Dong Jiing Doong

Quantity, distribution, and impacts of coastal driftwood triggered by a typhoon

Typhoon Morakot pounded Taiwan in 2009 with record-breaking rainfall, washing an unprecedented amount of driftwood into the sea that was partially deposited at the coastal areas. According to the satellite imagery analysis, more than three million trees fell and were washed away to occupy 83.2% of the Taiwanese coastline, including 52 fishing harbors. The amount cleaned-up was only 1/7 of the total coastal driftwood. It was found that the amount of coastal driftwood is not only related to the amount of precipitation but is also related to the distance from the location of the landslide to the river mouth and to the landslide area. The amount of accumulated coastal driftwood demonstrated log-profile declines with increasing distance to the river mouth. Nearshore current and wave motion are the critical factors for driftwood deposition. Much of the driftwood washed into the sea harmed the tourism and fishing industries, endangered navigation and oceanic activities, and impacted the marine environment and ecosystem.

Ocean remotely sensed image analysis using two-dimensional continuous wavelet transforms

Ocean remote sensing is a useful way to obtain ocean wave information. Due to possible inhomogeneities from remotely sensed images, the current work proposes issues concerning ocean wave image analysis using the two-dimensional continuous wavelet transforms (2-D CWTs) to calculate local wave image spectra from inhomogeneous images. To optimize the algorithm of the 2-D CWT for wave image analysis, this work explores ideal parameter values for the wavelet function. The current study also analyses the limits of spatial image resolution and wave image size. After implementing the 2-D CWT on satellite and X-band radar images, this study presents local image spectra and ocean wave information from all the ocean images. These local image spectra reveal the phenomenon of wave refraction and wave nonlinearity nearshore. Compared to real wave spectra, the wavelet spectra present accurate results to describe local wave features in the spatial frequency domain.

Wave Measurements Using GPS Velocity Signals

This study presents the idea of using GPS-output velocity signals to obtain wave measurement data. The application of the transformation from a velocity spectrum to a displacement spectrum in conjunction with the directional wave spectral theory are the core concepts in this study. Laboratory experiments were conducted to verify the accuracy of the inversed displacement of the surface of the sea. A GPS device was installed on a moored accelerometer buoy to verify the GPS-derived wave parameters. It was determined that loss or drifting of the GPS signal, as well as energy spikes occurring in the low frequency band led to erroneous measurements. Through the application of moving average skill and a process of frequency cut-off to the GPS output velocity, correlations between GPS-derived, and accelerometer buoy-measured significant wave heights and periods were both improved to 0.95. The GPS-derived one-dimensional and directional wave spectra were in agreement with the measurements. Despite the direction verification showing a 10° bias, this exercise still provided useful information with sufficient accuracy for a number of specific purposes. The results presented in this study indicate that using GPS output velocity is a reasonable alternative for the measurement of ocean waves.

Quantification of non-homogeneity from ocean remote-sensing images using two-dimensional continuous wavelet transforms

In this study, a two-dimensional continuous wavelet transform is applied to quantify the non-homogeneity from remote-sensing wave images. Our study shows that the non-homogeneity index, which is proposed here, is capable of identifying the degree of non-homogeneity from the wave-field images. However, the influence from the edges of the wave-field image should be considered in analysis. The calculated results from the simulated wave fields and from the natural wave-field images reveal that the non-homogeneity index is influenced by the bathymetry inside the area of the ocean-wave image. It conforms to the character of the natural wave non-homogeneity. After discussing the wave characteristics from the wave-field image with a high non-homogeneity index, an inhomogeneous algorithm is recommended for obtaining accurate and reasonable wave results from the wave image.

Buoy and Radar Observation Network around Taiwan

The coastal ocean monitoring center (COMC) was assigned by the government offices to establish an coastal ocean monitoring network around Taiwan. It is composed by data buoys, radars, tide stations and coastal weather stations. All field stations are operational and have real time data transmission function. The purpose of this paper is to present the frame of this observation network and show observation results especially during past typhoons. This paper focuses on the introduction of data buoy and radar systems. The hardware and analysis method of data buoy and radar system are presented in this study.

Searching for freak waves from in-situ buoy measurements

There are several criteria to determine a freak wave in the ocean. These criteria are independent but the results are often not the same. This study tests on four presented criteria including the ratios of H/H1/3 and H/Hs, the kurtosis k, and the groupiness factor GF, and add one more condition (Hs>1m) on the searching of freak wave in the database of data buoy during typhoon from 1998 to 2009. The water elevation is inversed from buoy measured acceleration signal by wavelet transform. It has still not enough knowledge to know how a freak wave looks like in the real world. The result shows more confident cases of freak waves by the joint assessment of 5 conditions. In this study, we find 18 freak wave cases within the 9122 time series during Typhoons.

