River Basin Management III 25 Preliminary estimation of the infrastructure change influence on flooding in 1 in the lower Mekong river delta L. T. V. Hoa 1, H. Shigeko 1, N. H. Nhan 2 & T. T. Cong 2 1 Institute of Natural Environmental studies, Graduate school of Frontier Science, University of Tokyo, Japan 2 National Hydro-Meteorological Center, Ministry of Natural Resource and Environment, Vietnam Abstract Besides causing severe damage, flooding in the Mekong river delta (MKRD) brings some benefit for this area by washing away acidic and salty water from the soil and therefore fertilizes it by the deposition of sediment. Thus, solving flood problems in this area is important. After the historical flood of 0, dike, embankment, and road systems in the MKRD in Vietnam were upgraded. This change affected to some extent the propagation of the extremely big flood in 1 that occurred immediately thereafter. This study aims to use HydroGis software with integration by a numerical hydrodynamic model and GIS tools to simulate and analyze the inundation influenced by this infrastructure change. Simulated results have showed obvious differences between the situations before and after systems upgrading. Keywords: flood, inundation, numerical model, Mekong river delta, hydrology. 1 Introduction The Lower MKRD occupies only 11% of the whole MKRD area, but it has to transport all water flow from the entire basin area; severe flooding occurs every year in this area. The delta is flat and inundation lasts several months, damaging crops and property that are insufficiently protected from flooding. Besides resulting in loss of life and property, however, flooding benefits agriculture in
26 River Basin Management III the area by washing away acidic and salty water while fertilizing soil by the deposition of sediment. During past decades, in order to mitigate flood damage for people who live with floods in this area, several plans for flood control structural and nonstructural measures have been studied. The National Science and Technology Center of Vietnam's geographical division [6] conducted research on flooding and proposed some methods for flood control in Dong Thap Muoi. Kazama S. et al. [3] studied the flood of 0 by numerical simulation and conducted a field survey in an area 100 km from east to west and 150 km from north to south around Phnom Penh. Hagiwara T. et al. [4] studied the change of inundation area under various controlled flood situations and irrigation by using numerical simulation in one limited downstream part of the lower Mekong basin. In this study, based on the data and information collected from field surveys, we applied the software HydroGis to simulate flooding in the Vietnamese part of the Mekong delta. Verification of accuracy for the 1 flood provided a series of results, such as water level, discharge, inundation depth, and time. The key objective was to conduct a preliminary analysis of changes in the flood regime influenced by infrastructure changes made between these two years. 2 Study area and data The Mekong River is the longest river in Southeast Asia and one of the largest rivers in the world. With a drainage area of 795,000 km2, it ranks as the 12 th, but in term of runoff of 475,000 million m 3, it ranks as the 8 th. The Great Lake in Cambodia is a natural regulating lake with a catchment area of 85,000 km 2 and a volume of 80 billion m 3 coming from the Mekong River during flood time and released to the Mekong River after floods [7]. The MKRD begins from Kompong Cham in Cambodia; the delta is bounded by the Gulf of Thailand on the West, by the South China Sea on the South, by the East Vaico River on the East. The delta also includes the triangle among north of Phnom Penh, Kompong Cham and Kompong Chnang in Tonle Sap River (see fig. 1). It covers a total area of 4.95 million ha, of which 3.9 million ha or 74% are located in Vietnam and the remaining 26 % in Cambodia. Annual rainfall in the lower Mekong river Basin is 1670 mm on average, varying from 1000 mm in less rainy areas to 4000 mm in heavy rain areas. The rain regime in the MKRD is almost entirely decided by the monsoon regime. The southeast monsoon prevails during the rainy season (May to October) and is the main cause of rains in the MKRD. Rainfall in August and September most strongly affects flooding. The land of the MKRD in Viet Nam is quite flat. A dense water channel network in Viet Nam is formed of both a natural river system and a man-made canal system. The MKRD is affected by two tidal sources, non regular semi diurnal with two rising tides a day from the South China Sea and irregular diurnal with one high tide a day from the Gulf of Thailand. The effect of the former with highest amplitude of fluctuation about 3.5-4 m is stronger than that of the latter with amplitude of 0.8-1 m.
