Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015September 7th -8th, Hoi An, Vietnam

FORECASTING THE EROSIONAL TREND OF THE NORTHERN SHORELINE OF LY SON ISLAND (QUANG NGAI, VIETNAM) DUE

TO CLIMATE CHANGE AND SEA-LEVEL RISE

TUNG, TRAN-THANH 1), TUYEN KIEU-XUAN 2), and DUNG LE-DUC 3)

1) Faculty of Marine and Coastal Engineering, Thuy Loi University,175 Tay Son, Dong Da, Hanoi, e-mail: tunghwru@gmail.com

2) Central Vietnam Institute of Water Resources, 132 Dong Da, Hai Chau, Danang, Vietnam, e-mail: kxtuyen@gmail.com

3) Vietnam Institute of Sea and Islands, 125 Trung Kinh, Cau Giay, Hanoi, Vietnam, e-mail: dung.ld.wru@gmail.com

Abstract

Many islands of various sizes exists along the central coastline of Vietnam; the islands play important roles in politics, national defense and sovereignty. In recent years, with higher population stress and impact of climate change and sea level rise (SLR), these islands withstand intensive disasters and frequent erosion with coastline retreat. This paper presents research output on erosional trends of the northern coastline of Ly Son island in Quang Ngai province, taking into account of sea-level rise. The research output, in programme KC.09/11-15, serves as a basis for planning, protection and sustainable development of Vietnamese islands in future

Keywords: Ly Son island, coastal erosion, SLR, MIKE 21, LITPROF

1. INTRODUCTION

With a coastline almost 2000 km long, accounting for over 2/3 the total length of Vietnam coast, the Central coastal zone with islands remarkably contributes to the socio-economic growth and security, national defense of Vietnam, and the Central region particularly. The location of these islands and distribution of local inhabitants plays an important role ine politics and country sovereignty. On several islands, the number of inhabitants reaches that equivalent to a district: Phu Quy island (Binh Thuan), Ly Son island (Quang Ngai), or equivalent to a commune: Cu Lao Cham island (Quang Nam), Con Co island (Quang Tri), etc. Therefore it is necessary to protect these island from adverse natural events, maintain safety and sustainable development of the local inhabitants, which is a key factor for developing a firm strategy of national defense.

Under the increasingly apparent impact of climate change and SLR, the islands in the Central region have been withstanding remarkable stresses from nature, aside from population and development stresses. For local sustainable development, active measures must be taken to prevent and mitigate natural impacts; such actions include building coastal protection works, building harbours for effective marine transportation, fishery and sheltering vessels from frequent storms.

This paper presents a research on effects of marine hydrodynamic factors e.g. wave, wind, littoral currents on Ly Son island (Quang Ngai, Vietnam) with prediction of erosional trend for this island, considering adverse effects of climate change and sea-level rise. The results from this research contribute to the scientific foundation for maintenance, protection, explotitation of natural environment as well as socio-economic development and national defense in Ly Son in the future.

2. MATERIALS AND METHODS

2.1 Study area and data used

Ly Son is an island located eastward of Quang Ngai province (15°22ʹ51ʺN, 109°07ʹE), 18 km from the mainland. A district-level administrative division, Ly Son comprises 3 communes: An Binh, An Hai and An Vinh with a total area of 9.97 sq. km, accounting for 0.19% the area of Quang Ngai.

The wave data used in this study is a long-term wave series measured at Capmia station (15°N, 109.5°E) by FUGRO company from 1-Jan-1996 to 31-Jan-2005 in the project of training My A inlet, Quang Ngai [4]. This wave series was analysed to find waves with equivalent energy [5] for various directions; the results are present in Table 1.

Table 1: Equivalent wave energy for various directions, Capmia station (1996-2005).Direction (°) Hs (m) Ts (s) tk (days) Direction (°) Hs (m) Ts (s) tk (days)

0-30 2.29 6.0 6 120-150 1.07 4.0 3230-60 2.26 5.8 118 150-180 1.08 4.0 4960-90 1.36 4.5 108 180-210 1.13 4.1 1690-120 1.05 4.0 30

The wave data in Table 1 shows that the coastal sea of Ly Son is influenced by seven major wave directions, of which prevailent waves are characterizes for Northeastern and Southwestern monsoon. The NE and ENE waves which are classified as 30°→60° and 60°→90° occur with the frequency of 118 and 108 days/year respectively. SE waves (150°→180°) occurs on 49 days/year. Those waves take main responsible for littoral currents and sediment transport in a year.

The wind time series measured at Ly Son station (1985-2012) was used for statistical analysis and calculation of equivalent wind energy similar to wave data. The results for equivalent wind energy can be referred to [1].

