HYDROLOGICAL MONITORING SYSTEM
FOR THE CAMBODIAN FLOODPLAINS
KANEHIRO MORISHITA
Team Leader of the JICA Study Team, CTI Engineering International Co., Ltd.,
4-2 Nihombashi Kodemmacho, Chuo-ku, Tokyo, 103-0001, JAPAN
HENRIK GARSDAL
Hydro-Hydraulic Modelling Expert of the JICA Study Team, DHI Water & Environment,
Agern Alle 11 DK-2970 Horsholm, DENMARK
TAKAO MASUMOTO
Advisory Committee Member of the JICA Study Team, National Institute for Rural
Engineering, 2-1-6 Kannondai, Tukuba-shi, Ibaraki-ken, 305-8609, JAPAN
The Cambodian floodplains, in particular Phnom Penh areas, are located in one of the
strategic areas in terms of water management along the lower Mekong mainstream. At
present the Mekong River Commission has been formulating the water utilization rules
for reasonable and equitable water use among the member countries. This document
presents the hydrological monitoring system to be necessary for flow management in the
Cambodian floodplains as a key area of the Mekong River system to well manage the
water utilization rules, based on the results of discharge measurements, filed observations
and simulation modelling.
INTRODUCTION
The “Agreement on the Cooperation for the Sustainable Development of the Mekong
River Basin” (the 1995 Agreement) was signed in April 1995 by the respective
plenipotentiaries of the four riparian countries: Laos, Thailand, Cambodia and Vietnam.
The Mekong River Commission (MRC) was established in 1995 based on the Agreement.
Formulation of the water utilization rules stipulated in the 1995 Agreement has been
one of the imminent issues for the MRC and the riparian countries since the MRC
establishment. To formulate the concrete procedural and technical rules on the Mekong
water utilization, the World Bank has assisted the MRC activities and announced to the
donors for technical assistances as some co-financed projects. In response to the
announcement, the Japan International Cooperation Agency (JICA) determined to
dispatch the study team to assist the water utilization program, which is one of the four
core programmes in the MRC, for formulation of the rule components and operation and
management of the rules after the completion. The JICA study, “Hydro-Meteorological
Monitoring for Water Quantity Rules in Mekong River Basin” [1] was one of the said
co-financed projects under the water utilization program of the MRC.
1
2
As reported in the proceedings of 1st conference of the APHW, “Hydrological and
Hydraulic Functions of the Cambodian Floodplains (Interim Report)” [2], the JICA study
had been continued for three years since March 2001 and was completed in March 2004.
In the course of the study, periodic and intensive discharge measurements were made in
the Phnom Penh areas for one and a quarter years. The Phnom Penh areas are called as
Chak Tomuk in Khmer language meaning four faces, where the Mekong River and the
Tonle Sap join and then bifurcate into the lower part of the Mekong and the Bassac River.
Clarification of the hydrological and hydraulic attributes of the areas and establishment of
their proper monitoring is essential for water management, following the water utilization
rules being formulated, in the lower Mekong River basin.
From the above-mentioned background, this document presents the hydrological
monitoring system that will be indispensable for the Cambodian floodplains as a key area
of the Mekong River system to well manage the water utilization rules, based on the
results of discharge measurements, filed observations and simulation modelling.
DISCHARGE MEASUREMENTS AND DEVELOPMENT OF RATING CURVES
From the viewpoint of data gaps in the discharge data, there exist big gaps in the
Cambodian territory. Since the early 1970s, any flow data have never existed in
Cambodia due to the long-lasting political instability. However, the upgrading process by
the Appropriate Hydrological Network Improvement Project (AHNIP) supported by
AusAID had progressed as a trunk telemetry system of water level gauging stations along
the mainstream. To avoid unfavorable overlapping with the AHNIP, the remaining
hydrological stations located in and around the Phnom Penh areas were selected. These
are Kompong Cham, Chrui Changvar (Phnom Penh Mekong), Koh Norea and Neak
Luong on the Mekong mainstream, Phnom Penh Port on the Tonle Sap, Monivong
Bridge (Chak Tomuk or Phnom Penh Bassac) on the Bassac River, as illustrated in
Figure 1. The study team conducted periodical and intensive discharge measurements at
the stations from July 2002 until October 2003.
Based on the discharge measurement results including those at Kratie (made by the
Ministry of Water Resources and Meteorology, Cambodia), rating curves had been
developed for the six hydrological stations mentioned above except for Phnom Penh Port.
Using the discharge hydrograph computed through these rating curves, it had been
verified that the discharge at Phnom Penh Port also could be estimated in a practical
manner during the wet season as shown in Figure 2 that illustrates the hydrographs at
various stations in the 2002 wet season. In this figure hydrograph estimated at Phnom
Penh Port well represents discharges measured for the reverse and normal flow periods
through some flow transitional period. As the result, the wet-season hydrological
monitoring system had been technically established from Kratie down to the Vietnam
border. The issue regarding the sustainability of monitoring will however remain
unsolved due to budgetary constraint.
