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Vulnerability to Flood in the Vietnamese Mekong Delta: Mapping and Uncertainty Assessment

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The Vietnamese Mekong Delta is located at the end of the Mekong River, one of the ten largest rivers in the world. It plays an important role, especially in terms of food security for not only Vietnam but also the world. However, the Vietnamese Mekong Delta is projected to be heavily affected by: (i) the annual (fluvial) flood, which would be changed in terms of time and spatial distribution after impacts of climate change scenarios (i.e. sharper hydrograph with shorter flood period); and, (ii) sea level rise. Such combination would result in significant changes of surface water resources, leading to consequent impacts on the existing farming systems in the Vietnamese Mekong Delta. Therefore, this paper presents a new approach of integrating a one-dimensional hydrodynamic model (ISIS-1D) with GIS analyses to: (i) identify priority areas for flood adaptation and mitigation; and, (ii) provide an insight to local decision-makers in the Vietnamese Mekong Delta in changes of future floods.
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Journal of Environmental Science and Engineering B 2 (2013) 229-237
Formerly part of Journal of Environmental Science and Engineering, ISSN 1934-8932
Vulnerability to Flood in the Vietnamese Mekong Delta:
Mapping and Uncertainty Assessment
Van Pham Dang Tri, Nguyen Hieu Trung and Vo Quoc Thanh
College of Environment and Natural Resources, Can Tho University, Can Tho City, Vietnam
Received: December 25, 2012 / Accepted: January15, 2013 / Published: April 20, 2013.
Abstract: The Vietnamese Mekong Delta is located at the end of the Mekong River, one of the 10 largest rivers in the world. It plays
an important role, especially in terms of food security for not only Vietnam but also the world. However, the Vietnamese Mekong
Delta is projected to be heavily affected by: (1) the annual (fluvial) flood, which would be changed in terms of time and spatial
distribution after impacts of climate change scenarios (i.e., sharper hydrograph with shorter flood period); and (2) sea level rise. Such
combination would result in significant changes of surface water resources, leading to consequent impacts on the existing farming
systems in the Vietnamese Mekong Delta. Therefore, this paper presents a new approach of integrating a one-dimensional
hydrodynamic model (ISIS-1D) with GIS (Geographic Information System ) analyses to: (1) identify priority areas for flood adaptation
and mitigation; (2) provide an insight to local decision-makers in the Vietnamese Mekong Delta in changes of future floods.
Keywords: Vulnerability, uncertainty, flood, Vietnamese Mekong delta.
1. Introduction
The Vietnamese Mekong Delta is located at the end
of the Mekong River (Fig. 1), one of the 10 largest
rivers in the world and running through six countries
(China, Myanmar, Thailand, Laos, Cambodia and
Vietnam) [1]. The Vietnamese Mekong Delta provides
annually 90% of national rice export and contributes
significant aquaculture production for the national and
global food market [2]. According to Thien [3], the
main causes of floods in the delta include: (1) flood
discharge from the upstream; (2) local heavy rainfall
(driven by monsoon or typhoons); (3) high tides in the
East Sea and West Sea. The annual fluvial floods (the
phenomenon considered in this paper) are a common
natural event and classified into two types, including:
(1) floods caused by the upstream discharge with long
flood inundation from 2 to 6 months (ranging from
July to December [4]); (2) tidal-induced flood driven
Corresponding author: Van Pham Dang Tri, Ph.D.,
research fields: hydrodynamic modelling, waterscape
management and (surface) water resource changes. E-mail:
vpdtri@ctu.edu.vn.
by tidal regimes in the East Sea and West Sea [3].
During the flood season, water enters the Vietnamese
Mekong Delta via the main river reaches (the
Mekong—running through Tan Chau and My Thuan
to the East Sea and Bassac—running through Chau
Doc and Can Tho to the East Sea) (Fig. 1) and
overflows across the common border between
Vietnam and Cambodia [5, 6].
The annual fluvial flood brings significant benefits
to the delta. During the flood season, the annual flood
conveys about 160 Mt of sediment per year [7] and a
large amount fish (in average, about 475.73 t) with
about 1,200 fish species in total to the delta [8]. In
specific, the 2000-flood (the 20-yr returned-period
event [9]) brought about 1.86 million tons of fish
(approximately 2,600 million USD) [10]. Besides,
annual flood plays an important role for wetland
protection and biodiversity conservation [4]. However,
the annual flood also causes negative impacts on the
livelihood of local residents (e.g., losses of life and
properties). For example, the 2000-flood caused
approximately
250 million USD of total damages
DAVID PUBLISHING
D
Vulnerability to Flood in the Vietnamese Mekong Delta: Mapping and Uncertainty Assessment
230
Fig. 1 The Vietnamese Mekong Delta [6] and its provinces.
