During a heavy downpour, runoff fluctuates rapidly in the upstream and a lot
of sediments, woody debris and other suspended load are transported and deposited.
Therefore it is important to prevent sediment from accumulating in the fishways.
To find an appropriate solution to this problem, a suitable fishway known as
the protruding boulder type fishway was designed. The authors investigated 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 various shapes of boulder could reduce the 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) There was a
suitable shape for boulders that could decelerate flow. In the case where the
upstream side of a boulder is flat, the velocity of upstream flow reduces. But
the downstream flow becomes turbulent in the case where the tail reach of the
boulder is flat. 2) As it is difficult for fish to find a route of ascent, boulders
should be closely arranged laterally for the upstream flow to dam up across
the entire channel. 3) In the case of dam]up flow, turbulent flow arising from
the boulder shape further makes fish ascent more difficult. To maintain sanctuaries
for fishes, it is necessary that the longitudinal distances between the boulders
are widened and lateral distances narrowed. It is also important that boulders
do not have the shape which disturbs downstream flow.
Key wordsFfishway, hydraulic model experiment, fish release test, arrangement of protruding boulders, shape of boulder