Evaluating on flooding area and tidal beach change in Quang Ninh coastal zone due to sea level rise

Use your smartphone to scan this QR code and download this article ABSTRACT Viet Nam is considered one of countries most affected by climate change and sea-level rise. It results in many negative effects, such as flooding, saline intrusion and beach erosion occurred in the coastal zones. Quang Ninh with more than 250 km coastline, located in the northeastern part of Vietnam, is one of the vulnerable coastal provinces under the heavily affected due to the sea level rise. In order to evaluate the changes of flooded areas and tidal beaches due to the sea level rise in Quang Ninh coastal zone a 2D numerical model is set up, using the 3-grids nesting technique. The numerical model is calibrated by using the harmonic constants of 8 tidal constituents at Hon Dau tide station and validated with the observed data. On the basis of the climate change scenarios (RCP4.5, RCP8.5) in the period from 2020 to 2100, the corresponding sea level values are used in the numerical modeling to calculate the changes of flooded areas and tidal beaches due to the sea level rise. The obtained results on changing of the flooded area and tidal beach in Quang Ninh coastal zone are not only statically by changing water sea levels but also due to changing of the tidal range in this area. The calculated results point out that districts under the most affected of the sea level rise are Quang Yen, Tien Yen, Hai Ha, Mong Cai.


INTRODUCTION
Viet Nam is considered as one of countries most af-

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Quang Ninh is a province along the northeastern 24 coast of Vietnam. There are more than 2,000 islands, 25 extending along the coast for more than 250 km. The 26 sea bottom topography of Quang Ninh isn't smooth, 27 and has an average depth of 20 m. The tide in Quang 28 Ninh sea area is diurnal with the maximum range 29 above 4m, the largest in the North of Vietnam. There-30 fore, Quang Ninh is one of the vulnerable coastal 31 provinces under the affects of inundation, saline in-32 trusion and beach erosion due to the climate change 33 and the sea level rise.

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In this study, on the basis of the climate change sce-35 narios issued by The Ministry of Natural Resources 36 and Environment in 2016, the changes of flooded ar-37 eas and tidal beaches due to the sea level rise after the 38 scenarios (averaged) RCP4.5 and (highest) RCP8.5 for 39 the period from 2020 to 2100 are calculated and eval-40 uated. The changes of flooded area and tidal beach 41 in Quang Ninh coastal zone are considered not only 42 statically by changing of water sea levels but also due 43 to changing of the tidal range in this area by using a 44 numerical hydrodynamic model for the East Vietnam 45 Sea and Quang Ninh coastal zone. (1) where, λ and φ are coordinates in the eastward and where u(z) and v(z) are velocity components in λ and 88 φ directions respectively at z level.

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The finite difference method is applied to solve 97 the above mentioned equations by using the non-98 collocated grid, the leap-frog difference scheme, nest-99 ing technique and wet-dry treatment 5 .

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In order to simulate the tide oscillation in the Quang 101 Ninh coastal area, a numerical model with two nesting 102 grids, as shown in Figure 2, is setup. The coarse grid 103 covers the whole of The Gulf of Tonkin, using the grid 104 step of 1/20 degrees (about 500 m) in longitude and 105 latitude respectively. The fine grid with the grid step 106 of 1/200 degrees concentrated on the nearby shoreline 107 of Quang Ninh Province.

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The numerical model has calibrated by comparisons 109 between the calculated water levels and the observed 110 data in the some specified time periods depending on 111 the data available. The calibration process is to mod-112 ify the value of the bottom friction coefficient and 113 tidal harmonic constants at open boundary reason-114 ably in order to obtain the calculated results in a good 115 agree with the observed ones. Because in the numer-116 ical model, the water levels are contributed by 8 tidal 117 constituents, therefore the calibration process is spit-118 ted into each individual constituent and its amplitude 119 and phase will be taken into the comparison Table 1. 120 presents the comparison between the calculated and 121 analyzed harmonics constants (amplitude and phase) 122 of 8 tidal constituents in the Hondau tide gauge sta-123 tion.

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Next, the numerical model has validated by calcu-125 lating the tide by using the chosen parameters and 126 comparing with the observed water levels at Hon Dau 127 from 01/9/2019 to 30/9/2019 as shown in Figure 3. 128 The compared results show that the numerical model 129 has been verified well enough in order to simulate the 130 tide oscillation in the Quang Ninh near coast area.