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Geographical Impact of Dyke Measurement for Land Use on Flood Water in the Mekong Delta
Abstract
The Mekong River Delta has an area of 3.9 million hectares and is home of 18 million inhabitants. It is one of the largest river deltas in the world which originates at the Tibetan plateau and crosses
China, Myanmar, Laos, Thailand, and Cambodia before entering the territory of Vietnam. Due to the strategy for intensive rice-based production issued by the Government of Vietnam, a multitude
of compartments enclosed by semi-dykes (to protect rice fields from flood water until middle August) and full-dykes (to protect the rice fields fully from flood water) have been built rapidly within the flooded areas in recent years. Consequently, they cause impacts on the flood situation in the Mekong delta. To explore comprehensively these influences, 1D hydrological model was used to analyse the impacts of several full-dyke compartments on the flood water along the main rivers based upon a Geographical Impact Factor. The authors found that different geographical compartments caused different rates of influences on the flood water levels along the main rivers.
... Several studies have mapped the high dykes in the VMD using remotely sensed images (e.g. Duong et al., 2016;Fujihara et al., 2016;Kuenzer et al., 2013). Using this method, the high dykes are identified via flooded and non-flooded areas. ...
... High dykes were rarely constructed in the VMD before 2000 (Duong et al., 2016). However, as previously stated, the year 2000 historical flood caused enormous damage to infrastructure and residents' properties. ...
... -Scenario 1 (Base) is the base scenario for the 2000 flood, without high dykes. The floodplains in the VMD were not protected by high dykes before 2000 (Duong et al., 2016); therefore, no high dykes are considered in this scenario. ...
... Several studies have mapped the high dykes in the VMD using remotely sensed images (e.g. Duong et al., 2016;Fujihara et al., 2016;Kuenzer et al., 2013). Using this method, the high dykes are identified via flooded and non-flooded areas. ...
... High dykes were rarely constructed in the VMD before 2000 (Duong et al., 2016). However, as previously stated, the year 2000 historical flood caused enormous damage to infrastructure and residents' properties. ...
... -Scenario 1 (Base) is the base scenario for the 2000 flood, without high dykes. The floodplains in the VMD were not protected by high dykes before 2000 (Duong et al., 2016); therefore, no high dykes are considered in this scenario. ...
... Several studies have mapped the high dykes in the VMD using remotely sensed images (e.g. Duong et al., 2016;Fujihara et al., 2016;Kuenzer et al., 2013). Using this method, the high dykes are identified via flooded and non-flooded areas. ...
... High dykes were rarely constructed in the VMD before 2000 (Duong et al., 2016). However, as previously stated, the year 2000 historical flood caused enormous damage to infrastructure and residents' properties. ...
... -Scenario 1 (Base) is the base scenario for the 2000 flood, without high dykes. The floodplains in the VMD were not protected by high dykes before 2000 (Duong et al., 2016); therefore, no high dykes are considered in this scenario. ...
... Several studies have mapped the high dykes in the VMD using remotely sensed images (e.g. Duong et al., 2016;Fujihara et al., 2016;Kuenzer et al., 2013). Using this method, the high dykes are identified via flooded and non-flooded areas. ...
... High dykes were rarely constructed in the VMD before 2000 (Duong et al., 2016). However, as previously stated, the year 2000 historical flood caused enormous damage to infrastructure and residents' properties. ...
... -Scenario 1 (Base) is the base scenario for the 2000 flood, without high dykes. The floodplains in the VMD were not protected by high dykes before 2000 (Duong et al., 2016); therefore, no high dykes are considered in this scenario. ...
