Underflows And Its Effects On Water Quality Of JinPen Reservoir (JPR) In Rainfall Events

To explore the succession process of underflows and its effects on water quality of JinPen Reservoir（JPR）in rainfall events,samples and data including feedwater in river and water column in reservoir were collected in rainfall events during 2017-2018 in JinPen Reservoir（JPR）.The objectives of this study are:（1）exploring the succession process of underflows in rainfall events;（2）assessing the impacts of the underflows on structure of water column and（3）the water quality in main reservoir.The main conclusions and results are as follows:（1）The density of the underflows was controlled by the temperature and the concentration of suspended solids（SS）of the feeding waters.When the flow rate and the concentration of SS are small,the density of the underflows was mainly controlled by water temperature.However,when the flow rate is relative big,the density of underflows was controlled by temperature and the increased concentration of SS.（2）Sufficient kinetic energy was required for the underflows reaching the main reservoir area.The SS was continuously settled in the upstream channel that the underflows less effected by SS if the flow rate was too small.According to the comparative test,the SS in the subsurface water will basically complete the deposition process in the upstream channel when the flow rate was less than 190.0 m³/s,and the turbidity of the water will be reduced to less than 10.0 NTU.In this case,the succession process of underflows was mainly controlled by temperature and the water quality was mainly influenced by the dissolved forms in feedwater.（3）It is no significant influence on the layered structure of reservoir water by the lower runoff（Q<190.0 m³/s）,while the layered structure of water column impacted when the runoff rate is bigger.The processes,underflows with higher flow rate（Q>190.0 m³/s）entered the main reservoir and intrude into the equal density layer,could decrease water stability and drive the thermocline migrates downward.When the rate of underflows reached maximum（Q>1750.0 m³/s）,the density of underflows gets much higher than that in bottom water due to the high sand content,the underflows could directly infiltrate into the bottom of the reservoir,forming the reverse water temperature stratification structure at the bottom of the reservoir and seriously damaging the stability of the water temperature stratification structure.（4）The runoff,especially in rainfall events,is the most important way to transport granular pollutants to the reservoir.Large amount of high turbidity,high pollution load of water into the main reservoir water body water quality deteriorate rapidly.A single field rainfall underflow was selected as an example,on 12 July 2018,the concentrations of TN,TP and COD_Mnn in water column were increased form 1.50,0.008 and 3.61 mg/L to 2.05,0.017 and 4.40 mg/L,respectively,where the grain forms of nitrogen and phosphorus were increased 25%and 50%due to the rainfall underflows at rate of 334.5m³/s.The results show that the high rate of underflows in rainfall events could largely impact the water quality and it is necessary to discharge the underflows at the mouth of reservoir at the intrusion processes and minimize the impacts of underflows.