3
Stung Treng
Kompong
Luong
Kratie
Prek
Kdam
Phnom Penh Port
Kompong Cham
Chrui Changvar
Chak Tomuk
Neak Luong
AHNIP Stations
WUP-JICA
Discharge
Measuring Stations
Figure 1. Major Hydrological Stations in Cambodia
Despite the issue on sustainability mentioned above, the dry-season monitoring
system had been proposed following the step-by-step approach in accordance with the
study results, namely, (1) It had been verified that the rating curve at Kompong Cham
could be used throughout the whole year; (2) The discharges at Chrui Changvar could be
estimated by the regression equation developed between Kompong Cham and Chrui
Changvar; and (3) The flow of Tonle Sap could be estimated through the developed
rating curve within the conditions of normal flow until around the end of April. For
bridging the limitation of the rating-curve development and establishing a
whole-year-round monitoring system in the Phnom Penh areas, the study team had
proposed the installation of direct measurement equipment for flow current along the
Tonle Sap and Bassac rivers. As a result, the hydrological monitoring system itself and
the reliability of observations had been enhanced for the dry-season monitoring as well as
the wet-season one in Cambodia.
DEVELOPMENT OF HYDRO-HYDRAULIC SIMULATION MODEL
The project of “Chaktomuk Area -Environment, Hydraulics, Morphology-” (Chak
Tomuk Project) [3] was carried out under the MRC from April 2000 to July 2001
supported by the Government of Japan. One-dimensional hydraulic model covering the
major channel system of the Mekong mainstream and tributaries from Kratie down to the
4
Vietnam border was constructed in this project to provide the boundary conditions for the
precise two-dimensional hydraulic model of the Chak Tomuk area.
Flow Hydrograph of Mekong System in the 2002 Wet Season
Discharge (m3/s)
55,000
Kratie
Kompong Cham
Chrui Changvar
Koh Norea
Neak Luong
Phnom Penh Bassac
50,000
45,000
40,000
35,000
30,000
25,000
20,000
15,000
10,000
5,000
28-Dec
21-Dec
7-Dec
14-Dec
30-Nov
23-Nov
9-Nov
16-Nov
2-Nov
26-Oct
19-Oct
5-Oct
12-Oct
28-Sep
21-Sep
7-Sep
14-Sep
31-Aug
24-Aug
17-Aug
3-Aug
10-Aug
27-Jul
20-Jul
6-Jul
13-Jul
29-Jun
22-Jun
8-Jun
15-Jun
1-Jun
0
Date/Month
Discharge (m3/s)
Flow Hydrograph at Phnom Penh Port in the 2002 Wet Season
12,000
Q_Estimated=Q_Koh Norea+Q_Monivong Bridge-Q_Chrui Changvar
10,000
8,000
6,000
4,000
Normal Flow
2,000
0
-2,000
Reverse Flow
-4,000
Q_Estimated
-6,000
Q_Observed
-8,000
1Ju
n
8Ju
15 n
-J
u
22 n
-J
u
29 n
-J
un
6Ju
13 l
-J
u
20 l
-J
u
27 l
-J
ul
3Au
10 g
-A
u
17 g
-A
u
24 g
-A
u
31 g
-A
ug
7Se
14 p
-S
e
21 p
-S
e
28 p
-S
ep
5O
c
12 t
-O
c
19 t
-O
c
26 t
-O
ct
2N
ov
9N
o
16 v
-N
o
23 v
-N
o
30 v
-N
ov
7D
e
14 c
-D
e
21 c
-D
e
28 c
-D
ec
-10,000
Date/Month
Figure 2. Computed Flow Hydrographs and Comparison Between Estimated and
Observed Discharges at Phnom Penh Port on the Tonle Sap, in the 2002 Wet Season
5
The study team had made modifications to the one-dimensional model prepared
under the Chak Tomuk Project so as to improve and utilize it as the analytical tool for the
study. The improvement of the model consisted of: (1) incorporating the rainfall-runoff
model of the sub-basins from Kratie down to the Vietnam border into the model; and
(2) incorporating the Great Lake model and the rainfall-runoff model of its sub-basins
into the model.
Furthermore, in the 2002 wet season the flooding mechanism over the Cambodian
floodplains had been precisely examined through intensive discharge measurement
campaign not only on the major river courses but also on the floodplains, and satellite
imaginary analysis in cooperation with the “Consolidation of Hydro-Meteorological Data
and Multifunctional Hydrologic Roles of Tonle Sap Lake and its Vicinities” Project
(TSLVP) [4 and 5], which succeeded the Chak Tomuk Project with financial assistance
from the Government of Japan. Based on the above observations and analyses in the 2002
wet season, the capability and reliability of the model description had dramatically
improved as depicted in Figure 3 as the results of the model calibration.