Deltas are low-lying plains which are formed when river sediments deposit in coastal environments. Deltas are nutrient-rich, and productive ecological and agricultural areas with high socio-economic importance. Globally, deltas are home to about 500 million people and are considerably modified by human activities. In addition, they are vulnerable to climate change and natural hazards like changing river flow and sediment supply, coastal flooding by storminess or sea level rise. To encourage better delta management and planning, it is of utmost importance to understand existing delta sediment dynamics. The objective of this study is to investigate the prevailing sediment dynamics and the sediment budget in the Mekong Delta by using a process-based model. Understanding sediment dynamics for the Mekong Delta requires high resolution analysis and detailed data, which is a challenge for managers and scientists. This study introduces such an approach and focuses on modeling the entire system with a process-based approach, Delft3D-4 and Delft3D Flexible Mesh (DFM). The first model is used to explore sediment dynamics at the coastal zone. The latter model allows straightforward coupling of 1D and 2D grids, making it suitable for analysing the complex river and canal network of the Mekong Delta. This study starts by generating trustworthy bathymetries based on limited data availability. It describes a new interpolation method for reproducing the main meandering channel topographies of the Mekong River. The reproduced topographies are validated against high resolution measured data. The proposed method is capable of reproducing the thalweg accurately. Next, this study describes the development of a Delft3D Mekong Delta model. The model is validated for hydrodynamics and sediment dynamics data for several years and focuses on describing near shore sediment dynamics. The model shows that sediment transport changes in the Mekong Delta are strongly modulated by seasonally varying river discharges and monsoons. The nearshore suspended sediment concentration (SSC) is significantly decreased due to a lack of wave-induced stirring when there is no monsoon. 3D Gravitational circulation effects limit the SSC field from expanding seaward in case of high river flow. In addition, the bed composition has an important role in reproducing sediment fluxes which were considerably decreased when a sandy bed layer is included. This happens due to effects of the initially mostly sandy mixing layer, where resuspension of the mud is proportional to the fraction of mud present. It takes time for an equilibrium bed composition to develop. Seasonally, the sediment volumes deposited in the river mouths increase regularly during the high flow season. During October they remain more or less constant and then, as wave action increases and discharges decrease, the deposited material is resuspended and transported southward along the coast. The DFM model explores the hydrodynamics and sediment dynamics in the fluvial reach of the Mekong River including the anthropogenic effect of dyke construction. After an extremely high flood in 2000 which caused huge damages, a dyke system has been built to protect agriculture in the Vietnamese Mekong Delta (VMD). These structures change hydrodynamic characteristics on floodplains by avoiding floodwaters coming into the floodplains. The DFM model shows that the high dykes slightly change hydrodynamics in the VMD downstream. These structures increase daily mean water levels and tidal amplitudes along the mainstreams. Interestingly, the floodplains protected by high dykes in Long Xuyen Quadrangle and Plain of Reeds influence water regimes not only on the directly linked Mekong branch, but also on other branches. Based on the validated hydrodynamic model, the model is validated against sediment data and used to derive a sediment budget for the Mekong Delta. For the first time, this study has computed sediment dynamics over the entire Mekong Delta, considering riverbed sediment exchange. The model suggests that the Mekong Delta receives ~99 Mt/year sediment from the Mekong River This is much lower than the common estimate of 160 Mt/year. Only about 23% of the modelled total sediment load at Kratie is exported to the sea. The remaining portion is trapped in the rivers and floodplains of the Mekong Delta. Located between Kratie and the entrance of the Mekong Delta, the Tonle Sap Lake receives Mekong River flow at increasing flow rates seasonally and returns flow when Mekong River flow rates decay. As a result Tonle Sap Lake traps approximately 3.9 Mt/year of sediments and explains the hysteresis relationship between water discharges and SSC at downstream stations. The VMD receives an amount of 79.1 Mt/year (~80 % of the total sediment supply at Kratie) through the Song Tien, the Song Hau and overflows. The model results suggest that the Mekong mainstream riverbed erodes in Cambodia and accretes in Vietnam. The results of this study advance understanding of sediment dynamics and sediment budget in the Mekong Delta. The model developed is an efficient tool in order to support delta management and planning. The validated model can be used in future studies to explore impact of climate change and human interference in the Mekong Delta.
... Several studies have mapped the high dykes in the VMD by using remotely sensed images (e.g. Duong et al., 2016;Fujihara et al., 2016;Kuenzer et al., 2013). By this method, the high dykes are identified via flooded and non-flooded areas. ...
... High dykes were hardly constructed in the VMD before 2000 (Duong et al., 2016). The year 2000 historical flood, particularly, caused enormous damage to infrastructure and residents' properties. ...
Building high dykes is a common measure to cope with floods and plays an important role in agricultural management in the Vietnamese Mekong Delta. However, the construction of high dykes cause considerable changes in hydrodynamics of the Mekong River. Therefore, this paper aims to assess the impacts of the high dyke system on water level fluctuation and tidal propagation on the Mekong River branches using a modelling approach. In order to consider interaction between rivers and seas, an unstructured modelling grid was generated, with 1D–2D coupling, covering the Mekong Delta and extending to the East (South China Sea) and West (Gulf of Thailand) seas. The model was manually calibrated for the flood season of the year 2000. To assess the role of floodplains, scenarios consisting of high dykes built in different regions of the Long Xuyen Quadrangle (LXQ), Plains of Reeds (PoR) and TransBassac were carried out. Results show that the percentage of river outflow at Dinh An sharply increases in the dry season in comparison to the flood season while the other Mekong estuarine outflows rise slightly. In contrast, the lateral river flows of the Mekong River system to the seas by the Soai Rap mouth and the LXQ decrease somewhat in the dry season compared to the flood season due to overflow reduction at the Cambodia–Vietnam border. Additionally, the high dykes in the regions that are directly connected to a branch of the Mekong River, not only have an influence on the hydrodynamics in their own branch, but also on other branches because of the connecting channel of Vam Nao. Moreover, the high dykes built in the PoR, LXQ and TransBassac regions are the most important factor for changing water levels at Tan Chau, Chau Doc and Can Tho, respectively. The LXQ high dykes result in an increase of daily mean water levels and a decrease of tidal amplitudes on the Song Tien (downstream of the connecting channel of Vam Nao). A similar interaction is also found for the the PoR high dykes and the Song Hau.
