Flow characteristic along the pools of a vertical slot fish pass

Abstract

Bangladesh is a riverine country which is located in the delta of Ganges, Brahmaputra and Meghna rivers. Bangladesh faces the cumulative effects of floods due to water flashing from upstream, the accumulation of the inflow of water from upstream catchments, and locally heavy rainfall enhanced by drainage congestion. During the monsoon period, the floodplains become the primary source of reproduction. Fishes move out into floodplains for feeding, grazing, growth and reproduction during floods. Inundation of the floodplains provide the spawning grounds, nursery areas and major feeding opportunities for a wide range of fish species. Fish migration upstream and downstream or in the floodplain depends on flow characteristics and surrounding flood plains connectively to the rivers. The construction of hydraulic structures parallel and across to the river for monsoon flood control and storage purposes of water for lean period results in an interruption of main river flow with the floodplains. Due to interruption in the natural sequence of flooding in floodplain of Bangladesh, food chain and life cycle of natural fish and other aquatic species have become vulnerable. It has become necessary to make an adjustment of the existing hydraulic structure, to enable the fish to move upstream or downstream without any obstruction. A fish pass is a hydraulic structure that enables fishes to overcome obstructions in the passage to the spawning grounds and to other upstream or downstream migrations and is built when it is required for ecological, economical or legal considerations. Some field investigations and research works have been performed by several institutes and researchers to analyze the performance of the existing fish friendly structures. But there is lack off detailed study specially with in fish pass turbulence with the local fish species in Bangladesh.

An experimental study has been carried out with a distorted physical model in the laboratory flume of Hydraulics and River Engineering Laboratory, Department of Water Resource Engineering, (DWRE) of Bangladesh University of Engineering and Technology (BUET), for this research work. The model structure consists of four pools. The dimension of each pool was approximately 1 m long, 0.762 m wide and 0.60 m height. The width of the opening of each pool was 0.127 m. The experiments were conducted by maintaining different upstream water level with different ranges of velocities. Velocities were in the range of 0.3 m/s to 0.8 m/s and discharges were in the range of 37 m3/hr to 145 m3/hr. Total 9 sets of experiments have been conducted for this study. For each set of experiments, three dimensional velocity data were collected at 0.4, 0.6 and 0.8 hydraulic depth by using an ADV (Acoustic Doppler Velocimeter).

Velocity [cm/s] components in The X and Y direction have been analyzed to plot the velocity vector fields and velocity contour maps to understand the flow circulation pattern. For turbulent kinetic energy T_x, T_y and T_z turbulence strength [cm/s] in the downstream, cross-stream, and vertical directions, respectively have been analyzed. Furthermore T_xy, T_xz and T_yz time-averaged local Reynolds stress [Pa] at 0.4, 0.6 and 0.8 hydraulic depth were calculated. The response of the selected fish species Rui (Labeo rohita), Catla (Giberlion catla) and Mrigel (Cirrhinus mrigala) of different sizes with respect to different hydrodynamic conditions provided has also been analyzed. It has been found that though the developed hydrodynamic conditions were favourable for the juvenile size of the selected species but were quite unfavourable for the fry and fingerling size of the selected species in most of the cases. The results that have been found from this study will be very much helpful to carry out further field and laboratory studies to understand fish characteristics in fishpass.