台灣週邊海域陸源沉積物的來源主要由極端降雨事件之逕流經由河口輸出，而懸浮沉積物於河口輸送陸源物質過程受潮汐影響甚大，考慮台灣海峽中段潮差達4-5m 潮流流速最大達1.5 m/s。本研究旨在探討潮汐對於懸浮沉積物於台灣海峽之輸送之影響。 本研究於2008年至2010年間濁水溪河口進行4次現場調查，首先探討利用聲學儀器推算懸浮沉積物濃度的可行性，利用聲學儀器推算懸浮沉積物濃度之準確性會受到懸浮沉積物粒徑以及儀器參數的影響，針對此兩項因素影響聲學儀器的不確定性，透過聲學儀器回聲強度平均後降低其不確定性並利用光學濁度值與採水樣本關係式間接推算懸浮沉積物濃度，希望透過此方法降低聲學儀器的不確定性影響，並以此估算懸浮沉積物濃度。 本研究調查結果發現濁水溪河口懸浮沉積物之粒徑大小分佈呈非線性韋伯分佈，據此本文以此分佈描述不同粒徑懸浮沉積物濃度之時空變化。本研究透過粒徑分佈形狀參數與尺度參數分別與採水樣本懸浮沉積物濃度進行比較，發現當懸浮沉積物濃度遞增時形狀參數遞減、尺度參數遞增，整體分佈向大粒徑方向偏極，說明懸浮沉積物濃度與粒徑分佈特性具有高度相關，且不受沉降或再懸浮機制之影響。在台灣海峽中的潮汐對於河口陸源沉積物之輸送影響方面：1.針對潮流流速引致的再懸浮部分由觀測與採水樣本結果中發現，流速增加將使底層之懸浮質濃度增加，其增加速度高於表層，增強近岸河口的層化效應；當流速高於0.8 m/s時，層化效應阻礙紊流的發展使底床剪應力減弱，限制底床懸浮質濃度的繼續增加。此外於2008年調查中直接觀測到懸浮沉積物於河口附近之沉降，其沉降速度為0.0013 m/s以及0.0022 m/s，跟史托克公式結果相符。 本研究發現陸源沉積物於極端事件與一般降雨事件有顯著差異：一般事件下濁水溪河口懸浮沉積物濃度之時空分布受潮流引致之再懸浮機制影響為主，懸浮質濃度與流速大小具有高度相關，其間存在約80分鐘之相位差。再懸浮發生時，粒徑為16 μm以下之細懸浮顆粒所佔之比例增加。而極端事件下濁水溪河口懸浮沉積物濃度受潮汐水位影響為主，低潮位時河口沉積物輸送通量增加，河口海域SSC隨之增加並形成水舌。 The annual yield of terrestrial sediment in the estuaries around Taiwan is dominated by fluvial discharge in extreme rainfall events. The transportation of terrestrial suspended sediment in the middle of Taiwan Strait is effected by tidal current, considering that the tidal range reachs 4-5 m while the current speed up to 1.5 m/s. The present study emphasize on the tidal effects on suspended sediment transport in Taiwan Strait. Four field surveys, including a typhoon case in the summers from 2008 to 2010 at Cho-Shui estuary were carried out. For the suspended sediment concentration (SSC) measurement, we discuss the validity of using acoustic echo intensity to infer the SSC. The associated accuracy is influenced by suspended sediment particle size and instrument parameters. In present study we establish the relationship of acoustic echo intensity with the optical back-dispersion turbidity to reduce the uncertainty of the estimation of SSC. We found that the Nonlinear Weibull distribution can be adopted to describe the probability distribution of suspended sediment particle size. It is found that the Weibull shape parameter is inversely proportional whilist the Weibull scale parameter is proportional to SSC. These results illustrate that the SSC feature high correlation to suspended sediment particle size distribution. Concerning to the tidal effects, resuspension by strong tidal current dominated the SSC in the estuary. The SSC at the bottom boundary layer increases with the current speed, moreover it increases more rapidly than in the sub-surface layer. Hence it contributes to the stratification strengthened in the estuary. As a result, when current speed reaches 0.8 m/s, the stratification effect damps the turburlence intensity, and reduce shear stress in bottom boundary layer, which limits the resuspension. This study also show a significant difference between normal condition and extreme events: In normal condition, temporal and spatial distribution of the SSC is dominated by the resuspension incluced by tidal current, with a phase lag of 80 minutes. In the Cho-Shui estuary, when the resuspension dominates, suspended sediment with grain size greater than 16 μm increases. In extreme events, the tidal sea-levels is responsible for the SSC temporal and spatial distribution, the estuary SSC increase rapidly by the low slack tide induced sediment discharge.