... The Mekong Delta is a crucial region for food security not only for Vietnam but also the world Trinh et al., 2014). However, the delta is facing an increasing vulnerability due to flooding (Schenk et al., 2022;Vu et al., 2021;Nestmann et al., 2016), land subsidence (Minderhoud et al., 2019), hydropower dam construction (Hecht et al., 2019), sediment reduction (Le Xuan et al., 2019), river bank erosion, salinity intrusion, and coastal erosion (Le Xuan et al., 2022b). Currently, the coastal shoreline along the Mekong Delta is eroding with a rate of 20-50 m per year, which is among the most serious challenges to sustainable development of the region (Le Xuan et al., 2022b). ...
Coastal protection, shoreline stabilization and mangrove forest restoration are pressing issues in such coastal regions worldwide as the Mekong Delta. A hollow triangle breakwater TC1 was developed to protect the coastline against erosion. The TC1 breakwater consists of holes arranged on the waveward and leeward sides, allowing for wave energy dissipation, water exchange, and sediment deposition to facilitate mangrove restoration. To evaluate comprehensively the working principles of the TC1, we investigate herein the wave-structure interaction under regular waves by means of an advanced computational fluid dynamics (CFD) platform, i.e. FLOW-3D. The numerical model was calibrated against experimental results with great agreement across three different water depths where the breakwater was tested. We also analyzed the velocity dynamics in the waveward and leeward sides of the breakwater and it revealed a significant difference of approximately 50% following the change in water depths from 0.66 m to 0.96 m. Furthermore, we examined the wave forces on both solid and hollow forms of TC1 structures, the wave forces on a hollow form were found to decrease by 20-30% in comparison with the forces on a solid form. In addition, the effect of liquid characteristics, i.e., density, viscosity and temperature showed slightly impact on wave transmission coefficients through the breakwater by 0.4-0.7 %.
... • Flooding data can be used/reused for flood risk management in the VMD, i.e., construction dike planning, calibration of flood extension for hydraulic models [6][7] , planning of residential areas, and rice development. Besides, a comparison of flooding management between the countries in the lower Mekong River Basin (i.e., Cambodia and Vietnam) could be evaluated based on the MODIS flooding maps, which cover the territories of Cambodia and Vietnamese Mekong Delta. ...
This paper compiles the data associated with a research article published in STOTEN [1]. We contribute a continuous dataset of land use/land cover (21 maps) and flood dynamics (567 maps) from 2000 to 2020 with detailed spatial data. The dataset represents figures, tables, and images illustrating the temporal and spatial distribution of land use and flood dynamics from 2000 to 2020 in the Vietnamese Mekong Delta (VMD). The MODIS products show a high validation with statistical data and radar satellites [1]. The datasets of flood map and land use, therefore, are available to scientists, engineers, and policy-makers in agricultural management associated with flood management in the VMD. They could be used for policy settings, household livelihood assessment as well as other economic analyses for the VMD region due to the change of land use and flooding dynamics.
... due to climate change and sea level rise (Jiang et al., 2019;Lauri et al., 2012;Shrestha et al., 2018), land subsidence (Minderhoud et al., 2018), hydropower development in upstream countries (Kuenzer et al., 2013a), drought and salinity intrusion Loc et al., 2021a), riverbed mining (Park et al., 2020), tidal effect (Duong et al., 2019), and coastal erosion (Chu et al., 2015). Besides, the VMD is annually influenced by flooding (Triet et al., 2020;Duong et al., 2016;Thanh et al., 2020). ...
