受熱帶暴雨影響的懸浮泥沙與河床質變化-以士文溪為例

The Variation of Suspended Sediments and Riverbed Material due to Tropical Storms – A Case Study of Shi-Wen River

席克懷(Samkele Sikhulile Tfwala)

指導教授 ： 王裕民 ； HASH(0xea11140) ； 顏才博

國立屏東科技大學/國際學院/熱帶農業暨國際合作系/碩士(2012年)

https://doi.org/10.6346/NPUST.2012.00022

摘要

台灣位處於易致水災，區域內其降雨特性是集中於夏秋季節的颱風和暴雨。這些暴雨量大約佔全年雨量的三分之二至四分之三。因此，在河川中產生較大的流量，導致水流中懸浮質增加，而部分的懸浮細粒成為了組成河床質顆粒。因此，本研究的目的在於評估暴雨對於士文溪懸浮泥沙量與河床質的影響。本研究共觀察六場暴雨，每場暴雨均使用普萊斯流速儀及全水深含沙取樣器（DH-59）作為量測流速及含沙濃度的工具，（研究期間係介於2011年7月至2012年8月）而暴雨中資料的觀測頻率為每2小時進行觀測乙次。此外，研究中蒐集水利署自2011年元月至2012年10月之旬流量與含沙量作為含沙量率定曲線準確度比較分析之用。為評估河床質在雨季前後特性的變化，研究中完成了粒徑分布，粒徑幾何平均值，標準差，分佈型態及糙率。本研究河床質調查係採用體積法和表面法，在研究中共調查11處，其中體積法約6公里1處，而表面法則約距離3公里1處。經由本研究所觀測的流速與含沙濃度資料得知以暴雨資料建立之含沙量率定曲線的判定係數（R2）0.87較以旬資料所建立者（R2=0.26）為高。為評估驗證所建立之模式之可用性，研究中利用南瑪都颱風實測資料與模擬結果進行比較。結果發現，以六場暴雨建立之模式所得結果低估18％，而以旬資料所建立模式則低估73％。再者，吾人亦發現在流量與含沙粒徑上，當流量增加時D50與D20有漸低的現象。反之，D90則有漸增的趨勢。由河床質在雨季前後的變化研究上發現研究河段表層河床質粒徑(表面法)之幾何平均質變大而底層(體積法)則變小。河床質的幾何標準差在雨季後則變小，顯示河床質趨向較為均勻。而底層材料分佈因暴雨而由雙峰轉變為多峰分佈，表層材料的峰級則由礫石轉為大卵石。體積法幾何平均值的變小將導致粗糙係數的降低。經由本研究結果可知每年的暴雨改變了河川特性與河床型態。最後，以10年與2年重現期流量利用徐爾滋數與雷諾數關係圖研究，顯示河床形態分別變為逆砂丘與過渡河床。

關鍵字

懸浮泥砂、輸砂量、暴雨、泥砂粒徑、河床質、士文溪

並列摘要

Taiwan, because of her location is a flood prone region and is characterized by typhoons and torrential rains, which concentrates during summer-autumn season. These storms bring about two-thirds to three quarters of the annual rainfall amount. Consequently, greater flow strengths will result in rivers, which in turn will increase the amount of suspended sediments and will entrain some fraction of the grains that constitute the bed. Therefore, the purpose of this research is to assess the impacts of these storms on suspended sediments and riverbed material. Six storms occurred and were observed during the study period. In each of these storms, Price meter and depth-integrating sampler, model DH-59 are used to measure flow velocity and suspended sediment concentration (SSC) in Shi-Wen River, Southern Taiwan. These data were collected every two hours in the study period, July 2011 to August 2012. Sediment rating curves (SRC) were developed from the observed discharge and sediment discharge for each of the six storms and all
the data observed. In order to determine the accuracy of the developed SRC by using the storm events data, 10-day interval flow and suspended sediment data was also collected from the Taiwan Water Resources Agency for the period of January 2011 to October 2012. To assess the riverbed material, characteristics of riverbed are observed prior to and after the typhoon season of 2012. These included grain size distribution, geometric mean and standard deviation, bimodality and roughness coefficient. Volumetric and grid methods are employed to sample bed material. In total, 11 sites are observed , located at about 6 km and 3 km from each other for volumetric and grid method, respectively. The results evidenced that sampling during these storms is sufficient to generate SRC with higher confidence (R2 = 0.87) in estimating suspended sediment discharge compared to the 10-day interval SRC having R2 of 0.26. To evaluate the applicability of the developed models, discharge of typhoon Nanmadol was applied to estimate sediment discharge and comparing results to the observed sediment discharge from the same typhoon. The SRC developed from the six storms underestimated suspended load from typhoon Nanmadol by 18% compared to 73% from the 10-day interval. Further, in the relationship between suspended sediment size and discharge, there is a decreasing trend of suspended sediment size for D50, D20, and an increasing trend for D90 with increasing discharge. Although these variations are observed, the relationship is not significant reflecting that sediment size is supply controlled rather than a function of flow. The characteristics of riverbed changed markedly after these storms. A decrease and an increase in geometric mean size of grains in riverbed were observed for subsurface and surface bed material, respectively. Geometric standard deviation decreased in all sites after typhoons, indicating a shift to a more homogeneous bed material. Subsurface was found to be bimodal prior to typhoons and polymodal after. For surface material modal class was in the gravel class, while after typhoons it shifts towards cobble class. The reduction in geometric mean resulted to a decrease in roughness coefficient. The research concludes that storms alter the stream properties and can change the bed form in a short time. Finally, the relationship of Shields and Froude number are studied by applying 10-year and 2-year return period simulated flows and a change in the bed form to antidunes and transition form is observed, respectively.

並列關鍵字

suspended sediments, sediment discharge, storms, sediment size, riverbed material, Shi-Wen River

參考文獻

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