[10]. With great impacts of the global climate change,
the local hydrological conditions in the Mekong Basin
in general and the delta in specific are projected to be
significantly changed [6], leading to a requirement of
vulnerability assessment to define priority areas for
flood mitigation and adaptation.
This paper is to characterize the hazard,
vulnerability and risk caused by the extreme historical
flood in the Vietnamese Mekong Delta (in 2000) and
the projected ones in 2050 [6] with changes of
upstream discharge and sea level rise.
2. Materials and Methods
2.1. Hydrodynamic Model Setup
In this study, the available ISIS-1D model
(developed by the Mekong River Commission) for the
entire Mekong Delta (including the Cambodia and
Vietnamese parts) is used with application of the
hydrodynamic component but not the hydrological
one. In fact, this model is used by the MRC (Mekong
River Commission ) to determine the annual fluvial
flood in the Mekong Delta. The hydrographs at the
upstream boundary (in Kratie, Cambodia) are
determined based on interpolated historical discharge
and predicted ones as per the climate change scenarios
CC1 and CC2 (Fig. 2). According to the CC1 climate
change projection, the Mekong River discharge in
Kratie is greater than the one of the CC2 projection; in
fact, with future developments (hydropower and
irrigation) in the upstream Mekong basin, less water
would arrive in the delta [6]. In addition, the sea level
in the future is the projected rise (+30 cm for both the
East Sea and West Sea with reference to that in 2000
[11]). Table 1 summarizes scenarios applied for the
hydrodynamic model in this paper.
Even though the ISIS-1D model is considered to be
able to provide a reasonable representation of the
hydrodynamics of the Cambodian floodplain and
Vietnamese Mekong Delta, MRC suggests that the
model should not be used for design purposes [12], but
rather to estimate the trend of changes when the
boundary conditions are modified. The reason for such
advise is that, for the large (deltaic) scale model, the
lack of details for a specific area will lead to under-
and/or over-estimation of the future events, and
consequently wrong calculations of the planned
construction.
Vulnerability to Flood in the Vietnamese Mekong Delta: Mapping and Uncertainty Assessment
231
Fig. 2 Measured hourly discharge in 2000 at Tan Chau and Chau Doc: (A) Annual hydrograph measured in 2000, historical
mean daily discharge (1985-2000) and projected annual hydrograph in 2050 (Scenarios 1 and 2); (B) Measured and projected
daily discharge in Kratie, Cambodia [6].
Table 1 Setup of the model boundary conditions.
Models setup Upstream boundary conditions Downstream boundary conditions
Base-line scenario Discharge hydrograph of the year 2000
Sea water level in year 2000 (SL 2000) for both the West Sea and East
Sea
Scenario 1a Projected discharge of the year 2050 (CC1) Sea water level in year 2050 (SL 2000 + sea level rise)
Scenario 1b Projected discharge of the year 2050 (CC2) Sea water level in year 2050 (SL 2000 + sea level rise)
2.2 Hazard Mapping
Flood hazard is categorized based on the level of
difficulties in daily life and/or damage of
properties [13]. The flood hazard maps are created
according to the flood depths (in comparison to the
local land surface elevation) (Table 2). According to
the previous study (e.g., Refs. [6, 14]), the fluvial
flood in the Vietnamese Mekong Delta should be
characterized with great attention paid to the upstream
discharge driven flood and tidal-induced flood;
therefore, two specific days are selected to create
inundated maps, including: (1) the highest flood stage
in the upstream section according to the
greatest measured stages at Tan Chau and Chau Doc;
(2) the largest flood extent at the deltaic scale
based on the greatest measured stage corresponding to
the greatest stages measured at Can Tho and My
Thuan.
Table 2 Classification of flood hazards.