This paper presents the first attempt to capture a comprehensive spatial view of land use change in the Vietnamese Mekong Delta (VMD) for a long period, i.e., from 2000 to 2020. It is aimed at monitoring holistically the land use change and flooding situation in the region, addressing the reasons for land use change, and assessing the impacts of land use change on hydraulic aspects and farmer livelihoods during the last 21 years. MODIS products, in particular, are used to study the dynamics of land use and floods after demonstrating high validation with statistical data and radar satellites, with R2 = 0.96 and R2 ≥ 0.97 for land use and flood maps, respectively. The results show that rice cultivation is the most dominant land use type, accounting for 40% to 46% of the delta area, while aquaculture accounts for 10% to 22%, respectively. The total rice cultivation area increased from 3764 thousand hectares (thous. ha) in 2001 to 4343 thous. ha in 2015 based on the intensive development of triple rice cropping in the upper zone, then decreased to 3963 thous. ha in 2020. In contrast, aquaculture areas are farmed mainly in the coastal area and remained relatively steady, increasing slightly from 619 thous. ha in 2001 to 856 thous. ha in 2020. The massive construction of dikes for triple rice cropping in the upper zone appears to cause a significant impact on the annual flooding regime. Land use policies have influenced the changes in land use patterns, flooding situations, and the livelihoods of local farmers.
... After this hugely damaging flood, rice crops were protected by a dyke system that consists of both low dykes and high dykes [ Thanh et al., 2020 ]. The VMD provinces rarely constructed high dykes before 2000 [ Duong et al., 2016 ] At first, the dyke system was positive for rice cropping. However it is now recognized that dykes also have significant negative effects (see Chapter 9). ...
This chapter provides an overview of the geological and hydrological characteristics of
Viet Nam’s Mekong Delta, as well as of the main anthropogenic drivers of change. We also
present the temperature and precipitation changes over the past four decades, and assess
future climate change according to different global climate scenarios, applying statistical
and dynamic downscaling methods.
Increasing temperatures are recorded at all stations in the Delta, with an average warming
trend of ~0.2°C/decade, while precipitation changes are more contrasted. By mid-century,
temperature is projected to increase by 1.3°C to 1.8°C and precipitation by 15% to 20%,
under climate scenarios RCP4.5 and RCP8.5 respectively. By the end of the century, the
temperature increase could reach 1.7°C to 3.7°C, and the precipitation increase 15% to
25%, depending on the global climate scenario.
Climate change is not the only threat to the Delta’s future: human activities in the delta
or upstream have strong impacts on hydrology and sedimentology and may exacerbate
climate change impacts, or in some cases pose an even greater threat in the short- to midterm. Sediment trapping by upstream dams and excessive fluvial sand mining are the main
drivers of enhanced saline water intrusions, while ground-water over-extractions also drive
high subsidence rates, and hence rapid relative sea-level rise.
Adaptation measures implemented up to now may be effective in terms of aquaculture and
agricultural production, but are not sustainable from a social, economic, or environmental
point of view. Therefore, a holistic approach is required to deal with future climate change
and anthropogenic pressures, and to develop sustainable agriculture and aquaculture in
the Delta.
... Development activities upstream also had to be considered, as these recently changed the hydrograph of the Mekong River area, significantly altering the frequency and intensity of annual floods. In exploring potential development strategies, the planning process divided the Mekong Delta into three zones according to Figure 6 Example situations in the Mekong Delta with high dikes and low dikes (Duong et al. 2016) hydrological regime and geographical characteristics5 (Royal HaskoningDHV et al., 2013). The location, climate, soil, and ecology of each zone was understood to be sufficiently distinct as to require its own development strategy. ...
... In contrast, the downstream boundaries are the time series of water levels in the East Sea and the West Sea estuaries. Boundary conditions and descriptions are discussed in previous studies, i.e. [9,10,20,34,35]. The hydrological data used as inputs for the boundary condition and model calibration process were collected from several sources. ...
This study aims at evaluating the geographical influences of rice-based protection dykes on floodwater regimes along the main rivers, namely the Mekong and the Bassac, in the Vietnamese Mekong Delta (VMD). Specifically, numerous low dykes and high dykes have been constructed particularly in the upper delta’s floodplains to protect the double and triple rice cropping against the annual flooding. For the whole deltaic domain, a 1D-quasi-2D hydrodynamic model setup was used to simulate seventy-two (72) scenarios of dyke construction development in the context of low, medium, and high floods that occurred in the VMD to examine the effects of different flood magnitudes on a certain dyke construction area. Based on the model simulation results, we established an evaluation indicator, the so-called Geographical Impact Factor (GIF), to evaluate the impacts of zone-based dyke compartments on the floodwater regimes along the main rivers for different kinds of floods. Our findings revealed different rates of influences on the floodwater levels along the Mekong and Bassac Rivers under different scenarios of zone-based high-dyke developments. GIF is a useful index for scientists and decision-makers in land use planning, especially in rice intensification, in conjunction with flood management for the VMD and for similar deltas worldwide.
