Multi-scale Mechanisms, Models and Behaviors of Bedrock Erosion
|關鍵字:||多重尺度;岩質河床;沖蝕機制;沖蝕模型;河床載磨蝕;岩塊抽離;遷急點倒退;multi-scale;bedrock riverbed;erosion mechanism;erosion model;bedload abrasion;plucking;knickpoint migration|
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.
|Appears in Collections:||Thesis|