The dynamic Remote Sensing monitoring of the east four outlets in Pearl river estuary and its application on offshore engineering

The dynamic Remote Sensing monitoring of the east four outlets in Pearl river estuary and its application on offshore engineering

Xu Xiangxiang, Chen Xuelian, Luo Dan
The Scientific Research Institute,
Pearl River Resources Commission,
Min. of Water Resources, China

Abstract
In this paper described are the dynamic monitoring of the flow field suspended sediment distribution beachland resources and its development by means of satellite remote sensing information processed with computer and its qualitative and quantitative interpretation .The recent change of water sediment transport in analyzed through a dynamic comparison of the features in multi time phase images The scoring and silting on the shoals at the outlets are analyzed through a comparison of historical survey data and present remote sensing data The effect of existing works on the water sediment transport and the hydro-dynamics at and out of the outlets is analyzed by means of monitoring the progress of the exclamation projects at the outlets . And proposals are put forward for outlet training reasonable development of the beachland and the optimal time of exploitation.

Introduction
The east four outlets (Humen, Jiaomen, Hongqimen and Hengmen ) and the shallow sea area out of them compose Lingdingyang Bay, which is the biggest estuary bay situated in front of the east part of Pearl River Delta with a trumpet it catches the water and sediment coming from the outlets lets the tidal water come in from and go out to the South China sea. The hydro dynamics is very complex resulting in an active sedimentation which yields rich beach land resources providing conditions for land extension but is unfavorable for flood disposal irrigation drainage and navigation therefore the improvement and development of the east four outlets is an urgent and heavy task .In recent years computer processing and optical processing the multi time phase suspended sediment images we analyze the image feature and their dynamic changes to research the flow field suspended sediment distribution and topography of shoals Some phenomenon and laws difficult to be find in traditional way are revealed and important information is provided for the planning and construction of estuary improvement works. Part of the results in this research work has been adopted by production sectors to adjust the plan of the projects certain social and economic benefit is yielded.

Remote Sensing information and its processing
The remote sensing information used in this research includes color infrared images in 16 time phase from MSS TM and China Land sat in Which TM CCT data and color infrared images taken in 1986 to 1988 are taken for information's about the present situation of the east four outlets while the MSS images taken in 1970's are taken for dynamic comparison information.

Computer image processing is involved in the quantitative analysis and mapping of the remote sensing information and survey data including radiation value rectification geometric rectification linear contrast stretch correlation analysis of remote sensing and field survey data digital enlargement digital mosaic and color composition In addition optical processing method is used in the color enlargement of images at the scales of 1:50000 and 1:100000 with DS 1840 color Enlarger and also in superposing of remote information on geographic information etc.

The qualitative and quantitative interpretation of flow field suspended sediment and topography of shoals

Interpretation Marks for the flow field at the outlets
The momentary situation of water surface and the sediment in the upper is recorded on remote sensing images The changes in flow fields will cause a change in the spectrum characteristics of the surface layer which can be shown in the images By means of inter lines and the difference among grey gradients flow filed features can be identified such as the main flood tide and ebb tide current the intersection of runoff flow and tidal currents the intersection of the flow out of neighbouring outlets with the dominant flow the mixing of salty water and fresh water etc.

The Quantitative Interpretation for Suspended in the Estuary
In this study the TM CCT received by China remote Sensing Ground Station (Beijing) are used to search the way for quantitative interpretation of surface suspended sediment concentration The experimental process and methods are as follows.

Synchronous and Quasi-Synchronous Survey
TM CCT data received on 8 December 1987 were taken for information source while synchronous and quasi-synchronous survey at the east four outlets and in the channel outside the outlets was undertaken in 8 and 24 December The main purposes of observation were flow direction, velocity, depth and suspended sediment concentration The survey time was 9 to 11 am Since the tidal pattern and the flow condition upstream of the outlets on 24 December are closely similar to those on 8 December when the satellite passed over the site the observation on 24 December can be considered as a quasi synchronous one.
Wave band Selection and correlation Analysis
Datum characteristics of different wave based on TM images are compared and analyzed in this study such as the brightness range bar chart etc it is shown that all the ratio of TM3 TM4 and TM4/TM2 can indicate the difference in suspended sediment concentration Taking the ratio wave bands for the variables in the correlation equation has the advantage that no effect of the changes in illuminance solar elevation and scan angle would occur further more, it can get rid of absolute radiation standardization for the data to be translated there for ratio TM4/TM2 is taken for correlation analysis with survey data of sediment load. Yielding:

Log S = 0.669E + 1.262

Where S is suspended sediment concentration (mg/1) E is the ratio Tm4/TM/2 in order to check the accuracy due to the equation a verification in undertaken using the survey data at 8 survey points on the image of another time phase it is shown that the accuracy can generally meet the need of application.

Table. 1Verification of the Equation

Ratio TM4/TM2	1.094	0.834	0.902	1.812	1.405	1.210	1.528	0.412
Calculated S (mg/l)	98	66	73	298	159	118	192	35
Surveyed S ( mg/l)	140	68	83	304	169	110	156	30
Error (%)	-30	-3	-12	-2	-6	7	23	7

Interpretations for Shoal Pattern at the Estuary
Interpretations of TM data in low flow and low tide period for the type and pattern of shoals at outlets and the present situation of offshore reclamation works can be undertaken by means of proper linear contract stretch and categorization of the wave band group TM4, TM5 , and Tm2 with computer In order to compare the present data with historical data about the evolution of shoals the border line between water and land is transitted into isobath for this more survey work was done at more points to get depth and level data when the satellite passing over the site so that the error to water surface gradient could be rectified.