2.2 Sea-level rise scenario used for simulationFor simulating shoreline erosion, the water level data was obtained from various scenarios of sea-level rise due to climate change; these scenarios were made publicly available in 2014 by Ministry of Natural Resources and Environment [6]. This study uses the sea-level rise data for Hai Van Pass – Dai Lanh Cape (which is the nearest location from Ly Son), with high emission scenario (listed in Table 2), to calculate erosion for the island. The choice of a high emission scenario is to consider adverse effects of sea-level rise on Ly Son island.

Table 2: Sea-level rise of the area by Hai Van Pass – Dai Lanh Cape, high emission scenario [6]Year 2020 2030 2050 2100

Scenario KB1 KB2 KB3 KB4 KB5

Sea-level rise (cm) 9 14 29 74 97

3. MODEL SETUP

To study the effect of marine hydrodynamic factors on Ly Son island, the research group conducted modeling major hydrodynamic processes for Ly Son island with the MIKE 21 software suite (Mike, 2007a) is used.

3.1 Computation domain and mesh and bathymetry

The computational domain (Figure 1) is the coastal area surrounding Ly Son, 30 km × 30 km in extent. The computational grid is constructed using a bathymetry chart scaled 1:2000 which was surveyed in Dec-2012 by Centre of Environmental Fluid Mechanics (University of Natural Sciences, Hanoi) in the framework of governmentally-funded Research Project [1] KC.09/11-15 (Figure 2).The geographic extents of the model in UTM projection are: 722500 m – 736000 m Easting and 1870600 m – 1894600 m Northing. The grid is unstructured with 4851 triangular cells.

Figure 1: Computational domain and grid Figure 2: Bathymetry surrounding Ly Son island

3.3 Model calibration and verification

The calibration and verification stage is necessary for producing a set of parameters suitable for area of interest. This will ensure better matching between simulation results and reality.

Calibration of water level: The water level data used for calibrating the hydrodynamic model was measured at the pier on Ly Son island (15°22ʹ46.74ʺN, 109°5ʹ45.48ʺE) from 0:00 1-Jan-2012 to 23:00 31-Dec-2012 with a sampling frequency of 1 hour.

Figure 3: Result of calibration for water level of Ly Son

Boundary conditions, initial condition and model parameters are reported in detail in Ref [1]. The result of water level calibration is shown in Figure 3. The error between simulated and measured water level is evaluated through Nash index (0.91).

Calibration of flow: The data of water current used for model calibration was measured offshore (15°23.3ʹN, 109°10.4ʹE), 5 km to the east of the island. The time of measurement was from 10:20 17-Dec-2012 to 9:50 29-Dec-2012, with a sampling frequency of 10 minutes. The boundary condition used were water level, waves and wind.

In terms of velocity magnitude, the calculated and measured flows differ at certain phases, especially during ebb tide. This difference is due to the 2-D model used; the calculated velocity is vertically-averaged over the whole domain, while the measured flow was obtained at a fixed depth. However, such difference is acceptable.

Figure 4: Result of calibration for flow of Ly Son

The details of boundary conditions, initial conditions and model parameters are reported in Ref [1]. The result of verification for flow is shown in Figure 4. The calculated flow is similar in phase with the measured flow.

4. CALCULATING THE TREND OF FORESHORE PROFILE DUE TO SEA-LEVEL RISE

Figure 5: Representative profiles for calculation

For an overview picture of shoreline evolution and changes in foreshore profile of the northern coastline of Ly Son island, the LITPROF model was used in tandem with MIKE SW model to calculate the evolution of representative cross-shore profiles. To facilitate modelling shoreline of the study area, the northern shoreline of Ly Son island (which experience continuous erosion) is divided into 4 segments (Figure 5) based on geomorphic features, briefly described as following:

- Segment D1-D2 composed of a coral reef and a sandy beach.- Segment D2-D3 mainly composed of sand. At D3 there is a mountain cape. - Segment D3-D4 has fairly complex composition, alternating between rocky and sandy coast

(mostly rock). The beach in this section is narrow and experiences little change; the seabed is mostly composed of coral.

- Segment D4-D5 is composed mostly from coarse sand. In the vicinity of D5, the coast is rocky seaward and sandy landward. Therefore, except for Segment D3-D4 which is almost unchanged due to rocky shoreline, the

other three segments are sandy coasts and frequently experience seasonal changes. Three profiles are selected for these three segment to analyse the temporal changes of the coast. The location of these profiles are shown in Figure 5. The representative profiles, being extended seaward to the limit of closure depth (depth contour -10 m), are 660 m long. Each profile is divided into 110 computational element, each having a size of 6 m. The simulation time for a profile is one year. The wave data used for this calculation is the equivalent wave energy listed in Table 1.

