Acknowledgement

1. 2008 OR-3 2006 OR-3 Water level Latitude (deg. N) Water level (cm) Longitude (deg. E) Salinity 2006 8/27 8/28 13:00 14:30 15:00 17:00 19:00 20:00 21:00 22:00 01:00 02:00 03:00 04:00 Depth (m) < 63 μm 10~63μm > 10 μm 05:00 06:00 07:30 08:00 09:00 11:00 12:30 13:00 13:30 14:00 Volume concentration (μl/l) Salinity 150~200μm Depth (m) 63~150μm 2008 10~63 μm 3~10 μm 05/23 05/24 < 3 μm 14:40 15:00 16:00 17:00 18:00 19:00 20:18 21:00 20:09 23:14 00:11 01:06 Depth (m) 02:06 03:05 04:08 05:04 06:02 07:00 08:00 09:00 10:00 11:00 12:00 13:05 14:00 15:00 Volume concentration (μl/l) Tidal Variation of the Grain-size Structure of the Gaoping River Plume Tidal Variation of the Grain-size Structure of the Gaoping River Plume Rick J. Yang, James T. Liu, and Kevin J. Chang Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan 80424, ROC Introduction Globally, rivers supply sediment for most coastal depositional systems. Hydrodynamics influence the sediments in their transport and appearance. The aim of this study is to understand how the hydrodynamics affect the suspended sediment plume off the month of a small mountainous river by looking at the grain-size structure of the river plume in the course of two semi-diurnal tidal cycles Gaoping River Gaoping Submarine Canyon 材料 a b b Fig 1. Bathymetric map of the study area Fig. 2 Time variability of surface salinity structure off the river mouth in 2006(a), 2008(b). The curve is the sea water level in Siaoliuciou buoy Materials and methods Two hydrographic surveys conducted on board R/V Ocean Research III at a location approximately 0.75 km off the Gaoping River mouth from August 27 and 28, 2006 and May 23 and 24, 2008 (Fig. 1). In each survey hourly hydrographic profiling using Sea-Bird SBE11 CTD and a LISST-100 was carried out. The volume concentration of thirty-two suspended sediment grain sizes were observed by LISST-100, so we can compare the volume concentration to the salinity. Results and discussion The results show that in both years, there is a surface nepheloid layer (SBL). The SBL in Aug. 2006 is thicker than that in May 2008 (Fig. 3 and 4) because August is the month of maximum river discharge in a year. Negative linear relationships between grain-size concentrations and the salinity suggest that they are associated with the river plume and the sediment-laden river plume is the major contributor to the SBL. That means river plume dispersal is the most important process controlling the SBL movements. Within the river plume, there is distinctive vertical variation of grain-size composition. In general, sediment sizes coarser than sand comprise most of the suspended sediment population in the river plume, whose temporal structures show clearly tidal influence. Size classes finer than the sand (including coarse silt to sand, very fine to medium silt, and clay) whose percentages do not vary strongly with the tide. Fig. 3 Time variability ofvolume concentration profiles in 2006 Fig. 4 Time variability ofvolume concentration profiles in 2008 Conclusion In Gaoping River plume coarse particles (larger than 63μm), whose temporal structures show clearly tidal influence. The structures of finer size than sand (including coarse silt to sand, very fine to medium silt, and clay) do not vary strongly with the tide. Acknowledgement This study was funded by the National Science Council of Taiwan under grant numbers NSC95-2611-M-110-018 and NSC96-2611-M-110-011.