Discussion

1. Results represented a distinct diurnal and semi-diurnal mixed tides, dominated by the semi-diurnal tide. The maximum significant wave height (Hsig) was only 65 cm and modulated by tides. The main currents were long-shore currents. Due to the warmer and fresher water discharged from the Dapeng Bay via the inlet, the salinity and temperature time series showed the opposite fluctuations following semi-diurnal frequencies. The turbidity and total volume concentration also synchronously increased as the water become warmer and fresher. According to the sea surface fluctuation harmonic analysis, the form number (Nf) and tidal to total energy ratio (ER) are 1.44 and 98%, res-pectively. The result Fig. 1 Fig. 2 Suspended particle-size distribution and their dynamics off the tidal inlet of the Dapeng Bay (a lagoon in Southern Taiwan) Introduction Dapeng Bay is a high productivity and sensitive lagoon in Southern Taiwan. This lagoon was filled with aquaculture farms for oysters and fishes and received some city waste water. Since suspended particles are vehicles carrying minerals, nutrients, contaminants from land to the ocean. Therefore, it is important to understand how suspended sediments exchange between the lagoon and the coastal ocean. In a previous study, suspended particles in the lagoon effluent was transported to the coastal ocean through the inlet. And then, transported by long-shore tidal currents. Re-suspension by waves was a driving force to move the sediments in the coastal zone. This study focused on the responses of different grain sizes to the coastal forcing. Instrument tripod deployment A tripod was deployed at the average depth of 9 m off the inlet of the Dapeng Bay (fig. 1) from June 5-15, 2001. The tripod contained three instruments (fig. 2), including LISST-100 (measuring suspended particle-size spectra, temperature, and pressure at approximately 0.6 m above the bed), SP2100 (measuring incident wave height, direction, period, current direction and speed, tidal height, and turbidity (OBS) at about 1 m above the bed), and TD-410 (measuring temperature, conductivity, DO, and turbidity at approximately 0.6 m above the bed). The sampling intervals of these three instruments were synchronized at 30-min intervals. The burst mode (scheduled for wave and OBS) in SP2100 had 4096 data points sampled at the rate of 0.5 Hz. The tidal mode collected 32 data points per burst at 1 Hz. Twenty-six water samples were collected at 30-min intervals from 19:00 6/5 to 7:30 6/6, 2001. These samples were analyzed for suspended sediment concentration (SSC) in the laboratory using filtration method. The SSC values were than used to calibrate the OBS and LISST measurements for total concentration. Wave mode Discussion In order to understand how these co-varying forcings act on different grain-sizes of particles, the EOF (empirical orthogonal function) analysis of time series of 32 grain sizes with corresponding forcings was applied to distinguish the correlations among grain sizes with hydrodynamic parameters. The first three eigenmodes explain more than 83 % of the correlations. The first mode (identified as lagoon mode) explains 66.27 % of total correlations (fig. 3). Since all the time series data were modulated by the tide, some cause-effective relations are obscured the tidal signals. In order to enhance the influence of waves, a low pass filter was applied to remove by tidal signal. The EOF analysis of de-tided the data (fig. 4) indicated that the second mode (recognized as wave mode) increases to 24.4 %. In fig. 4, it shows there is a size-fractionation phenomenon at the size about 26.74 mm. This mode indicates that grain sizes are separated into two groups at 26.74 m, a threshold of wave sorting. The coarser group (> 26.74 m) are positively reacted to waves but the finer group are negitive. Lagoon mode Fig. 4 Fig. 3 Low Pass filter Remove tide signal Ray T. Hsu and James T. Liu* Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan, R.O.C. ray@mail.nsysu.edu.tw, james@mail.nsysu.edu.tw* • Conclusions • The major forcing for transporting suspended sediments from lagoon to near-shore through the inlet was tidal currents. • Major suspended sediment source in the near-shore off the Dapeng Bay inlet was the sediments carried by the effluent from the lagoon. Fine-grained suspended particles (smaller than 26.74 mm) are more important in this source. • The second source was coarse grained (> 26.74 mm) sediments resuspended in-situ by waves.