Statistical analysis on the long-Term observations of typhoon waves in the Taiwan sea

Seventeen data buoys were deployed in the Taiwan Sea since 1997. These buoys have made measurement for more than 100 typhoons. The purpose of this paper is to study the statistical characteristics of the observed typhoon waves. High resolution directional wave spectra are obtained by analyzing the buoy data. The Significant Typhoon Wave Height (SWTH) and the Duration of Large Waves (DLW) are proposed to indicate the sea severity of typhoons. The joint effect of the SWTH and the DLW is used to assess possible impact by the typhoon waves to the coast, using the damage curve calibrated from historical events. In addition analysis on the slope of the equilibrium range on the high-frequency side of the typhoon wave frequency spectra shows that its slope is less than 3.5 in absolute value for coastal water and larger than 3.8 for the deep sea. The JONSWAP spectrum model is then used to fit the mean wave spectra of the largest sea state in typhoons. It is found that the peak-enhancement factors  obtained were 2.48 and 2.19 for the deep sea and the coastal ocean, respectively, showing that typhoon waves in Taiwan sea are in the developing stage.

Numerical Study of Monsoon Effect on Green Island Wake

ABSTRACT Hsu, T.-W.; Doong, D.-J.; Hsieh, K.-J., and Liang, S.-J., 2015. Numerical study of monsoon effect on Green Island wake. The wind effect on Kuroshio-induced island wake downstream of Green Island, Taiwan, is studied using a depth-averaged shallow-water model. The seasonal monsoon effect on spatial–temporal scales, such as the aspect ratio, dimensionless width, and Strouhal number, as well as the relative propagation speed of Green Island vortices, is studied. It is found that a NE monsoon has a more pronounced effect on wake characteristics than a SW monsoon does in terms of net speed and vorticity, aspect ratio, and dimensionless width. Vortices are pushed back and packed close to the north of the island with a smaller area of recirculation when Kuroshio flows against the NE monsoon; vortices are pushed farther downstream with a larger area of recirculation when Kuroshio flows in favor of the SW monsoon. However, the wind effect on temporal variation of island wake seems less significant. The period of vortex shedding is between 14.2 and 15.3 hours. The corresponding Strouhal number is between 0.116 and 0.125, and the relative propagation speed of vortices is nondimensional, between 0.51 and 0.56. These values are in good agreement with the results of previous reports.

Development of a GNSS Buoy for Monitoring Water Surface Elevations in Estuaries and Coastal Areas

In this work, a Global Navigation Satellite System (GNSS) buoy that utilizes a Virtual Base Station (VBS) combined with the Real-Time Kinematic (RTK) positioning technology was developed to monitor water surface elevations in estuaries and coastal areas. The GNSS buoy includes a buoy hull, a RTK GNSS receiver, data-transmission devices, a data logger, and General Purpose Radio Service (GPRS) modems for transmitting data to the desired land locations. Laboratory and field tests were conducted to test the capability of the buoy and verify the accuracy of the monitored water surface elevations. For the field tests, the GNSS buoy was deployed in the waters of Suao (northeastern part of Taiwan). Tide data obtained from the GNSS buoy were consistent with those obtained from the neighboring tide station. Significant wave heights, zero-crossing periods, and peak wave directions obtained from the GNSS buoy were generally consistent with those obtained from an accelerometer-tilt-compass (ATC) sensor. The field tests demonstrate that the developed GNSS buoy can be used to obtain accurate real-time tide and wave data in estuaries and coastal areas.

Bathymetry Determination From Marine Radar Image Sequences Using the Hilbert Transform

This letter presents an image processing technique based on the theory of the Hilbert transform to determine the coastal bathymetry from marine radar image sequences. Use of the Hilbert transform enables the difficulties and complications of inhomogeneous image analysis to be avoided. In addition, a number of steps and complex computations can be avoided using the numerical algorithm of the Hilbert transform. Because the Hilbert transform is suitable for only monocomponent signals, we first applied the image procedure to decompose the irregular wave patterns into different single wave period images. Analysis of the simulated wave field and radar image sequences demonstrated that there was a high correlation between the estimated depths and reference depths. The causes for errors in the bathymetry estimations are also discussed.