| The characteristics of fish swimming over the crests of three experimental weir designs were examined in this study. Ayu (Plecoglossus altivelis altivelis) Freshwater Sculpins (Cottus pollux) and Whitespotted char (Salvelinus leucomaenis) behaviors were recorded using two ultra high-speed cameras in a full-scale experimental weir. The swimming loci of the fish were traced by motion capture system and their movement distance, speed and acceleration in the nappe were estimated. In addition, two-dimensional flow velocity distribution and flow fluctuations are measured using a PIV (Particle Image Velocimetry) system. To compare the experimental result and actual phenomenon, these experimental methods were applied to the fishway at the Naruka Weir on the Kuzuryu River in Fukui Prefecture, Japan, which have observation window. The findings revealed that both of the high velocity characteristics and their stream lines of the nappe prevented fish from effectively using the weir as a fish ladder. The comparison of the swimming loci shows that differences of the stream line on nappe resulting from the different weir shapes were effectively for the passage route of small fish. The boundary line of the nappe flow acts as a barrier to relatively weak swimmers, such as ayu and sculpins. The separated flow of the nappe in the low levels can produce a flow on the spot of the weir that is sufficiently weak for these weak swimmers to overcome. However, with large-size fishes, few influence on these difference. In addition, in order for relatively small fish species such as ayu to swim upstream the high velocity flow repeat swimming performance accompany body vibration which duration is short time. The results showed that weir shape is an important consideration when designing weirs that will be positioned along rivers that are inhabited by either small sizes fish or bottom swimming fish, as these fishes are typically weak swimmers. |