Water depth (m) Hazard ranking Hazard category
0-0.2 0-0.04 Very low
0.2-0.5 0.04-0.1 Low
0.5-1.0 0.1-0.2 Medium
1.0-2.0 0.2-0.4 High
> 2.0 0.4-1 Very high
Vulnerability to Flood in the Vietnamese Mekong Delta: Mapping and Uncertainty Assessment
232
2.3 Vulnerabilities and Risks Assessment
The Coastal City Flood Vulnerability Index (based
on exposure, susceptibility and resilience to coastal
flooding indicators) [15, 16] are modified and
applied to meet the actual conditions of the
Vietnamese Mekong Delta and each province in the
delta is identified as a calculated unit. The
considered components to evaluate the
vulnerabilities and risks of each calculated unit in the
delta include the hydro-geological and climatic
components (i.e., sea-level rise, river discharge, soil
subsidence, number of cyclones, storm surge),
socio-economic components (a part of the
socio-economic system) and politico-administrative
components (the administrative and institutional
system) [16]. With limitation of the available data of
socio-economic for the future, the vulnerability of
different provinces in the delta is calculated for the
current socio-economic conditions in conjunction
with projection of upstream discharge in 2050 and
sea level rise of 30 cm (with the reference of daily
measured sea level in 2000).
The calculated indicators are standardized after Eqs.
(1) and (2) for the positive and negative impacts,
respectively. The standardized indicators are
multiplied with weights (ranging from 1 to 10) of each
indicator based on their importance to vulnerability in
specific conditions of the Vietnamese Mekong Delta
(according to the available publications and local
reports, e.g., Refs. [17, 18]). All indicators are then
aggregated to identify vulnerability of each calculated
unit Eq. (3). Vulnerability uncertainty assessment is
done by adjusting the applied weight according to Eq.
(4) and the following rules: (1) For each iteration,
weight of one indicator would be adjusted while the
others would be remained; (2) The iteration would
keep going until all assigned weights are changed.
According to Ref. [19], risk value is the product of
vulnerability and hazard with 0 and 1 corresponding
to the lowest and greatest.


(1)
x

1
x
x

(2)
Vulnerability Exposure  Susceptibility
 Resilience
(3)
New
_
Weight Assigned_Weight  1
(4)
3. Results
3.1 Simulated Flood
Fig. 3 presents the simulated stages of the fluvial
flood at the Tan Chau, Chau Doc, My Thuan and Can
Tho gauging stations from September 15 to October
15, 2000. The greatest (simulated vs. measured) stages
in Tan Chau (5.18 m vs. 5.06 m above mean sea level)
and Chau Doc (4.73 m vs. 4.90 m above mean sea
level) appeared on September 23, 2000 (Fig. 4a) while
such the greatest in Can Tho (2.31 m vs. 1.79 m above
mean
sea
level)
and
My
Thuan
(2.23
m
vs.
1.80 m
Fig. 3 Simulated stages at Can Tho, My Thuan, Chau Doc and Tan Chau; msl is mean sea level.
Vulnerability to Flood in the Vietnamese Mekong Delta: Mapping and Uncertainty Assessment
233
Fig. 4 Inundated map at the peak flood (a) and greatest inundated area (b) in the Vietnamese Mekong Delta.
Fig. 5 Hazard maps at the greatest stage in the upstream (a) and greatest flood extent in the Vietnamese Mekong Delta (b).
above mean sea level) appeared on September 28,
2000 (Fig. 4b) [6, 10, 20]. Even though there are
differences between the simulated and measured
stages, the differences are among the accepted ranges
[6].
3.2 Hazard Map
The hazard maps of flood in 2000 are shown in Fig.
5. Despite the greatest stages appeared in Tan Chau
and Chau Doc, influenced area is not the greatest (Fig.
5a) as the flood is still remained in the upstream
section of the delta. Flood hazard covers about 48% of
the entire Vietnamese Mekong Delta, including very
low, low, medium, high, and very high hazard area
corresponding to 7.89%, 5.71%, 8.77%, 12.93% and
12.71% of total area, respectively. The high and very
high hazards are mainly found in the upstream, while
there is improbable hazard in the coastal area.
However, when the flood is expanded largest in the
delta (Fig. 5b), the medium, high, and very high
hazard areas do not only expand in the upstream but
also in the coastal areas. The influenced area are
5.98%, 12.67%, 17.84%, 25.84% and 14.29% of the
entire delta, accounted for very low, low, medium,
high, and very high hazard, respectively.
In 2050, simulated greatest flood stage would
appear in the upstream section of the delta on October
16
and the flood extent would be the largest in the
Vulnerability to Flood in the Vietnamese Mekong Delta: Mapping and Uncertainty Assessment
234
Fig. 6 Hazard maps in Scenario 1 at the greatest stage in the upstream and greatest flood extent in the Vietnamese Mekong
Delta.
following days. As the flood dynamics are not
significantly different between the two scenarios (of
the projected upstream discharge), Fig. 6 presents
hazard maps of the greatest flood stage in the
upstream section of the delta and at the greatest extend
according to Scenario 1 only. At the greatest flood
stage in the upstream section, the hazard area would
cover 60.38 % of the entire delta, including the very
low, low, medium, high and very high hazard
corresponding to 7.52%, 15.31%, 16.15%, 16.83%
and 4.58% of the entire area, respectively. The high
and very high hazard would be found also in the
upstream section of the delta. Similarly, in Scenario
2, the greatest flood stage in the upstream section of
the delta and the greatest flood extent would also
appear on the same day to Scenario 1. The hazard
areas in Scenario 2 (59.68%) would be smaller than
those in Scenario 1 as the upstream discharge in
Scenario 2 is projected to be smaller than that in
Scenario 1.
