探討臺灣河川輸砂傳輸特性的時空變異

Abstract

河中輸砂是了解流域地形演育的重要因子，而懸浮載（suspended load）的推估對於輸砂的估算至關重要。許多研究經常以乘冪函數（aQb）擬合河中懸浮載與流量之間的關係，並據此估計懸浮載的輸出量。本研究以全臺灣15條主要河流為對象，利用水利署於1948至2018年所觀測的懸浮載與流量數據，擬合得出乾季（11至5月）、溼季（6至10月）、一年和三年時間尺度的乘冪係數，藉以探討每條河流的乘冪係數隨時間變化的原因，並了解究竟是何種因素導致不同河流在輸砂傳輸特性（sediment transport regime）上之差異。結果發現近年來多數河川的乘冪係數有a值增加、b值減少的趨勢，臺灣輸砂行為於年內的時間分配上趨於不極端；西部河川的輸砂效率逐年降低、東部河川則逐年升高，推論受到地震空間分布影響；颱風對輸砂的影響時間集中且大量，而地震對輸砂的影響則較為長久。每條河流每季率定曲線的乘冪係數繪製於散佈圖中，會大致落於一條迴歸線上，每條迴歸線即描述該河長期平均之輸砂特性，不同時期乘冪係數與迴歸線的距離反應其短時期輸砂效率的變化，可作為瞭解不同流域間輸砂空間差異與單一流域輸砂時間變異的實用工具。Fluvial sediment discharge, estimated by suspended sediment load, is a key to understanding the topographical evolution in river basins. The rating curve method, Qs=aQb, is applied to correlate observed suspended sediment load (Qs) with river discharge (Q) and estimate hourly Qs with input of hourly river discharge. The Q and Qs data from 15 rivers during 1948-2018 in Taiwan, measured by Water Resources Agency, are used to derive the rating parameters at different time scales, including dry period (November to May), wet period (June to October), one year and three years, attempting to understand their temporal and spatial variations and relationships between the variations and sediment transport regime. Results show that rating parameters in most of the rivers have been changing to bigger “a” and smaller “b”. The efficiency of sediment transport becomes lower in the western rivers while getting higher in the eastern, which is perhaps related to the spatial distribution of earthquakes. Compared to earthquakes, typhoons lead to a shorter but enormous influence on sediment transport. The rating parameters derived from seasonal rating curves of a river more or less follow a regression line describing the long-term sediment transport regime. The distance between rating parameters and the regression line changes with time, which reflects the change in the short-term sediment transport efficiency. It is found that the rating parameters can be a practical tool to understand the sediment transport in terms of spatial differences among rivers and the temporal variation in a single river.