量化極端事件在輸砂上之影響

Quantifying the Impacts of Extreme Events on Sediment Discharge

林穎東(Ying-Tong Lin)

指導教授 ： 黃誌川

國立臺灣大學/理學院/地理環境資源學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202000310

摘要

率定曲線是一個常見的輸砂計算方式，其利用兩個參數，代表了沉積物供給a係數與代表輸送能力的b 冪次項，來描述沉積物濃度與河流之間的關係。但是對於極端事件對輸砂變化的影響仍缺乏完整了解。地震與颱風在形塑地景上扮演了關鍵的角色，反應了地球表面活動之間的交互關係與回饋。台灣是地震與颱風的熱區，提供一個很好的舞台來分析極端事件後的輸砂變化。然，率訂曲線的參數卻會因見解與方法的不同而有不小的差異，本研究選擇台灣濁水溪上下游集水區與高屏溪下游做為案例，使用三種方法來計算率定曲線: (1)率定曲線a係數與b 冪次項皆不固定; (2) 率定曲線a係數不固定，b 冪次項固定在事件前期平均; (3)率定曲線a係數固定在事件前期平均，b 冪次項不固定，藉由比較三者參數與輸砂量變化結果，了解沉積物輸送量的變化，以及輸送能力的改變。
結果顯示固定一個參數來比較另外一個參數事件前後期變化，對於事件更加敏感。但是不固定任一參數去計算出之率定曲線，在估計輸砂量上有較好的統計精度，具有較低的RMSE與較高的R2。輸砂輸送體系顯示在濁水溪上游在集集地震、高屏溪下游在莫拉克颱風後，不管在事件尺度或是年尺度上，輸砂濃度上升但單位輸送能力下降；相反的濁水溪下游的輸砂輸送區域則維持穩定。濁水溪上下游在集集地震之後高流量事件帶來的輸砂分別增加了約663Mt與230Mt，但高屏溪下游未有顯著增加；而莫拉克颱風之後，僅濁水溪下游略增加28Mt，其餘兩者並未增加，可能是因為b 冪次項減少，輸送能力下降導致。崩塌地比例與位置可能扮演一個重要的角色，其可能和a參數有正相關，與b 冪次項有負相關。

關鍵字

地震 ； 颱風 ； 輸砂傳輸

並列摘要

Rating curve method, which used two parameters, a and b, representing sediment supply and kinetic transport to describe the relationship between sediment concentration and streamflow, is widely used in estimating sediment discharge. However, it still lacks fully understanding about sediment discharge changing on extreme events. Earthquake and typhoon activities play a critical role in landscape shaping which response to the interactions and feedbacks among earth surface processes. Taiwan is the hotspot of earthquake and typhoon, which provides a good platform for analyzing sediment discharge changing after extreme events. However, rating curve parameters may be difference by different opinions and calculated methods. This study selected Choshui river, both upstream and downstream gauges, and Kaoping river, a downstream gauge, as our study sites. Calculated the rating curve parameters by three methods: (1) Both parameters, a and b, are free; (2) Only parameter a is free and parameter b is determined by the pre-event period, and (3) Only parameter b is free and parameter a is determined by the pre-event period. By comparing three methods parameter and sediment discharge changing, can understand more about the change of sediment amount and the transport power of the river.
The results show that fixing one parameter and calculating other parameters is a more sensitive way to determine events than other methods. However, calculating rating curve without fixing any parameter has better statistical accuracy on estimating sediment discharge, which has lower RMSE and higher R2. Sediment transport regime shows that sediment concentration increases but the unit transport ability decreases in upstream Choshui after Chi-Chi earthquake and downstream Kaoping after typhoon Morakot at either event scale or annual scale, while the sediment regime in downstream Choshui keeps constant. Event-induced sediment discharge in Choshui upstream and downstream after Chi-Chi earthquake was increased about 663Mt and 230Mt, respectively, but downstream Kaoping sediment discharge didn’t has significant increase. After typhoon Morakot, only downstream Choshui increased about 28Mt, the others basins didn’t increase, which may because b exponent decrease and transport ability decrease. Landslide ratio and location may play a crucial role in the river system where landslide may have a positive relation with a coefficient and a negative relation with b exponent.

並列關鍵字

Earthquake ； Typhoon ； Sediment transport

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