A hydrodynamic characterization of a reservoir: Case study Yongdam Reservoir

Kenneth Muniina

April 2006

Abstract

Reservoirs, like Yongdam in South Korea, have a transient water quality that is very sensitive both to its catchments and its water usage. Further more, traditional water quality monitoring approaches to control the water quality are inadequate to provide the required background for a strategic water quality assessment. To make matters worse, the response of reservoirs to anthropogenic inputs is very dependent upon their hydrodynamic behaviour, and cannot be induced or implied from previous or other reservoir studies. This makes reservoirs very delicate systems that require a detailed water quality study with aid of a more advanced tool: mathematical modelling, to provide a background for strategic water quality assessment.

A detailed water quality assessment of environmental water bodies involves three sequential analyses: hydrodynamic, physico-chemical and biological. The hydrodynamic study is the most important and is a prerequisite for the other two studies since the physical chemistry and biology are both advected by the water transport (hydrodynamics) at a variety of scales. This research addresses such a hydrodynamic study of Yongdam reservoir.

Hydrodynamic modelling of a reservoir, especially a reservoir affected by eutrophication, requires the response of the reservoir to its forcing functions be understood at a variety of spatial and temporal scales. Unfortunately, the modelling systems available on the market (like one-dimensional vertical and three-dimensional models) are proficient in one of the scales and limited in others, but not in all scales. In this study it was decided to use both CE-QUAL-W2 (1DV) and MIKE 3 (3D) to get around this obstacle. However, MIKE 3 was originally developed as an “ocean” model that is being extended for use in reservoirs, which produces a severe strain to the modelling system.

Calibrated models of Yongdam were developed in CE-QUAL-W2 for the period January 1^st to August 31^st 2005, and in MIKE 3 for the summer period July 19^th to August 28^th. An analysis is carried out of the simulations right from the velocities in the reservoir to the turbulence levels in the bottom currents. The analysis reveals that Yongdam can be divided into two hydrodynamic zones in the horizontal, that is, littoral zones and pelagic zones, with quite different hydrodynamic behaviours. In the vertical, Yongdam is stratified in the pelagic zones and unstratified in the littoral zones.

Not only, was a hydrodynamic built and applied in the research, but also an assessment of MIKE 3 (an ocean model) to its suitability to reservoir modelling, in terms of its scientific approach to reservoir hydrodynamics.

Keywords: hydrodynamics, water quality, stratification, turbulence, eddy viscosity, mixing etc

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