In comparison to the 2000-flood, the flood hazard
in the future would open further to the Ca Mau
Peninsula as the consequence of the projected sea
level rise. The simulated hazard areas would increase
about 12.39% and 11.68% in Scenario 1 and 2,
respectively. Although hazard areas would increase,
very high hazard area would decrease according to the
lower flood peaks in the future scenarios. At the
greatest flood extent in the future, the hazard area
would cover 81.45 % and 79.21 % of the entire delta
in the Scenario 1 and Scenario 2, respectively. The
high flood hazard does not only concentrate in the
upstream but also open to the downstream accounting
for about 30% area of the entire delta.
3.3 Vulnerabilities
Fig. 7a introduces the vulnerability of each province
in the Vietnamese Mekong Delta in the base-line
scenario with the calculated integrated Coastal City
Flood Vulnerability Index. In general, differences in
flood-induced vulnerability between provinces are
relatively small as the provinces share a common
system of the delta (e.g., cultural heritage,
unemployment level and population density). The Soc
Trang Province would be most vulnerable area as the
area is facing directly to the Bassac River and the East
Sea, having a dense of the canal network and less ratio
of investment over the total gross domestic product.
The Bac Lieu Province is the second most vulnerable
to flood because of high population growing, a dense
canal network and facing directly to the East Sea. The
following vulnerable province is accounted for Can
Tho and Hau Giang with a dense river network and
less
ratio of investment over the total gross domestic
Vulnerability to Flood in the Vietnamese Mekong Delta: Mapping and Uncertainty Assessment
235
Fig. 7 Vulnerability of the Vietnamese Mekong Delta (a) and range of flood vulnerability (by province) after changes of the
assigned weight (b).
Fig. 8 Risk maps of Scenario 1 at the peak flood (a) and greatest flood extent (b) in the Vietnamese Mekong Delta.
product; in addition, Can Tho is facing directly to the
Bassac River. Other coastal provinces (Ca Mau, Ben
Tre, Tra Vinh and Tien Giang) would be directly
affected by sea level rise and the increase of storm
surge in the future but with less magnitude in
comparison to the other provinces. In addition, local
residents of those provinces receive little impacts (if
any) from the annual flood, so they would not have
sufficient experiences for flood adaptation. The low
vulnerable province is Ben Tre with the low
population density considered as the most important
indicator. An Giang and Dong Thap are the two least
vulnerable provinces as they are not facing directly to
the sea and therefore the sea level (rise) does not
introduce strong influences on the area [6, 14, 21]. In
addition, local residents of the two provinces have
sufficient knowledge and experiences on
living-with-flood.
Fig. 7b illustrates the range of vulnerability by
province after the changes of the assigned weight. Soc
Trang, Bac Lieu, Tien Giang, Tra Vinh and Ben Tre
are considered to be highly vulnerable, while An
Giang and Dong Thap are of the low vulnerability
values. Although there are the changes of flood
vulnerability, Soc Trang is also of the greatest
vulnerable area according to the annual fluvial flood.
As the flood dynamics generated for Scenarios 1
and 2 are not significantly different, Fig. 8 only
presents the risk map of Scenario 1 at the peak and the
greatest flood extent. At the greatest flood stage in
Tan Chau and Chau Doc, the percentages of flood risk
are of about 60% and 59% area of the entire delta in
Vulnerability to Flood in the Vietnamese Mekong Delta: Mapping and Uncertainty Assessment
236
Scenarios 1 and 2, respectively. When the flood
expands largest spatially, the flood risk would cover
most of the entire area of the entire delta. In addition,
the high category of flood risk would concentrate
along the main water-ways (including the Mekong and
Bassac River).
4. Conclusions
With the application of the deltaic-scale model,
differences between the measured and simulated
stages are still large. In addition, the hydraulic
constructions system and river bathymetry are not
updated fully leading to a need to update the model,
especially (1) the full-dyke system in the upstream
section (to prevent flooding the intensive rice field); (2)
sluices along the coastline (to prevent saline intrusion
and also to confine the flood discharge loaded to the
sea); (3) current status of the river network and actual
status of bathymetry.
