Visibility of River Plume Fronts with an X-Band Radar

A land-based X-band radar was employed to observe river plume fronts at the mouth of the Tenryu River, Japan. Time-averaged radar images captured fronts extending offshore from the river’s mouth as bright streaks. Comparisons between satellite optical images and radar images confirm that streaky features in the radar image represent color river plume fronts. Further corroboration comes from field observations of water temperature, salinity, and turbidity conducted simultaneously with the radar measurements. When a survey ship crossed the front, the measured properties varied discontinuously, suggesting that water from the river and sea converged there and also that a downwards current was present. Variation of visibility of the fronts was assessed and compared with the rate of variation of water level and the wind speed and direction. The radar is able to image fronts when the water level is decreasing during ebb tide and the wind speed is over 3 m/s along shore. Surface ripple waves are generated by the local wind, and if they propagate across the front, wave heights increase, causing higher backscatter of the emitted radar beam. This observation gives further evidence on the imaging mechanism of river plume fronts with X-band radars in relation to wind direction

Automatic Shoreline Position and Intertidal Foreshore Slope Detection from X-Band Radar Images using Modified Temporal Waterline Method with Corrected Wave Run-up

Automatic and accurate shoreline position and intertidal foreshore slope detection are challenging and significantly important for coastal dynamics. In the present study, a time series shoreline position and intertidal foreshore slope have been automatically detected using modified Temporal Waterline Method (mTWM) from time-averaged X-band radar images captured throughout the course of two-week tidal cycle variation over an area spanning 5.6 km on the Hasaki coast between 12 April 2005 and 31 December 2008. The methodology is based on the correlation map between the pixel intensity variation of the time-averaged X-band radar images and the binary signal of the tide level ranging from −0.8 m to 0.8 m. In order to ensure the binary signal represented each of the water levels in the intertidal shore profile, determining the water level direction-wise bottom elevation is considered as the modification. Random gaps were detected in the captured images owing to the unclear or oversaturation of the waterline signal. A horizontal shift in the detected shoreline positions was observed compared to the survey data previously collected at Hasaki Oceanographical Research Station (HORS). This horizontal shift can be attributed to wave breaking and high wave conditions. Wave set-up and run-up are the effects of wave breaking and high wave conditions, respectively. The correction of the wave set-up and run-up is considered to allow the upward shift of the water level position, as well as shoreline position, to the landward direction. The findings indicate that the shoreline positions derived by mTWM with the corrected wave run-up reasonably agree with the survey data. The mean absolute bias (MAB) between the survey data and the shoreline positions detected using mTWM with the corrected wave run-up is approximately 5.9 m, which is theoretically smaller than the spatial resolution of the radar measurements. The random gaps in the mTWM-derived shoreline positions are filled by Garcia’s data filling algorithm which is a Penalized Least Squares regression method by means of the Discrete Cosine Transform (PLS-DCT). The MAB between survey data and the gap filled shoreline positions detected using TWM with corrected wave run-up is approximately 5.9 m. The obtained results indicate the accuracy of the mTWM with corrected wave run-up integrated with Garcia’s method compared to the survey observations. The executed approach in this study is considered as an efficient and robust tool to automatically detect shoreline positions and intertidal foreshore slopes extracted from X-band radar images with the consideration of wave run-up correction

Estimation of Shoreline Positions by Combining X-band Radar and SAR Observations

Shoreline positions of Kashima Coast facing the Pacific Ocean, which is approximately 16 km long with Hasaki Fishery Port at the south end and Kashima Port at the north end, have been observed with four land-based X-band radars and Synthetic Aperture Radar (SAR) satellite. X-band radars observe shoreline positions continuously in time but do not cover the whole coast. On the other hand, SAR covers the whole spatial domain, but data is available only a few times in a year. The purpose of the present work is to propose a data fusion method which combines different shoreline data observed by X-band radars and SAR satellite with the help of Garcia\u27s method, a Penalized Least Square (PLS) regression based on Discrete Cosine Transform (DCT). Garcia\u27s method is initially applied to shoreline positions dataset derived from X-band radars, and its performance has been checked for this dataset with artificial gaps. Then Garcia\u27s method is executed to combine Radar and SAR shoreline positions dataset together. The data fusion result is verified by survey data, and we confirm that our fusion method performs reasonably well to process shoreline data set

SAR衛星による高頻度観測結果を用いた鹿島灘南部の汀線変動解析

合成開口レーダ（Synthetic Aperture Radar）衛星はマイクロ波レーダを搭載した衛星であり，雲天時，夜間にも観測可能である．そのため光学（可視）衛星に比べ同一地点のシーンを高頻度に取得できる．年に複数回取得されたシーンより汀線位置を読み取り，漂砂系内の季節的な汀線変動を追跡した．高頻度に観測されたデータの解析結果を示し，その優位性を検討する．具体的には地球観測衛星ALOS (PALSAR)（運用期間：2006～2011年）とALOS-2 (PALSAR-2)（2014年～現在）が取得した鹿島灘南部（鹿島港～利根川河口）の32シーンを分析した．2006年から2016年にかけての約10年間の汀線変動の特徴を議論し，年に1回取得される観測結からは抽出できない汀線変動特性を説明する

