RELATIONSHIP BETWEEN INUNDATION AREA AND

RELATIONSHIP BETWEEN INUNDATION AREA AND IRRIGATION AREA
ON FLOOD CONTROL IN THE LOWER MEKONG
TERUMICHI HAGIWARA
Department of Civil Engineering, Tohoku University
SO KAZAMA and MASAKI SAWAMOTO
Department of Civil Engineering, Tohoku University
[email protected]
Abstract-Flood and inundation are annual event,
which is utilized for agriculture in the Lower Mekong
basin. Inundation area can be associated with
agricultural land. A numerical simulation was carried out
to understand change of inundation area by flood
control, and irrigation area estimated from reserved
water. The decrease of inundation area caused by flood
control is much smaller compared with potential
irrigation area. Although decrease of inundating area
influence agricultural land, that can be made up for by
irrigation. However it is hard to prevent inundation of
floodplain even if there is large-scale water control
system.
I.
INTRODUCTION
Now development of the Mekong river basin is one
of the most notable planning all over the world. The
Mekong river is an international river and its basin
spreads six countries. Its development planning has
various problems. For some decades of conflict and
disruption over the basin countries, development in the
Mekong is the lowest level in the world.
The flood in rainy season and salinity intrusion from
South China Sea in dry season are the most important
annual events in the Mekong River. Flood and
inundation analysis and assessment of salinity
intrusion in the Mekong delta have been made with a
numerical method ([1] and [2]). The subjects of those
studies are not whole Mekong Delta but a part included
in Vietnam. The Cambodian part studies are hardly made
for the reason of difficulty in getting available data.
Although annual flood and inundation have negative
point such as disaster, that also has some merits.
Inundating water is traditionally used as the water for
agricultural use in this region. Inundation can be
regarded as natural irrigation area, fish habitant, and
grand water recharge. Water resource development in
the Mekong basin may control annual flood and reduce
the inundation area in the Mekong Delta. However it
has not researched how the change of inundation area
influences society and economy in term of water
resource. Particularly agricultural area may be decided
by decrease of inundation area. It is an important
problem how much area can be irrigated with the water
stored in the upstream dams or reservoirs by controlling
a flood. That is useful in discussing the validity of
development in this region.
This paper estimates the change of inundation area
under the various flood-controlled situation by using
numerical simulation and irrigation area from stored
water in upstream dams and from inundated water for
natural irrigation in downstream.
II. THE STUDIED REGION
The Mekong River has its source in the southeastern Himalayan mountains and flows 4200 kilometers
to its mouth at the South China Sea, making it the
world’s twelfth longest river. It ranks tenth in terms of
dis charge.
The studied region of this study is a part of the
Lower Mekong basin (Fig. 1). Annual precipitation in
the Lower Mekong is strongly seasonal, as is the
annual flow of the Mekong. About 85% of the
precipitation falls during the rainy season. Annual
precipitation averages about 1680 mm across the basin.
The Mekong flood season lasts from July through
December with an average discharge of 25,000 m3/s. The
low flow season lasts from January to June with an
average discharge of 6,000 m3/s.
In this region Mekong has two large tributaries,
Bassak and Tonle Sap, and these rivers joins at Phnom
Penh, which is capital city of Cambodia. The end of
Tonle Sap river is the Great lake(the Tonle Sap lake),
which has a retarding function. Therefore the
complicated flow is formed around Phnom Penh. Here
spread low and flat land called Mekong Delta and
floodplain cover a wide area.
III. DATA SET
As the boundary conditions in the flood simulation
of rivers, water level data at Kampong Cham, upstream
of the Mekong, at Prek Kdam of Tonle Sap, at Tan Chau,
downstream of the Mekong and at Chau Doc of Bassak
are used (Fig. 2). Water level data at Phnom Penh is
used for validation of the simulation. Those data were
periodically obtained by the Mekong river Commission
(MRC). The simulation period is from July 6 to October
12.
18
Kampong Cham
16
water level (m)
14
12
Prek Kdam
10
8
Tan Chau
6
4
Chau Doc
2
(a) Lower Mekong
0
7/6
7/20
8/3
8/17
8/31
9/14
9/28 10/12
date
Tonle Sap
Kampong Cham
Fig.2. Water levels for boundary condition
Prek Kdam
Mekong
IV. BASIC EQUATIONS AND M ODEL
Phnom Penh
A. Flood calculation
Flood water levels of rivers are estimated by 1-D
dynamic wave model, which consists of following a
continuity equation and a momentum equation.