Observed and simulated discharge at Chaktomouk Junction, 2002
40000
Chrui Changvar
35000
30000
25000
Observed and simulated discharge at Kompong Cham, 2002
50000
Discharge [m3/s]
Discharge [m3/s]
60000
Koh Norea
20000
15000
Monivong bridge
10000
40000
30000
20000
10000
5000
0
27-Jan
20-Jan
13-Jan
06-Jan
30-Dec
23-Dec
16-Dec
09-Dec
02-Dec
25-Nov
18-Nov
11-Nov
04-Nov
28-Oct
21-Oct
14-Oct
07-Oct
30-Sep
23-Sep
16-Sep
09-Sep
02-Sep
26-Aug
19-Aug
12-Aug
05-Aug
29-Jul
22-Jul
15-Jul
08-Jul
01-Jul
0
-5000
Simulated
Phnom Penh Port
Observed
-10000
27-Jan
20-Jan
13-Jan
06-Jan
30-Dec
23-Dec
16-Dec
09-Dec
02-Dec
25-Nov
18-Nov
11-Nov
04-Nov
28-Oct
21-Oct
14-Oct
07-Oct
30-Sep
23-Sep
16-Sep
09-Sep
02-Sep
26-Aug
19-Aug
12-Aug
05-Aug
29-Jul
22-Jul
15-Jul
08-Jul
01-Jul
Simulated Chrui Changvar
Observed Chrui Changvar
Simulated Koh Norea
Observed Koh Norea
Simulated Phnom Penh Port
Observed Phnom Penh Port
Simulated Monivong Bridge
Observed Monivong Bridge
Figure 3. Model Calibration Results in the 2002 Wet Season
FLOOD ATTENUATION MECHANISMS OF THE CAMBODIAN
FLOODPLAINS
The team conducted longitudinal discharge measurements along the major river courses,
Mekong, Tonle Sap and Bassac, every other week from July 2002 until January 2003.
The results were combined with the discharge measurements and water level
observations on the floodplains under the TSLVP. Utilizing and analyzing the data
6
measured, the following facts were clarified in the hydrological functions of the
Cambodian floodplains.
In the stretch of Kompong Cham down to Neak Luong, 12 longitudinal discharge
measurements were conducted in the 2002 wet season. A part of the results is illustrated
in Figure 4. The flood discharges of 45,100 m3/s on August 29 and 44,800 m3/s on
September 26 are the observation data nearest to the peak of the 2002 flood at Kompong
Cham. The large discharge of some 45,000 m3/s was regulated through overbanking flood
and outflow into the flood paths of the floodplains. Some 25% of the discharge was
reduced on the way down to Chrui Changvar.
In addition flow divergence conditions down to the Mekong at Chak Tomuk junction
dominantly depend on the absorbing capacity of the Great Lake.
Figure 5 depicts the relationship between discharges at Kompong Cham and Chrui
Changvar. The flooding might start at Kompong Cham when discharge is about
25,000 m3/s (gauge height: 11 m in the rising stage). Beyond the discharge of 35,000 m3/s
(gauge height: 13 m in the rising stage), extensive flooding might occur. In addition, the
flood flow below 25,000 m3/s can be conveyed smoothly down to Chrui Changvar
without flooding.
Discharge (m3/s)
Longitudinal Flow Changes along the Mekong Mainstream
2002/7/18
2002/8/15
2002/8/29
2002/9/26
Chrui
Changvar
40,000
2002/10/10
2002/11/21
30,000
2002/12/19
20,000
Kompong
Cham
10,000
Neak
Luong
40,000
Discharge of Chrui Changvar (m3/s)
50,000
30,000
20,000
10,000
0
0
0
10,000
20,000
30,000
40,000
50,000
Discharge of Kompong Cham (m3/s)
250
300
350
400
450
500
Distance from the Mouth (km)
Figure 4. Longitudinal Flow Changes
along the Mekong Mainstream
Figure 5. Relationship between
Flood Discharges of Kompong
Cham and Chrui Changvar
Flood balance between Kompong Cham and Phnom Penh was computed utilizing a
series of the work results, such as flood hydrographs at major stations, water balance in
the floodplains, and hydro-hydraulic simulation outputs. Figure 6 summarizes the flood
balance in the 2002 wet season.
From the figure the flood attenuation elements were estimated as follows: (1) flow
conveyance; flood peak of 37,900 m3/s conveyed in the channel between Kompong
Cham and Phnom Penh even though accompanying overbanking flooding, (2) flood
divergence; flood flow of 11,900 m3/s (= 37,900 - 26,000) into two channels, Tonle Sap
7
and Bassac, 31% of flow reduction at the Chak Tomuk junction, (3) overbanking
flooding; flood flow reduction of 11,500 m3/s (= 49,400 – 37,900), equivalent to 23%
reduction. Thus regarding flood attenuation functions, both of flood divergence at the
Chak Tomuk junction and overbanking flooding between Kompong Cham and Phnom
Penh can be almost equal from the 2002 flood events.
