An Experimental Study on the Influence of a Fissure on Bedrock Channel Erosion

A rock mass has fissures, which play an important role in erosion. In order to understand the influence of fissures on bedrock erosion by running water, two kinds of experiments were conducted. One was an erosion experiment using artificial rock with a slit. The other was a visualization experiment of flow in the vicinity of a slit on a fixed bed.<br> A re-circulating flume was used, which is 250cm long, 16cm wide and 16cm deep. The gradient of the flume was 0.2 and the discharge was 3.81/sec. The velocity was 1.8m/sec and the flow depth was 1.3cm; the flow was supercritical and turbulent. The erosion experiment was designed to exam ine erosion of artificial rock samples with different strengths (the compressive strength ranged from 200gf/cm² to 900 gf/cm²). These samples had a slit simulating a fissure, which was placed perpendicular to the flow direction. The artificial rock was made of a mixture of sand, cement and water.<br> The following results were obtained: (1) erosion was more rapid near the slit, especially at its lower edge (Fig. 2); (2) erosion became severe as the slit became wider (Fig. 2); and (3) irrespective of the material strength, the difference of erosion speed between the upper and lower edges made a step and the step height increased until it reached a critical value, which became constant thereafter (Fig. 3), and was closely related to the slit width (Fig. 4).<br> In the flow visualization experiment, many polystyrene beads (specific gravity 1.05, mean diameter 1.2mm) were used as neutral buoyant floats to visualize flows on the fixed bed. As the slit width was increased, part of the main flow over the slit entered the slit more intensely. The difference in step height causes two types of vortices. One was a clockwise vortex in the slit, seen from the righthand side, called the A-type vortex (Photo 1). The other was characterized by a two-vortices system: a clockwise vortex formed in the downstream section of the lower edge and anti-clockwise vortex in the slit, called the B-type vortex (Photo 2). When the slit width was constant, an A-type vortex was formed under conditions smaller step height than the critical value, whereas for higher values of step height a B-type vortex was formed.<br> A physical process of the present experiment can be explained qualitatively as follows. When some of the running water flows into the slit, it exerts compressive force on the lower edge, which brings about more rapid erosion than in other places. As the slit width increased, more water particles impact the lower edge. This is the reason why erosion increases with increasing slit width. Two types of vortices play an important role in erosion of the lower edge. When the A-type vortex is formed, water particles impact the lower edge, resulting in more intense erosion than that of the upper edge where only shearing force acts. This difference in erosional intensity causes the step height to increase. When the step height attains a critical value, however, a B-type vortex is formed instead of an A-type vortex. While the B-type vortex is being formed, no water particles exert compressive force on the lower edge. Both edges are under the action of shear force. The rates of erosion at both edges are not significantly different, with the result a constant step height.