【Institution】: National Sun Yat-sen University, Taiwan

【Contact】: Chen Jingyan, chenjy@xmu.edu.cn

Abstract:

Zhoushui River has the largest sediment load (50-60 Mt) whose mouth is located on a mesotidal coast in central Taiwan.  Because of the strong tidal currents and seasonal monsoon waves in the Taiwan Strait, the sediment exported by the river formed a small fan-shaped delta at the mouth and a large tidal basin located immediately north of the river mouth.  This tidal system contains tidal ridges separated by large tidal channels.  Swathbars separated by cross-bar channels are large secondary depositional features more or less perpendicular to the main tidal channel.  The delta and tidal basin form a single complex sedimentation system. It is hypothesized that this system is the immediate/temporary sink of sediment exported by the river though plume-tide interaction.  The sediment might be transported out of the tidal basin during ebbing tide into the littoral drift and further dispersed by alongshore currents and tidal/coastal current systems. To test the hypothesis surfacial sediment samples from the river delta and the tidal basin were taken in May (the end of dry season) and September (end of the flood season) 2010.  The samples were analyzed for the grain-size composition of the original, the lithogenic and non-lithogenic fractions.  Six grain-sizes classes were used in the analysis, including clay, silt, very fine sand, fine sand, medium sand, and coarse sand.  The samples were also analyzed for their organic (TOC, TN) content and C/N ratio.  Statistics-based trend analysis techniques (McLaren Model and Transport Vector) and the multivariate EOF analysis technique combined with ‘digital filters’ were used to decipher the information contained in the spatial grain-size distribution patterns.  According to the McLaren Model, sediment transport directions in the study area similar for the dry and wet season data sets.  Common themes in these two

scenarios are the bi-directional transport along the surf zone on the outer edge of the complex, which is due to the tidal currents and seasonal littoral drift.  In the dry season, the influence of the river is weak, and on the side-lobes of the delta the sediment transport patterns are bi-directional (landward and seaward) due the influence of the river effluent and tide.  The sediment mainly moves from the river delta to the tidal basin. In the more energetic wet season where the water level is higher over the complex, bi-directional transport exists along the main tidal channel and along the upper tidal flat, the net sediment movement is still from the river into the tidal basin.  Over the delta, due the increased river influence, the sediment transport patters are seaward except on the south side of the delta where secondary flood transport exists. Results from filtering and EOF technique show among the possible forcing of river discharge, tidal transport, wave sorting, and littoral drift, none of which is the dominant factor that influenced the spatial grain-size patterns observed in the dry season. In the wet season, the northbound littoral drift appears to be the dominant factor/forcing.  This is because the incident monsoon waves are from the SW in the summer. Other forcing is of secondary importance.  Despite of the large sediment load in the summer, most sediment discharged by the river bypasses the delta-tidal basin complex and is transported further away from the study area.  However, the river is the primary source for organic material in the complex.  Through grain-size affinity, sediment that is organic-rich and has stronger terrestrial signals (high C/N) is associated mostly with the distribution of clay in the upper tidal flat and river delta in both seasons.