Paper Template for HIC2004

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. 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 3 border. The issue regarding the sustainability of monitoring will however remain unsolved due to budgetary constraint.

Stung Treng

Kompong Luong Kratie

Kompong Cham Prek Kdam

Phnom Penh Port 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 4 major channel system of the Mekong mainstream and tributaries from Kratie down to the Vietnam border was constructed in this project to provide the boundary conditions for the precise two-dimensional hydraulic model of the Chak Tomuk area.

Discharge (m3/s) Flow Hydrograph of Mekong System in the 2002 Wet Season 55,000 Kratie 50,000 Kompong Cham Chrui Changvar Koh Norea 45,000 Neak Luong Phnom Penh Bassac 40,000

35,000

30,000

25,000

20,000

15,000

10,000

0 l l l l t t t t v v v v v c c c c n n n n n g g g g g p p p p c c c u u u u c o o o o o e e e e u u u u u u u u e u u e e e J J J J O O O O J J J - - J J - - N N N N A A A A S S N D D A S S D D ------6 3 0 7 5 2 9 6 1 8 5 2 9 2 9 6 3 0 3 0 7 4 1 7 4 1 8 7 4 1 8 1 2 2 1 1 2 1 2 2 1 2 3 1 1 2 3 1 2 2 1 2 2 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

4,000 Normal Flow 2,000

-2,000 Reverse Flow -4,000 Q_Estimated

-6,000 Q_Observed

-8,000

-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

25000 Observed and simulated discharge at Kompong Cham, 2002 60000 Koh Norea ]

s 20000 50000 / 3 ] s / m [ 3 40000

15000 [

g r e a

g 30000 r h

Monivong bridge a c h s c i 10000 20000 s i D D 10000 5000 0 0 0 1 2 2 0 1 1 2 0 0 1 2 3 0 1 2 2 0 1 1 2 0 0 1 2 3 0 1 2 2 1 8 5 2 9 5 2 9 6 2 9 6 3 0 7 4 1 8 4 1 8 5 2 9 6 3 0 6 3 0 7 ------J J A A S S S O O O N N D D D D J J J J J J A A S S O N N D J u u a a a u u u a u u u u e e e e e c c c o o e e e e 0 c o o e l l n n n l l l n g g p p p g g p p t t t v v c c c c t v v c

-5000 Simulated Observed Phnom Penh Port -10000 0 0 1 2 2 0 1 1 2 0 0 1 2 3 0 1 2 2 0 1 1 2 0 0 1 2 3 0 1 2 2 1 8 5 2 9 5 2 9 6 2 9 6 3 0 7 4 1 8 4 1 8 5 2 9 6 3 0 6 3 0 7 ------J J J J J A A A A S S S S S O O O O N N N N D D D D D J J J J u u u u u a a a a u u e e u u e e e c c o o e e e c c o o e e l l l l l n n n n g g p p g g p p p t t v v c c c t t v v c c Observed and simulated discharge at Chaktomouk Junction, 2002 40000 Sim ulated Chrui Changvar Obs erved Chrui Changvar Chrui Changvar Sim ulated Koh Norea Obs erved Koh Norea 35000 Sim ulated Phnom Penh Port Obs erved Phnom Penh Port Sim ulated Monivong Bridge Obs erved Monivong Bridge 30000

25000 Koh Norea 20000 Figure 3. Model Calibration Results in the 2002 Wet Season 15000 Monivong bridge FLOOD ATTENUATION MECHANISMS OF THE10000 CAMBODIAN 5000 FLOODPLAINS 0

-5000 The team conducted longitudinal discharge measurementsPhnom along Penh Port the major river courses, -10000 0 0 1 2 2 0 1 1 2 0 0 1 2 3 0 1 2 2 0 1 1 2 0 0 1 2 3 0 1 2 2 1 8 5 2 9 5 2 9 6 2 9 6 3 0 7 4 1 8 4 1 8 5 2 9 6 3 0 6 3 0 7 ------J J J J A A S S O O N N D D J J Mekong, Tonle Sap and Bassac, every other week from J JulyA A 2002S S S O untilO N JanuaryN D D D 2003.J J u u u u a a u a a u u u u e e e e e c c o o e e c c o o e e e l l l l n n l n n g g p p g g p p p t t v v c c The results were combined with the discharge measurementst andt v v waterc c c 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 50,000 2002/8/15 2002/8/29 40,000 ) s /

Chrui 2002/9/26 3 m

40,000 (

Changvar 2002/10/10 r 30,000 a v

2002/11/21 g n a 30,000 2002/12/19 h C

i 20,000 u r h C

20,000 o Kom pong e 10,000 g Cham r a h c s 10,000 i Neak D 0 Luong 0 10,000 20,000 30,000 40,000 50,000 0 Discharge of Kompong Cham (m 3/s) 250 300 350 400 450 500 Distance from the Mouth (km)

Figure 4. Longitudinal Flow Changes Figure 5. Relationship between along the Mekong Mainstream 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.: Mekong River 12 BCM Vol.: 26 BCM Kompong Vol.: 14 BCM Cham

Tonle Sap to Tonle Sap Qp: 49,400 m3/s Vol.: 338 BCM Q: 6,500 m3/s Vol.: 29 BCM(R) 14 BCM(N) Chrui Phnom Penh Port Changvar Qp: 37,900 m3/s Vol.: 280 BCM Monivong Br. Vol.: 32 BCM

Q: 26,000 m3/s Vol.: 222 BCM Q: 5,400 m3/s Legend Vol.: 43 BCM Bassac River Mekong River : 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). 9

[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).