A Channel Experiment on the Interaction Between Woody Debris and Mobile Bed

In recent years, the occurrence of natural disasters in Taiwan has turned into compound disasters. For example, a landslide disaster combined with large woody debris could be conveyed by water flow to damage the piers of bridges. Therefore, to understand the patterns, characteristics and hydrological conditions of woody debris is key for preventing woody debris related disasters. In this research, the dynamics of wood debris is observed in a moveable bed channel using single logs. The interaction between woody debris, channel bed, movement mechanism, and surface flow fields are investigated. Wood debris were presented by logs of 3 different diameters, 4 lengths and 3 different orientations; parallel, oblique and perpendicular to flow. The results show wide variation on the woody debris-movable bed interaction. When the log is parallel to flow direction, two kinds of wood movements are observed. First, there is large channel bed scour in front of the log. The accumulation form into a strip and V-type, which increases the stability of the logs and the log move by pivoting. Secondly, the log is entrained by a small discharge, resulting to less significant scour and sliding movement. For logs oblique or vertical to flow direction, two kinds of movement are observed. First, the logs bear height hydraulic resistance, resulting to both scour and deposition. The log will simultaneously pivot and slide. Secondly, the logs at both angles are entrained by an even smaller discharge (when compared to logs parallel to flow), and roll downstream. When the log length increases, the stability of woody debris parallel to flow direction also increased. Moreover, in logs oblique and perpendicular to flow, length show poor correlation with stability. Log diameter on logs oriented oblique and perpendicular to flow is the key factor for bed variation. Furthermore, increasing log length on these orientations yielded more scour depth and more deposition, while scour area and volume decreased with length. Finally, from the studied logs, none moved by floating, only pivoting, sliding and rolling were observed. The results of this study can provide a reference basis for understanding the dynamics of logs in a moving bed channel.近年來台灣所發生之天然災害，從原本的單一災害轉為複合式天然災害，如單純之土砂災害中，若同時搬運大量的漂流木運動，漂流木移動會撞擊橋墩及破壞橋樑結構。因此了解漂流木堆積在河道之型態、特性及啟動條件，對於防治漂流木災害是很重要的關鍵。本研究為了瞭解漂流木在河道中的運動方式，利用渠槽試驗模擬單根漂流木在動床渠道之啟動情形，使用不同漂流木之長度、直徑及與水流方向夾角之條件，並探討漂流木與泥砂的互動關係、運動機制、表面流場和流速變化。 試驗結果顯示，漂流木啟動時會與泥砂發生互動，在漂流木擺放方向為θ=0°時，會有兩種運動方式，第一種是漂流木前端發生淘刷，會堆積成長條狀及V型堆積，進而增加漂流木的穩固性，使其以軸轉動方式啟動，第二種是漂流木的啟動流量較小時，對底床的淘刷現象較不明顯，使漂流木以滑動的方式啟動。在漂流木擺放方向為θ=45°與90°時，會有兩種運動方式，第一種是淘刷與堆積會發生在漂流木承受較大水力阻力之位置，而漂流木後方形成三角形堆積物，會使漂流木的運動方式為軸轉動與滑動同時進行，第二種是啟動流量小，以滾動方式啟動。漂流木的長度增加會提升水平方向漂流木的穩固性，而漂流木傾斜與垂直時，漂流木長度增加使淘刷深度與堆積高度有正向關係，但對淘刷堆積範圍和體積量則是負向關係。漂流木與水流方向平行、傾斜及垂直時，直徑是影響地形變化的重要因子。利用漂流木福祿數與漂流木浮力的比較，可以劃分出漂流木運動方式的條件，以說明本試驗的漂流木運動方式，本研究之結果可提供未來模擬漂流木在動床上對河道影響之參考依據。摘要 i Abstract ii 目錄 iv 圖目錄 vi 表目錄 viii 符號說明 ix 第一章前言 1 第二章文獻回顧 4 2.1 漂流木之定義 4 2.2 漂流木對河道形貌之影響 4 2.3 漂流木對泥砂運移之影響 6 2.4 漂流木之運移 9 2.5 漂流木之運動機制 10 第三章試驗設計與方法 15 3.1 試驗目的 15 3.2 試驗設計 15 3.2.1 試驗渠槽設計 15 3.2.2 漂流木設計 17 3.2.1 泥砂條件 19 3.2.2 水流條件 20 3.2.3 雷射地形掃描 21 3.2.4 測量表面流場及平均流速 23 3.2.5 座標轉換方法 25 3.3 試驗流程 27 第四章試驗結果與分析 31 4.1 漂流木之啟動方式 31 4.1.1 漂流木0°方向的啟動方式 32 4.1.2 漂流木45°方向的啟動方式 37 4.1.3 漂流木90°方向的啟動方式 42 4.2 漂流木對底床之影響 46 4.2.1 漂流木與水流方向夾角對底床變化之影響 47 4.2.2 漂流木長度對底床變化之影響 48 4.2.3 漂流木直徑對底床變化之影響 54 4.3 漂流木對表面流場及流速之影響 59 4.3.1 漂流木影響表面流場 59 4.3.2 漂流木影響表面流速 62 4.3.3 漂流木福祿數對漂流木運動方式之影響 64 第五章結論 66 參考文獻 68 附錄 1漂流木地形掃描結果 72 附錄 2漂流木周圍流速資料 7