Collapse and recovery process of the sand spit at the Tenryu River mouth on the Pacific Coast of Japan

This paper investigates the collapse and recovery processes of the sand spit at the Tenryu River mouth on the Pacific Coast of Japan, when two characteristic typhoons, Man-Yi and Fitow, passed over the study site in the year 2007. Although these two typhoons caused equally high storm waves, these two events were different in principal wave directions and in the amount of river discharges. As a result, Man-Yi collapsed the sand spit, while Fitow rather enhanced the recovery of the sand spit. Successive still images recorded by six field cameras were analyzed to investigate the dynamic morphology change of the river mouth for 2 months during which these two events had occurred. Comparisons of obtained topography changes and various hydrodynamic characteristics yielded several findings: (i) the sand spit was breached approximately 6 h after the peak of flow velocity and 2 h after the peak of the water head difference across the sand spit; (ii) the breached part of the sand spit was refilled by wave-induced shoreward sediment transport; and (iii) a core sample showed three clear layers of graded sedimentary structures of gravel, which correspond to the number of observed high waves overtopping the sand spit

Observation of Whole Flushing Process of a River Sand Bar by a Flood Using X-Band Radar

Morphological changes during a flood event in July 2010 were observed with X-band marine radar at the mouth of Tenryu River, Shizuoka, Japan. Radar images were collected hourly for more than 72 h from the beginning of the flood and processed into time-averaged images. Changes in the morphology of the area were interpreted from the time-averaged images, revealing that the isolated river dune was washed away by the flood, the width of the river mouth increased gradually, and the river mouth terrace expanded radially. Furthermore, image analysis of the radar images was applied to estimate the migration speed of the brightness pattern, which is assumed to be a proxy of bottom undulation of the river bed. The migration was observed to be faster when the water level gradient between the river channel and sea increased

Shoreline Change around a River Delta on the Cox’s Bazar Coast of Bangladesh

A recent erosional problem around a river delta on the Cox’s Bazar coast was analyzed in this study. The coastline extends from south to north. Rapid erosion has affected some portions of a 24-km road along the coast, and local authorities have attempted to protect the road via revetment. However, the structure was soon buried with sediment because of a growing sand spit along the river delta, and a new area was eroded. Shoreline positions for a 44-year (1972–2016) period were digitized using Landsat images. From the time stack images, we observed a sand spit growing in a northward direction from 2000 to 2015, and the adjacent erosion area extended in the same direction. We employed a numerical model (MIKE21FM SM) for the computation of wave-driven currents and sediment transport along the coast, and attempted to reproduce recent erosional processes. The numerical result shows that net littoral drift is dominant in the northward direction along the coast, which is the same direction of the spit growth observed in the satellite images. A higher amplitude spit induces higher sediment transport compared to a low amplitude spit because of the difference in local incident wave angles resulting in greater positive gradient of the longshore sediment flux distribution, causing erosion in the downcoast

Automatic Shoreline Position and Intertidal Foreshore Slope Detection from X-Band Radar Images Using Modified Temporal Waterline Method with Corrected Wave Run-up

Automatic and accurate shoreline position and intertidal foreshore slope detection are challenging and significantly important for coastal dynamics. In the present study, a time series shoreline position and intertidal foreshore slope have been automatically detected using modified Temporal Waterline Method (mTWM) from time-averaged X-band radar images captured throughout the course of two-week tidal cycle variation over an area spanning 5.6 km on the Hasaki coast between 12 April 2005 and 31 December 2008. The methodology is based on the correlation map between the pixel intensity variation of the time-averaged X-band radar images and the binary signal of the tide level ranging from −0.8 m to 0.8 m. In order to ensure the binary signal represented each of the water levels in the intertidal shore profile, determining the water level direction-wise bottom elevation is considered as the modification. Random gaps were detected in the captured images owing to the unclear or oversaturation of the waterline signal. A horizontal shift in the detected shoreline positions was observed compared to the survey data previously collected at Hasaki Oceanographical Research Station (HORS). This horizontal shift can be attributed to wave breaking and high wave conditions. Wave set-up and run-up are the effects of wave breaking and high wave conditions, respectively. The correction of the wave set-up and run-up is considered to allow the upward shift of the water level position, as well as shoreline position, to the landward direction. The findings indicate that the shoreline positions derived by mTWM with the corrected wave run-up reasonably agree with the survey data. The mean absolute bias (MAB) between the survey data and the shoreline positions detected using mTWM with the corrected wave run-up is approximately 5.9 m, which is theoretically smaller than the spatial resolution of the radar measurements. The random gaps in the mTWM-derived shoreline positions are filled by Garcia’s data filling algorithm which is a Penalized Least Squares regression method by means of the Discrete Cosine Transform (PLS-DCT). The MAB between survey data and the gap filled shoreline positions detected using TWM with corrected wave run-up is approximately 5.9 m. The obtained results indicate the accuracy of the mTWM with corrected wave run-up integrated with Garcia’s method compared to the survey observations. The executed approach in this study is considered as an efficient and robust tool to automatically detect shoreline positions and intertidal foreshore slopes extracted from X-band radar images with the consideration of wave run-up correction