Bassak
Chau Doc
TanChau
(b) Around Phnom Penh
Fig. 1. The studied region
∂A ∂Q
+
−q= 0
∂t ∂x
2
1 ∂v
1 ∂v 2
∂H n v v
+
+
+
=0
g ∂t 2 g ∂x
∂x
h 4/ 3
(1)
(2)
Where A : cross-sectional area (m2), Q : discharge
(m3/s), g : gravity (m2/s) v : velocity (m/s) H : water
table (m), n : Manning coefficient, h : water depth (m).
The q refers inflow per unit area such as tributary
Riverbed elevation data were not available along
rivers but were available at only hydrological stations.
These elevations are –0.93m at Kampong Cham, -1.02m
at Phnom Penh, 0.08m at Prek Kdam and 0m at Tan Chau
and Chau Doc. Riverbed elevation were interpolated by
using these data.
Elevation data of floodplain were used for the
inundation simulation. The United States Geological
Survey (USGS) made digital elevation data in all over
the world with 1km Square. However this data has
wrong value at some point compared with a map, which
named Cambodia Topographical Maps, edited by Japan
International Cooperation Agency. Thus elevation data
of USGS was corrected and interpolated at some point
according to the map.
inflow or rainfall.
This model was solved by finite difference method
that time interval is 30 second and ground resolution is
1km. It was supposed that all rivers have wide
rectangular open channel, river width was set 1000m for
the Mekong, 500m for the Bassak and Tonle Sap, and
Manning coefficient is 0.02.
B. Inundating flow
For floodplain, the following 2-D continuity equation
and momentum equation in two directions are applied.
∂h ∂M ∂N
+
+
=0
∂t
∂x
∂y
(3)
∂M
∂H gn2 M M 2 + N 2
= − gh
−
∂t
∂x
h 7 /3
(4)
∂N
∂ H gn 2 N M 2 + N 2
= − gh
−
∂t
∂x
h 7 /3
(5)
C. Overflow
For the overflow discharge formula combining rivers
and the floodplain, the following equations are used.
For the complete overflow,
(6)
water level (m)
11
Where M = uh , N = vh : discharge flux in x-and ydirection (m2/s), respectively. Although these equations
originally include nonlinear term, we ignored this term
for the reason of less influence of this term and
avoidance complicated calculation.
Also in this model, time interval and ground
resolution are 30 second and 1km. Manning coefficient
of the floodplain is 0.05.
Q = 0. 35 Bh1 2 gh1
12
10
9
8
observed
7
culculated
6
7/6
7/20
8/3
8/17
8/31
9/14
9/28
10/12
date
Fig. 3. Comparison of calculated and observed water
level at Phnom Penh
Kampomg Cham
For the submerged overflow,
Q = 0 .91 Bh2 2 g ( h1 − h 2 )
(7)
Where B refers the width of overflow. h are water
stages from the crest of overflow on levee, and h1 is
the higher.
V. SIMULATION RESULT OF 2000 FLOOD
Calculation was performed in order of flood, overflow
and inundating flow. Fig. 3 shows comparison the result
of calculation with measured value. Although there are
some errors early time due to initial condition, good
results are obtained in a long time.
The inundation areas gained from calculation and
satellite image at November 4 are compared in Fig. 4.
Both show good agreement except for around Kampong
Cham, northern side of Mekong and western side of
Tonle Sap. This discrepancy is occurred for the wrong
elevation data. More correct data and field survey are
necessary for more accurate calculation. The
distribution of inundation depth could not compare
with actual value for the lack of available data. However
the range of inundation can be sufficiently expressed
by this model.
VI. RELATIONSHIP BETWEEN INUNDATION A REA AND
IRRIGATION A REA
A. The change of inundation area
Flood control caused by basin development may
observed
calculated
Fig. 4. Comparison of inundation area
decrease inundation area. We simulate the change of
inundation area under assumed flood controlled
condition. Here water level at boundary condition was
changed for expressing controlled situation.
As shown in Fig. 5, different water levels were
assumed to change with some percent decrease to the
2000 flood for flood control. The ratio range of control
is from 10% to 50% with 10% interval. Boundary water
levels at other points were given as the same control
ratio.
