現地調查與模型實驗模擬堰塞湖演化機制:以那瑪夏堰塞湖為例

Abstract

本論文利用野外現地調查以及實驗模型，預測山區堰塞湖演化發展。莫拉克風災後，南台灣山區崩塌情形嚴重，大量崩塌土方堆積於主支流交匯處產生堰塞湖，為觀察堰塞湖現地實際變化情形，以旗山溪與二溪交匯處那瑪夏堰塞湖為研究對象，於2011年一月及2012年二月同地點進行野外量測，在兩期調查中取得旗山溪上游河道縱剖面高程與現地淤積土體取樣分析。為解釋現地河道形貌反應，故根據過去現地資料及條件，利用系列一維動床實驗在不同底床坡度、支流砂量及流體濃度條件下，模擬堰塞湖地形變化。比較現地狀況與實驗結果得知，初始坡度與主支流砂量比主要影響堰塞湖規模大小；而流體濃度則控制底泥厚度多寡與堰塞湖存在時間長短；且經過實驗參數率定驗證後，可提供現地河川未來形貌演化模擬與預測。

Using field surveys and laboratory experiments, the present study examines the evolution of a debris-dammed lake in montane Taiwan. During Typhoon Morakot in August 2009, a tributary called Creek No. 2 supplied abundant debris to its confluence with the Chishan River, causing the formation of Namaxia Lake. To document the subsequent evolution of the debris-dammed lake, two surveys were conducted in January 2011 and February 2012. In particular, long profiles were measured and the distribution of fine and coarse sediment material was observed. To model the river response observed in the field, experiments were conducted in a narrow flume. Different runs examined the influence of initial slope, tributary influx, and fine sediment concentration. Initial slope and tributary influx are found to greatly influence the observed long profiles. Fine sediment concentration, by contrast, exerts little influence on long profiles, but affects the composition of lake deposits and the duration of lake existence. By adjusting these experimental parameters, it is possible to obtain river responses similar to those observed in the field.