Research On Emergency Control Scheme Of Tributary Water Bloom Of Three Gorges Reservoir Based On Water Residence Time

Since the water storage of the Three Gorges reservoir,the water level upraised,the flow velocity slowed down and the time of water retention is prolonged.The reservoir area is gradually transformed from the original river type habitat to the lake type habitat,and the newly formed habitat environment has gradually stabilized.Algal blooms dominated by different algae occurred in most of the tributaries.Most of the outbreak of water bloom is the process of a large number of dominant algae dispersed in the water body under the suitable hydrologic and hydrodynamic conditions.How to prevent and control algal blooms is the key problem to be solved in Three Gorges Reservoir.As an effective measure to improve river water environment,reservoir ecological regulation has been widespreadly concerned and explored by someone who concerned.In this paper,the algal bloom in Xiangxi River of the Three Gorges Reservoir is taken as a research object.On the basis of the monitoring data in the field in 2015,the laboratory control experiments are carried out,then analysis of the monitoring results and formate the preliminary laws and theoretical assumptions,finally using CE-QUAL-W2 model to do the numerical simulation.That is to say,the research plan included field observation,rule analysis,control experiment,numerical simulation and other research methods should be combined orderly to analyse the variety of water residence time,and analyse the influence of water residence time on the growth of algae,then deeply discussed the control scheme of water bloom of the Three Gorges Reservoir based on the water residence time.The concrete results are as follows:1.On the basis of field monitoring and the hydrological and climatic data of Xiangxi bay,the water bloom and hydrodynamic conditions of Xiangxi River in 2015 are analyzed,and the water environment of Xiangxi bay is preliminarily understood.The water in Xiangxi River is in the state of eutrophication,and the frequence of the outbreak of water bloom is high.The residence time of Xiangxi River depends on the change of water level of the Three Gorges reservoir.The outbreak situation of water bloom in reservoir bay is obviously different from that in storage and drainage,during discharge is obviously higher than that in storage period.In the early stage of discharge,the residence time of water body was larger and the concentration of chlorophyll a decreased;in the middle and late stage of discharge,the residence time of water body gradually decreased and the concentration of chlorophyll a increased obviously,while during the storage period,its in the opposite,the residence time in the water body before and during the storage was smaller,the residence time in the later period increased gradually,the concentration of chlorophyll a in the reservoir bay was low,and the water bloom was obtained.In the process of discharge,the reservoir bay produced the surface inversion of the surface water,which resulted in the residence time of the surface water of the Bay became smaller,and it had a certain effect on the inhibition of the occurrence of water bloom.2.Taking low concentration of Chlorella as the research object,study the effect of water residence time on algal growth in laboratory environment.The study showed that when the initial concentration of algae was low,the concentration of chlorophyll a increased with the increase of water residence time,the stability of algae growth are exponentially related with the reciprocal of the water residence time.To a certain extent,the residence time of water can restrain the growth of algae and reduce the peak value of chlorophyll a in the outbreak of algal bloom.The threshold of water residention time for Inhibition of algal growth is 4.5d.3.Taking high concentration of Chlorella as the research object,study the effect of water residence time on algal growth in laboratory environment.When the time of water residence is shorter,the algae is in negative growth,and the concentration of algae drops obviously.Compared with the low initial concentration of algae,the effect of the water residence time is more obvious in high concentration algae.The concentration of chlorophyll a was reduced by 50%,the threshold of water retention time was 4d,the concentration of chlorophyll a was reduced by 70%,the threshold of water retention time was 2.5d,the concentration of chlorophyll a was reduced by 90%,and the thresho ld of water retention time was 1.25d.Based on the hypothesis analysis,the horizontal migration equation of algae based on the residence time of water is:（?）The general solution is:C=Ke^MT4.Analysing the changes of residence time and chlorophyll a concentration in the bay under different scheduling conditions by using CE-QUAL-W2 mode.1)During the process of scheduling,in the suface and middle layer body of the Xiangxi River,the water residence time from the upstream to the downstream gradually decrease,and the chlorophyll a gradually decreases too.At the bottom layer body of Xiangxi River,from upstream to downstream,the residence time gradually increased,and the concentration of chlorophyll a firstly increased then decreased.On the whole,the concentration of chlorophyll a was lower than that of the surface.Chlorophyll a concentration is consistent with the trend of water residence time.2)The discharge process can shorten the water residence time in upstream of Xiangxi River and inhibit the growth of algae in the upstream.During the discharge process,the discharge intensity increases,and the flow also increases,the water residence time in the upper reaches of the Bay decreases,and the water body with high algae concentration in upstream rolled into the middle and lower reaches of the Bay from bottom,thus reducing the algae concentration on the Bay.3)Daily storage of 2 meters can significantly reduce the water residence time in the middle and lower reaches of the bay.During this water storage scheduling,the water body enters into the tributaries from the surface layer and exchanges with the tributary water,which reduces the residence time of the middle and lower reaches of the tributary Bay,and the water body with high algae concentration plunged through the bay from the bottom.The impoundment process has significant effect on controlling algal blooms in the middle and lower reaches of the reservoir.4)The storage and drainage alternation scheduling process has little influence on the residence time of bay,and the effect of this scheduling is not obvious.