Abstract


Contribution of remote sensing technology in studying morphological change of a bar–built estuary in a part of South Central coast of Viet Nam, Loc An Estuary

Pham Bach Viet
Institute of Tropical Biology
Collaborator of Remote Sensing Division
85, Tran Quoc Toan,
3rd District, HCMC, Vietnam
E-mail: herbarium@hcm.vnn.vn, vientham@hcm.vnn.vn, bach_viet@hotmail.com


1. Introduction
Using remote sensing data to study change of estuarine areas usually focuses on deltaic – shaped estuaries characterized by muddy flats and often extending in area, by process of accumulation of materials. This extension can make an accretion in coastline or shallow channel of river. Identification of the change does not meet much difficulties. Other type as bar – built estuary is less interested because in terms of environment and economic, using resources of this type is not diverse as the former. However, morphological change of this type does not happen as the pattern of accretion and erosion. This makes prediction of change become more difficult because its variation is affected by various factors such as hydrology, sea – ocean current, geomorphology, local climate condition, particularly if it is located on coastal zones facing open sea. Loc An estuary is located in the furthermost southern part of central coastal zones of Vietnam characterized by cliff, sandy beaches with existence of mouths of short rivers and small estuarine lagoons. This estuary area with a mixed shape of bar - built and estuarine lagoon is a small one, its coastline for the whole area is about 6 kilometers length and less than 1 kilometer width. Loc An estuary can also considered as typical one for this morphology type in central coast of Vietnam.

This study presents an application of multiple – date remote sensing data including aerial photos and satellite images to study shape changing over past 50 years to contribute understandings on complex morphological change of a bar - built estuary and to clarify the changes in order to use the resources in this area more effectively. Data used in this study include aerial photos, digital satellite imageries of Landsat MSS, TM and ETM+. The aerial photos take in 1953, the earliest remotely sensed data in this area and digital satellite data of MSS acquired in 1973 from Landsat 1 are useful historical data for tracking changes.

Totally there are eight benchmark times of data from 1953 up to 2001 were used to identify shape changes. The available data are aerial image data of 1953, 1979 and 1990 and satellite images in 1973, 1989, 1993 and 2001. The topographic map at scale 1: 50.000 published in 1966 was used as baseline and for georectification of all imageries after the UTM projection. Only the MSS data at spatial resolution 80m was resampled to TM data with resolution 30m, all the rest data were kept on the original resolution.

The two approaches of change detection as imagery analysis are applied to supplement each other. That is combination of single band of each date to identify changes and processing separate data then shoreline extracted and comparison on GIS to get more accuracy outcome. Aerial images of different scale were rasterized and treated as remotely sensed data, shorelines were extracted directly.

2. Results
The result shows a great change between today and the past 50 years. From 1953 to 2001, at least there were 5 times of significant movement of sand dunes – spits. These changes do not follow a certain pattern of accretion and erosion observed. Process of shorterning, splitting, extending in length have occurred. Inlet position moves from place to another and there is a tendency that previous status (shape of the lagoon in the past) could re-occur at some positions of spit.

It is necessary to notice that in the study area, historically, before 1980s there were not clear evidences on relation between human activities and shape change. In the whole area, from river mouth area to upper stream and adjacent zones, vegetation cover rate is very hight, all is dense forests. Before 1980s shape and size of spits were change 3 times. This is mentioned in order to consider natural factors in change process in the past.

From imageries of 1993 and 2001, on front face of inlet appeared submerged bar blocking the waterway. This could make inlet changes position again.

3. Conclusions
Although the resolution of the used imageries is not unique (aerial photos at different scale, Landsat MSS, TM and ETM+), studying morphological changes of this research did reveal significant changes of this river mouth over different periods.

Natural factors seem to be major ones. Sand dunes originated from oceanic process in the past are still on affect.

Intervention of human activities such as harbor construction, blocking up a part for aquaculture, removing mangrove and related constructions in the area, dam making and vegetation cover clearance in upper stream will make processes of change more complex.

50 years is considered to be a short moment in term of geological age. The change shows an unstable status of shorelines and river mouth with this type of structure bar – built estuary located in this coastal zone – central to south central coast zone.

The change of this estuary indicate an unstable status. A relatively stable period could prolong only about 10 years. Uses of any kind of resources in this area have to consider carefully.