... The upper part of the MD is divided into 22 compartments based on the natural topography, existing canals and rivers and the range of flood depth. Duong et al. (2016) analysed comprehensively the relation of full-dyke compartment and water level along the Mekong river based on Geographical Impact Factor (GIF). Flood maps from MODIS satellite and online hydrological data from the MRC website are analysed and collected to combine with GIF and the results from 1D model to estimate the flood level along the Mekong river. ...
With an area of about 5% of the Mekong River basin and low-lying topography (mostly below +2 m a.s.l.), the Mekong delta (MD) is annually flooded due to upstream inflows from the Mekong River. Therefore, flood level along the Mekong River is an important factor that causes flooding in the downstream region. However, there are only two hydrological gauges of Tan Chau and Vam Nao on Mekong River that have online data at the website of the Mekong River Commission (MRC). A new approach of Flood Level Estimation Method (FLEM) along the Mekong and Bassac Rivers is presented with fast and precise results and simple calculation.
Mục tiêu nghiên cứu nhằm theo dõi và đánh giá tác động của đê bao ngăn lũ lên hiện trạng canh tác lúa vùng Đồng Tháp Mười (ĐTM) năm 2000 và 2019. Phương pháp nghiên cứu sử dụng ảnh LANDSAT áp dụng thuật toán ảnh chỉ số khác biệt nước (NDWI) và trích rút đường bờ và bộ dữ liệu ảnh MODIS áp dụng chuỗi ảnh chỉ số thực vật (NDVI) và phân loại phi giám sát (ISODATA) trong 2 năm 2000 và 2019. Độ tin cậy kết quả phân loại ảnh khá cao với độ chính xác toàn cục >85% và hệ số Kappa >0,7 cho 2 năm 2000 – 2019. Kết quả nghiên cứu cho thấy diện tích canh tác lúa trong vùng đê bao ngăn lũ đã tăng thêm khoảng 126.139,40 ha (19,36%). Vùng được bao đê ở ĐTM không còn canh tác lúa 1 vụ và hầu hết đã chuyển đổi sang canh tác 2 hoặc 3 vụ lúa và gia tăng lần lượt năm 2019 là 81.229,47 ha (39,18%) và 126.142,15 ha (60,82%) so với năm 2000. Vùng chuyển đổi nhiều nhất là huyện Tháp Mười và Cao Lãnh tỉnh Đồng Tháp, huyện Mộc Hóa tỉnh Long An, huyện Cái Bè và Cai Lậy tỉnh Tiền Giang.
Building high dykes is a common measure of coping with floods and plays an important role in agricultural management in the Vietnamese Mekong Delta. However, the construction of high dykes causes considerable changes in hydrodynamics of the Mekong River. This paper aims to assess the impact of the high-dyke system on water level fluctuations and tidal propagation in the Mekong River branches. We developed a coupled 1-D to 2-D unstructured grid using Delft3D Flexible Mesh software. The model domain covered the Mekong Delta extending to the East (South China Sea) and West (Gulf of Thailand) seas, while the scenarios included the presence of high dykes in the Long Xuyen Quadrangle (LXQ), the Plain of Reeds (PoR) and the Trans-Bassac regions. The model was calibrated for the year 2000 high-flow season. Results show that the inclusion of high dykes changes the percentages of seaward outflow through the different Mekong branches and slightly redistributes flow over the low-flow and high-flow seasons. The LXQ and PoR high dykes result in an increase in the daily mean water levels and a decrease in the tidal amplitudes in their adjacent river branches. Moreover, the different high-dyke systems not only have an influence on the hydrodynamics in their own branch, but also influence other branches due to the Vam Nao connecting channel. These conclusions also hold for the extreme flood scenarios of 1981 and 1991 that had larger peak flows but smaller flood volumes. Peak flood water levels in the Mekong Delta in 1981 and 1991 are comparable to the 2000 flood as peak floods decrease and elongate due to upstream flooding in Cambodia. Future studies will focus on sediment pathways and distribution as well as climate change impact assessment.
