FLOOD DAMAGE ASSESSMENT IN THE MEKONG DELTA, VIETNAM

FLOOD DAMAGE ASSESSMENT IN THE MEKONG DELTA, VIETNAM ASHIM DAS GUPTA Water Engineering and Management, School of Civil Engineering Asian Institute of Technology, P. O. Box 4, Klongluang Pathumthani 12120, Thailand MUKUND SINGH BABEL Water Engineering and Management, School of Civil Engineering Asian Institute of Technology, P. O. Box 4, Klongluang Pathumthani 12120, Thailand PHAM NGOC Water Engineering and Management, School of Civil Engineering Asian Institute of Technology, P. O. Box 4, Klongluang Pathumthani 12120, Thailand Information on depth, duration and spatial extent of inundation, and estimation of damages caused by floods are needed for planning proper flood mitigation measures. The Vietnam River Systems and Plains (VRSAP) model is calibrated with the year 2000 flood data and the model is subsequently used to predict depth, duration and extent of inundation at different return periods. Flood damage assessment is based on the actual flood damage information available for the year 2000 flood and the potential flood damages to different categories of land use activities in the delta obtained through questionnaire survey. INTRODUCTION The Mekong River is one of the largest international rivers in the world and the largest in Southeast Asia. It originates in snow-covered mountains of the Tibetan Plateau, Yunnan Province of China covering a portion of China, Myanmar, Thailand, and Vietnam and almost the entire territories of Laos and Cambodia along its stretch of 4,200 km. The total drainage area of the Mekong basin is 795,000 km 2, of which 24% lies in the upper Mekong River Basin comprising China and Myanmar and the remaining 76% is in the four riparian countries in the Lower Mekong River Basin: Laos, Thailand, Cambodia and Vietnam. The Vietnamese part of the Lower Mekong River Basin consists of the Mekong River Delta in Vietnam (the Delta), the Upper Sesan and Upper Srepok tributaries in the Central Highlands of Vietnam and a small area in the Nam Rom tributary in the northwest of the country. The delta is a flat, low-lying area of 3.9 million ha and its rich natural resources are of vital importance to the country. The mean annual rainfall in the

delta ranges from approximately 2,400 mm in the western part to 1,600 mm in the eastern and 1,300 mm in the central with an average of 1,500 mm/yr across the delta and 90% of this occur in the wet season lasting from May through October. The average discharge of the Mekong River is 25,000 m 3 /s and 6,000 m 3 /s during the wet and dry season respectively. Floods are a recurring event in the Lower Mekong Basin resulting in loss of life and property, causing damage to agriculture and rural infrastructure, and disrupting social and economic activities. On the other hand, flooding of the mainstream and tributaries of the Mekong River is an important source for the wealth of biodiversity, abundance of fish and soil fertility in the basin. The flood management and mitigation has become a priority issue at the national and regional levels, particularly in the aftermath of the disastrous floods of 2000 (MRC, 2003). In line with this, the objective of the present study is to derive damage-frequency relationships for various types of land use based on the estimated flood damage caused by floods of different return period. These are needed for rapid damage estimation in planning for relief works. Flood damages for different types of land use are estimated based on the predicted floods of different return period and the field surveyed data of the year 2000 flood. The Vietnam River Systems and Plains (VRSAP) model (Dong, 2000) is calibrated with the year 2000 flood data and the model is subsequently used to predict flood information of spatial extent, depth and duration of inundation at different return periods. FLOODING IN MEKONG DELTA Flooding in the Mekong Delta is mainly by high discharge in the Mekong River due to heavy rainfall over the upper catchment caused by typhoon or tropical low pressure, heavy rains in the delta itself and the tidal effects from the South China Sea further aggravate the situation. Flooding frequently inundates the entire floodplain of the delta to an aerial extent of 1.4 to 1.9 million ha, normally for duration of 2 to 6 months. The level of Mekong starts to rise in May and reaches its peak in mid-august or early September in the upper reaches, and in mid-september or early October in the delta region. For the Mekong Delta, the flood season is divided into 3 periods. The first period is from July to August with the water level in the main river rising at a high rate, flooding the area between Mekong and Bassac Rivers. The second period is considered when water level at Tan Chau exceeds 4.0 m and at Chau Doc exceeds 3.8m. During this period, flooding is from the main river channel (about 80-85%) and from the surface runoff through the Vietnam-Cambodia border (about 15%-20%). The third period is from October to December when the overflow through the border is reduced and the flood level recedes by the month of December (Thuc, 2000). The recorded flood water levels at Tan Chao station for the year 1961, 1978, 1996 and 2000 are provided in Figure 1. The observed flood peak levels are 5.12, 4.72, 4.78, 4.86 and 5.06 m, respectively in year 1961 (October), 1978 (August), 1978 (October), 1996 (October) and 2000 (October). The Hydro-meteorological Department of Vietnam

