Year

1998

Degree Name

Doctor of Philosophy

Department

Department of Civil, Mining and Environmental Engineering

Abstract

Reservoirs are becoming an increasingly important source of freshwater for potable, irrigation, and industrial purposes. Water supply reservoirs just as natural lakes, are subject to the effects of loadings of silt, depletions of dissolved oxygen, and nutrient releases. The water quality in reservoirs is significantly affected by these processes. The deterioration of water quality in reservoirs is a global problem. The reservoirs are in great need of management and protection. The knowledge on how these problems arise and how they can be managed is critical. Reservoir water quality modelling can provide great understanding of the interaction between physical mixing processes and water quality parameters. A time-variable vertical mathematical model (RESMOD) has been developed and applied to the Avon reservoir. It comprises of a hydrodynamics model ( R E S M O D - H D ), a thermal stratification model (RESMOD-ST), a dissolved oxygen model ( R E S M O D - D O ) and iron and manganese model (RESMOD-FE/MN). The model can be used to successfully simulate the thermal stratification, reduction in hypolimnion dissolved oxygen concentration, dynamic fluxes of iron and manganese over an annual cycle under the reservoir operation and the effect of change in meteorological conditions. The major processes considered in the dissolved oxygen budget model are photosynthesis and respiration of biota, carbonaceous and nitrogenous biochemical oxidation of dissolved oxygen, surface reaeration and sediment oxygen demand. The model quantified the contributions of individual components (sources and sinks) to the overall oxygen budget. The processes included in the modelling of the iron and manganese cycles are the dissolved iron and manganese fluxes from the sediment, the dissolved iron and manganese oxidation, and particulate iron and particulate manganese removal by coagulation from the water column. The basic descriptive statistical analysis was undertaken on the field data obtained Avon reservoir in 1989 and 1990 to draw preliminary conclusions. It were also used to determine the validity of the model and to estimate the rate constant values for the processes. Optimal values of the rate constants were determined by comparison of simulated results with field data and were in general agreement with literature values. The predicted profiles of water temperature, dissolved oxygen, total iron and manganese, filterable iron and manganese, particulate iron and manganese are shown to agree closely with measured data. Sensitivity analysis and comparison of individual processes indicated that the sediment oxygen demand was the most important sink of the oxygen budget in the hypolimnion of the reservoir. The importance of light intensity and photosynthetic production in the D O budget was demonstrated. The results exhibited the seasonal variations of filterable iron and manganese fluxes from the sediment and the critical role of oxygen content of the waters. The two important differences in the redox chemistries of Fe and M n were indicated. The model provided a greater understanding and highlighted the gap in knowledge of the Fe and M n cycles in a dynamic lake system. The model can be successfully used as tool for management of water quality in a water supply reservoir. It can be used to study the effect of hydrological and meteorological factors on thermal stratification processes in a reservoir. Further, the model lend itself for optimising several management options including a selective intake and destratification. It is able to predict water supply quality for different operation schemes at offtake tower. The model R E S M O D with bubble plume theory can be used to assess possible aeration strategies and design optimal air flow rates for the summer period. It displayed its potential as a valuable research and management tool for the Avon reservoir.

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