the behaviour during its propagation and its effects on structures. This can obviously be accomplished through numerical and physical model studies or combination of both. The characteristics of a tsunami wave can approximately be same as that of a solitary wave which is basically a shallow water wave. Hence, the studies on the characteristics of shallow water waves have become an emerging topic of interest. An important aspect of a mitigation effort is to predict the tsunami wave kinematics. A combination of experimental and numerical simulation of tsunami represented by the solitary wave was studied and their comparison is discussed in this paper. The details of the numerical approach, methodology, instrumentation and measurement adopted for the present study are reported. The disagreement of the experimental simulation of solitary wave elevations with that of numerical simulation has been addressed and the possible discrepancies are overcome, the procedure of which are briefly discussed. The dynamics of the tsunami wave propagation over an uneven topography is studied using the developed numerical model.

Understanding the effects of seawall construction using a combination of analytical modelling and remote sensing techniques: Case study of Fansa, Gujarat, India:

The effect of seawall on the adjacent beaches and coastal dynamics has not been well documented in literature. The purpose and function of coastal structures, especially seawalls, have often been misunderstood, as in some cases, seawalls lead to coastal erosion, contrary to protecting the shoreline for which they are generally constructed. Seawalls have been reportedly causing changes in the near-shore process, specifically the sediment dynamics by affecting the onshore/offshore and, to some extent, the longshore sand transport. Therefore, it becomes imperative to understand the effect of seawalls on the adjoining beach to make sure more informed decisions are made on their installation. This article discusses the effects of seawall construction along the coast of Fansa, South Gujarat, India. A numerical model has been used to estimate the wave parameters along the selected coast, the results of which are subsequently utilized in an analytical model (parabolic shape model) to predict the end-wall effect. Independently, remote sensing datasets of CARTOSAT 1 with spatial resolution of 2.5 m are used to understand the shoreline change dynamics in this region, post-construction of this seawall. It is found empirically that the net longshore sediment transport rate is approximately 1.9 Mm3 per year along the coast. The results of the analytical model predict a maximum landward erosion of about 20 m and an alongshore erosion of 200 m on the down-drift side of the seawall. These estimations agree with those obtained by the remote sensing–based analysis, which estimates an erosion of approximately 40 m by the year 2014.