Diurnal And Seasonal Variations In The Thermal Structure Of A Deep Run-of-the-river Reservoir

Multiple reservoirs have been constructed or are currently under construction in Southwest China,the common characteristics of which are substantial water depths,large volumes,and massive flow rates.In this study,these special types of reservoirs are referred to as “deep run-of-the-river reservoirs”.The Xiluodu Reservoir is a quintessential deep run-of-the-river reservoir and exhibits a thermally stratified structure along with regular seasonal fluctuations.In this study,on-site observations and numeric simulations are combined to determine the diurnal and seasonal patterns of the thermal regime both within the Xiluodu Reservoir and its outflow.The thermal structures of multiple cross-sections along the Xiluodu Reservoir were measured monthly by a CTD instrument from 2013 to 2016.In addition,a thermistor chain and water surface weather station were installed at a cross-section near the dam.Water temperature and meteorological data were continuously collected at this crosssection from November 2016 to September 2017.A two-dimensional(longitudinalvertical)numeric model of the Xiluodu Reservoir based on the hydrodynamic software CE-QUAL-W2 was established.This numeric model was calibrated using observed data and then used to simulate velocity,temperature and water age of the Xiluodu Reservoir from 2013 to 2015.Validation revealed that this model exhibited high accuracies.The diurnal fluctuations of thermal stratification and the growth patterns among the individually stratified layers within the Xiluodu Reservoir were analyzed and summarized to ascertain the dominant controlling mechanisms.When the daily averaged total heat flux is positive,the vertical temperature gradient in the diurnal mixing layer is positive.When the daily averaged total heat flux is negative,the vertical temperature gradient in the diurnal mixing layer is also negative,which in turn induces vertical convection.Vertical convection is the dominant factor in the cooling process in the upper epilimnion.During seasons when the daily averaged total heat flux is negative,inflows always enter the lower epilimnion and form interflow.This interflow is the dominant factor in the cooling process within the lower epilimnion.The mechanisms governing the formation and deepening process of stratification within the Xiluodu Reservoir were revealed.Warm inflow entering the reservoir as the overflow is the main factor controlling the formation of thermal stratification in deep runof-the-river reservoirs.Due to the relatively rapid longitudinal movement of water within the epilimnion,the cumulative effect of positive surface heat flux on the heating of the epilimnion is not as significant as inflow temperatures.When the water is stratified,the metalimnion will form a slope in the upstream direction and the slope changes with the ups and downs of the inflow rates.The outflow rates of the Xiluodu Reservoir exhibit significant hydropeaking.Under stratification conditions,it was found that hydropeaking at Xiluodu generated an internal wave with a period of 24 hours.This wave propagated upstream along the metalimnion and the wave speeds agree well with the solution of Kelvin-Helmholtz wave theory.Due to metalimnetic tilt and wave motion,longitudinal dispersion induces mixing between different layers,so that the longitudinal dispersion coefficient plays an important role in the numeric model.There is a positive correlation between the hypolimnetic deepening rate and the square of mean velocities above the hypolimnion.Hypolimnetic deepening rates are also influenced by outflow withdrawal elevations.Inflow is also the dominant factor controlling the destratification of the thermal structure.Annual phase method was introduced in this study to evaluate temperature delay at the outflows of both the Xiluodu Reservoir and its downstream reservoir,the Xiangjiaba Reservoir.In 2014,the delays of outflow temperatures in Xiluodu and Xiangjiaba compared with the inflow temperatures of Xiluodu were 16.5 and 27.1 days,respectively.In 2015,the temperature delay for Xiangjiaba was 22.3 days.Before the impoundment of the Xiluodu Reservoir,the river water mainly absorbed heat and temperatures increased about by 0.54? while flowing through the reservoir section.After dam impoundment in 2014 and 2015,the volume weighted temperatures in the inflow were higher than the volume weighted temperatures in the outflow.This means that the reservoir retained part of the heat that entered through the inflow.Part of this heat was used to warm up the reservoir water while the rest of it was lost at the water surface.