Key Words: Freshening Process, Landsat TM, Atmospheric Correction, Numerical Simulation, Convective Mixing

Abstract
The hydraulic characteristics of the estuarine reservoir constructed in the mouth of a river, which is influenced by the complex mechanisms such as convective mechanism, vertical diffusion, and mixing action in the freshening process. The convective mixing mechanism is the most important element in the freshening of estuarine reservoirs introducing desalted fresh water for the control and improvement of water quality. This paper describes for the convective mixing characteristics using satellite image data and a numerical simulation of flows. The satellite image data consist of two kinds of Landsat TM images acquired in different times separately, which are classified for the study area including the Yongam lake and its bay after the geometric and atmospheric correction processing. These results are also compared to a numerical simulation related to the velocity-vector distribution of influent water and field measurement values that show the temporal variations of water quality in the freshening process. Finally, in the analysis hydraulic characteristics in freshening process, we examined the applied possibility of Landsat TM Image data.

1. Introduction
The Yongam lake located in the southwestern coast of the Korean peninsula (Fig.-1), is a standard estuarine reservoir which had been completed on Dec. 31, 1993 as a part of Yongsan river comprehensive agricultural development projects (?-1stage). The hydraulic characteristics of the reservoirs constructed in the estuary, which has been influenced by the complex mechanisms, such as the convective freshening in an upper fresh water layer, the vertical diffusion through a density interface of fresh-salt water, and the mixing action from the bottom soil of a lake. The convective mechanism caused by the mixing of inflows, is very important element in the freshening process of the estuarine reservoir as the Yongam lake introducing desalted fresh water (surplus water) through link canal from adjacent lake.



This article deals with the convective mixing mechanism in the freshening process using Landsat Thematic Mapper(TM), a numerical simulation of flows, and field observations. But, in the use of Landsat image data, three visible bands (485nm, 560nm, and 660nm) observed in the wave length range of visible spectrum have been powerfully affected by the presence of atmosphere, and the analysis of original data becomes difficult. The atmospheric influence must be especially removed in study cases of very small difference of radiance, or surface reflectance or absolute values of temperature, or in researches related to water pollution. And, the atmosphere correction of satellite image data is not much performed because the radiative transfer of electromagnetic wave follows very complicated process.

And then, this study uses the Second Simulation of the Satellite Signal in the Solar Spectrum (6S) of radiative transfer code to make the atmosphere correction possible on the image data without the various information about atmosphere, which is the improved version of 5S developed by the Laboratoire d'Optique Atmospherique. That is a pre-processing process of TM data to analyze the hydraulic characteristics of estuarine reservoir. The corrected data were classified, which were generally compared to the results of the two dimensional numerical simulation of flow fields and field surveys.

2. Study Area And Topographical Characteristics
The study area of this study is the Yongam lake located in the middle of three estuarine reservoir in Yongsan river comprehensive agricultural development projects, as shown in Fig. 1. The enclosure dike of Yongam lake was completed on April 8, 1991. The length of the enclosure dike is 2.2 km while the water resources developed in this desalted lake is 153 million m3 and the water area reclaimed land is 6,730 ha. And the area of the agricultural land adjacent to the water area reclaimed land offered for improvement of irrigation and land consolidation is 5,470 ha. The water resources of Yongam lake is controlled and managed by the management office of Yongam lake opened on May 3, 1994.

The Yongam lake is introducing the desalted fresh water through link canal from Yonsan estuary lake because the river basin is narrow and the inflow is small in comparison to the storage capacity of Yongam lake. The excess water of lake is also recycled as fresh water to desalt the salt water of high concentration in Kum lake. At the present time, the link canal to Kum lake has been completed, the desalinization is progressing well by introducing surplus water desalted from Yongam lake.