defined the extent of flood based on the flood water level at Tan Chau station as small flood when the flood peak is less than 4.0 m, moderate flood when the flood peak is between 4.0 and 4.5 m, and big flood when the flood peak is more than 4.5 m. The flood event in 2000 had two peaks, the first one in early August with the water level reaching over 4.0 m followed by a second peak of 5.06 m in October, very close to the highest peak observed in 1961. This prolonged flood occurrence resulted in severe economic and social losses in the delta. ANALYSIS PROCEDURE The flood flow analysis in the Mekong delta is done with the help of VRSAP model. The VRSAP model has been extensively used by the researchers of the Institute for Water Resources Planning in a number of local and nation-wide water control studies in Vietnam and the Mekong Delta. The model is based on the numerical solution of the one-dimensional Saint-Venant equations using an implicit finite difference scheme. For model application, the river and canal networks are discretized by segments connected by nodes, each segment being specified with the cross-sectional area and the roughness of the representative river reach and floodplains are represented by storage cells of specified area connected to specific nodes. The version of the model calibrated with the flood data of 1996 is used for this study. The analysis is done for the entire Mekong River system from Kratie to the sea including the Great Lake, Tonle Sap and the Bassac River system with main tributaries, primary and secondary canal systems. The model is further calibrated to simulate the flood event of 2000. The calibrated model is then used to predict the expected flooding conditions due to floods of different return periods. Recorded peak discharges at Kratie station for the period from 1935 to 2000 have been used for flood frequency analysis to estimate the peak discharges for different return periods. Based on the simulated water levels along the main rivers and canals and the ground surface elevations, inundation maps for flood events of different return periods are prepared. Flooding durations are estimated based on the flood warning system adopted in the Mekong Delta by referring to the water level at Tan Chau station. This information on flood inundations and durations of flooding is used for assessing flood damage. Flood damage assessment is as well based on the actual flood damage information available for the year 2000 flood and the potential flood damages to different categories of land use activities in the delta obtained through questionnaire survey. Land use activities are classified into four groups namely residential, commercial, agriculture and infrastructure. Direct damages to all land use activities in urban and rural areas are considered when the floodwater level at Tan Chau reaches 4.5 and 3.5 m, respectively as per the alarm levels of the flood warning system. As detail land use maps were not available, classification of land use activities in the Mekong delta is based on the country-level percentage of use by different categories. In addition to direct damages, indirect damages such as economic losses due to interruption of economic activities, intangible damages such as anxiety, inconvenience, ill health and loss of