Calibration of hydrodynamic models is – compared to other disciplines like e.g. hydrology – still underdeveloped. This has mainly two reasons: the lack of appropriate data and the large computational demand in terms of CPU-time. Both aspects are aggravated in large-scale applications. However, there are recent developments that improve the situation on both the data and computing side. Remote sensing, especially radar-based techniques proved to provide highly valuable information on flood extents, and in case high precision DEMs are present, also on spatially distributed inundation depths. On the computing side the use of parallelization techniques brought significant performance gains. In the presented study we build on these developments by calibrating a large-scale 1-D hydrodynamic model of the whole Mekong Delta downstream of Kratie in Cambodia: we combined in-situ data from a network of river gauging stations, i.e. data with high temporal but low spatial resolution, with a series of inundation maps derived from ENVISAT Advanced Synthetic Aperture Radar (ASAR) satellite images, i.e. data with low temporal but high spatial resolution, in an multi-objective automatic calibration process. It is shown that an automatic, multi-objective calibration of hydrodynamic models, even of such complexity and on a large scale and complex as a model for the Mekong Delta, is possible. Furthermore, the calibration process revealed model deficiencies in the model structure, i.e. the representation of the dike system in Vietnam, which would have been difficult to detect by a standard manual calibration procedure.
A one-dimensional (1D) hydrodynamic model for the river network of the Long Xuyen Quadrangle, Vietnamese Mekong Delta, was developed in HEC-RAS based on: (i) Available data of river network and cross-sections deployed in the ISIS-1D hydrodynamics model for the whole Mekong Delta (including the Vietnamese and Cambodia parts); and, (ii) Field-based data to update the existing river network and full-dyke systems. Developed scenarios included: (i) Scenario 1: The measured geometric data in 2000 (no dykes constructed), and upstream discharge and sea level measured in 2000; (ii) Scenario 2: The developed full-dyke systems, and upstream discharge and sea level measured in 2000; and, (iii) Scenario 3: The geometry and upstream discharge remained similar to Scenario 2 while the sea level was supposed to be 30 cm greater than that in 2000 (in both the East and West Sea). By comparing scenarios 1 and 2, possible impacts of the full-dyke systems to the area could be examined while by comparing scenarios 2 and 3, impacts of sea level rise would be evaluated in the context of the deployed full-dyke systems.
Keywords: One dimensional (1D) hydrodynamics model, flow dynamics, full-dyke systems, HEC-RAS, and Long Xuyen Quadrangle.
Satellite remote sensing is a valuable tool for monitoring flooding. Microwave sensors are especially appropriate instruments, as they allow the differentiation of inundated from non-inundated areas, regardless of levels of solar illumination or frequency of cloud cover in regions experiencing substantial rainy seasons. In the current study we present the longest synthetic aperture radar-based time series of flood and inundation information derived for the Mekong Delta that has been analyzed for this region so far. We employed overall 60 Envisat ASAR Wide Swath Mode data sets at a spatial resolution of 150 meters acquired during the years 2007–2011 to facilitate a thorough understanding of the flood regime in the Mekong Delta. The Mekong Delta in southern Vietnam comprises 13 provinces and is home to 18 million inhabitants. Extreme dry seasons from late December to May and wet seasons from June to December characterize people’s rural life. In this study, we show which areas of the delta are frequently affected by floods and which regions remain dry all year round. Furthermore, we present which areas are flooded at which frequency and elucidate the patterns of flood progression over the course of the rainy season. In this context, we also examine the impact of dykes on floodwater emergence and assess the relationship between retrieved flood occurrence patterns and land use. In addition, the advantages and shortcomings of ENVISAT ASAR-WSM based flood mapping are discussed. The results contribute to a comprehensive understanding of Mekong Delta flood dynamics in an environment where the flow regime is influenced by the Mekong River, overland water-flow, anthropogenic floodwater control, as well as the tides.
This paper briefly introduces an integrated remote-sensing methodology based on MODIS time-series imagery for detecting spatiotemporal changes in Vietnamese Mekong Delta (VMD) farming systems. The integrated methodology consists of six parts, and uses a wavelet-based filter to smooth MODIS-derived time-series indexes (EVI, LSWI, and DVEL) from 2000 to 2008. Through a proposed decision tree using the smoothed indexes, we classified several farming systems, viz., triple rice cropping, two types of double rice cropping, shrimp-rice farming (rotational cropping), and inland aquaculture (monoculture). The MODIS-derived estimate of the total rice-planted area, shrimp- rice farming area, and inland aquaculture area agreed well with statistical data at the province level (R2 ≥ 0.96). However, in some provinces, the estimate has a large margin of error, probably because of the mixed-pixel effect due to the moderate spatial resolution of MODIS (250 m). According to the estimated spatial pattern of the farming systems in the whole VMD, inland aquaculture and shrimp- rice farming areas are distributed mostly in the coastal provinces. The areas of the farming systems steadily expanded until 2007, and double rice cropping systems in both upper and coastal regions were replaced by triple rice cropping because of infrastructure improvements. The proportion of the triple rice cropping area peaked in 2005 and then declined steadily over the next 2 years. We discuss the advantage of the proposed methodology for detecting the spatiotemporal changes of land use patterns, especially of farming systems in a regional area.
