Entin A. Karjadi

Obliquely incident irregular waves in surf and swash zones

A time-dependent numerical model is developed for predicting the free surface elevation and depth-averaged cross-shore and alongshore velocities in the swash and surf zones under obliquely incident irregular waves. The assumption of small incident angles is made to simplify the finite-amplitude, shallow-water equations and allow the computation of the cross-shore fluid motion using the existing one-dimensional model for normally incident waves. The developed numerical model is compared with available field data. The time-dependent model is shown to be capable of predicting the cross-shore variations of the root-mean-square wave height and longshore current in the surf zone. The numerical model predicts large cross-shore and alongshore velocities near the shoreline. The causes of these large velocities are examined but cannot be ascertained for lack of velocity data near the shoreline.

The Effects of Surface Gravity Waves on High-Frequency Acoustic Propagation in Shallow Water

A realistic 2-D time-evolving ocean surface model has been combined with an existing acoustic ray-based model to simulate the effects of sea surface roughness on acoustic wave propagation in coastal regions. Rough sea surface realizations are generated and used as sea surface boundaries in the acoustic model. An approach to achieve high resolution and accurate results while maintaining computational efficiency of a ray-based model is applied. The results are then compared against a unique set of experimental data collected in 15-m water depth in Delaware Bay. These data include simultaneous environment and acoustic propagation (1-18 kHz) measurements. Modeled arrival time-angle fluctuations compare well with data and suggest that there are physical processes which need to be included to improve the model, such as bubbles and turbulence as well as 3-D scattering effects.

SWASH DYNAMICS UNDER OBLIQUELY INCIDENT WAVES

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

Impact of surface gravity waves on high‐frequency acoustic propagation in shallow water.

Sea surface roughness is one of several factors that significantly influences high‐frequency (1–50‐kHz) acoustic wave propagation in shallow water. The evolving sea surface introduces several variability effects including Doppler shift. Data analyses from high‐frequency acoustic experiments show high‐correlation between time, angle, and intensity fluctuations of received signals and varying sea surface conditions. In order to assess detailed acoustic signal interactions with the sea surface, a realistic wave model is developed and combined with an acoustic ray‐based model. Model validity is evaluated by comparing the results with data from multiple experiments. [Work supported by ONR 321OA.]

High‐frequency broadband acoustic current tomography in shallow water

To study current tomography in very shallow water regions a simultaneous oceanographic and broadband (1‐25 kHz) acoustic experiment was conducted in the Delaware Bay. The mean water depth was 15 m and the source‐receiver range was 760 m. In this paper, we discuss the feasibility of using reciprocal acoustic transmission for current tomography applications. A beamforming technique is used to resolve the arrival time of direct and surface‐bounced rays since in shallow water the received acoustic signals are more complicated due to multiple interactions with bottom and sea surface. Using the experimental data, the accuracy of travel time measurements for variable environmental conditions is examined for different center frequencies and bandwidths. The current velocity prediction results are compared with ADCP measurements to determine the feasibility of current tomography in shallow water.

Sound intensity variations in the presence of shallow‐water internal waves passing through acoustic track

Fluctuations of low frequency pulses (LFM signals in 270‐330 Hz band) in the presence of internal solitary wave (ISW) packet during the SW06 experiment are analyzed to quantify the interaction of sound with ISW field. Three situations during approximately 2 hours (20:30 ‐ 22:30 GMT of August 17, 2006) are considered: a period when ISW was absent, a period when ISW started to intersect the acoustic track, and a period when ISW occupied the acoustic track. The propagation direction, velocity, and amplitude of the ISW were estimated from the on‐board radar images recorded by two research vessels along with temperature records of sensors moored at the source, at the receiver, and between the source‐receiver track. Modeal and frequency filtering of received pulses was carried out as well as analysis of temporal variations of the field depth distribution. This analysis allows us to identify two different acoustic fluctuation mechanisms: horizontal refraction and adiabatic variability, and it confirms the previo...

High‐frequency current tomography in coastal regions

In deep‐water tomography refracted ray paths can provide accurate account of travel time for the inverse problem. However, in shallow coastal regions, due to the geometry of the source‐receiver as well as the waveguide dimensions, most acoustic energy is a multiple reflection from sea surface and sea bottom. This makes the forward problem which deals with resolving the multipath structure of the arriving signals difficult. In this paper, we discuss the limits of reciprocal acoustic transmission as a function of waveguide temporal and spatial parameters for the purpose of tomography application in coastal regions. Results from a high‐frequency (0.5–24 kHz) quasi‐reciprocal tomography experiment in a very shallow‐water coastal region are presented. Using these data, the accuracy and limit of current tomography for variable sea states are examined for different center frequencies and bandwidths. [Work supported by ONR.]