Under the above hydrological mechanism, the area downstream of Kompong Cham,
in particular, the Capital City of Phnom Penh, is protected by the natural flood
attenuation functions; namely, the flood peak reduction by overbanking flooding in the
floodplains and the flood risk dispersing through floodwater divergence into three
channels.
In addition, flood flow conveyance to the Great Lake is as well a crucial natural
function in terms of conservation of the environment of the Great Lake. An almost
equivalent volume of overbanking floodwater compared with the Tonle Sap reverse flow
occurs in the same period. Therefore, these natural functions shall be conserved for the
protection of human lives and assets in the cities and towns against floods, as well as
protection of the natural environment and resources of the Great Lake and floodplains
against unregulated development. These functions are indispensable for the sustainable
development in Cambodia.
to Tonle Sap Lake
via Floodplain
Vol.:
12 BCM
Mekong River
Vol.: 26 BCM
Vol.:
14 BCM
Tonle Sap
Kompong
Cham
to Tonle Sap
Qp: 49,400 m3/s
Vol.: 338 BCM
Q: 6,500 m3/s
Vol.: 29 BCM(R)
14 BCM(N)
Phnom Penh Port
Chrui
Changvar
Qp: 37,900 m3/s
Vol.: 280 BCM
Monivong Br.
Q: 5,400 m3/s
Vol.: 43 BCM
Vol.: 32 BCM
Q: 26,000 m3/s
Vol.: 222 BCM
Bassac River
Mekong River
Legend
: Channel Flow
: Flooding Flow
Note:
Qp or Q indicates river flow on 24 August as a flood peak at
Kompong Cham in the 2002 flood season.
Vol. indicates total flow or flooded volume
for 6 July to 20 October 2002, showing in billion cubic meters.
Figure 6. Water Balance between Kompong Cham and Phnom Penh in the 2002 Wet
Season
8
IMPENDING ISSUES FOR SUSTAINABLE MONITORING
The aim of the hydrological network is to provide timely, sufficient and reliable
hydrological data/information to the agencies and activities concerned. The team
recognized the gaps on the hydrological monitoring activities among the riparian
countries in the course of the JICA study. After completion of the formulation work on
the water utilization rules, the next stage of flow management should start with the
full-functioning of the hydrological monitoring systems of the four riparian countries. To
keep the mutual trust built up for a long time as the “Mekong Spirit,” the line agencies
shall make efforts to provide timely, sufficient and reliable data to each member country
and the MRC. In particular the hydrological network in Cambodia shall be improved
further based on guidelines/master plan to be developed, in which clear goals and
stepwise improvement strategy of the network in parallel with institutional strengthening
and capacity building of the line agencies shall be described in detail.
ACKNOWLEDGMENTS
The 2002 wet season was the highlighted period in clarification of flood attenuation
mechanisms of the Cambodian floodplains. In this period both teams of the JICA and the
TSLVP worked closely and collaboratively together. The authors gratefully acknowledge
the cooperative work of TSLVP team and also wish to express our appreciation for the
support with their deep understanding from the JICA Headquarters and its Cambodia
Office and the MRCS. Staffs from the Department of Hydrology and River Works,
Ministry of Water Resources and Meteorology, Cambodia, were extremely supportive in
discharge measurements. This paper could not be written without their painstaking efforts.
REFERENCES
[1] CTI Engineering International Co., Ltd. and Nippon Koei Co., Ltd. (Japan
International Cooperation Agency), “Hydro-Meteorological Monitoring for Water
Quantity Rules in Mekong River Basin”, Final Report, (2004).
[2] H. Fujii, K. Morishita, H. Garsdal and K. Odawara, “Hydrological and Hydraulic
Functions of the Cambodian Floodplains”, Proceedings of the 1st International
Conference on Hydrology and Water Resources in Asian Pacific Region, (2003).
[3] DHI Water & Environment (Mekong River Commission), “Chaktomuk Area Environment, Hydraulics, Morphology-”, Final Report, (2001).
[4] CTI Engineering Co., Ltd. and DHI Water & Environment (Mekong River
Commission), “Consolidation of Hydro-Meteorological Data and Multi-functional
Hydrologic Roles of Tonle Sap and its Vicinities”, Final Report, (2003).
[5] H. Fujii, H. Garsdal , P. B. Ward, M. Ishii, K. Morishita and T. Boivin,
“Hydrological Roles of Cambodian Floodplains of the Mekong River”, Journal of
River Basin Management, Vol. 1, No. 3 (2003).