The Mekong River Delta (MRD) is one of two primary rice‐growing areas in Vietnam. Flooding in the Mekong River is a recurrent event and is not only one of the most destructive natural disasters but also a natural resource in this area. The cultivated surface soil layer in the Mekong Delta has a thickness of only about 50 cm, and is heavily polluted by acidic water infiltrating from deeper soil layers during the dry season. The annual floods carry fertile silt to farmland and fresh water to wash away the acidic water and provide the water needed to grow vast rice fields. The flood water carries with it various fish species that facilitate aquaculture development in the area. The floods also wash away polluted water and provide the whole delta with clean water.
Owing to these different factors, the flooding in this area has a positive impact on agriculture and a negative impact on regional planning. Recent infrastructural changes designed to mitigate flood damage and protect crops and residents' lives make the inundation regime more complicated. To understand the role of infrastructure in the flood regime in this area as well as the mechanism of the flood regime, it is necessary to apply an integrated method of study including numerical modelling, a geographic information system (GIS), and statistical analyses. This study includes a brief presentation of the measured data analysis of flood variation trends over the 43‐year period from 1961 to 2004 and an analysis of the hydrological effects of infrastructure changes associated with human activities in the period from 1996 to 2001 based on the integrated hydraulic model known as HydroGis. Copyright © 2008 John Wiley & Sons, Ltd.
Calibration of hydrodynamic models is – compared to other disciplines like e.g. hydrology – still underdeveloped. This has mainly two reasons: the lack of appropriate data and the large computational demand in terms of CPU-time. Both aspects are aggravated in large-scale applications. However, there are recent developments that improve the situation on both the data and computing side. Remote sensing, especially radar-based techniques proved to provide highly valuable information on flood extents, and in case high precision DEMs are present, also on spatially distributed inundation depths. On the computing side the use of parallelization techniques brought significant performance gains. In the presented study we build on these developments by calibrating a large-scale 1-D hydrodynamic model of the whole Mekong Delta downstream of Kratie in Cambodia: we combined in-situ data from a network of river gauging stations, i.e. data with high temporal but low spatial resolution, with a series of inundation maps derived from ENVISAT Advanced Synthetic Aperture Radar (ASAR) satellite images, i.e. data with low temporal but high spatial resolution, in an multi-objective automatic calibration process. It is shown that an automatic, multi-objective calibration of hydrodynamic models, even of such complexity and on a large scale and complex as a model for the Mekong Delta, is possible. Furthermore, the calibration process revealed model deficiencies in the model structure, i.e. the representation of the dike system in Vietnam, which would have been difficult to detect by a standard manual calibration procedure.
The Mekong Delta, Vietnam is one of the largest river deltas in the world with 39.712 km 2 and home to over 18 million inhabitants. It is located in the lower Mekong River, which originates at the Tibetan plateau and crosses China, Myanmar, Laos, Thailand, and Cambodia before entering the territory of Vietnam. The delta was formed by the deposition of sediments from the Mekong River over thousands of years. It is considered as a rice bowl of the whole country with a contribution approximately 52% of the national rice production, and 70% of national rice exportation, putting Vietnam at the second place in the world rice market. With an area of only about 5% of the total catchment area of the Mekong River and low natural ground (mostly below +2m a.s.l.), the Mekong delta is annually impacted by flooding from the upper part of the delta. Extreme land-use occupation for rice intensification in the upper provinces of Mekong Delta in recent years with double and/or triple rice cropping, has been significantly influencing the seasonal flood regime of the Delta. Due to progressive poldering of the flood plains (to protect the rice crops from flood waters), the flood plains are increasingly restricted. In this line, the present study is aimed at analyzing the impacts of land-use change to flooding situation in the Mekong delta based on hydraulic modeling and satellite products.
