Satellite observation of south coast of Bohai Bay

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.

Deposition and erosion along the south Bohai Bay Coast
Deposition and erosion are various from place along the south Bohai Bay coast. we therefore divide the coast into two segments: one from the Tiaohe river to west, and the other from the Taiohe river to east.

West Segment
This segment belongs to weakly retreat-by - erosion type coast, extending from the Taoer river to the Tiaohe river 9 Fig. 1). The Huanghe river emptied from this segment into sea from 1904 - 1926. After then, retreat-by-erosion environment has developed Because of the long period of stagnant, and retreat rate has become lower.

This segment can be still subdivided into two parts: from Walagou to west, the beach is broad and gentle, its maximum width being up to more than 20 k, and its slope only about 1:10,000; from Valagou to east, the beach width is about several kilometers and slope about 4:10,000. along the western part, tide difference is larger, and storm surge effect is obvious, retreat mainly results from tide and wind effects.

The high tidal boundary , therefore , has strongly gone back. according to the theoretical zero-depth contour from 1975 to 1985, the low tide zone has widened , demonstrating that the coast became gentler and wider by erosion and retreat. The resultant coast topography will reduce erosion and tend to make the coast stable. The eastern part has been influenced by sludge supply by the Huanghe River from 1974 to 1976, and has propagated with silting up. Since then, eastern part has retreated by erosion and showed the common development trend with the western part, but to a lower degree. Retreat of erosion cliff displayed in the satellite images before 1980 and after 1983 shows that the high tide zone went back slowly. The theoretic zero depth contour shows that the retreat rate changed with time (Fig. 2) and reached up to maximum from 1978 to 1980, become lower in recent years.

Fig. 1 A diagram showing deposition and erosion along the south Bohai Bay Coast

Fig. 2 A diagram showing propagation and retreat rate.

East Segment
This segment, extending from the Tiaohe River to the Wuhaohuang, belongs to strongly retreat-by-erosion type. The Huanghe River emptied from this segment into sea for three times from 1926 to 1929. Because of the latest stopping of water current and short stagnant period, retreat-by-erosion is strong and retreat rate along the whole segment is larger than that along the west segment.

	Image 1.	Obtained , May 22, 1975. The tide level is lower than the mean sea level.
	Before the 1975 flood season coming, the river mouth is smaller.
	Image 2.	Obtained, Dec. 17, 1976. The tide level is lower than the mean sea level, the tide situation is similar to image 1.
	After undergoing the 1975 flood season, coast -line changed evidently Towards sea the river mouth is in convex bank.
	Image 3.	Obtained, April 2, 1981. The tide level is lower than the mean sea lovel, the tide situation is similar to both image 1 and image 2.
	Since the huanghe River channel changed its direction in 1976, the erosion caused the shape of the River mouth changing evidently
	Image 4.	Obtained, Nov. 25, 1995. The tide level is lower than the mean sea level, but it is little lower than image 1, 2, and 3.
	Erosion caused the coastline changing in more smooth shape.

We take E118045' as a boundary to subdivide this segment into two parts. The western part was the main depositing area of the sludge from the Huanghe River from 1974 to 1976,' beach propagated seaward significantly. Steep frontal slope and loose deposits resulting from fast deposition have caused strong retreat-by - erosion after the water-way change of the Huanghe River May, 1976. Retreat-by-erosion effected the whole segment, and achieved the maximum rate during the period from 1978 to 1980. The theoretical zero depth contour of 1980 shrunk landwards of the zero depth-contour before the flood of 1975, and sub water slope of this segment became gentler and width increased. After 1980, retreat rate has tended to slow 1976, change of the mean high tidal line has kept pace with that of the zero depth contour and the retreat magnitude is not lower than erosion magnitude of the zero depth contour. and the retreat magnitude is not lower than erosion magnitude of the zero depth contour. From the vertical view, depth of down erosion is not very large from 1976 to 1985, the maximum depth of down erosion was no more than 4 m, showing that main result of retreat-by- erosion is lateral coast retreat. Satellite images can show the result directly (Images 1,2,3,4). Compared with the west part. sludge deposition stopped earlier along the east part. Which has ex- perienced erosion for a longer time, and has a loser retreat rate. Retreat-by-erosion can be observed clearly in both vertical and lateral views. From 1976 to 1985, an obvious center of down erosion had developed in the underwater area, the erosion depth reached up to more than 6 m. This is the cause often washing at the platform bases in this area. The embankment built recent years for petroleum development has been eroded and collapsed locally. In addition , there is a non-tide area with small tidal difference and strong tide current. The coast adjacent to this non-tidal area has been more strongly eroded and become narrower.

Main conclusion
Based on the above analysis, we can conclude as follows:

The south Bohai Bay coast can be divided into two segments : weak retreat-by-erosion segment and strong retreat-by-erosion segment . The topography of the former one changes slowly with a broad beach, a gentle slope, weak erosion and a stable erosion cliff. In contrast, the topography of the later one laterally changes strongly with a narrow beach and a steep slope, and without a stable erosion cliff..
Erosion at the protruding part and deposition at the elepressing part are one of main manifestation of retreat-by-erosion. Erosion at the protruding river mouth makes the coast smoother and retreat rates along different part of the coast be similar.
Another important expression of retreat-by-erosion is the change in beach topography ----that beach becomes gentler and wider, and tend to be not faver to erosion.
On a profile view, erosion occurs at the upper part of the whole coastal slope, and deposition does at the lower part. The transition between erosion and deposition lies slightly above the coastal slope break. Toward land from the transition are located erosion areas.
Coast retreat landwards and water deepening by erosion make petroleum development difficult. Only with the full knowledge of deposition and erosion in this area, we can make reasonable plans to avoid accidents.

References

Gao Shanming et al., Formation and Sedimentary Environment of the Huanghe Delta, Science Press, Beijing , 1989.
Chen Shupeng et al.., Application of Satellite Information to General Territory Survey, Science Press, Beijing, 1989.
Hydrology & Hydrodynamics Lab of the Tianjing University, Marine Petroleum Engineering Environment; Petroleum Industry Publishing House, Beijing, 1983.