Long-term sediment distribution calculation taking into account sea level rise and the development of Day estuary

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1 VNU Journal of Science, Natural Sciences and Technology 28, No. 3S (212) Long-term sediment distribution calculation taking into account sea level rise and the development of Day estuary Nguyen Xuan Hien 1, Duong Ngoc Tien 1, Le Quoc Huy 1, Nguyen Tho Sao 2, * 1 Vietnam Institute of Meteorology, Hydrology and Environment 2 VNU University of Science, 334 Nguyen Trai, Hanoi, Vietnam Received 15 July 212 Abstract. The deposition in Day estuary tends to increase strongly due to abundant sources of sediment from the Red River system and the estuary is located in a concave coast where strong impacts of wave are eliminated. In this study, MIKE model developed by Danish Hydraulic Institute is employed to simulate, evaluate and forecast hydrodynamic regime as well as erosion, sedimentation and shoreline change processes in Day river mouth located in Ninh Binh. Keywords: Sediment transport, Red river, Day estuary, MIKE model. 1. Introduction Red River is the largest river of northern Vietnam. Sediment conveyed by this river is the vital source to enrich Red River Delta. Three main rivers that play key role in sediment transport in the system are Red, Ninh Co and Day. The process of dynamic interactions between rivers and sea causes the deposition and erosion in coastal areas. River mouth of Day situated in Ninh Binh province has significant changes in the alluvial deposition with a rapid rate. This, on the one hand can enlarge natural land, it also affects hydrodynamic regime and flood drainage capacity on the other. Moreover, the phenomenon of sea level rise has been causing great impacts on Vietnam in Corresponding author. Tel: saont@vnu.edu.vn 57 general and Ninh Binh province in particular. Sea level rise could lead to huge consequences for livelihoods and welfare of residents in these areas, valuable lands could also be lost. Shellfish pools may be removed and the coastal fisheries could disappear. The temporal mangrove areas surrounding these pools may be affected and no longer suitable for agricultural production. The diversity of the flora and fauna in coastal estuaries of coastal areas may be decreased. Mangrove forests the important ecosystem in estuaries and coastal areas may be reduced in size or completely disappeared, etc. 2. Model setup and input data This study applies Mike package developed by DHI with module Mike 11 for flow and concentration of sediment coming from river, module MIKE 21 SW for wave simulation,

58 N.X. Hien et al. / VNU Journal of Science, Natural Sciences and Technology 28, No. 3S (212) 57-62 MIKE 21 HD for hydraulic simulation and MIKE 21 ST for sediment transport.

Domain stretches from 19 o 46'N to 2 o 21'N in latitude and 15 o 56'E to 16 o 45'E in longitute.

2 58 N.X. Hien et al. / VNU Journal of Science, Natural Sciences and Technology 28, No. 3S (212) MIKE 21 HD for hydraulic simulation and MIKE 21 ST for sediment transport. Topographical data for model MIKE 21 is obtained from nautical maps with scales ranging from 1:1, to 1:1,, by Vietnam Navy. Domain stretches from 19 o 46'N to 2 o 21'N in latitude and 15 o 56'E to 16 o 45'E in longitute. Maps with large scale are used for coastal areas and islands, while small scale maps are used for offshore areas. In this study the finite element mesh from the sea to the shore is shown in Fig. 1b. Smallest area of an element is 125m 2 in region of river estuaries such as Day, Ninh Co and BaLat, the largest area is 25km 2 in offshore region. The domain includes 2879 nodes with the coarse resolution of 5m for offshore and the fine resolution of 5m for nearshore areas. Figure 1. a) Bathymetry of the study area, b) Mesh used. Flows from river calculated by MIKE 11 model are used as river mouth boundaries. This study applies 3 models namely MIKE 21 SW, FM MIKE 21 HD, MIKE 21 ST [1] for simulations. At first, MIKE 21 SW model is used for wave spectrum calculation, then the outputs from wave model are used as input data for the remaining two models. Inputs for MIKE 21 SW model include: wave height, wave period and wave direction for the offshore boundaries. Sea levels obtained from harmonic analysis are used as boundary conditions for MIKE 21 HD model. Boundary condition for the model MIKE 21 ST is the sediment concentration balance. Meteorological field is taken from meteorological records at VanLy station in the "Handbook of continental hydro-meteorological features of Vietnam" [2]. Grain size (d 5 =.17mm) and sediment concentration are derived from the data measured at Day estuary in 2 surveys (July and November, 29), and from previous studies on geology and geomorphology of this area in [3, 4]. 3. Model calibration 3.1. One dimensional model MIKE 11 MIKE 11 model which has been using for operational forecast of Red - Thai Binh river system is used to simulate the flow and sediment concentration at the hydrological stations near river mouths such as Ba Lat, Nhu Tan and Phu Le. Because observed sediment data in Red river and Ninh Co river are not available, only sediment concentration data