cultural significance are considered. Finally, with the estimated damages for different types of land use for floods of different return periods, damage-frequency relationships for different land uses are developed. RESULTS AND DISCUSSION The 1996 version of VRSUP model is further calibrated through several simulation runs by adjusting the roughness coefficients and area of flood storage cells in river reaches till the computed water level hydrographs at five selected stations along the river system agree reasonably with the observed hydrographs for the year 2000. For model runs, the upstream boundary conditions are observed streamflows at six gauging stations from July to December 2000 and the downstream boundary conditions are the observed tidal level at four gauging stations in the South China Sea and the Gulf of Thailand during flood season of 2000. The maximum rainfall of ten days form July 1 to December 31, 2000 at 17 rainfall stations in Vietnam and 3 rainfall stations in Cambodia are considered as local rainfall input to the model. The computed water levels compared with the observed water levels of flood year 2000 at two selected stations, Chau Doc and Cao Lanh are shown in Figure 2(a) and 2(b), respectively. For the two upstream stations, Chau Doc and Tan Chau, the magnitude, shape and phase of the computed and observed flood hydrographs match quite satisfactorily (refer to Figure 2(a) for Chau Doc), while for the three gauging stations in the lower part of the model domain, the computed maximum water levels match very well with the peak water levels observed, as seen from the comparison indicated in Figure 2(b) for one of the stations. It can be observed that even though the temporal trend of water level change is reproduced, the magnitude of the computed water levels during the rising part of the flood hydrograph is much higher than the observed values. However, the calibrated model is acceptable for predicting the flooding conditions in the delta for flood discharges of different return periods. The correlation coefficient and the root mean square error for the comparison at Chau Doc and Cao Lanh stations are 0.99 and 0.17 m, and 0.89 and 0.24 m, respectively. Recorded peak discharges at Kratie station for the period from 1935 to 2000 have been used for flood frequency analysis to estimate the flood magnitude for different return periods. Analysis with different frequency distribution function indicates that the Pearson type 3 distribution fits well with the recorded peak discharge data at Kratie (Ngoc, 2003). The streamflow hydrographs of different return periods are then obtained based on the observed flood hydrograph at Kratie for the flood year 2000 and the discharge ratio factor defined on the basis of linear system theory. The discharge factor is defined as the ratio of the peak discharge of design flood obtained from the flood frequency analysis to the peak discharge of actual flood observed at Kratie in 2000. The streamflow hydrographs of different return period at Kratie deduced from the observed flood hydrograph of 2000 are shown in Figure 3. The total submerged area ranges from 1.5 million ha to 1.95 million ha for the range of return period of 2 to 1000 year. The

areal extent of flooding with time is reproduced based on the limited satellite observations during flooding season of year 2000. The predicted flooding extent of year 2000 (1.57 million ha) agrees well with the actual submerged area based on satellite image (1.52 million ha). Based of the field survey, households in urban areas are grouped in three classes as per their face values, ranging from US $ 1,333.00 to more than US $ 6,670.00. On average, the monitory equivalence of the year 2000 flood damage is 14.3 and 12.3% of the face value for Class 1 and 2 households, while it is negligible in case of Class 3. The rural population spends US $ 13.0 per household to cope with the flood and another US $ 33.0 in post-flood measures. However, the loss of asset has been found to be negligible in rural areas while in urban areas the loss of indoor assets is estimated as 21% of the asset value. In addition to direct damage, the indirect cost of flood damage is attributed to additional cost for living and loss of income due to flood which is estimated at US $ 5.0 per household per flood season for rural household, while for urban area, it is 27% more in the flood season than that in the dry season. Economic loss due to intangible damage like health effect is found to be of appreciable amount in the urban residential areas, which is estimated at US $ 16.5 per month per household. Commercial damages are economic losses due to flood damage to industrial establishment and commercial households. As per statistics of year 2000, there are 84,392 industrial establishments in the delta. Responses to the questionnaire survey indicate that the flood damage to the commercial household is negligible; however, the industrial establishments suffer flood damage when the inundation depth is higher than 1.5 m. Questionnaire survey indicates that the damage to goods in commercial households in economic term is 18% of the total value of goods, while damage to products and goods stored in the industrial establishments is 10% of their economic value. The average cost for coping with flood in case of commercial household is estimated at US $ 490.0 per enterprise and for industrial establishment, it comes out to be 3.4% of their total asset value. In addition to direct damages, the industrial establishments suffer loss because of reduction in output and the survey results indicate that, on average, economic value of output loss for an establishment is about 53% of the average annual value of the output. Agricultural production such as paddy, fruits and fisheries are subjected to flood damage. Of the three paddy crops grown, viz. winter-spring (W-S) paddy, summerautumn (S-A) paddy and autumn-winter (A-W) paddy, both S-A and A-W paddy are prone to flood damage as their growing season falls under the flood season in the delta. S-A paddy is found to be cultivated to a great extent in the flood prone areas, which covered about 48% of the total area of the delta in 2000. The average yield of the S-A paddy is 3.72 t/ha and the average market value of rice is US $ 100.0 per ton. On the basis of the inundation depth, duration and time of flood, the reduction in crop yield is estimated depending on crop growth stage. In case of fruit orchard, the flood damage area of fruit cultivation is estimated as 10 to 20% of the total agricultural submerged area, which is also a standard practice adopted by the local authorities. Average loss of