The characterization of the historical flood hazard,
vulnerability and risk are quantified. Even though the
flood depth and duration in the upstream section of the
Vietnamese Mekong Delta are greater than those in the
downstream section, the risk of local residents due to
the annual flood in the upstream is lower than that in
the downstream. In fact, local residents living in the
upstream section have great experiences to eliminate
negative impacts and earn the most benefits from such
natural events. In contrast, local residents living in
downstream section of the delta are not used to
tidal-induced floods leading to negative impacts (e.g.,
damages of the agriculture and aquaculture [22],
leading to negative impacts on the local livelihood [23,
24].
The hazard in 2050 would increase, especially in
the downstream section of the delta due to the
projected sea level rise. The results of this study will
help local stakeholders and decision-makers
understand what might happen in the future in terms
of hydrological changes and consequent impacts (i.e.,
risk) on each province. The obtained results also
introduce local areas that need specific attention for a
detailed study in the future.
The coastal areas, especially those along the East
Sea, are projected to be highly affected due to the
hydrological changes while there is still limitation of
awareness for flood adaptation. Therefore, future
research is required to provide suitable set of
indicators that meet the actual conditions of the
Vietnamese Mekong Delta and detailed study should
be done to meet specific local settings.
Finally, the study paid great attention to the
magnitude of flood and its impacts, however, does not
take into account the timing distribution of flood
dynamics. Such dynamics, which may have great
impacts on the agriculture and aquaculture activities
of the local residents, should be studied in more
details in the near future.
Acknowledgments
This paper is developed with financial support from
the Ministry of Economic Affairs from the
Netherlands with the project of Developing
Agriculture, Aquaculture and Environment based
Climate Change Adaptation strategies for the Mekong
Delta Plan of Vietnam. This project was carried out
with support of experts from Wageningen University
and Research, the Netherlands and Can Tho
University, Vietnam.
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The coastal region in Vietnam is the region most directly affected and severely damaged by climate change and sea-level rise, including in the Mekong Delta. The low-income coastal community is one of the most vulnerable populations to disaster risks because of limited adaptability and unsustainable livelihoods. Since 2005, people living along the 56 km coastline of Bac Lieu have faced high tides tending to rise over alarming levels III, damaging to life and production. This study used the community-based disaster risk assessment framework to identify the extent of vulnerabilities and its factors driving forces. It was conducted using mixed methods, including in-depth interviews of nine experts, questionnaire interviews of 233 households, and six focus groups at six coastal communes in Bac Lieu province. The study finds that the low-income coastal households are exposed to high levels of vulnerability about livelihood, residence safety, access to clean water and sanitation, food, and medical services due to a lack of adaptive capacity to the climate changes.
... For FBs, these toxins were produced by F. proliferatum in rice grains [28,37], producing FBs at 20-30 • C [38]. Moreover, in winter-spring, most fields are invaded by seawater and drought conditions [39]. However, some paddy lines cannot tolerate seawater and drought conditions, resulting in vulnerability to crop disease (e.g., Fusarium diseases) [40]. ...
... In fact, the Can Tho farmers cultured only one type of paddy line (e.g., Jasmine or DT8) for three season crops, including winter-spring, summer-autumn, and autumn-winter per year. As mentioned above, in winter-spring crops, most fields in Can Tho were invaded by seawater and drought conditions [39]. However, Jasmine or DT8 is not a seawater and drought tolerating paddy variety, resulting in susceptibility to crop disease (e.g., Fusarium diseases) [40]. ...
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The current study aimed to evaluate the impact of the crop season, cultivation region, and traditional pre- and post-harvest agricultural practices on mycotoxin contamination in the Mekong Delta rice chain of Vietnam. The results showed that aflatoxins (AFs) and fumonisins (FBs) were predominantly detected in both paddy (n = 91/184, 50%) and white rice (n = 9/46, 20%). Aflatoxin B1 (AFB1)-contaminated paddy samples (n = 3) exceeded the regulatory threshold (5 µg·kg−1). The contamination of paddy with AFs and FBs was not significantly different by growing seasons and cultivation localities. Evidently, in the winter–spring season, fumonisins frequently occurred in paddy planted in Can Tho, while AFs were found in paddy planted in regions Dong Thap and An Giang, and such toxins were absent in Can Tho. Furthermore, the selection of paddy varieties strongly impacted the occurrence of these toxins, especially AFs, for example, line DT8 and Jasmine were susceptible to AFs and FBs. In addition, poor pre- and post-harvest practices (such as crop residue-free fields, fertilizer application, unsanitary means of transport, delayed drying time) had an impact on the AFs and FBs contamination. Our findings can help to understand the dynamics of AFs and FBs in the rice chain in the Vietnamese Mekong Delta, leading to the mitigation of the contamination of AFs and FBs in rice.