This paper presents the methodology used to detect temporal changes in the extent of annual flooding within the Cambodia and the Vietnamese Mekong Delta (VMD) based on MODIS time-series imagery (Wavelet-based Filter for detecting spatio-temporal changes in Flood Inundation; WFFI). This methodology involves the use of a wavelet-based filter to interpolate missing information and reduce the noise component in the time-series data, as proposed in a previous study. The smoothed time profiles of Enhanced Vegetation Index (EVI), Land Surface Water Index (LSWI), and the Difference Value between EVI and LSWI (DVEL) are obtained from MOD09 8-day composite time-series data (resolution: 500 m; time period: 2000–2005). The proposed algorithm was applied to produce time-series inundation maps (WFFI products) for the five annual flood seasons over the period from 2000 to 2004. The WFFI products were validated via comparisons with Landsat-derived results and inundation maps based on RADARSAT images, hydrological data, and digital elevation model data. Compared with the RADARSAT-derived inundation maps at the province level, the obtained RMSE range from 364 to 443 km2 and the determination coefficients [R2] range from 0.89 to 0.92. Compared with Landsat-derived results at the 10-km grid level, the obtained RMSE range from 6.8 to 15.2 km2 and the determination coefficients [R2] range from 0.77 to 0.97. The inundated area of flooded forests/marsh to the northeast of Tonle Sap Lake were underestimated, probably because of extensive vegetation cover in this area. The spatial characteristics of the estimated start dates, end dates, and duration of inundation cycles were also determined for the period from 2000 to 2004. There are clear contrasts in the distribution of the estimated end dates and duration of inundation cycles between large-scale floods (2000–2002) and medium- and small-scale floods (2003 and 2004). At the regional scale, the estimated start dates for the southern part of An Giang Province during 2003 and 2004 was distinctly later than that for surrounding areas. The results indicate that these triple-cropping areas enclosed by dikes increased in extent from 2003 to 2004. In contrast, the estimated end dates of inundation at the Co Do and Song Hau State Farms were clearly earlier than those for surrounding areas, although the estimated start dates were similar. Temporal changes in the inundation area of Flood pixels in the Dong Thap and Long An Provinces are in excellent agreement with daily water-level data recorded at Tan Chau Station. The estimated area of Long-term water body increased in size from 2000 to 2004, especially in coastal areas of the Ca Mau and Bac Lieu Provinces. Statistical data for Vietnam indicate that this trend may reflect the expansion of shrimp-farming areas. The WFFI products enable an understanding of seasonal and annual changes in the water distribution and environment of the Cambodia and the VMD from a global viewpoint.
The objective of this study is to measure the technical, allocative, and cost efficiency for farmers who grow crops following either rice-monocultural patterns or crop-rotation patterns in the non-flooded and flooded areas of the Mekong River Delta in Vietnam. The non-flooded areas are located within irrigated boundary systems, and the flooded areas are located outside these systems. In addition, the determinants of household income and productive efficiencies are identified in the study.
Related to productive efficiency, the measured results show that the crop-rotation farmers are more efficient in terms of technical and cost efficiency than the continuous-rice farmers and vice versa for allocative efficiency, for the case of non-flooded areas. Similarly, the mean efficiency score is greater with respect to technical, allocative, and cost efficiency for farmers employing the crop-rotation pattern in comparison with farmers following the continuous-rice pattern.
Regarding the factors influencing efficiencies, in the case of non-flooded areas, the estimated results show that although there are some differences in determinants of each component of total productivity, sex, age, education, share of female labor, and farming pattern are found to be the main factors driving changes in most of the components such as technical and allocative efficiency. In the case of flooded areas, all components of total productivity in terms of the technical, allocative, and cost efficiency are impacted by the variation in sex, age, and education.
The Mekong River delta plays an important role in the Vietnamese economy and it has been severely impacted during this century by a series of unusually large floods. In the dry season the delta is also impacted by salinity intrusion and tides. These effects have caused severe human hardship. To mitigate these impacts, a large number of engineering structures, primarily dykes and weirs, have been built in the delta in recent years and are still being built, mainly to control floods and saltwater intrusion. These control measures are still being upgraded. A GIS-linked numerical model shows that the flood levels in the delta depend on the combined impacts of high river flows in the Mekong River, storm surges, sea level rise, and the likely, future siltation of the Mekong Estuary resulting from the construction of dams in China as well as many other dams proposed throughout the remaining river catchment. The model suggests that the engineering structures in the delta increase the flow velocities in the rivers and canals, increasing bank erosion, and cause the water to be deeper in the rivers and canals. This increases flooding in the non-protected areas of the delta and increases the risk of catastrophic failure of the dykes in the protected areas. The model also predicts that a sea level rise induced by global warming will enhance flooding in the Mekong River delta in Vietnam, and that flooding may worsen in the long term as a result of estuarine siltation resulting from the construction of dams. At the scale of the Mekong River basin, a multinational water resources management plan is needed that includes the hydrological needs of the delta. At the scale of the delta, a compromise is needed between allowing some flooding necessary for agriculture and preventing catastrophic flooding to alleviate human suffering.
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