岩質河床多重尺度之沖蝕機制、模型及行為

Abstract

岩質河床與河岸沖刷常會改變河道的地形地貌，影響跨河構造物基礎的安全。過去學者大多經由歷史事件或模型試驗，建立評估岩床沖刷深度或速率的經驗式及物理基本模型，然而對岩床沖蝕力學機制的基礎研究仍相當有限。本研究由多重尺度的觀點，探討主控岩床沖蝕的力學機制及其力學行為。床面水流剪應力所導致之磨蝕及河床載彈跳撞擊所導致之磨蝕是發生於完整岩石材料「顆粒尺度」之沖蝕機制。動態波動水壓力所造成的弱面連通，進而導致岩塊上舉抽離乃為發生於節理發達岩體「岩塊尺度」之沖蝕機制。至於河道遷急點(局部坡度變陡處)之倒退以及深槽高陡岩質側壁的岩體穩定性則可視為改變河道地形地貌的「岩體尺度」沖蝕機制。 針對顆粒尺度之材料磨蝕行為，本研究透過顆粒流分析進行微觀模擬，基本上將該模擬視為小尺度之「虛擬材料沖刷試驗」，探討岩床表面承受作用力而造成顆粒尺度之沖蝕過程，以了解軟岩河床沖蝕之機制。將岩床材料視為顆粒集合體，由微觀角度探討河床載撞擊岩床材料過程中的能量變化、轉移與顆粒間鍵結破壞之原理；探討岩石材料破壞之漸進過程，描述岩床反覆受到撞擊所累積的損傷效應對沖蝕行為的影響。經分析發現，河床載撞擊後轉移至岩床材料的動能約有2/3以摩擦功的型態消耗，1/3用來破壞材料內部顆粒間的鍵結，造成部分體積沖蝕並伴隨鄰近區域損傷。本研究透過一連串的虛擬撞擊試驗結果，將河床載撞擊傳遞至岩床材料的能量對岩床材料的最大彈性應變能正規化，並建立河床載彈跳撞擊之磨蝕沖蝕量與損傷範圍迴歸式。 針對岩塊尺度之岩塊抽離行為，本研究首先由顆粒流分析模擬岩體內部不連續面的岩橋連通過程，繼而經理論推導建構可考慮不同幾何形狀、摩擦係數的岩塊，承受波動水壓力作用之力學行為與運動模式，進而探討影響岩塊抽離的條件及不同的岩塊上舉型態。分析結果顯示，於低重現期的洪水事件下，岩體內部的不連續面會在很短的時間內連通，形成獨立岩塊。岩塊的運動行為以及上舉抽離潛勢會與岩塊的摩擦係數及寬高比都有密切關係。當岩床表面岩塊承受相同之波動壓力，岩塊間摩擦阻抗越大，岩塊越容易因波動壓力作用而累積上舉位移量。岩塊之寬高比也影響岩塊抽離行為甚大，扁平型之岩塊較容易在單一動態水壓力週期內即瞬間發生上舉抽離(暴衝形岩塊抽離行為)，瘦高狀之岩塊則較容易隨著動態水壓力作用週期逐漸上舉抽離(累積型岩塊抽離行為)。本研究提出一正規化摩擦係數以評估相鄰岩塊間的摩擦阻抗的效應，除摩擦係數之外，此正規化摩擦係數將岩塊厚度、波動壓力水頭、岩塊單位重等都納入考量，進而可迴歸正規化摩擦係數與岩塊累積上舉抽離門檻之關係。本研究運用此模型模擬大漢溪義興壩下方之沖刷坑及壩下游河道之下切變化，以此案例驗證本研究所提出之分析模型，模擬所得結果與河道實際沖蝕變化相當吻合。 針對岩體尺度之遷急點倒退行為，本研究考慮河道遷急點(knickpoint)多具有高陡之曝露坡面，若弱面發達，坡面之塊體有機會在重力與水流作用力聯合作用下失去穩定而發生岩體尺度之平面滑動、岩塊抽離、或翻倒，則一次性之體積脫離沖蝕速率可遠高於「顆粒尺度」與「岩塊尺度」之沖蝕機制所造成之沖蝕速率。 本研究從多重尺度探討岩床受到河床載磨蝕、岩塊抽離、遷急點倒退的沖蝕行為與機制，希望能由微觀至巨觀不同的尺度，探討岩質河床的沖蝕機制，更貼切地評估台灣西部軟岩河川的整體沖刷現象。