... Climate change sea-level rise has been having a significant influence on agricultural development of the Mekong delta in general and Long An province in particular by extreme weather conditions, erratic rain, prolonged heat, saltwater intrusion deep into the field (Le, 2019;Phan et al., 2020;Pham et al., 2020). Besides, the exploitation of hydroelectric resources of the upstream zone also contributes to increasing the impact of natural factors affecting farmers' agricultural production (Turner et al., 2009;Tri et al., 2013;Hong et al., 2019;Tran., 2021). ...
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Background: This study aims to estimate the factors affecting the agricultural production process on the problem soil of Tan Thanh district as a case study to develop solutions for sustainable agriculture. Methods: Through consultation with 150 households and 60 experts, Four primary and 18 secondary factors were identified that affect the agricultural land use of the community. Result: The results show the factors of the consumption market, government organization, profit and soil quality that are the factors that receive much attention from experts by the “multi-criteria evaluation” method. On the contrary, the aspects of problem soil, irrigation capacity, flooding and drought time are of little interest to experts. The results have also proposed 12 groups of structural and non-structural solutions to improve the district’s agricultural land use efficiency. Which focuses on the consumption market, cost, profit and government organization.
... Different studies have already highlighted the vulnerability of the VMD to climate change [ e.g. Van et al., 2012;Tri et al., 2013;Chapman [ Figure 7.2 ] The Mekong Delta's flood zones and its complex canals network Source: Thanh et al., 2020. and . ...
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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.
... According to recent assessments, 40 deltas in the world are projected to be at risk in terms of coastal erosion due to a decrease of sediment supply and due to sea level rise in combination with subsidence (Wong et al., 2014). Among these, the Mekong Delta is one of the deltas that are extremely vulnerable to predicted sea level rise by the year 2050 (Tri et al., 2013). The main reason for this vulnerability is that deltas are highly dynamic areas that can be significantly impacted by changing environmental conditions. ...
... According to recent assessments, 40 deltas in the world are projected to be at risk in terms of coastal erosion due to a decrease of sediment supply and due to sea level rise in combination with subsidence (Wong et al., 2014). Among these, the Mekong Delta is one of the deltas that are extremely vulnerable to predicted sea level rise by the year 2050 (Tri et al., 2013). The main reason for this vulnerability is that deltas are highly dynamic areas that can be significantly impacted by changing environmental conditions. ...
... According to recent assessments, 40 deltas in the world are projected to be at risk in terms of coastal erosion due to a decrease of sediment supply and due to sea level rise in combination with subsidence (Wong et al., 2014). Among these, the Mekong Delta is one of the deltas that are extremely vulnerable to predicted sea level rise by the year 2050 (Tri et al., 2013). The main reason for this vulnerability is that deltas are highly dynamic areas that can be significantly impacted by changing environmental conditions. ...
... According to recent assessments, 40 deltas in the world are projected to be at risk in terms of coastal erosion due to a decrease of sediment supply and due to sea level rise in combination with subsidence (Wong et al., 2014). Among these, the Mekong Delta is one of the deltas that are extremely vulnerable to predicted sea level rise by the year 2050 (Tri et al., 2013). The main reason for this vulnerability is that deltas are highly dynamic areas that can be significantly impacted by changing environmental conditions. ...
Thesis
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.
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Worldwide, there is a need to enhance our understanding of vulnerability and to develop methodologies and tools to assess vulnerability. One of the most important goals of assessing coastal flood vulnerability, in particular, is to create a readily understandable link between the theoretical concepts of flood vulnerability and the day-to-day decision-making process and to encapsulate this link in an easily accessible tool. This article focuses on developing a Coastal City Flood Vulnerability Index (CCFVI) based on exposure, susceptibility and resilience to coastal flooding. It is applied to nine cities around the world, each with different kinds of exposure. With the aid of this index, it is demonstrated which cities are most vulnerable to coastal flooding with regard to the system's components, that is, hydro-geological, socio-economic and politico-administrative. The index gives a number from 0 to 1, indicating comparatively low or high coastal flood vulnerability, which shows which cities are most in need of further, more detailed investigation for decision-makers. Once its use to compare the vulnerability of a range of cities under current conditions has been demonstrated, it is used to study the impact of climate change on the vulnerability of these cities over a longer timescale. The results show that CCFVI provides a means of obtaining a broad overview of flood vulnerability and the effect of possible adaptation options. This, in turn, will allow for the direction of resources to more in-depth investigation of the most promising strategies.
