Arrangement of protruding boulders in a slanted fishway

Abstract

During a heavy downpour, runoff fluctuates rapidly in the upstream and allows a lot of sediments, woody debris and other suspended load to be transported and deposited. Therefore it is important that sediment deposition does not occur in fishways. To find an appropriate solution to this problem, a suitable fishway known as the protruding boulder type fishway was designed. The authors tried to investigate the characteristics of this type of fishway through a hydraulic model experiment and the fish release test. The purpose of the hydraulic model experiment was to verify whether boulders could reduce flow velocity while the essence of the fish release test was to observe the behavior of fishes around the boulders. The experiment revealed the following findings: 1) Flow in the channel accelerated without the boulders, so the function of the boulders was to decelerate flow. 2) There were two types of flow. One occurred as the boulders were submerging, while the other occurred as the boulders were emerging from the stream. In the case where the boulders sank, the velocity of flow was fast. For the fishes to ascend the cannel, the mean velocity must be below the cruising speed. To satisfy this, the slope of the channel should be less than 2%. 3) In the case where the boulders rise to the surface, it partially dams up the upstream flow. When the spaces between the boulders (arranged along the width of the channel) are narrow, the water surface becomes level and appears as steps. To maintain sanctuaries for fishes, it is necessary that the longitudinal distances between the boulders are widened and lateral distances narrowed.