3 N.X. Hien et al. / VNU Journal of Science, Natural Sciences and Technology 28, No. 3S (212) measured at hydrological station NhuTan (on Day river) from 1/11/29 to 3/11/29 are used for model calibration. Results show good agreement between simulated and observed sediment concentration in terms of phase and magnitude. m Calculation Obtained from harmonic analysis 9/1/21 9/11/21 9/21/21 1/1/21 Time g/l.2.16 Caculation Observation Figure 4. Calculated water levels and water levels obtained from harmonic analysis at Day estuary (16 35 E, 2 13 N) /11/29 : 6/11/29 : 11/11/29 : :: :: :: Time Figure 2. Sediment concentration at NhuTan station on Day river Two dimensional model MIKE 21 FM The model is calibrated with measured water levels at Day river mouth. These data were obtained during the 7-days survey under the project "Research on prediction methods for riverine tidal boundary" conducted by Center for Marine and Ocean-Atmosphere Interaction Research (from 19-VIII to 26-VIII-21) m Caculation Observation 8/19/1 8/21/1 8/23/1 8/25/1 Time Figure 3. Observed and calculated water levels at Day estuary ( E, N). The model is validated against water levels obtained from harmonic analysis (from 1-IX to 1-X-21) at three nearshore locations. Fig. 4 show the results at testing locations KN (near Day estuary). It can be seen that there is a good agreement between calculated by model with water levels obtained by harmonic tidal prediction: coincidence of phase and amplitude (correlation coefficient is.88). The results can be now used as input to MIKE 21 ST model for simulating sediment transport. 4. Sea level rise scenarios for Day estuary area Greenhouse gases emission scenarios are selected for calculation and development of sea level rise scenarios for Vietnam including low emission scenario (B1, medium (B2) and the highest (A1F1). In this study, sea level rise for Day estuary are taken from the scenarios published in 212 by the Ministry of Natural Resources and Environment. Details of scenarios for Day estuary are shown in in Table 1 [5]. Table 1. Sea level rise scenarios at Day estuary (cm) Time High emision A1F1 Medium emision B2 Low emision B1 Upper limit Lower limit Upper limit Lower limit Upper limit Lower limit

To the scenario B2, the sea level could rise by 2-24cm and by 49-65cm, respectively.

4 6 N.X. Hien et al. / VNU Journal of Science, Natural Sciences and Technology 28, No. 3S (212) According to the low emission scenario (B1), sea level in the Day estuary could rise by cm by middle 21st century, and by 42-58cm at the end of the century. To the scenario B2, the sea level could rise by 2-24cm and by 49-65cm, respectively. Under high emissions scenario A1FI, sea level could rise by cm by mid-21st century and by 66-86cm by the end of the century. 5. Results Considering sediment transports in Day estuary as long-term processes, the duration of simulation is chosen as 2 years (1/1/21-31/12/23) Update the sea level rise in the model As mentioned above, sea level has an increasing trend during the period from 21 to 23. Therefore, in this study, results of sea level rise projection was added to water levels obtained from harmonic analysis to use as input of MIKE 21 HD model. As in Tab. 1, sea level in the study area could rise by 7-9 cm by 22 and by cm by 23 under different scenarios (compared to 5.4. Calculation of sediment distribution baseline period ). So, the average increase during is about.5 cm/year. Therefore, the calculation added the value of.5 cm/year to the water level time series each year River discharge This study assumes that the flow pattern in 3 rivers: Red, Ninh Co and Day from 21 to 23 is consistent with the pattern in the period Thus, river discharges observed at hydrological stations: Ba Lat, Phu Le, NhuTan in period were used as input to Mike Hydrodynamic calculation (m) Elevation 1/1/21 1/1/215 1/1/22 1/1/225 1/1/23 Figure 5. Water levels at location near Day estuary (16 5 E; 19 5 N). Figure 5 shows the increasing trend of water level (red line). Magnitude of the increase is consistent with the increase projected in the sea level rise scenarios in section 4. Time Figure 6. Bathymetry:a) bed level, b) bed change after 5 years.