Simulation results of the five sea-level rise scenarios are compared with present trend of shoreline evolution at the three repsentative cross-shore profiles of Ly Son (Table 3). The results mainly focused on evaluation of the maximum shoreline retreat, the erosion depth of the foreshore at both breaker

zone and swash zone. The simulation results show a general picture on shoreline retreats in the five scenarios.

Figure 6: Results of bathymetric changes of 3 profiles in 5 scenariosTable 3: Summarize of simulation results on for 5 scenarios at 3 cross-shore profiles

  Present SLR 9cm

SLR 14cm

SLR 29cm

SLR 74cm

SLR 97cm

Shoreline retreat due to SLR (m)

MC1   12 18 30 60 100

MC2   12 18 36 60 100

MC3   6 12 18 33 36

Maximum erosion depth at breaker zone (m)

MC1 -0.88 -1.12 -1.28 -1.25 -1.4 -1.19

MC2 -1.43 -1.98 -2.61 -1.98 -2.27 -1.4

MC3 -2.44 -2.2 -2.22 -2.2 -2.32 -1.22

Maximum erosion depth at swash zone (m)

MC1 -0.39 -0.47 -0.53 -0.42 -0.49 -0.38

MC2 -0.44 -0.54 -0.5 -0.44 -0.52 -0.38

MC3 -1.28 -1.37 -1.18 -1.27 -1.19 -1.12

Also from the simulation results, the shoreline retreat at profiles 1 and 2 are not much different, whereas Profile 3 shows much less retreat. On the contrary, the erosion depth and foreshore lowering

at Profile 3 are much greater than those of Profiles 1 and 2. This difference is due to the fact that Profile 3 is composed of coarse sand with steeper slope and has different behaviour under the impact of sea level rise.

The shift of shoreline at different profiles tends to form submerged sand bars of new equilibrium profiles adapting to the present local wave climate. Calculation results show maximum seabed change at breaker zone (bed elevation from -6 m to -8 m). For Profiles 1 and 2, due to mild bed slope, relatively small waves reach the shore, the erosion mainly occurs at the breaker zone, hence this zone experience the largest change. For Profile 3, the steep slope allows wave to penetrate close to shore, therefore not only at the breaker zone, waves also affect the swash zone wth greater impact compared to the case of a more gently beach slope as in Profiles 1 and 2. A more gentle slope and a protected foreshore are very important in wave reduction and mitigation of erosion at construction toes, especiall in the context of climate change and sea-level rise.

4. CONCLUSIONS

The results of study show that the coastline erosion and retreat trends of Ly Son island is intensified due to climate change and sea-level rise. For the erosion trend, higher sea level causes the more impact of hydrodynamic factors to the beach which leads to more severe erosion and lowering of foreshore. For coastline retreat, both area and distance of retreat depend on sea-level rise scenarios and local bathymetry. At gently sloped and low sandy beaches, the effect of erosion and the amount of retreat are larger. Evaluating and predicting the trends of erosion and coastline retreat of Ly Son is very important to the future planning, protecting and sustainable development of the island when impacts of climate change and sea-level rise are taken into account.

However, due to limited time and data, in this study only calculation, analysis and evaluation on erosional trend and shoreline retreat of Ly Son island had been performed; while there are 3000 island in Vietnam in need for protection and sustainable development, especially nowadays with growing impact of climate change and sea-level rise. Therefore further research is needed for the whole system of islands in the country.

5. ACKNOWLEDGEMENTS

A part of this study was financially supported by a National research project “Evaluation of changes in extreme values of oceanographic factors; their effects on environment, socio-economic development, with recommendation of preventive solutions for populous islands in Central Vietnam (mainly Ly Son and Phu Quy islands). The authors thank the project managers for kind support

6. REFERENCES

Tung T.T., Dung L.Đ., (2015) Technical Report Computation of shoreline changes and beach evolution for Ly Son island, Quang Ngai. Water Resources University. 2015.

DHI. (2007). Mike 21 Flow Model FM, Hydrodynamic Module. User Manual. DHIDHI. (2007). Mike LITPACK- LITPROF. User Manual. DHIFUGRO Oceanor. (2006). Calibrated wave parameters off Cap Mia in Vietnam. Trondheim, Norway.Tung T.T., Dũng L.Đ., (2012). Computation of nearshore wave energy for the Central coast of

Vietnam. Journal of Water Resources and Environmental Engineering, Vol 39 , pp46-53.MONRE. (2012). Climate change, sea level rise scenarios for Vietnam. Ministry of Natural Resources

and Environment, Hanoi.