Study on the Initiation Mechanism of Debris-Flow Along the Gravel Slope

Abstract

Debris flows were defined that the none compaction sediments or the weathering rocks and rocks debris in slope, because of the torrential rain or runoff influence, had arrived saturated condition after water content was increase, the mixure with gravel and soil had the obvious liquefied phenomenon, under coordination of the terrain slope, impelled to form flowing. Occurrence analysis on initiation of debris flow, the slope failure caused landslide, which produced movement with the contact of upstream runoff, made the landslide material fluidize, and form debris flow, some scholars call the slope-type debris flow. For discussing the occurrence environment of slope-type debris flow, suppose the debris flow mobilized from landslides, in fact, the domestic and foreign correlations research, the initial mechanism still limited to understand. In order to analysis the initiation mechanism of debris flow, in the past many scholars that apply to the theorical or the experimental analysis carries on debris flow research, by means of field data collection with hydrology and topograpy, established the experience and the semiempirical relationship on initial mechanism. This paper was considered initial mechanism of debris flow, by the experience and the semiempirical formula, derived physico-mathematics formula. And the description of characteristics replaced by qualitative and statistical explanation and for field investigation. Therefore this paper discussed the occurrence characteristics and initial mechanism of debris flow along gravel slope. This paper contribution may states as follows: 1. Forcus on gravel slope which was easy to form debris flow, and drafted the operation flowchart included field investigation, GIS management, statistics and laboratory experiment. 2. For the feature of debris flow, according to areophoto judgement, field investigation, to analysis the topographic characteristics along gravel slope.And confirmed the difference of sharp and space distribution with landslide, slope type and channel type debris flow. 3. Using laboratory experiment to adjust discharge runoff upstream, water content in earth, the slope and the grain size composition, to simulate havior of debris flow from landslide, in order to realize initial process of debris flow along gravel slope. 4. Combining revelant theory, field investigation, the statistical analysis, the laboratory experiment, analysis initial mechanism of debris flow along gravel slope. This capital research mothod, first was collection that happened case analysis in Taiwan, and research area designation; second was theory establishment about debris flow from landslide; third was the research area investigation, analysising the occurrence characteristics about debirs flow along gravel slope; Finally the confirmation of occurrence characteristics from experiment, establishment and revision initial mechanism of debris flow along gravel slope. According to the above research mothod, after carries on the analysis and the discussion below, obtains the conclusion as follows: 1. Because water permeability was good in conglomerate formation, the runoff was easy to form seepage into the landslide block base, has the destruction to the landslide block. In addition to the higher porosity in conglomerate formation, had supply enough space to cause flowing with the water intensive mixing, and the pattern of slope had converge concave sharp, made landsliding mass in the short distance to form debris flow. 2. The debris flow occurs depend on critical slope of slopeland, is related with soil and water depth, Flow Froude number and Grain Froude number, the debris flow has the critical slope value in 26~40%. 3. If the correlation parameters with critical slope for doposit block in channel, used for doposit block in slope, it is considerd the effect of seepage force to doposit block, then must increase correction coefficient A0, its value approximately is 0.6.

4. The initiation mechanism on debris flow form landsliding mass, was considerd that runoff upstream enters landsliding mass, formed the liquefied layer to the bottom of landsliding mass, when liquefied layer was flowing, created the above landsliding mass destruction, the relations with critical slope form landsliding mass to flow, and correlation parameters indicate:

5. If the slope is bigger than 15, the distance of sliding landslide mass is over thickness of the landslide mass 4.5 time of above the distance, also has the sufficient water volume supply, the landslide mass has liquefied phenomenon to form debris flow usually.

土石流為斜面上未固結之沉積物或風化之礫石、岩屑等，因豪雨或逕流集中的影響，在土體含水量到達飽和狀態下，土壤形成明顯之流體化現象，當地形坡度配合達到發生流動的條件，而後形成流動之現象。在土石流發生的原因中，邊坡崩塌造成土石崩滑，所產生的動力與後方逕流接觸時，因崩塌土體含水量增加，使土石沿坡面發生流體化，形成土石流，有些學者稱之坡面型土石流。至於坡面型土石流發生的環境，崩滑土體於坡面上能否轉變成流動型態，及其形成流體化的過程。實際上國內外之相關研究，其瞭解仍相當有限，為能更瞭解土石流之發生機制與發生臨界條件，過去有很多學者透過理論或實驗的分析來進行土石流發生之研究，或藉由降雨及現場地文資料的收集分析，來建立土石流發生條件之經驗及半經驗關係式。 如何將土石流發生條件，由經驗及半經驗關係式，推導成具有數理基礎公式。或將野外調查得到土石流定性描述，利用統計方法取得定量的特徵，為本文主要研究的起因。因此本文的研究目的，係以探討礫石坡面土石流之特性與發生機制。 本論文的貢獻可列述如下： 1. 針對易形成坡面型土石流環境之礫石坡面著手，擬訂一套礫石坡面土石流野外調查、地理資訊系統、資料統計與試驗分析之調查作業流程。 2. 針對土石流之特性，藉航照判釋與現地調查，分析礫石坡面土石流之地形特性，以確認坡面型土石流與崩塌、溪流型土石流形貌與分布區位間之差異。 3. 利用室內試驗，調整後方逕流量、土體含水量、坡度與粒徑組成等條件，模擬邊坡土體崩塌產生坡面流動的過程，以期對礫石坡面引發土石流有更深一層的暸解。 4. 結合相關理論、野外調查、統計分析、室內試驗，綜合分析礫石坡面土石流發生之機制。 本文主要的研究方法，首先為資料蒐集，進行礫石坡面土石流歷年發生案例分析，與研究區選定；其次建立礫石坡面崩塌形成土石流之土體破壞流體化理論；然後，配合研究區現場調查，分析發生特性，及坡面型土石流與崩塌、溪流型土石流形貌與分布區位間之差異；最後結合室內試驗資料的驗證，建立並修正礫石坡面土石流發生之機制。 根據以上的研究方法，進行分析與討論後，得到以下的結論： 1. 礫石層本身因透水性良好，地表逕流容易滲透至崩塌土體成為地下滲流，容易對土體產生破壞；加上有較大的孔隙率，土體內有足夠的空間使其與水充分混合流動；再加上多為內聚凸坡的坡型，使沖蝕或崩塌土體在較短距離內即可形成土石流。 2. 土石流發生與否可藉由臨界坡度評估決定，並同時以土體深度與逕流水深比值、水流福祿數、與顆粒福祿數來決定。 3. 溪床中堆積土體臨界坡度關係式，若運用至坡面崩積土體臨界坡度關係式，可考慮滲流力影響，並增加一地形修正係數A0，其值約為0.6。

4. 滑動中土塊發生流動的機制，為逕流進入滑動土體中，於底部形成液化層，液化層的流動，造成上方土體的破壞，滑動中土塊臨界坡度關係式可以下式表示：

5. 若坡度大於15°，土塊滑動距離超過土塊厚度4.5倍以上的距離，且有充足之水量供應，土體有發生液化現象而形成礫石坡面土石流之可能。