17
Without control
Without control
water level (m)
16
wat 15
er
14
lev 13
12
50%
control
50% control
11
11
0
20
20
40
60
60
80
100
100
day
day
Fig. 5. Assumed water level at Kampong Cham
Fig. 6 shows the distribution of inundation depth at
80th day after the beginning calculation, with maximum
inundation area, under flood control of 0%, 10%, 30%,
and 50%. Inundation area and depth decrease, as flood
control is getting more. Practically reduction of
inundation area is remarkable in eastern part of the
Mekong. It may become impossible to continue a
traditional agricultural form in this area. There is large
area that water depth reaches more than 5 meters in the
case of 0% control. In the case of 50% control, most of
the inundation area could not reach 1 meter of water
depth. However the region between the Mekong and
Bassak, and eastern part of lower the Mekong still
inundate more than 2 meters depth. Therefore it is hard
to prevent large scale of inundation even if there is
large-scale flood control system.
a) without control
b) 10% control
c) 30% control
d) 50% control
5m
0m
0m
5m
Fig. 6. Distribution of inundation depth
45000
40000
dischage (m3/s)
35000
30000
25000
20000
15000
without control
10000
50% control
5000
0
0
20
40
60
80
100
day
Fig. 7. Controlled discharge at Kampong Cham
irrigation area
inundation area
80000
70000
60000
area (km2 )
B. Irrigation area
Difference of inflow from upstream between the case
without control and the case of controlled situation was
used for estimating stored water in upper dams or
reservoirs. Fig. 7 shows discharge at Kampong Cham in
the case of 0% and 50% control. The peak discharge is
considerably controlled, discharge change smoothly.
This difference of discharge is assumed to be stored in
upper dams. Therefore amount of stored water was
estimated by integrating the difference of discharge at
Kampong Cham between the case of 0% control and of
each controlled situation. Irrigation area was estimated
by dividing the amount of total reserved water by
necessary water head for rice, which assumed 0.5m per
unit irrigation area 1m2 since necessary water for
irrigation is 1l/second/ha according to [3], and the
irrigation period was set for two months. In Fig. 8,
irrigation area is compared with inundation area that is
the maximum value in each control ratio. This can be
seen as the comparison of natural and artificial irrigation
area.
Despite higher flood control, inundation area could
not change so much, while irrigation area increase
rapidly. At the point of 10 % control the irrigation area
reaches equivalent value to inundation area. This result
shows that it become possible to get larger irrigation
area than natural irrigation in this region with more than
10% of flood control. However the irrigation area is
over-estimated in this study for ignoring other purpose
of stored water use. Actually irrigation area will be much
smaller value. Moreover dam construction has some
problems such as environmental disruption.
50000
40000
30000
20000
10000
0
VII. CONCLUSIONS
The wide area of inundation in the lower Mekong is
reduced by flood control. In this region, inundation area
relates to agricultural land since inundating water is
traditionally utilized as water for agricultural use. In this
paper the change of inundation area and distribution of
inundation depth caused by flood control are estimated
by a numerical method, and irrigation area is gained
from stored water in upper dam or reservoir. The result
shows that traditional irrigation style becomes
impossible in some region. While amount of stored
water in upstream dams or reservoirs make it possible to
irrigate large area. And wide range of inundation is hard
to prevent even the case where large-scale flood control
was made.
However this irrigation area is maximum value in the
case that all reserved water is used for irrigation. Since
reserved water is used for various purposes such as
drinking or industry, irrigation area will likely be smaller
value. Moreover there are many places where
inundation is not expressed by calculation for the lack
of available elevation data. Comparing more accurate
map and field survey will solve this problem.
Inundation areas are also important in this region for
ground water charge and fishery. Ground water is useful
0
10
20
30
40
50
control rate %
Fig. 8. The relationship between inundation area and
irrigation area
for water supply in developing country in dry season.
Inundation fishery is a chance for farmers to make cash.
This simulation results are effective for deeper such
discussion.
A CKNOWLEDGMENTS
The writing of this paper was made possible largely
through a grant from the Sumitomo Foundation and
through the Grants-in-Aid for Scientific Research from
Japan Society for the Promotion of Science (JSPS)
(Encouragement of Young Scientist 13750481), and we
would like to acknowledge here the generosity of these
organization.
REFERENCES
[1] K.Inoue, K.Toda and O.Maeda (2000) A mathematical
model of overland inundating flow in the Mekong Delta in
Vietnam. Ecosystem and flood
[2] N. Huu-toi and A.D.Gupta (2001) Assessment of water
resources and salinity intrusion in the Mekong delta. IWRA,
Water International, Vol.26, No.1
[3] H. Hori (1997) The Mekong river - development and
environment-.
[4] J.W.Jacobs(1997) Adjusting to climate change in the
Lower Mekong. Global Environmental Change, Vol.6, No.1
[5] S.Herath and D.Dutta(2000) Mekong basin study.
Proceeding of the AP FRIEND Workshop
[6] MRC(1997) Lower Mekong Hydrologic Yearbook.