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.
To evaluate the contribution of bedrock groundwater seepage to stormflow generation in a granitic headwater catchment, runoff and water chemistry of springs and streamflow were observed simultaneously at a headwater catchment, Ibi, central Japan. In the catchment covered with a natural closed forest, there were about 20 springs along the first-order stream, one from soil-bedrock interface, and the other from cracks in the bedrock. Hydrologic and hydrochemical observation indicates that bedrock springs at the catchment can be divided into two types; relatively responsive to rainfall, and low SiO2 concentration and EC (“Type A”) ; little runoff fluctuation and relatively high SiO2 concentration and EC (“Type B”). The SiO2 concentration indicated the source of “Type A” spring was shallower than “Type B”. Hydrologic observation indicates that the runoff from soil-bedrock interface is dominant to rapid and large peak runoff that coincided with the rainfall peak (45mm, 110mm). However on the heaviest rain event (287mm), bedrock groundwater seepage is dominant to delayed and large peak discharge. This peak flow was three times greater than the initial peak flow, which was conjunction with the peak of subsurface storm flow.
A debris flow due to typhoon No. 11 occurred at Higashinokawa river basin of Wakayama Prefecture southern part in 2001. Debris flow was trapped and the expansion of the overflowing debris flow was controlled by the trees in debris flow deposit area. The form of the trapping of debris flow by the tree is divided into two types. One is that boulders are trapped by boulders interlocking each other between the trees (type 1). The other is that boulders are trapped by boulders by log crosspieces due to splintered trees and fallen trees (type 2). To control overflowing of debris flow in this basin, It was found out that 30m of average forest length along stream line was necessary.