Study On Hydrodynamic Characteristics And Ecological Flow In The Middle And Lower Reaches Of Hanjiang River Based On Bloom Control

Water bloom is easy to occur in lakes,reservoirs and other static water environment.With the rapid development of urbanization and social economy,a large number of hydropower projects are built on rivers,which leads to the destruction of natural connectivity,the slowing down of water flow velocity the increase of hydraulic retention time,and creating suitable conditions for the occurrence of water bloom.The mechanism of river blooms is affected by many factors,among which hydrodynamic conditions play an important role in the accumulation and transportation of algae.The integrity of river ecosystem structure and function can be maintained by clarifying the response relationship between river blooms and hydrodynamic forces and deducing the river ecological flow under the control of river blooms.This paper takes the middle and lower reaches of Hanjiang River as the research object,based on the series data of hydrology,topography,water environment and water ecology collected from 2015 to 2019;1)The dominant species of algae were identified by dominance index,and the community structure and transport characteristics of algae in different periods were analyzed.The key control factors of algal blooms were identified by principal component analysis(PCA)and redundancy analysis(RDA).2)By constructing the hydrodynamic and hydroecological model of the study area,the change trend of algae under different hydrodynamic conditions was studied,and the response relationship between algae and hydrodynamic factors in each period of bloom occurrence was clarified.3)Based on the established hydrodynamic ecological model,the hydrodynamic threshold of water bloom was identified,and the ecological flow of water bloom control section in different periods was calculated by using the velocity inhibition method and section flux method.Then,the rationality of water bloom control flow of each section was analyzed,and the section water diversion volume was further calculated.The main contents and conclusions are as follows:(1)Based on the analysis of the characteristics of algal blooms in the middle and lower reaches of Hanjiang River from 2015 to 2019,it was found that the occurrence period of algal blooms is mainly concentrated in the dry season in spring,and the main algal population was diatoms.There was the possibility of algal blooms in the wet season.Green algae and diatoms are the main dominant species of algae.The algal community structure presents the phenomenon of seasonal succession in the dry season,and the dominant population gradually changes from diatoms to green algae.The algal density and biomass of each section were at a high level during the low water period,and the main control section was below the Shayang section.The algal density and biomass of each section along the river were low during the high water period,and there was no continuous bloom on the river section.Flow rate,temperature,total phosphorus and chemical oxygen demand were the common driving factors in dry season,and the contribution rate of flow rate was 40.4%.(2)By establishing hydrodynamic and hydroecological model in the middle and lower reaches of Hanjiang River by EFDC software,the results showed that the simulated values of water level and chlorophyll a in the study area were in good consistent with the observed values.The certainty coefficients of the research indexes in the simulation series were more than 70%,and the relative error was less than 30%.The calibrated and verified model can accurately simulate the change process of regional hydrodynamic and water quality.The response relationship between water bloom and hydrodynamics showed that the flow velocity is low in dry season.Under the action of slow hydrodynamics,the algae continuously get new nutrients,which promoted the growth of algae.At this time,the algae density was at a high level,reaching the threshold of water bloom.When the flow velocity increases gradually and exceeds 0.432 m/s at any time,under the condition of strong hydrodynamic force in the wet season,the algae in the cross-section are transported,which is not easy to cause aggregation and can not reach the threshold of water bloom.(3)In order to control the occurrence of water bloom in the middle and lower reaches of Hanjiang River during the dry season,it is suggested that the ecological flow threshold of from December to April should be 695～890 m~3/s in Shayang section,742～920 m~3/s in Qianjiang section,750～957 m~3/s in Xiantao section and 865～985 m~3/s in Hanchuan section.According to the current hydrological conditions and water resources situation in the middle and lower reaches of Hanjiang River,in order to meet the demand of water bloom control in each section during the dry season from December to April,it is suggested that the water diversion of Hanchuan section in the middle and lower reaches of Hanjiang River should be61 m~3/s and 71 m~3/s in December and April respectively,and that of Qianjiang River,Xiantao River and Hanchuan section should be 12 m~3/s,66 m~3/s,63 m~3/s and 26 m~3/s,32 m~3/s and 42m~3/s in January and February respectively.In the dry season of March,only Xiantao section needs 112 m~3/s of water.