River Basin Management III 27 Figure 1: Mekong river delta. Cross-sectional hydraulic digital data from hydrometric readings and planning data are available from the Vietnamese Ministry of Transportation's inland waterway administration from 1980 to 0. Elevation data on the Vietnamese part of the lower Mekong delta are based on a 1:25000-scale map made in 1996 and on the Cambodia part based on a 1:100000-scale map (date unknown). Hydrological data are available from the Hydro-meteorological Service of Vietnam. Information about changes in dikes and embankment elevations are available from local offices. Before the flood of 1, some constructions and roads were upgraded or renewed, some canals were dredged, and many resident areas were constructed. It is significant to consider about some roads such as the Road 30 along the Mekong river from Cao Lanh - Thanh Binh, road along the canals from Hong Ngu to Tan Hong, from Kien Binh to Moc Hoa that had been upgraded over the flood of 0 s level, Road 80 along the West sea (the Gulf of Thailand) were being upgraded, some canals to drain water flow to the Gulf of Thailand on the West and some canals in Dong Thap Muoi had been dredged. The hydraulic dams Tha La and Tra Su in the Vinh Te canal (boundary between Cambodia and Vietnam) were opened when the water level at Chau Doc exceeded 4.2 m. 3 Methodology Water flows in rivers and floodplains in the Lower MKRD are naturally unstable. Hydraulic models for simulation and assessment of flood are the standard method. In this study, we used for this purpose the hydraulic model HydroGis developed by Nguyen H.N. s group at the National Hydro- Meteorological Center of Vietnam. It integrates a numerical hydrodynamic
28 River Basin Management III model and GIS tools. The hydrodynamic model includes a 1-D model for river networks (with full Saint-Vernant equation), a 2-D model for floodplain cell network (with hydrodynamic equations without inertial term) and infrastructure network (with energy equation) [1]. The GIS tools are used for managing, visualizing, and analyzing input/output data. There are 13245 river cross-sections and 2281 floodplain cells. The flood of 1 was simulated to verify the accuracy of the model and compared to result simulated with the situation in 0. This validated model was then used to analyze the impact of infrastructure change on flood characteristic during the flood of 1. Water level, cm 550 500 450 400 350 Observed 250 Simulated with infrastructure change simulated without infrastructure change Figure 2: Water level at Tan Chau in 1. 500 Water level, cm 450 400 350 250 Observed Simulated with infrastructure change simulated without infrastructure change 150 4 The results Figure 3: Water level at Chau Doc in 1. In this study, all information about infrastructure change that was obtained from local offices, in some previous studies and the residents was used to verify the accuracy for flood 1. The hydrograph of water level at Tan Chau, Chau Doc - stations in the main river and some main stations in Dong Thap Muoi such as Moc Hoa and Hung Thanh are in Figs. 2, 3, 4, 5. The results show that without
River Basin Management III 29 human activities, the simulated water level would have been higher than it was in Dong Thap Muoi. In the main rivers, event though simulated water levels that were done with and without infrastructure change in the main rives are insignificantly different. But the positive difference at Tan Chau and negative at Chau Doc show that in case of having these changes, water flow from Mekong River to Bassac River might be lesser. In Dong Thap Muoi the flood water level simulated with situation, in which there is no infrastructure change after 0, was much higher than observed values during the peak time. The difference between observed value and simulated value without infrastructure change at Moc Hoa is 23 cm, and at Hung Thanh 21 cm. During first stage of flood season, flooding would have been worse in downstream of Dong Thap Muoi if there had been no re-enforcement of the embankment after the flood of 0. Water level, cm 400 350 250 150 100 50 0-50 simulated without infrastructure change Observed Simulated with infrastructure change Figure 4: Water level at Moc Hoa in 1. 400 Water Level, cm 380 360 340 320 280 260 240 220 180 160 140 120 100 80 Simulated without infrastruture change Simulated with infrastruture change 60 Observed Figure 5: Water level at Hung Thanh in 1. In Long Xuyen quadrant, the difference of water level is very little in the raising limb of the hydrograph, see Fig. 6. The construction of embankments for preventing early flooding to protect the Summer-Autumn crop in An Giang province slowed flood propagation in some parts of this area.
