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

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 chang

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

Nguyen Xuan Hien1, Duong Ngoc Tien1, Le Quoc Huy1, Nguyen Tho Sao2,*

1

Vietnam Institute of Meteorology, Hydrology and Environment

2

VNU University of Science, 334 Nguyen Trai, Hanoi, Vietnam

Received 15 July 2012

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: 84-4-38584945

E-mail: saont@vnu.edu.vn

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,

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 1:10,000 to 1:1,000,000 by

Vietnam Navy Domain stretches from 19o46'N

to 20o21'N in latitude and 105o56'E to 106o45'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 1250m2 in region of river estuaries such as Day, Ninh Co and BaLat, the largest area is 25km2 in offshore region The domain includes 2879 nodes with the coarse resolution

of 5000m for offshore and the fine resolution of 50m 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 (d50 = 0.17mm) and sediment concentration are derived from the data measured at Day estuary

in 2 surveys (July and November, 2009), 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

measured at hydrological station NhuTan (on

Day river) from 1/11/2009 to 30/11/2009 are

used for model calibration Results show good

agreement between simulated and observed

sediment concentration in terms of phase and

magnitude

g/l

0

0.04

0.08

0.12

0.16

0.2

1/11/2009 0:00 6/11/2009 0:00 11/11/2009 0:00 16-11-2009 00:00:00 21-11-2009 00:00:00 26-11-2009 00:00:00 Time

Caculation Observation

Figure 2 Sediment concentration at NhuTan station

on Day river

3.2 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-2010)

-1.5

-1

-0.5

0

0.5

1

1.5

m

Caculation Observation

Figure 3 Observed and calculated water levels at

m

-1.5 -1 -0.5 0 0.5 1 1.5

9/1/2010 9/11/2010 9/21/2010 10/1/2010

Time

Calculation Obtained from harmonic analysis

Figure 4 Calculated water levels and water levels obtained from harmonic analysis at Day estuary

The model is validated against water levels obtained from harmonic analysis (from 1-IX to 1-X-2010) 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 0.88) 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 (B10, medium (B2) and the highest (A1F1) In this study, sea level rise for Day estuary are taken from the scenarios published in 2012 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

According to the low emission scenario

(B1), sea level in the Day estuary could rise by

19-23 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 20-24cm and by

49-65cm, respectively Under high emissions

scenario A1FI, sea level could rise by 22-27 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 20 years (01/01/2010 -

31/12/2030)

5.1 Update the sea level rise in the model

As mentioned above, sea level has an

increasing trend during the period from 2010 to

2030 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 2020 and by 11-14 cm

by 2030 under different scenarios (compared to

baseline period 1980-1999) So, the average increase during 2010-2030 is about 0.5 cm/year Therefore, the calculation added the value of 0.5 cm/year to the water level time series each year

5.2 River discharge

This study assumes that the flow pattern in

3 rivers: Red, Ninh Co and Day from 2010 to

2030 is consistent with the pattern in the period 1990-2010 Thus, river discharges observed at hydrological stations: Ba Lat, Phu Le, NhuTan

in period 1990-2010 were used as input to Mike 11

5.3 Hydrodynamic calculation

El evati on

-2 -1 0 1 2

1/1/2010 1/1/2015 1/1/2020 1/1/2025 1/1/2030

Ti me (m)

Figure 5 Water levels at location near Day estuary

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

5.4 Calculation of sediment distribution

Figure 6 Bathymetry:a) bed level, b) bed change after 5 years

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 10m

Figure 7 Bathymetry:a) bed level, b) bed change after 10 years

Figure 8 Bathymetry:a) bed level, b) bed change after 15 years

Figure 9 Bathymetry:a) bed level, b) bed change after 20 years

6 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

Transport Module, User Guide, DHI Softwave

2005

,

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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, 2004

và nnk, Địa chất khoáng sản biển nông ven bờ

(0-30 m nước) Việt Nam tỷ lệ 1/500.000 Lưu

trữ ĐC, Hà Nội, 2001

đổi khí hậu và nước biển dâng cho Việt Nam,

2011

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

, Lê Quốc Huy1, Nguyễn Thọ Sáo2 1

Viện Khoa học Khí tượng Thủy văn và Môi trường

2Trường 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