G. M. Jahid Hasan

OBSERVATION OF A STORMY WAVE FIELD WITH X-BAND RADAR AND ITS LINEAR ASPECTS

An X-band nautical radar system was employed to observe the wave field during a storm around the research pier HORS in Hasaki, Japan. The radar system provides instantaneous distributions of wave crests in the near-shore region, with image sequences comprising a clear spatial and temporal variation of the wave pattern. The spectra of radar echo signals and water surface elevations from wave gauge records were compared, and a relatively high coherence for a particular bandwidth was observed. A sequence of radar images was filtered at the dominant frequency, which was used to estimate the wavenumber distribution of the refraction field. The Fast Fourier Transform (FFT) and Maximum Entropy Method (MEM) were used to estimate the wavenumbers. The estimated wave angles were then compared with linear refraction analyses. The water depths along the pier were also estimated using the linear dispersion relationship and compared with the surveyed depths. The linear estimates of hydrodynamic parameters during the passage of the storm are presented in this paper to demonstrate the potential of radar measurements in capturing essential characteristics of coastal dynamics.

Numerical Study on Mixing and Stratification in the Ebb-Dominant Johor Estuary

ABSTRACT Hasan, G.M.J.; van Maren, D.S., and Hin Fatt, C., 2013. Numerical study on mixing and stratification in the ebb-dominant Johor Estuary. A three-dimensional hydrodynamic model is used to investigate intratidal spring-neap variations of turbulent mixing and stratification in the Strait of Singapore and its adjacent Johor estuary area. The waters in the Johor estuary are ebb dominant because of interaction of the O1–K1–M2 tidal constituents. In most estuaries mixing rates during the flood are larger than during the ebb because of a flood-dominant tidal asymmetry. The Johor estuary is an example where tidal asymmetry supports stronger mixing during ebb tides, whereas similar to other estuaries, tidal straining promotes stable stratification during the ebb. Therefore an analysis of intratidal variation in stratification reveals the relative importance of both mechanisms. Vertical profiles of salinity, flow velocity, and eddy diffusivity show a marked asymmetry between flood and ebb tides. Small changes in the tidal currents substantially influence the eddy diffusivity, energy dissipation rate, and build up of stratification, which is explained using the nondimensional Richardson number. The estuary is found completely mixed during flood tide and slightly stratified during ebb tide, suggesting tidal straining (generating ebb stratification) is more important for the intratidal variation in mixing and stratification than tidal asymmetry (flood stratification).

Destroying hills in the northeastern part of Bangladesh: A qualitative assessment of extent of the problem and its probable impact

Indiscriminate cutting of hills in the Sylhet region has become a major environmental issue. The nature and life style of Sylhet intimately related with the hills are thus under the threat of a drastic imbalance in its ecosystem. Due to such hill cutting the mostly affected sectors of this region will be its weather and climate, geomorphology and hydrology, and the indigenous flora and fauna. As a result the frequency of natural calamities like earthquake, flash flooding etc may increase considerably. Deforestation and resulting increased soil erosion, decreased ground water recharge and deteriorated water quality might also be as consequences of such hill cutting. This paper investigates the cause and extent of the problem along with its probable impact and finally suggests actions for conservation of hills for ecological balance of the region.

WAVE RUN-UP ANALYSES UNDER DISSIPATIVE CONDITION USING X-BAND RADAR

An X-band nautical radar system was employed to examine wave run-up during a typhoon around the research pier HORS in Hasaki, Japan. Analyses on radar echo images were done to estimate the spatio-temporal variation of water fronts by manually digitizing cross-shore time stack images. Digitized instantaneous water fronts were validated with wave gauge measurements with an acceptable agreement. Longshore distribution of mean shoreline positions and inter-tidal foreshore slopes were then estimated using time-averaged images. Run-up, the height of discrete water level maxima, was estimated from the digitized water fronts with the help of foreshore profile. Run-up variations under dissipative condition were parameterized with surf similarity parameter. Low frequency variances in the run-up motion were observed, which travelled in the longshore direction.

Numerical Simulation: Hydro-Morphology of Meghna Estuary:

The Meghna Estuary is formed by the combined flows from three great rivers in the south-east Asia; the Ganges, the Brahmaputra and the Meghna. This estuary experiences dynamic hydro-morphological changes due to very high discharge of water and sediment from these three mighty rivers. Erosion and accretion occur simultaneously at very high rates; such that, the flow and sediment discharge through the estuary are the third highest and the highest, respectively, in the world. On the other hand, the tidal current itself is also strongly affected by the dynamic morphology changes. It is thus essential to understand the hydraulic behavior and interactive features of tides and morphology change in the estuary. With this end in view a two dimensional general model of Bay of Bengal was developed under cyclone shelter preparatory study (csps) undertaken by Bangladesh Government and the model was updated later during other projects. The present article focuses on development of model, its set-up, boundary conditions and few calibration results of this two dimensional hydrodynamic and morphology model. The model applications clearly show the variation of the flow structure; their speed and direction separately for monsoon and dry season around the model area which covers the northern part of Bay of Bengal more specifically the Meghna Estuary.

Foreshore Applications of X-band Radar

The shallow waters of a nearshore region are dynamic and often hostile. Prediction in this region is usually difficult probably by our limited understanding of the physics or by availability of accurate field data. It is a challenge for traditional in situ instruments to provide these inputs with the appropriate temporal or spatial density at a reasonable cost. Remote sensing provides an attractive alternative. An X-band nautical radar system was employed for this study to examine alongshore propagation of low frequency run-up motion around the research pier HORS in Hasaki beach, Japan. Analyses on radar echo images were done to estimate longshore distribution of shoreline positions and inter-tidal foreshore profile using time-averaged images. Spatio-temporal variation of water fronts were digitized manually from cross-shore time-stack images. Run-up heights were then estimated from the digitized water fronts with the help of foreshore slope. Run-up variations under dissipative condition were parameterized with surf similarity parameter. Low frequency variances in the run-up motion were observed, which were traveling in the longshore direction. Longshore structures of this motion were examined and compared with different wave incidences during two typhoon events in the Pacific Ocean. Estimates of morpho-dynamic parameters during passage of different storms were analyzed and are explained in this chapter to demonstrate the potential of radar measurement in capturing essential characteristics of foreshore dynamics.