30 River Basin Management III Water Level, cm 280 260 240 220 180 160 140 Observeb 120 simulated without infrastructure change 100 simulated with infrastructure change 80 Figure 6: Water level at Tri Ton in 1. Figure 7: Maximum inundation depth in two situations (a) without and (b) with infrastructure change. Comparison of the two simulated maps of inundation in 1 (with and without infrastructure change after the flood of 0) in Fig. 7 show that: (1)
River Basin Management III 31 human activities for infrastructure changes have effectiveness to flood control in Vietnamese part of MKRD, (2) the water flow can drain to the Gulf of Thailand faster, thus inundation depth in Long Xuyen quadrant was lower in case of having infrastructure change (3) the maximum inundation depth that was simulated with infrastructure change was lower than that in case of no change of infrastructure about 0.3 to 0.4 m in the downstream of Dong Thap Muoi. Raising of the embankment along canal from Hong Ngu to Tan Hong and canal from Tan Thanh to Lo Gach after the flood of 0 delayed flood draining to downstream from this area. Strengthening Road 30, which is along the Mekong river before flood season 1 decreased run-off capacity of flood returning to the main (Mekong) river. However, influence on the flooding from this change is not significant, because the total area of the region equipped with flood prevention embankments is small compared to the whole MKRD area. 5 Conclusion The results of modeling using HydroGis software to simulate flooding of 1 in the MKRD have shown that human activities to prevent flood have some impact on flood characteristic in the delta. These changes can effect positively on flood prevention, but cause inundation deeper during flood peak period in downstream of Dong Thap Muoi, in which the flood drain direction is to Vam Co river, so that a combination of embankment re-enforcement and canal dredging to drain water flow to Vam Co river may drain floodwater more efficiently. However, compared to the period 1996-0, the infrastructure change between two these years was not significant, and thus the effects on flooding was little. This study may provide valuable information of flood regime for flood prevention decision and for the mitigation of damage in the lower MKRD. References [1] Nguyen H.N., Tran T.C., Ho N.D., HydroGis software: technical instruction, Ho Chi Minh Water Resource Univ., Oct., 2. [2] Nguyen H.N., Tran T.C., HydroGis Software, Proc. of conference Development and application of new technology in the period 1993-3, HMS of Vietnam, p 50-58, 3. [3] Kazama S., Muto Y., Nakatsuji K., Inoue K., Study on the 0 flood in the lower Mekong by field study and numerical simulation. Proc. of the 13 th IAHR-APD congress. Singapore 6-8 Aug, Volume I, 534-539p, 2. [4] Hagiwara T., Kazama S., Sawamoto M. Relationship between inundation area and irrigation area on flood control in the lower Mekong, Proc. of the 13 th IAHR-APD congress, Singapore 6-8 Aug, Volume II, 590-595p, 2. [5] Kazama S., Sawamoto M., Integrated evaluation of the Mekong river flood using benefit calculation, J. of Hydroscience and Hydraulic Engineering, Vol. 21, No. 2 November, p 85-92, 3.
32 River Basin Management III [6] The Hochiminh geographical division, The scientific basic for flood control methods in Dong Thap Muoi. Vietnamese National project, National Science and Technology Center, November, 1998. [7] Mekong river commission, Flood control planning for development of the Mekong delta (basinwide): volume 2, Water Resource Planning and engineering study, Koica, July, 0. [8] Le T.V.H, Haruyama S., Nguyen H.N, Tran T.C., Simulation and assessment of flood 0 in lower Mekong river using hydraulic model HydroGis. Proc. of the General meeting of the Association of Japanese Geographers, No. 66, p. 75, 4. [9] Le T.V.H, Haruyama S., Le B.H., Bui D.L., Historical flood/inundation and hydrological regime in the Mekong river delta in Vietnam. Environmental change and social environment of the Large River-the Mekong river, p. 38-50, 5. [10] Le T.V.H, Haruyama S., Le B.H., Bui D.L., Historical flood in recent years in Mekong river delta. Proc. of the General meeting of the Association of Japanese Geographers, No 67, p. 87, 5.