摘要:瞭解陸源沈積物跨越大陸邊緣海的機制與路徑是海洋科學研究的重要課題之一,因為此傳輸過程對元素(例如碳)的循環、海岸形態與生態系統、以及土地資源管理具有關鍵性的影響。近年來,島嶼型河川(以台灣為例)對全球懸浮沈積物通量的貢獻已受到重視,然而,其沈積物輸出通常集中於短暫的颱風暴雨,且懸浮沈積物傳送過程受到陸棚不同外力作用如恆流、往復潮流、波浪等,而有不同的地域特性。因此,我們對這類型河川與陸棚的沈積物離岸傳輸機制瞭解仍然有限。
基於以上原因,本計畫擬以理想化的數值模式探討高懸浮沈積物濃度河川水舌(sediment-laden river plume)的動力及離岸傳輸過程。為達此目的,目前已使用 Regional Ocean Modeling System (ROMS) 建立了理想化的河口−陸棚數值模式。水舌動力之探討將依懸浮沈積物濃度區分為高密度(hyperpycnal; negatively
buoyant)與低密度(hypopycnal; positively buoyant)流。依此,本計畫目標為:一、瞭解重要地形(陸棚坡度、海底峽谷的存在)與沿岸外力(河川流量、潮汐、風、波浪)等因子對水舌三維結構的影響;二、量化與比較各離岸傳輸機制; 三、量化臨界坡度。當陸棚坡度超過某一臨界值,hyperpycnal flow 的底部剪應力 (shear stress)可否因避免懸浮物之沈積而增加其離岸傳輸距離;四、探討河口鹽度鋒面(salinity front)產生的動力過程與其對懸浮沈積物聚集(trapping)的作用。 最後,我們將結合所有環境因子並探討當水舌中懸浮沈積物連續變化時,各離岸傳輸機制的變化與交互作用。此計畫將可做為日後現場觀測的設計與數值模式驗證的基礎。
Abstract: Understanding the pathways of terrestrial sediment fluxes across the Earth’s continental margins (Source-to-Sink) has a first rank priority in ocean science because this transfer processes play a key role in the cycling of elements such as carbon, in ecosystem and coastal morphological responses to climate change and sea-level rise, and in resource management of soil and wetland. The small mountainous rivers, as exemplified by Taiwanese rivers, deliver an exceeding amount of sediment to the coastal oceans, making their contribution of global significance. Due to the episodic nature of sediment delivery and the modulation by complex coastal forcing, the across-shelf transport pathway of these highly turbid river outflows remains poorly understood.
We propose a process-based, idealized modeling study on the dynamics of sediment-laden river plume off small mountainous rivers. The overall objectives are to determine how the sediment-laden outflows respond to key bathymetric (e.g. slopes, presence of a submarine canyon) and forcing (e.g. river discharge, tides, wind, waves) parameters and to quantify the mechanisms of across-shelf sediment transport under various physical settings. We have constructed idealized river-shelf domains at the scales of actual river outflows, using a well-validated hydrodynamic model ROMS. The outflow dynamics will be systematically explored according to the river-borne sediment concentration:
When the sediment concentration exceeds 30~40 kg/m3, the bulk density of the river plume becomes greater than the receiving seawater. The so-called hyperpycnal plume descends to the seabed and moves offshore as an undercurrent. We will determine: (1) the responses of hyperpycnal plume structures such as thickness, width, across-shelf extension, density anomaly, and velocities to varying slopes, discharge, sediment concentration, and sediment settling velocities; (2) the critical slope over which the plume generates large enough stress to sustain its density anomaly (self-accelerating); (3) the influences of advection and mixing by tides and upwelling- and downwelling-favorable winds on 3D plume structures; (4) the conditions at which surface forcing exerted by wind and waves has negligible impacts on bottom-hugging, hyperpycnal flows; (5) the influences of a submarine canyon on plume behaviors. Numerical experiments will be guided by the configurations of Kaoping submarine canyon, and the insights gained from this work will be linked with the NSC-sponsored
field program FATES.
When river-borne sediment concentration is dilute (~1 kg/m3), hypopycnal (i.e. surface) plume prevails. Sediment trapping by a bottom salinity front beneath the plume is hypothesized to focus the highly erodible fine particles. The ambient forcing
including tides and wind/waves then ignites the subsequent across-shelf transport as gravity currents. We will determine (1) the trapping processes, in particular the interaction between mixing, flow convergence, and sediment-induced stratification; (2) the influences of tidal, wind/wave mixing on the hypopycnal plume structure and frontal formation; (3) the rate of across-shelf sediment flux due to current- and wave-supported gravity current.
Finally, the transition from hyperpycnal to hypopycnal plume will be investigated, and the integrated effects of various across-shelf transport pathways will be quantified. The results of this study will lay the groundwork for the design of future field programs and comprehensive model-data comparisons.