Satellite observation of south coast of Bohai Bay
Fan Zhaomu, Wu peihong
Remote Sensing Department, P.O. box, 910, Beijing
Wang Zhihao
North China Sea Sub-bureau State Oceanic Administration,
Qindgao China
Abstract
The beach and shallow waters along the south coast of the Bohai Bay are dominantly influenced by oceanic effects and deposition of sludge from the huanghe river. Sludge deposition during high water level and erosion during low water level make the shoreline frequently migrate.
Slopes of different coastal segments have been estimated based on satellite images obtained in different times since 1975 and available data abut tide, hydrology and so on. Translating instantaneous shoreline into zero water-depth contour makes image comparison be reasonable, really reflect erosion and deposition along coast. Authors' observation makes up the lack in measurement along beach and shallow waters, and provides important information for petroleum exploration, development, and engineering.
It is discovered from observation that different segments of he south Bohai Bay coast not only have different characteristics during different periods of time but also show common tend in variation.
Introduction
The studying area is beach and shallow-waters extending from Taoer River in west to Wuhaozhuang in east along the south Bohai Bay coast..
The Huanghe River transports volume silt into the Bohai Bay every year. The statistical data by Lijing Hydrometric Station show that up to 10 billion tons of silt flow through the river mouth each year from 1950 to 1988. Five mouths have developed along the south Bohai Bay coast after 1904. Their total life span amounts to 43 years, during which, deposition of large amount of silt made shoreline frequently propagate. After the Huangbe River changed its course in 1976, fresher water has emptied into the south Bohai Bay coast and oceanic effect has become dominate, the south coast has been eroded.
The achievement in petroleum exploration along the south Bohai bay coast in past years demands more and more development engineering and surface building, and hence more and more knowledge of the oceanic environment. For this purpose, we have studied deposition and
erosion, and their effects on petroleum engineering and surface building by the explaining many satellite images covering this area.
Data processing
More than 30 images obtained during the period from 1975 to 1987 are selected for our goal, most of which are satellite images by U.S.A. Landsat. Because the shoreline on a satellite image just show the instantaneous water boundary corresponding to the formation time of the image, tidal effect must be able to influence the result of studying on migrating trend of the coast. for example, there are obvious difference between water-boundary of the image of May 21, 1975 and that of the image of May 22, 1975, and this difference does not demonstrate the change in deposition and/or erosion. The tidal effect, therefore, must be excluded when we study deposition and /or erosion with a satellite image. we have estimate a standard tidal level based on various instantaneous water-boundaries, and take it as a correlative criterion of image comparison for studying deposition and / or erosion.
Our estimation method includes: working out the celestial tidal level (h
°) corresponding to the instantaneous water-boundary of a satellite image, base on tide measurement by tide stations in this area; estimating coast slope (p); by combining the known tidal level 9ho) corresponding to theoretical zero-depth contour, e.g. the extrapolating distance (
DL ).
DL = ( h - h° ) / P
where , both two tidal levels h and h
° are relative to the mean sea level in site; By reference to the estimated extrapolating distance along each section, we can map the theoretical zero-depth contour corresponding to an instantaneous water-boundary. Estimations of tidal level and coastal slope are of very importance. We use harmonic analysis to estimate tidal levels, and combine resultant estimations of many satellite image of different times and measurement of water depth to recover the coastal topography , and finally to estimate the coastal slopes. this processing not only makes the correlation of satellite images of different times reasonably quantified, but also effectively complete hydrometric measurement along the south Bohai Bay coast.
We can also observe deposition and /or erosion by comparison of two images corresponding to the same tidal level. For example, the two satellite images obtained in November 29, 1976 and November 28, 1983 respectively show similar tidal condition , the difference between water-boundaries of these two images reflects deposition and/or erosion.
