Monitoring and predicting the shoreline change in Can Gio area in condition of the sea level rise

Locating on the domestic and international navigation routes (Long Tau-Tac Dinh Cau route, Soai Rap route) the Can Gio area is impacted by waterway traffic activities. The seasonal movement of sand bars on the Can Gio is significantly impacted by hydrodynamic of the river mouth. With the important roles of the area, the authors consider the coastal morphology processes under the hydrodynamic. In this paper, the authors has inherited previous studies combined the satellite image analysis to detect the shoreline changes from 1973 to 2013. Besides, numerical modeling was also applied to predict the shoreline changes under impacts of the sea level rise. Results show that, the Can Gio shoreline prolonging from Can Thanh to Dong Hoa is seriously eroded, with average of 7-10 m/year, maximum to 15 m/year. It is found that, beach erosion at Can Gio is a kind of surface erosion impacted by human trigger (shrimp ponds, beach encroachment...). In addition, natural factors (wave, long-shore current, littoral materials) also contribute to increase the erosion rate. Predicted results with sea level rise scenarios show that, the Dong Hoa and Can Thanh will be seriously eroded while the 30-4 beach will be annually deposited.


INTRODUCTION
Locating on the Southwest, Can Gio called "green lung" of the Ho Chi Minh City.The mangrove forest system is deemed the "priceless treasure" of the downstream of Sai Gon -Dong Nai river system.Several years ago, along with the development of socioeconomic and the impact of global climate change, the coastal erosion occurred seriously, which lead to narrow the mangrove forest, negatively impact on beach tourist and the local aquaculture.To assess the erosion mechanism, many researches have been carried out by applying of various methods: satellite image analysis, numerical modeling… In this paper, the authors have assessed the shoreline change at Can Gio beach (around 13km from the Dong Hoa to Can Thanh cape).Figure 1 illustrates the study area.

Geographic Information System (GIS)
To assess the shoreline change, the satellite data from USGS (Landsat image) have been used.Parameter of satellite used are given in Table 1 below: The authors have combined the Band 5 (1.55-1.75μm), 4 (0.76-0.9 μm) and 3 (0.69 -0.63 μm), to determine the changes of land-cover through many years.After that, authors determined the location of shoreline by using images of Band 5 (1.55 -1.75 μm) and Band 2 (0.76-0.90 μm) of Landsat/TM and evaluated the parameter TM5-2 defined as the following equation (Deguchi et al., 2005): By using the TM5-2 images, the radiance values in the pixel between land and sea are changed suddenly so that the shoreline change can be extracted easily.Besides, the land-cover analyzing may assess the change of land cover through many years.

Numerical modelling
To assess the potential impact of the sea level rise on the shoreline change, a model of "littoral processes and coastline kinetics" (LITPACK) was established.The main modules of the LITPACK are as the followings: Non-cohesive sediment transport (LIST); Long-shore current and littoral drift (LITDRIFT); Coastline evolution (LITLINE); Cross-shore profile evolution (LITPROF) and Sedimentation in trenches (LITTREN).
The LITLINE is developed based on the equation of the continuity for sediment volumes Q(x) Where: yc (x) is the coastline position; t is time; Q(x) is the volume of long-shore sediment transport; x is the position; h (x) -height of the active cross-shore profiles; ∆x is long shore discretization step; Q sou (x) is the volume of source/sink term.
The general long-shore transportation is mapped out in Fig. 2

Before 1995
Satellite data analyzing shows that, the beach eroded seriously in average of 11.5 m/year.In period of 1973-1989, erosion occurred with a catastrophic rate (25-30 m/year) at the Can Thanh (section A-B) and 15 m/year at section C-D.While, the erosion rate at Dong Hoa is around 15 m/year (E-F section).Fig. 4 presents the shoreline change analyzed results at Dong Hoa and Can Thanh cape in this period.Fig. 5 assesses the shoreline change at Can Gio beach from 1989 to 1995, results show that, the Ly Nhon (green highlighted) and the An Thanh (yellow highlighted) is significantly deposited while erosion occurred at the Dong Hoa (red highlighted).Since 1982, the Can Gio beach was protected by structure; however, the protective ability of these structures had been limited due to its rude technology.

Period of 1995-2004
By effect of the tropical storm Linda in 1997, the Can Gio sea dike had been damaged.To 1998, the sea dike had been upgraded to "hard structure" by quarry structures and the groin system completed in 2002.Effectiveness structures protected the beach from the erosion.To 2006, the Durian attacked to Can Gio area and damaged the sea dike.Fig. 6 presents the shoreline analysis in this period.

Period of 2004-2013
Downgrading of the sea dike combined with the tropical storms, beach erosion at Can Gio occurred more frequently.Fig. 7 compares the shoreline change in Dong Hoa and Can Thanh from 2004 to 2013.It is shown that, seasonal surface erosion is predominant in this period, i.e. the beach was eroded in the winter and then it was replenished in the summer.

Conceivable mechanism of the shoreline change
Erosion at Can Gio beach occurs by multi-impact of wave and high tide.Littoral material composition (fine sand and mud) also contributes to general beach erosion.Fig. 9 presents the simulated current at the Can Gio area.It is shown that, the long-shore currents are dominant to the cross-shore current.Fig. 10 presents the wave interaction at Can Gio beach.In the summer (Southwest monsoon season) South wave is dominant with maximum wave height is around 0.3 m. in the winter, the Northeast generates the higher wave height, maximum at 0.5 to 0.7 m.Wave height causes erosion whenever the tide is high.
The Can Gio significantly affected by the semidiurnal regime tidal from the estuary, on the rainy season (summer) the average discharge from Soai Rap and Long Tau rivers is around 600 to 1000 m 3 /s.This trends to generate the sand bars in front of the estuaries and the Can Gio foreshore.These sand bars take effect as the "natural breakwaters" in front of the shore and relocate the breakwater line (H=1.28h) to offshore.The beach therefore is accelerated in this period.Fig. 11a simulates the bed thickness change on the summer.On the dry season (winter), low discharge from rivers (less than 100 to 300 m 3 /s) leading to decrease the sediment source to the shoreline.By affected of Northeast monsoon season, wave height in this season is around 0.5 to 1.25m, the breakwater line is close to shoreline and potentially damage the shoreline whenever the tide is high.Fig. 11b simulates the bed thickness change on the winter.
Besides, human activities (aquaculture, coastal structure, tourist, hydroelectric power…) also contribute to the balance of the coastal morphology.Fig. 12 presents the conceivable mechanism of the beach erosion due to aquaculture.It is shown that, by cutting of the mangrove forest for aquaculture, the aggressively easily occurs.

Prediction of shoreline change under the sea level rise
Simulated shoreline change due to sea level rise is established with increment of the mean sea level in accordance with climate change for Vietnam, published on 2012 by Ministry Of Natural Resource and Environment (MONRE).Fig. 13 presents the simulated results for shoreline change.It is shown that, by impact of sea level rise, the Dong Hoa and Can Thanh cape eroded seriously.

CONCLUSION
Coastal process in Can Gio area varies seriously, rate of erosion various depends on state of the related activities.It is concluded that, erosion at the Can Gio beach is the surface erosion impacted by the wave interaction and long-shore current.Moreover, human activities (aquaculture, coastal structure, hydroelectric power...) are also main contributors to the balance of the erosion/deposition at the area.In the tendency of sea level rise due to climate change, potential erosion, inundation... in the Can Gio coastal zone is very likely.Therefore, consideration of the change on urbanization, tourist development in the lowland area is necessary.This may both mitigate the negative impacts from the nature phenomena and ensure a sustainable development for the area.