探討系列梳子壩土石流之衝擊力與規模關係之研究

Abstract

台灣位處亞熱帶地區，每當颱風來襲，暴雨劇烈，加上河川源短流急，地質脆弱，若遇豪雨極易使堆積於河床之土石與暴漲之溪水混合後傾瀉而下，對下遊造成嚴重災情與損失， 921集集大地震後，對中部山區造成廣泛破壞，其中又以土石流災害發生最為頻繁，如民國93年七二水災導致台中縣和平鄉嚴重之土石流災害，嚴重影響下游地區人民生命財產與安全，是故對土石流特性了解及災害防治刻不容緩。 本研究針對土石流規模與衝擊力之關係進行探討，於室內試驗中以等重量替代法模擬松鶴現地粒徑分佈曲線，進行土石流防治工法之水槽模型試驗，採用90度及60度兩種型式梳子壩體，並於水槽上下游裝設系列梳子壩，觀測上下游各種不同土石量之流動狀況、堆積情形與對壩體產生之衝擊力，將兩種不同壩體之量測結果進行分析比較，以探討土石規模與衝擊力之關係，並於梳子壩前設置具有緩衝性質材料之輪胎觀察是否有其減緩土石流衝擊力之特性。 由室內試驗結果顯示：水槽坡度越大時，土石堆積高度越高，所形成之自然堆積角度亦愈大。在開口寬度與最大粒徑比（B/Dmax）為1.5情況下，兩種梳子壩貯砂效果相似，貯砂率會隨著水槽坡度增加而減少。梳子壩壩型90度所受之壩體震動與位移量皆比梳子壩壩型60度大，此乃壩型60度之側向彈性勁度K較梳子壩90度為大。而當壩體角度越大，承受土石流正面撞擊之可能性越高，故梳子壩壩型90度所受之平均衝擊力較壩型60度大。衝擊力減緩方面，在壩體前加裝緩衝性質之輪胎時，上游梳子壩大約有降低15~20% 衝擊力趨勢，下游梳子壩，大約降低25~40%衝擊力。另外，由頻率域分析結果所得之衝擊力分別對不同壩體形式三種不同河道坡度與規模進行分析，結果顯示土石流規模愈大或水槽坡度愈大，衝擊力亦愈大。

Taiwan is an island located in the subtropical zone. It is generally known that typhoons often bring heavy rainfalls whenever they occur in this area. In addition, the streams flow rapidly and the geological condition is fragile. Especially after the 921 Chi-Chi earthquakes, the geological condition of the mountain area located in the central part of Taiwan tends to be more susceptible to natural disasters. For example, in 2004, flood caused the debris flow disaster at the Xin-Yi Village in Nantou County and at the Song-Her Village in Taichung County. The flowing condition of the debris flow was found to be very special and vigorous and its destructive power was so strong that it has seriously damaged the property and affected the quality of lives of the people in the downstream. Therefore, it is of great importance to understand the characteristics of debris flow and to develop the preventive measures and countermeasures for the disaster caused by debris flow. The purpose of this study is to develop the relationship between the impact force and the magnitude of debris flow. The study was conducted by first selecting an appropriate site at the Song-Her Village for the observation. The data of characteristics of debris flow was recorded by various auto-recording equipment, such as pluviometer, water pressure gages, earth pressure gages, load cells and accelerometers. Accordingly, the grain size distribution (GSD) for the laboratory tests was stimulated with the one of the in situ test at the Song-Her Village by equal weight substitution method. The flume model test was then performed on the two types of slit dams by measuring the impact force of debris flow on the model dams in different flowing situations to obtain the relationship between the impact force and the magnitude of debris flow. The results of the laboratory tests show that the accumulation angle increases with an increase in slope of the flume, thereby increasing the soil pressure to the bottom of the dam. when the ratio (B/Dmax) of the open width and maximum size of the debris flow is the same, the two types of slit dam have the similar entrapped rate. The vibration and displacement caused by debris flow to the slit dam with a front face batter of 90 degrees are greater than those to the one with 60 degrees, because the impact force of the 90 degree type of slit dam is greater than that of the 60 degree type. Besides, a direct impact force exerted on the 90 degree slit dam by debris flow is found to be greater than the 60 degree slit dam. From the results of test, it is found that when tires were put before the slit dam, the maximum impact force of the upper slit dam is reduced about 15~20% due to the damping effect of the tires. In addition, the impact force analyzed in the frequency domain, at variance slopes and scales of the rivers also increase with an increase in the magnitude of debris flow and the slope of the flume