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Urban development increases flood risk in cities due to local changes in hydrological and hydrometeorological conditions that increase flood hazard, and also to urban concentrations that increase the vulnerability. The relationship between the increasing urban runoff and flooding due to increased imperviousness better perceived than that between the cyclic impact of urban growth and the urban rainfall via microclimatic changes. The large-scale, global impacts due to climate variability and change could compound these risks. We present the case of a typical third world city - Can Tho (the biggest city in Mekong River Delta, Vietnam) - faced with multiple future challenges, namely: (i) climate change-driven sea-level rise and tidal effect, (ii) increase river runoff due to climate change, (iii) increased urban runoff driven by imperviousness, and (iv) enhancement of extreme rainfall due to urban growth-driven micro-climatic change (urban heat islands). A set of model simulations were used to assess the future impact of the combination of these influences. Urban growth of the city was projected up to year 2100 based on historical growth patterns, using a land-use simulation model (Dinamica-EGO). A dynamic limited-area atmospheric model (WRF), coupled with a detailed land-surface model with vegetation parameterization (Noah LSM), was employed in controlled numerical experiments to estimate the anticipated changes in extreme rainfall patterns due to urban heat island effect. Finally, a 1-D/2-D coupled urban-drainage/flooding model (SWMM-Brezo) was used to simulate storm-sewer surcharge and surface inundation to establish the increase in the flood risk resulting from the changes. The results show that, if the city develops as predicted, the maximum of inundation depth and area in Can Tho will increase by about 20%. The impact of climate change on inundation is more serious than that of urbanization. The worse case may occur if the sea level rises 100 cm and the flow from upstream happen in the high-development scenarios. The relative contribution of causes of flooding are significantly different at various locations; therefore, detailed research on adaptation are necessary for the future investments to be effective.
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The Vietnamese Mekong Delta (VMD) has an important role in terms of food security and socio-economic development of the region; however, it is one of many areas of the world vulnerable to floods resulting from sea level rise (SLR) due to climate change. Therefore, management of flooding is a priority at national and regional levels in Vietnam. The Long Xuyen Quadrangle is the most important region in the VMD in terms of agriculture and economy. In the present work, flood hazard, vulnerability and risk were assessed and mapped to identify the priority areas in the Long Xuyen Quadrangle for flood mitigation. A hydrodynamic model was used to simulate the flood event of 2000 when a flood of 20-year return period occurred and caused loss of human lives and extensive damage. The calibrated model was then used to simulate a possible flood event in 2050 due to SLR. The resulting flood depth of the simulation was used to prepare inundation maps and to analyse flood hazard in this region, as well. The flood vulnerability of the region was assessed using the coastal areas flood vulnerability index (FVI) method. The FVI was determined by district, and flood vulnerability maps were developed based on these data. The results indicate that the major part of the study area (35.4%) can be classified as being at high risk. It was also found that 32.7% of the area is under medium risk and only about 18.4% is under very low and low risk; 10.2% of the total area is not subjected to flood risk.We show that district level flood vulnerability maps are potentially useful for decision makers and the public in planning better measures for adaptation and mitigation of the negative impacts of flooding.
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Worldwide, there is a need to enhance our understanding of vulnerability and to develop methodologies and tools to assess vulnerability. One of the most important goals of assessing coastal flood vulnerability, in particular, is to create a readily understandable link between the theoretical concepts of flood vulnerability and the day-to-day decision-making process and to encapsulate this link in an easily accessible tool. This article focuses on developing a Coastal City Flood Vulnerability Index (CCFVI) based on exposure, susceptibility and resilience to coastal flooding. It is applied to nine cities around the world, each with different kinds of exposure. With the aid of this index, it is demonstrated which cities are most vulnerable to coastal flooding with regard to the system’s components, that is, hydro-geological, socio-economic and politico-administrative. The index gives a number from 0 to 1, indicating comparatively low or high coastal flood vulnerability, which shows which cities are most in need of further, more detailed investigation for decision-makers. Once its use to compare the vulnerability of a range of cities under current conditions has been demonstrated, it is used to study the impact of climate change on the vulnerability of these cities over a longer timescale. The results show that CCFVI provides a means of obtaining a broad overview of flood vulnerability and the effect of possible adaptation options. This, in turn, will allow for the direction of resources to more in-depth investigation of the most promising strategies.
