Effects of thermal stratification and mixing on reservoir water quality

Abstract

In this study, the effect of thermal stratification on water quality in a reservoir has been investigated by field observations and statistical analysis. During the summer period, when stratification is evident, field observations indicate that the observed dissolved oxygen concentrations drop well below the standard limit of 5 mg l -1 at the thermocline, leading to the development of anoxia. The reasons for variations in the dissolved oxygen concentrations were investigated. Variations of air temperature and other meteorological factors and lateral flows from side arms of the lake were found to be responsible for the increase of dissolved oxygen concentrations. It was also observed that turbidity peaked mostly in the thermocline region, closely related to the location of the maximum density gradient and thus low turbulence stabilizing the sediments in the vertical water column. Relatively cold sediment-laden water flowing into the lake after rain events also resulted in increased turbidity at the bottom of the lake. Nondimensional analysis widely used in the literature was used to identify the strength of the stratification, but this analysis alone was found insufficient to describe the evolution of dissolved oxygen and turbidity in the water column. Thus correlation of these parameters was investigated by multivariate analysis. Fall (partial mixing), summer (no mixing), and winter (well mixed) models describe the correlation structures between the independent variables (meteorological parameters) and the dependent variables (water-quality parameters). Statistical analysis results indicate that air temperature, one day lagged wind speed, and low humidity affected variation of water-quality parameters.

Physical limnology of a typical subtropical reservoir in south China

“…The water level and inflow volume had clear seasonal variations, with a large interannual difference in inflow between 2008 and2009 (Fig. 2).…”

“…In stratified lakes, vertical water movement is hindered and the thermocline behaves as a barrier to DO transport (Elci 2008), a condition seen in Liuxihe Reservoir. In addition, the thoroughly mixed period is so short (Fig.…”

“…Weak turbulence is expected in water layers with a high vertical density gradient for stabilization of the water column, which leads to high turbidity (Elci 2008). After March, the thermocline reached outlet level (205-210 m) and stayed there until mixing occurred (Fig.…”

“…In addition to chemical and biological factors, some physical factors also affect water quality. Studies have found that hydrodynamics play an important role in nutrient flow, related ecological processes, and water quality (Alavian et al 1992, An and Jones 2000, de Cesare et al 2001, Chen and Wu 2003, Romergo and Imberger 2003, Chen et al 2006, and thermal stratification alters velocity profiles and hinders oxygenation in the hypolimnion, thus affecting reservoir water quality (Elci 2008). In addition, turbid water with suspended sediment is known to influence physical, chemical, and biological processes in lakes and reservoirs (Lind et al 1997, Faithful and Griffiths 2000, Doyle and Smart 2001, Effler et al 2006, McCullough et al 2007, Cheng and Kao 2008.…”

Using numerical models and acoustic methods to predict reservoir sedimentation

“…Soil granulometry of the rivers indicated negligible concentration of noncohesive sediments. Constant initial suspended sediment concentration of the lake was defined based on the turbidity values measured by water quality meter and was specified as 15 mg/L at each layer of the water column (Elçi 2008). Constant sediment settling velocity was defined as 5 × 10 −5 m/s based on (Yang 1996):…”