Erosion Control of Steep Open Channels Using Articulated Concrete Blocks
Abstract
Overtopping flow can undermine the stability of hydraulic structures including embankments and spillways through the shear stress and the subsequent soil erosion. Increased flow velocity can cause scour, which can result in the failure of dams, bridges, and overall open channel hydraulic structures. Sediment transport is another crucial issue in open channels that can cause severe socioeconomic and environmental consequences. Two general approaches are commonly considered for scour prevention. The first approach involves flow modifications to minimize the corresponding effects on the structures. The second approach is associated with bed armoring and placing physical barriers on the natural bedding such as the Articulated Concrete Block system (ACBs). The Articulated Concrete Block (ACB) system includes a series of concrete blocks or slabs that are interconnected using steel or synthetic cables, which are commonly employed for the bed and bank protection of open channels. Two flow-induced failure mechanisms can undermine the performance of ACBs: Direct failure, which is attributed to the direct impact of free-surface flow on concrete blocks leading to the overturning or rolling-up of the edge of the ACB. Indirect failure is associated with the scouring of the underlying bedding that can result in the deformation of the ACB structure and its failure. The main objective of this study is to analyze the ACBs protection layer, understand the factors that can influence scouring, and develop measures to reduce the failure risks and minimize the amount of sediment transport of the bedding and the subgrade. The indirect failure mechanisms of ACBs laid over the downstream face of an embankment are explored through a suite of laboratory experiments (∼ 40 tests). The effects of substrate structure, soil particle size, the slope of the embankment, and different types of permeable filters (i.e., geotextile, geogrid, and filter cloth mats) on the incipient motion of the bedding and the amount of soil loss are assessed. The stability of the embankment is investigated through various configurations with different water depths and velocities. Results show that overtopping flow with high velocity causes soil erosion and sediment transport in a very short period of time with the absence of a protection layer.
ACB can noticeably improve channel stability and reduce sediment transport due to its perfect contact with subgrade and its flexibility. However, the risk of ACB failure increases if the subgrade is not designed and implemented properly. ACBs are to be in close contact with the subgrade and the fabrics and spaces between the blocks should be optimized otherwise, the overtopping of the blocks and sediment transport happen. The fabrics placed in between the layers can prevent the suction of the finer particles and reduce the risk of failure. Coarser and well-grade particles underneath the ACB structure can drain more water due to their higher hydraulic conductivity and improve the stability of the embankment. Further, risks of failure are higher near the downstream toe, which can be partly controlled by sufficient flow drainage and anchoring the downstream segments. The subgrade beneath the ACB is to be uniformly compacted so that the water flow passes through the blocks and over the subgrade steadily. The median diameter and the slope of the embankment played a key role in mitigating the amount of soil loss.