Distribution Of Hydraulic Habitat Indexes In Typical Compound Channels With Fish Spawning Habitats In The Upper Reaches Of The Yangtze River

A compound channel comprising the main channel and floodplains exists in the upper reaches of the Yangtze River widely and is susceptible to the formation of eddy structures due to the main flow,secondary flows,and the flow exchanges between floodplains and the main channel.The special and complex flow structure of the compound channel has an important impact on fish eggs’ movement and cross-sectional distribution.Hydraulic habitat indexes are the key parameters to determine the abundance and distribution of fish,but few studies have been conducted on the structural characteristics of natural compound channel flow,and the effects of compound channel flow characteristics on the distribution of fish hydraulic habitat indexes have not been adequately studied.The study on the distribution of fish hydraulic habitat indexes in typical compound channels with spawning habitats in the upper Yangtze River is of great scientific significance for the conservation of fish resources in the upper Yangtze River and is also of great practical value in the ecological operation of reservoirs and river ecological restoration.This study uses a combination of three-dimensional numerical simulation of water flow and theoretical analysis to study the structural characteristics of water flow in compound channels with fish spawning grounds under the characteristic flow and highflow pulse process during the fish spawning period and to explore the intrinsic connection between the characteristic hydraulic indexes of compound channels and the distribution of hydraulic habitat indexes of spawning grounds,and the main research results are as follows:The inlet section of this study is a curved compound channel,the outlet section is a straight compound channel,and the intermediate transition section is a single-section channel;the water in the study area has obvious stratification characteristics,and the transverse water surface gradient along the course shows an increase then decrease and finally stabilization.The water of the compound channel section is influenced by the flow exchanges between floodplains and the main channel,the distribution of transverse water surface and transverse vertical average velocity are concave at the junction of floodplains and the main channel,and the obvious vortex structure appears in floodplains under small flow rate;when the flow rate increases,transverse water surface gradient increases,but the concave degree of transverse water surface and transverse vertical average velocity slightly decreases;the concave degree of transverse water surface and transverse vertical average velocity is relatively larger in the section with a larger slope coefficient of the main channel or smaller width of the floodplain.The distribution of the habitat suitability index of the four major Chinese carp will change with the flow,when the flow rate is small,the larger value of the habitat suitability index is in the main channel,and with the increase of the flow rate the larger value of the habitat suitability index is in the floodplain;the larger value of the habitat suitability index of the schizothorax prenanti is basically concentrated in the floodplain of the compound channel,and the rise and fall of flow rate mainly affect the size of its distribution area.Under the same flow conditions,the water depth and flow velocity of the floodplain are basically smaller than those of the main channel,and the kinetic energy gradient and vortex are larger than those of the main channel;vertically,the kinetic energy gradient and vortex of the bottom flow area are larger.Under different high-flow pulse conditions,the fluctuation of the hydraulic index of the floodplain is generally larger than that of the main channel,the fluctuation of water depth is the smallest and the kinetic energy gradient is the largest.During the high-flow pulse in a certain flow interval,the flow rate increases,the ideal weighted usable area of habitat suitability index greater than 0.8 of the four major Chinese carps and the schizothorax prenanti increases,and the connectivity of ideal habitat patches also increases;under different high-flow pulse conditions,the curves of weighted usable area in compound-section channel and single-section channel were basically similar,and the curves of the weighted usable areas of the schizothorax prenanti of compound-section channel were basically above the curves of the weighted usable areas of single-section channel,and the larger the width of floodplains,the larger ideal weighted usable area is.