Influence of W-shaped Submerged Dam Groups on Flow Structure and Aggregation Behavior of Typical Fish Species

[Objective] Submerged dams, as an important component of waterway regulation projects, can prevent from river and coastal erosion and also alter river flow patterns and improve river habitat environments. This study selects juvenile crucian carp as the target species to investigate the effects of W-shaped submerged dams with different particle size compositions on flow velocity distribution and the aggregation of target fish. It aims to clarify the habitat improvement effectiveness of W-shaped submerged dams and provide a theoretical basis for the construction of ecological conservation zones in cut-off river sections. [Methods] Crucian carp, a dominant fish species in the Pinglu Canal, were selected as the experimental species. Experiments on the influence of W-shaped submerged dam groups on flow structure and typical fish aggregation behavior were conducted in an annular flume. The W-shaped submerged dams were set with three particle size compositions ([5,10] mm, [5,20] mm, [10,20] mm), and four flow rate conditions (0.03, 0.045, 0.06, 0.09 m3/s) were set up. Under each test condition, 30 juvenile experimental crucian carp were placed, and the entire experimental process was recorded using a Nikon D7500 camera. Tracker software was used to collect and record the positions, frequencies, and total time where the experimental fish stayed for more than 1 minute. The average fish aggregation rate was used to evaluate the attraction effect of the W-shaped submerged dams on fish, and the flow pattern diversity index was used to quantify changes in flow patterns. [Results] Under the action of W-shaped submerged dams of the same particle size, as the flow rate increased from 0.03 m3/s to 0.09 m3/s, the area and flow velocity of the backwater zone upstream of the weir in the study area continuously increased, while the area of the triangular recirculation zone formed downstream of the weir gradually decreased. Under the action of W-shaped submerged dams of different particle sizes, those composed of larger particle sizes (10-20 mm) exhibited better permeability, smaller velocity gradients compared to smaller particle size groups, and higher flow pattern diversity. Analysis of the influence of W-shaped submerged dam groups on the aggregation behavior of experimental fish showed that at a flow rate of 0.09 m3/s, the W-shaped submerged dams composed of 10-20 mm particle sizes had the highest number of stays in fish aggregation areas, reaching 85 times. These dams composed of larger particle sizes (10-20 mm) had high habitat diversity, providing better habitat and shelter for experimental fish. As the flow pattern diversity index increased, the average fish aggregation rate also showed an increasing trend. The river flow pattern diversity index and the average fish aggregation rate exhibited a linear relationship. [Conclusion] W-shaped submerged dams composed of larger particle sizes have high habitat diversity and can provide better habitat and shelter for experimental fish. In the construction of ecological conservation zones in cut-off river sections of artificial canals, it is recommended to select large-particle-size submerged dam schemes with better permeability effects. A good correlation is observed between the average fish aggregation degree and the flow pattern diversity index, enabling quantitative assessment of habitat diversity and providing theoretical support for research on river habitat heterogeneity and ecological optimization design. In the future, the long-term effects of different submerged dam design parameters on the ecosystems of target river sections can be further explored, and more influencing factors can be incorporated to investigate the ecological development of canals.