Fig. 1 TM Image of Lingdingyang Bay in low flow
and low tide (08 / 12 / 1987 )

Dynamic recent change analysis for the water sediment transport at the outlets
Remote Sensing images of one time phase can show the characteristics such as flow pattern of runoff and tidal current horizontal distribution of surface suspended sediment and its gradient change which can objectively indicate the interaction increase and decrease of various hydro dynamics under given conditions However comparing and analyzing the dynamics characteristics of multi-time-phase images the characteristics of the changes in water sediment transport with time and space and its dynamic mechanism can be revealed According to this analysis following are the main characteristics of dynamic recent change in water sediment transport at he east four outlets .

It is found an arc shape inter line from the top of the Bay to the mouth exists on the images in the middle stage of flood tide and ebb tide both in medium flow and flood periods this line divides the Bay in to two parts the east part and the west part The west part is a water sediment transport zone dominated by the dynamics of runoff from Jiaomen, Hongqimen and Hengmen Outlets, while the east one dominated by the dynamics of tidal current passing through Humen Outlet. This remarkable inter line shown on the images is quite stable in horizontal location main training alignment in the west part of lingdingyang Bay in the improvement planning That is if the alignment is planned to the west of this inter line the tidal current dynamics in Lingdingyang Humen will not be severally affected.

Fig. 2 TM Image of Lingdingyang Bay in Flood Flow
and Flood Tide (03 / 07 / 1988 )
Recently, the water-sediment runoff coming easterly out from the North Branch of Jiaomen Channel has a large effect on the water sediment transport at Humen out let and in the east part of the Bay IN medium flow and periods the easterly flow coming from North Branch of Jaiomen C Channel severely impacts the tidal current at Humen Outlet in the middle stage of flood or ebb tide in the last stage of ebb tide as the tidal current in the east part of the Bay is considerably weakened the flow coming from the North Branch of Jioamen Chuanbi Channel into east part of the Bay (Fig. 3).

Fig. 3 Interpretation of Flow Field and Sediment Concentration
Lingdingyang Bay (08 / 12 / 1987 )
Due to the growth of the sand at Jiaomen outlet and the construction of the reclamation projects the sediments runoff coming through the North Branch transport recently less through the gully between the shoal southeasterly. The suspended sediment gathering zone fore casting due to the intersection of the flow going downstream from Jioamen outlet and the tidal current going upstream has recently been moved to the vicinity of the entrance shoal and the tail of Masha Shoal the easterly dominant flow resulting the combination of the flows from Hongqimen outlet and North Branch of Hengmen Channel does not go easterly across shoal In to the mouth of the South Branch of Jiaomen channel any longer but a part of them downward water- sediment flow goes in to the shaxia of lingding channel through the gully between the entrance shoal and the tail of Shaxianwei Shoal it will be harmful to lingding channel if developing naturally.

The progess monitoring of the offshore reclamation project its application
Since 1908's large scale reclamation and exploitation has been implemented on the shoals and beached at the outlets on the west of he Bay using remote sensing we have monitoring the progress of the work and got a good result in application on Jiaomen Reclamation project is described as follows.

The Project is composed of Jibaosha Polder and Masha Polder the underwater riprap started in 1985 to set the alignment TM information on July 30 1986 December 8 1987 July 1988 December 10 1988 were collected and MSS images and survey data before the construction were also collected the riprap line and the flow filed characteristics can be seen on the TM image on July 30 1986 it is shown that the implemented work of Jiaosha East Polder has expounded the planned outline has remarkable effect on the water sediment transport at the outlet it appears that the main easterly from the outlet diverts northeasterly toward Chuanbi Channel due to the resistance area by tee North Dykeline the pattern of shoals and channels in or out of reclamation area as well as the area of shoals surrounded by -1 m isobaths can be seen on the Tm image on December 8 1987 The comparison with the topography surveyed in 1984 shows that no remarkable deposit occurs on the east part of Jibaosha Polder and Masha Polder in concentration it was partly scoured in the past 3 years instead further more the beach land in front the North decline was also severely scoured due to the resistance to the main flow the east part of Jibosha Shoal and the east side of Masha Shoal are in the flow intersection zone where part of runoff from the outlet and the tidal current going upstream backwater each other and the dynamics weakening is favorable to deposit After riprap and raising the underwater dyke the flow intersection zone has moved southerly due to the change of boundary condition which is one of the causes of why there is no remarkable promotion of deposit in the reclamation area recently other causes are the plan of the North and east Dykelines is not very reasonable and the under water dykes were raised too early.

Jiaomen Reclamation Company adjusted the plan in the working stage of embankment raising the under water dykes over the water surface according to the results of this monitoring and analysis the east Dykeline east section of the North Dyke and part of the East Dyke were collapsed in the flood period in 1988 after the adjustment this fact indicates that larger economic loss would have been constructed according to the original Dykeline the location of collapse and the flow filed characteristics shown on the principal cause of the collapse is the resistance of the dykes to the disposal flow the comparison of the flow field before and after the adjustment shows tat the North Dyke and the under water riprap ex dyke still divert the runoff from Jiaomen Outlet to Humen outlet the above result of dynamic monitoring of the reclamation project implementation have provided important scientific basis for the optimization of the project plan.

Fig. 4 TM Image of Jiaomen & Humen Outlets in Flood Flow
and Ebb Tide (03 / 07 / 1986 )

Fig. 5 Interpretation of Evolution and Exploration of
Shoals at Jiaomen Outlet

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