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

Abstract

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.

Full text

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
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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.
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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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