submerged orange garden and mixed fruit garden the two types of cultivation practiced in the fruit cultivation in the area- is estimated at US $ 87.0 and 17.4 per ha respectively. In case of fishery, the financial loss due to flooding is calculated based on average fish production (2.63 t/ha), average market price (US $ 1000.0 per ton), defined percentage of the fish culture in total area, and the total submerged area under flooding condition of different return period. Direct damage to infrastructures such as road networks, schools, health establishments and offices of other utilities are estimated based on the statistics of submerged national and provincial roads, classrooms, health establishments and offices corresponding to annual flood levels observed at Tan Chau. The unit loss factors used are the average cost of road construction taken as US$ 66,667.0 per km, the average cost of one submerged health establishment as US$ 2,800.0, and the average cost of one classroom/office room taken as US$ 67.0. The computed average damage to different types of land use caused by floods of different return periods are presented in Table 1. For flood with return period of 5 year and higher, economic loss from flood damage is mainly from the commercial sector, contributing to about 97% of the total damage. Based on these results, damagefrequency function of each type of land use is developed, as shown in Figure 4. The damage-frequency functions for residential and commercial sectors are of linear form, while the functions for agricultural and infrastructure sectors are of power form. These functions are relationship between probability of flood at Kratie and economic damage. Even though the estimates are based on limited field survey data, the average value of distribution of different type of land use in the delta and the average economic loss factor, these functions can be used for rapid flood damage assessment, which can serve as guide for preparedness and relief work. Table 1. Estimated damage of different land use Probability of design flood (%) Residential Commercial Agriculture Infrastructure Total damages 50 24 0 272 388 684 20 218 48,202 299 538 49,257 10 232 52,830 322 613 53,995 5 243 54,262 341 678 55,522 2 260 61,450 353 749 62,810 1 280 75,235 403 795 76,712 0.2 295 76,072 413 939 77,719 0.1 305 80,610 432 975 82,321 CONCLUSION The VRSAP model, calibrated with the observed flood data of 2000, is used to predict depth, duration and extent of inundation for flood events of different return periods.

Recorded peak discharges at Kratie station are used for flood frequency analysis to estimate the flood magnitude for different return periods. Inundation maps for various flood events are then prepared based on the simulated water levels along the main rivers and canals and the ground surface elevations. Flooding durations are estimated based on the flood warning system adopted in the Mekong Delta by referring to the water level at Tan Chau station. With this information on flood inundation and duration for floods of different return periods, flood damage assessment is done using the actual flood damage data available for flood of 2000 and the estimates of the potential flood damages to different categories of land use activities in the delta obtained through questionnaire survey. Based on these results, damage-frequency functions are developed; functions are of linear form for residential and commercial sectors and are of power form for the agricultural and infrastructure sectors. These functions can be used for rapid flood damage assessment, which can serve as guide for preparedness and relief work. Figure 1. Water levels at Tan Chau station for selected flood events Figure 2 (a). Computed and observed water level hydrograph at Chau Doc Figure 2(b). Computed and observed water level hydrograph at Cao Lanh

Discharge m 3 /s Time (Date/month) Figure 3. Simulated streamflow hydrographs at Kratie for different return periods Figure 4(a). Damage frequency function for residential sector Figure 4(b). Damage frequency function for commercial sector Figure 4(c). Damage frequency function for agricultural sector Figure 4(d). Damage frequency function for infrastructure sector

REFERENCES [1] Dong, T. D., VRSAP model and its application, Proc. Hydrological and Environmental Modelling in the Mekong Basin, Mekong River Commission, Phanom Penh, Cambodia, (2000), pp 236-245. [2] Mekong River Commission (MRC), State of the Basin Report 2003, Mekong River Commission, Phanom Penh, Cambodia, (2003). [3] Ngoc, P., Flood Damage Assessment in the Mekong Delta, Vietnam, M.Eng Thesis, Asian Institute of Technology, Pathumthani, Thailand, (2003). [4] Thuc, T., Overview of Research Activities in Vietnam on Lower Mekong Basin, Proc. APFRIEND Workshop on Mekong Basin Studies, Asian Institute of Technology, Bangkok, Thailand, (2000), pp 139-151.