The erosive process in a rock riverbed is an irreversible process. Intense river-bed erosion can take place in soft rock or heavily jointed rock mass. Intense erosion may expose the foundation of a structure across or along a river, which threatens structural stability. Although there are a variety of existing methods aiming at evaluating the incision depth of a bedrock channel, not many models are mechanics-based. The erodibility and erosion rate of rock riverbed are largely dependent on the mechanisms of rock erosion. Field observation data reveals that there is usually a dominant mechanism controlling the erosion on bedrock; various mechanisms may be different in the scale of erosion. Accordingly, it should be reasonable to establish multi-scale erosion models by taking the scale of erosion into account. The abrasion by bed shear or by saltating bed load is in “grain scale”. For soft rock without abundant discontinuities, saltation abrasion is often the major mechanism of rock erosion during a large flood. Plucking is an erosion mechanism usually occurring to sub-meter rock blocks in a bedrock channel subjected to intense water current. Plucking erosion is in “rock-block scale”. In case of a steep rock outcrop (e.g., near a knickpoint or a steep river bank), the mass loss on the bedrock surface may be in “rock-mass scale”. For abrasion in grain scale, this study conducts particle flow simulation to identify the major factors affecting the abrasion of bedrock due to the impact from saltating particles. The simulations may be regarded as a “virtual erosion test”; the modeled abrasion process is a result of particles’ release due to inter-particle bond break subjected to multiple particle impacts. The continuous impacts by bedload particles may result in the accumulative damage in a rock material near its surface and cause the broken rock fragments to detach from the intact rock. By the decomposition of kinetic energy, it appears that the extent of damage and amount of erosion can well correlates with the normal component of kinetic energy. This study attempts to adopt the dissipate energy as a damage index for describing the degree of accumulated damage in the rock material as a consequence of continuous particle impacts, and to find the relationship between the initial particle kinetic energy and the eroded volume due to impact. For plucking in rock-block scale, the study develops a theoretical model capable of estimating the required conditions for plucking. This model considers the kinetics and kinematics of a rock block subjected to an inclined jet flow. The removal of a rock block because of plucking can be either a result of impulsive plucking or accumulative plucking. A plate-like block is more likely to escape owing to impulsive plucking, while a pillar-like block is more likely escape because of accumulative plucking. The potential of impulsive plucking can be evaluated by examining the maximum uplift displacement within the period of the maximum upward impulse load acting on a typical rock block in a rock mass. A non-dimensional index APP is formulated to assess the potential of accumulative plucking. In addition, a comprehensive approach is proposed to evaluate both the scour-hole depth in a plunge pool and the incision depth in its downstream channel, subjected to a prescribed jet flow passing a spillway or an overflowing weir. A case study adopting the proposed method demonstrates the applicability of the comprehensive approach. Intense knickpoint migration may involve structural failure of jointed rock masses along preexisting weak planes. In this study, the factor of safety for a rock block subjected to the gravitational and hydraulic loads against various failure modes (including uplift, sliding, and overturning) are formulated and examined for a variety of conditions. The dominant mode of rock-mass instability can be identified by examining the factor safety for each failure mode. The structural failure in the rock mass can lead to the loss of a large rock volume during a large flood event; this rock-mass scaled volume loss can well explain the high erosion rate near a knickpoint in jointed rock masses.