Study On Fish Habitat Creation Techniques In A Typical Sharp Bend And Branching River Reach Of The Three Gorges Fluctuating Backwater Area

In order to meet the development of shipping,a 4.5m water depth channel construction project has been implemented in the Chaotianmen-Fuling river section of the Three Gorges fluctuating backwater area,which involves the national aquatic germplasm resource reserve of four domestic fish in Chongqing section of Yangtze River.To respond to the ecological protection of Yangtze River and promote the synergistic development of shipping and ecology,it is proposed to implement fish ecological compensation measures in the Chaotianmen-Fuling waterway treatment project to expand the area of suitable habitat for fish in the protected area.In this paper,the habitat creation technology of a typical sharp bend and branching river section in the Three Gorges change backwater area is studied by taking the Dajiantan river section as an example.The two-dimensional hydrodynamic model and habitat suitability evaluation model were established to calculate and analyze the hydrodynamic characteristics of the habitat and the effect of the habitat creation plan in the study section,and the optimal habitat creation plan was recommended.The main contents of the thesis and the research results obtained are as follows:(1)The study river section shows the characteristics of reservoir channel during the impoundment period,with the characteristics of high water level,small specific drop,and large water depth and small flow velocity;the hydrodynamic characteristics of this river section are similar during the abatement and flood periods,showing the characteristics of natural channel,with the characteristics of low water level,large specific drop,and small water depth and large flow velocity;the maximum value and range of variation of water depth,flow velocity,and specific drop in the left branch of the river section in these three periods are larger than those in the right branch of the river section.According to the hydrodynamic characteristics and topographic features of the studied river section,it can be divided into six types of geomorphic units,such as river core,river floodplain and side beach slow flow area,and eight types of hydrodynamic units,such as slow flow area,fast flow area and rapid flow area,etc.The type and distribution position of geomorphic units are basically unchanged in each period,while the type and distribution position of hydrodynamic units will change significantly with the change of flow.The distribution characteristics of geomorphic units affect the distribution of hydrodynamic units,and the more complex the geomorphic units are,the more complex the hydrodynamic units are;the left branch of the river section is mainly deep grooves,stone beams,slow flow areas of the side beach and other geomorphic units,and the right branch of the river section is mainly slow flow areas of the side beach,river manholes and other geomorphic units;in the impoundment period,the hydrodynamic units of the river section are single,and the left and right branches of the river section are mainly deep grooves and slow flow areas,and in the abatement and flood periods,the types of hydrodynamic units increase,and the left branch of the river section is mainly fast flow areas,rapid flow areas,flat flow areas,slow flow areas of the side beach The left branch of the river section is mainly fast flow area,rapid flow area,advection area,side beach slow flow area,etc.,and the right branch of the river section is mainly slow flow area,side beach slow flow area,etc.(2)The water depth and flow velocity were selected as the key influencing factors for the four major fish species,and the water depth and flow velocity suitability curves of the typical four major fish species in the study river section were determined,and the hydrodynamic model and habitat suitability evaluation model were established,and three habitat creation schemes were proposed.The two-dimensional mathematical model was used to calculate and analyze the hydrodynamic characteristics and habitat suitability of each habitat creation plan.The results showed that the suitability index of water depth and flow velocity in the right branch of the river section was higher and the habitat quality was better when the flow rate was greater than 22,000 m~3/s during the storage and flooding periods,which could be used as a suitable habitat for the four big fish.(3)Comparing the habitat quality ratio,habitat quality area growth rate and habitat weighted area of each habitat creation scheme,the results show that the best habitat creation scheme with an average dam length of 96m is the optimal one.The implementation of the habitat creation program expands the range of flow velocity changes in the typical cross-section and increases the complexity of the flow pattern in the habitat creation area,and the longitudinal flow velocity in the habitat creation area shows obvious high and low changes which are more conducive to the exchange of water substances.In addition,the implementation of the habitat creation plan effectively improved the flow speed suitability under high flow conditions,increased the overall high and medium quality habitat area in the river section,and improved the hydrodynamic conditions and habitat quality in the study river section.