This cause the filling in Day estuary. As a consequence, flow is changed.

It can be seen that the areas, where greatest change occurs, cover channel of

5 N.X. Hien et al. / VNU Journal of Science, Natural Sciences and Technology 28, No. 3S (212) Fig. 6-9 show that the deposition trend is dominant in the first 15 years. This cause the filling in Day estuary. As a consequence, flow is changed. Flow coming out from the Day river no longer exists after 15 years due to the filling of the river mouth. Later 15 years, changes only happens in the sea bed outside the river mouth. It can be seen that the areas, where greatest change occurs, cover channel of Day river mouth and the area outside Ninh Co rive mouth. The magnitude of change can be over 1m. Figure 7. Bathymetry:a) bed level, b) bed change after 1 years. Figure 8. Bathymetry:a) bed level, b) bed change after 15 years. Figure 9. Bathymetry:a) bed level, b) bed change after 2 years.

6 62 N.X. Hien et al. / VNU Journal of Science, Natural Sciences and Technology 28, No. 3S (212) Conclusions Day estuary always receives large amounts of sediment from Day river and deposition tends to happens at this area. As a result, Day river mouth will be filled in near future. To avoid bad consequences caused by the filling Day river mouth, dredging and widening of the river are of great importance. Ninh Binh province implements the project "Dredging the Day river segment from Gian Khau bridge to the river mouth to improve flood drainage for Hoang Long river, Ninh Binh province" to avoid problem of filling in Day river mouth. Calculations show an agreement with recent other studies. Results also provide evidence for the importance of annual dredging of Day river mouth for the navigation activities. The results explain why coastal lands in Nghia Hung district (Nam Dinh) and Kim Son (Ninh Binh) are extended every year. The issue of sediment transport and changes in morphology of rivers, estuaries and coastal areas is still a difficult problem both in theory and practice. This study applied the MIKE pakages to analyze trend of accretion and erosion as well as marine-continental interactions in the Day estuary. However, this study only considered and analysed trends and changes of coastlines in some specific cases. In addition, the process of shifts and changes of coastlines was not considered in this study. Therefore, to assess the phenomenon fully and comprehensively, other studies are needed. References [1] MIKE 21 ST Non-Cohesive Sediment Transport Module, User Guide, DHI Softwave 25. [2] ế ),, 2. [3] Đỗ Minh Đức, Nghiên cứu sự hình thành và biến đổi quá trình bồi tụ-xói lở ở đới ven biển Thái Bình-Nam Định. Luận án Tiến sĩ chuyên ngành Địa chất công trình, 24. [4] Nguyễn Biểu, Vũ Trường Sơn, Dương Văn Hải và nnk, Địa chất khoáng sản biển nông ven bờ (-3 m nước) Việt Nam tỷ lệ 1/5.. Lưu trữ ĐC, Hà Nội, 21. [5] Bộ Tài nguyên và Môi trường, Kịch bản biến đổi khí hậu và nước biển dâng cho Việt Nam, 211. Tính toán phân bố trầm tích dài hạn có xét đến ảnh hưởng của biến đổi khí hậu và quá trình phát triển cửa Đáy Nguyễn Xuân Hiển 1, Dương Ngọc Tiến 1, Lê Quốc Huy 1, Nguyễn Thọ Sáo 2 1 Viện Khoa học Khí tượng Thủy văn và Môi trường 2 Trường Đại học Khoa học Tự nhiên, ĐHQGHN, 334 Nguyễn Trãi, Hà Nội, Việt Nam Vùng cửa sông Đáy có xu hướng bồi tụ mạnh do nguồn trầm tích dồi dào từ hệ thống sông Hồng và do cửa sông nằm ở vùng bờ biển lõm, tránh được các hướng sóng chính có tác động mạnh. Trong nghiên cứu này, mô hình MIKE của Viện Thủy lực Đan Mạch được sử dụng để tính toán, đánh giá chế độ thủy động lực cũng như xói lở, bồi tụ và quá trình biến đổi đường bờ tại khu vực cửa sông Đáy thuộc tỉnh Ninh Bình. Từ khóa: Vận chuyển trầm tích, sông Hồng, cửa Đáy, mô hình MIKE.

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