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Study on the numerical simulation of flow pattern in freshening reservoir using satellite image data

Choi, Moon-Soo1), Takashi Hoshi2), Kiyoshi Torii3)
1)Graduate School of Science and Engineering, Ibaraki University,
4-12-1 Nakanarusawa, Hitachi, 316-8511, JAPAN
Tel : (81)-294-38-8235 Fax : (81)-294-23-7586
E-mail : [email protected],ibaraki.ac.jp
2)Department of Computer & Indormation Sceinces, Faculty of Engineering,
Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, 316-8511, JAPAN
Tel : (81)-294-38-5133 Fax : (81)-294-37-1429
E-mail : [email protected]
3)Division of Environmental Sciences & Technology, Graduate School of Agriculture,
Kyoto University, Kitashirakawa, Sakyo, Kyoto, 606-01, JAPAN
Tel, Fax : (81)-75-753-6459
E-mail : [email protected]

Abstract
This study carries out a numerical simulation of inflow and outflow by grid generation in order to examine the mixing process of sea water and fresh water, which applied to Yongam lake in the southwestern of Korea. And then, the Landsat TM imagery are used to analyze freshening process and mixing pattern using the different reflective features between sea water and fresh water. As a result, the simulation is performed successfully using computational grids, those generated for the case of before and after of the enclosure in Yongam lake. The Landsat TM data classification interpreted the freshening process and the mixing pattern of influent water, stagnant water and sea water et at. In freshening reservoir.

Introduction
The comprehensive tidal land reclamation projects in Korea have been implementing for satisfying the demand of land and water resources increased since the beginning of 1970s. The Projects are also construct a freshening reservoir which supply irrigation water to the reclaimed land and existing surrounding farmland. However, the construction of freshening reservoir is in conflict with the preservation of natural environment. This study is associated with the analysis of the sequence of changes and phenomena including environment in the freshening reservoir.

A standard freshening reservoir consists of three kinds of mechanisms comprising convective freshening in an upper layer, the vertical diffusion through a spring layer and the mixing action in the bottom of a lake in freshening process1). The first mechanism of convection by inflow is very important phenomenon that relates to freshening process and water quality.

This study carries out a numerical simulation of fluid flows and image classification using Landsat TM data to analyze the convective mechanism of freshening reservoir. The former shows the flow simulation of inflow and outflow by grid generation in order to examine the situation of the mixing process of sea water and fresh water. The later analyzes the freshening process and mixing pattern using the different reflective featues between sea water and fresh water.

These methods are applied to Yongam lake in the south-western of Korea, which have been executing the comprehensive tidal land reclamation. This paper also examined the method to investigate the influent flow pattern and the relation to the water quality of Yongam lake.

Computational grid generation of study area

Study area
The study area of this paper is the Yongsam lake of Yongsan river comprehensive agricultural development projects third stage, located in the southwestern coast of the Korean peninsula(Fig.1). The length of enclosure like in this district is 2.2 km, the water resources to be developed by desalination is 153 million m3 and the area of water reclaimed land is 6,730 ha. The river basin of Yongam lake has a lot of small stream influent only in the rainy season. The inflow of all the stream is calculated in three places and five places respectively, the time before and after the completion of enclosure dike in Yongam lake, as shown in Fig. 3 The catchment area is relatively small compared with the area of lake, and the inflow also in insufficient to desalt. Therefore, the connection canal to desalt by introducing fresh water from adjacent Yongsan river estuary has been already completed and operated voluntarily according to inflow without intervals.

Fig 1. Location of the study area

Fig 2. Flow chart showing various Steps of this study

Grid generation
The grid generation used the transfinite interpolation for the different times in Yongam lake and applied to boundary adaptive curvilinear coordinate grid. The outline of the grid generation is shown in Fig. 2 and these steps are follows.

First, the representative sides are generated by linking the representative points decided to locations which have topographical features and the changes of water flows. Second, The grid points are generated onto the representative sides of equal and unequal intervals.. If it have an equal interval, the grid number is divided in proportion to distance between the representation point of both sides. However, the division of unequal intervals is used the linear interpolation using Robert’s equation. Third, smoothing is executed using Lagrangian interpolation on the whole area. The smoothing has two kinds of method whether smoothing will perform or not for grid point onto the boundary. Fouth, the Jacobian evaluation of the grid generation put into practice to examine overlapping representative sides on the same axis. Finally, the masking is described as a black mark these not included computational area.

The grids generated by these steps are 194 points in vertical plane and 55 points in horizontal plane for the direction of the mouth of a river in the area before enclosure, 210 points and 56 points separately in the area after enclosure, as shown in Fig. 3.

Fig3. Gird generated in the area of A) before and B) after enclosure

Fig4. Contour line of eater depth in the area
Numerical simulation of inflow flows
The long wave equation utilized as equation for the numerical analysis of flow fields is defined as follows
Where h : displacement of water surface
q: liner discharge vector,
t : time ,
h: water depth,
g: acceleration of gravity,
n: kinetic viscosity
fe : coefficient of friction,