Research On Distribution Characteristics Of Fish Spawning Hydraulic Habitat Indexes In Meandering River

In recent years,with the development of a series of large-scale human activities,such as dam building,the boundary conditions of the Yangtze River have changed,and the ecological balance of the water in the basin has been affected.As the most important economic fish in the Yangtze River Basin of China,the spawning grounds of the four major domestic fishes have special hydraulic characteristics.The change of river boundary conditions affects the hydraulic structure of the spawning grounds of the four major fishes,which leads to the reduction of the spawning grounds,and then affects the population of the domestic fishes.In order to protect the domestic fishes stock and restore the ecological balance of water in the basin,it is necessary to study the hydraulic habitat index of the spawning grounds of the four major domestic fishes,which is of great significance to the restoration of the spawning grounds of the four major domestic fishes and the construction of ecological channel projects in the future.Taking four major domestic fishes spawning of hydraulic habitat index as the research object,the thesis adopts the method of numerical simulation and theoretical analysis,summarizes the four hydraulic habitat index classification of all fish spawning grounds,to study the flow velocity,velocity gradient,vorticity,kinetic energy gradient and hydraulic habitat suitability index.The distribution characteristics of hydraulic habitat indexes along the path and along the groove width in the bend are discussed,and the response relationship between the distribution of hydraulic habitat indexes and the geometric shape of the bend is analyzed.The results are as follows:(1)The area of the suitable spawning velocity area of four major domestic fishes was directly proportional to the change of the flow;In the low-flow condition,the suitable area increases first and then decreases before and after the curved roof.With the increase of the flow,the suitable area presents a distribution characteristic of"increasing first,then decreasing and finally increasing"along the river.(2)The velocity gradient G presents an"S"shape on the longitudinal profile of the convex bank of the generalized bend.Under the influence of the mainstream movement,the maximum value of the index appears in front of the bend roof break,and the minimum value appears on the sections with bend angles of 50°and 60°.In the natural bend,the G value of the vertical section of the deep depth is less than that of the two sides,the difference is about 10 times.In the curves with two different boundaries,the areas with larger G values are distributed on the side wall and the variation trend is basically the same.(3)The distribution law of kinetic energy gradient M in the generalized bend is similar to that of G;In natural curves,the larger areas of M_y value are mainly distributed on the convex bank wall and the concave bank wall at the inlet section of the curve,and a small amount are distributed near the stone beam of the middle plate.The amplitude of M_x and M_y is the smallest before and after the curved crest in the concave bank area,and four major domestic fishes consume less energy swimming here,which is conducive to their growth and reproduction.The mean values of M_x in the three longitudinal profiles were 0.00514,0.0044 and 0.0056,which were relatively close;The mean values of M_y are 0.0148,0.00287 and 0.00419 respectively,and the values of M_x and M_y on both sides are larger than those on the longitudinal section of the depth.Combined with the distribution of the velocity gradient,it is speculated that the suitable spawning area of the four major domestic fishes may be the upstream of the Zhongpanzi stone beam and the upstream and downstream of the Duiwotan stone beam in the concave bank.(4)The vorticity increases along the three longitudinal profiles of the generalized bend;In the natural bend,the vorticity distribution of the three longitudinal profiles fluctuated sharply due to the influence of topography.The topography of the concave bank longitudinal profile was the most complex,but the vorticity variation range was smaller than that of the other two longitudinal profiles.On the plane,the distribution area of the reverse vortexes is much larger than that of the direct vortexes.The reverse vortexes are mainly distributed in the curved inlet of the convex bank to the middle plate stone beam section,while the direct vortexes are distributed in the convex bank wall,the downstream of the middle plate and the opposite nest beach.In the exit cross-section,the average value of the vorticity cross-section is the smallest,and the part with the smaller absolute value of?z is mainly distributed on the convex bank,while the four major four major four major four major four major domestic fisheses are more likely to spawning on the concave bank and the deep groove.(5)In the generalized bend,the SU>0.8 area in the deep layer of middle water is the largest under the condition of small flow,which is more suitable for spawning of four major domestic fishes;Under the condition of large flow rate,the area of SU>0.8 was distributed in high water depth,and four major domestic fishes preferred to spawn in high water layer under the condition of large flow rate.(6)In the natural bend,the area with the larger velocity suitability SV value under the condition of small flow is in the deep groove,and the area moves to the side beach after the flow increases;Under the multi-year average flow in natural curves,the area of SH>0.8mainly concentrates on the banks and the concave shore beach.The habitat suitability index SH on both banks was mainly affected by SD.The statistical results showed that the correlation coefficient R~2 between SH and SD was 0.57 and 0.91,respectively,and SH was linearly positively correlated with SD.