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The present paper investigated the extent of the flood propagation in the Vietnamese Mekong Delta under different projected flood hydrographs, considering the 2000 flood event (the 20-yr return period event, T. V. H. Le et al., 2007) as the basis for computation. The analysis herein was done to demonstrate the particular complexity of the flood dynamics, which was simulated by the 1-D modelling system ISIS used by the Mekong River Commission. The floods of the year 2050 are simulated using a projected sea level rise of +30 cm. The future flood hydrograph changes at Kratie, Cambodia, were also applied for the upstream boundary con-dition by using an adjusted regional climate model. Two fu-ture flood hydrographs were applied at the upstream part of the delta, the first one in a scenario of climate change with-out considering developments in the Mekong Basin,and the second one in a scenario of climate change taking into ac-count future development of the delta. Analyses were done to identify the areas sensitive to floods, considering the uncer-tainty of the projection of both the upstream and downstream boundary conditions. In addition, due to the rice-dominated culture in the Vietnamese Mekong Delta, possible impacts of floods on the rice-based farming systems were also analysed.
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The flood pulse is a key element characterizing the hydrology of the Mekong River and driving the high ecosystem productivity in the Lower Mekong floodplains, both in the Cambodian lowlands and the Mekong Delta in Vietnam. This paper assesses the impacts of climate change, both in terms of changed basin water balance and sea level rise, on the Lower Mekong flood pulse. The impacts were simulated by a three-dimensional hydrodynamic model using the projected changes in sea level and the Mekong mainstream discharge under the influence of climate change as boundary conditions. The model simulations projected that average and maximum water levels and flood duration increase in 2010–2049. The most consistent and notable changes occurred in the average and dry hydrological years. Sea level rise had the greatest effects in the Mekong Delta, whereas the impacts of changed basin water balance were more notable in the upper areas of the Mekong floodplains. The projected impacts were mostly opposite to those resulting from regional water infrastructure development. Higher and longer flooding could cause damage to crops, infrastructure and floodplain vegetation, and decrease the fertile land area. On the other hand, it might boost ecosystem productivity and enhance dry season water availability.
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The current flood vulnerability index (FVI) methodology developed by the authors uses 71 indicators in its calculation. However, it is recognized that some of these indicators may be redundant or have no influence on the results. This paper presents the results of analysis carried out to select the most significant indicators in order to establish parsimonious usage of the FVI. As with the original methodology, this is applicable at three different spatial scales (river basin, sub-catchment and urban) and to the various components of flood vulnerability (social, economic, environmental and physical). For the FVI methodology to be sustainable, improvements were made by analysing the indicators' relevance and by studying the main indicators needed to portray reality of the fluvial floods in an effective way. For this purpose, mathematical tools (a derivative and a correlation method) and expert knowledge (via a questionnaire) were used. Finally, all these methods were combined in order to select the most significant indicators and to simplify the FVI equations. After reducing its complexity, the FVI can be more easily used as a tool for education, improvement of decision making and ultimately reduction of flood risk.
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Flood hazard and risk assessment was conducted to identify the priority areas in the southwest region of Bangladesh for flood mitigation. Simulation of flood flow through the Gorai and Arial Khan river system and its floodplains was done by using a hydrodynamic model. After model calibration and verification, the model was used to simulate the flood flow of 100-year return period for a duration of four months. The maximum flooding depths at different locations in the rivers and floodplains were determined. The process in determining long flooding durations at every grid point in the hydrodynamic model is laborious and time-consuming. Therefore the flood durations were determined by using satellite images of the observed flood in 1988, which has a return period close to 100 years. Flood hazard assessment was done considering flooding depth and duration. By dividing the study area into smaller land units for hazard assessment, the hazard index and the hazard factor for each land unit for depth and duration of flooding were determined. From the hazard factors of the land units, a flood hazard map, which indicates the locations of different categories of hazard zones, was developed. It was found that 54% of the study area was in the medium hazard zone, 26% in the higher hazard zone and 20% in the lower hazard zone. Due to lack of sufficient flood damage data, flood damage vulnerability is simply considered proportional to population density. The flood risk factor of each land unit was determined as the product of the flood hazard factor and the vulnerability factor. Knowing the flood risk factors for the land units, a flood risk map was developed based on the risk factors. These maps are very useful for the inhabitants and floodplain management authorities to minimize flood damage and loss of human lives. Copyright © 2005 John Wiley & Sons, Ltd.