Analysis and attribution of trends in water levels in the Vietnamese Mekong Delta

Abstract

In the Vietnamese Mekong Delta (VMD), water levels at some stations have increased. However, the factors that cause this rise in the VMD have not been identified. We considered four factors that may have contributed to the water level rise: (1) increased runoff from upstream, (2) sea-level rise, (3) land subsidence, and (4) decrease in flood mitigation function because of construction of high dykes. We analysed daily maximum and minimum water levels, and mean daily water levels from 24 monitoring stations from 1987 to 2006. Using daily and annual water level differences, we classified the delta into two groups: one is dominated by flows from upstream, while the other is tide dominated. We then tested the trends of annual maximum and minimum water levels using the Mann-Kendall test, and identified the slope of the trend using the method of Sen. The areas of dyke construction were estimated using the Enhanced Vegetation Index (EVI) and Land Surface Water Index (LSWI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) data. Results show (1) river inflow has little impact on rising water levels in the VMD, (2) the influence of high dykes on water level rise could not be quantified in this study, and (3) both maximum and minimum water levels significantly increased in the tide-dominated area. Trend of annual minimum water level can be considered as the sum sea-level rise and land subsidence. Therefore, we attribute 6.05 mm year À1 (80%) to land subsidence and 1.42 mm year À1 (20%) to sea level rise, indicating that inundations have been severe in the VMD, caused primarily by land subsidence. Figure 9. Dyke construction areas and trends of annual maximum water level (mm year À1 ). Red areas highlight where high dykes have been constructed 844 Y. FUJIHARA ET AL.

Assessing impacts of dike construction on the flood dynamics of the Mekong Delta

“…Previous studies report water level increases of +60 to +100 cm concentrated in the upper reaches in the LXQ (Hoa et al, 2007;Duong et al, 2014;Fujihara et al, 2015) and limited increases (4-5 cm) downstream (Duong et al, 2014). Nonetheless, these large increases in water levels and flow velocities in the upper delta point to a heightened risk of bank erosion and catastrophic dike failures there (Hoa et al, 2007).…”

“…Some authors suggest that a greater flood risk downstream might be linked to the prevalence of high dikes on the upper VMD floodplains (Hoa et al, 2007;Duong et al, 2014;Marchand et al, 2014;Fujihara et al, 2015). Using a 1-D hydrodynamic model combined with a quasi2-D approach (following Dung et al, 2011), we quantified the impacts of extensive high dike construction on floodwater levels and flood risk across the VMD.…”

“…Wassmann et al (2004) concluded based on a hydraulic model that the higher water levels in the delta were caused by sea level rise in association with climate change. Fujihara et al (2015) investigated the impacts of upstream runoff, sea level rise, and land subsidence on flood levels. They found that flood depths would be significantly increased in 19 tidedominated areas, and that land subsidence and sea level rise would worsen inundation.…”

“…Duong et al (2014) and Marchand et al (2014) proposed that the higher downstream river water levels observed during the 2011 floods could be due to the construction of higher dikes. Fujihara et al (2015) pointed out the need for more research to understand the impacts of high dike construction. Despite the rapid expansion of high dike systems for triple rice cultivation in the upper Mekong Delta, few modeling studies have as yet assessed the implications of such dikes for floodwater regimes.…”

Sea-Level Rise and Land Subsidence: Impacts on Flood Projections for the Mekong Delta’s Largest City

“…Water surface elevation at Dinh An, which faces the sea, is considered to be relatively independent of river flow, while water levels at Chau Doc, 190 km inland from the river mouth, are predominantly determined by river discharge and less influenced by ocean tides [24].…”