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Purpose This study aims to investigate aspects related to the changing trends of the rainfall extremes in the entire Mekong Delta in the period of 32 years (1984-2015) applying rainfall extreme indices. First, the homogeneity tests were applied to assess the quality of observed rainfall data series. The authors, then, investigated three rainfall indices including the number of very heavy rainfall days 20 mm (R20), number of days above 50 mm (R50) and number of days above 100 mm (R100) applying the Mann-Kendall test and Sen’s slope estimate. Design/methodology/approach First, the homogeneity tests were applied to assess the quality of observed rainfall data series. The authors, then, investigated three rainfall indices including the number of very heavy rainfall days 20 mm (R20), number of days above 50 mm (R50) and number of days above 100 mm (R100) applying the Mann-Kendall test and Sen’s slope estimate. Findings The results of R20 pointed out that an insignificant upward tendency was found in the coastal provinces, whereas an insignificant downward tendency was also recorded in the inland provinces. Regarding the number of R50, a similar trend to R20 was recorded with five stations slightly increased and five stations slightly decreased. For the number of R100, the results recorded an absence of significant trends over the entire study area. Approximately 58.5% of stations show a slightly decreasing trend, while 41.5% of the remaining stations recorded a slightly increasing trend. Originality/value For the number of R100, the results recorded an absence of the significant trends over the entire study area. Approximately 58.5% of stations show a slightly decreasing trend, while 41.5% of the remaining stations recorded a slightly increasing trend. Of note is the fact that the number of R100 occurred more frequently in the northern provinces, which means the northern region is facing a high risk of flooding.

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Shear stress is a key parameter that plays an important role in sediment transport mechanisms; therefore, understanding shear stress distribution in rivers, and especially in river bends, is necessary to predict erosion, deposition mechanisms and lateral channel migration. The aim of this study is to analyze the shear stress distribution near a river bed at 90-degree channel bend using a depth-average method based on experimental measurement data. Bed shear stress distribution is calculated using the depth-averaged method based on velocity components data has been collected from a 3D-ADV device (three-dimensional acoustic doppler velocity) at different locations of a meandering channel. Laboratory experiments have been made at the hydraulic laboratory of the RCRFIDF (Research Center for River Flow Impingement and Debris Flow), Gangneung-Wonju National University, South Korea to provide data for simulating the incipient motion of the riverbed materials and then predicting the river morphological changes in the curved rivers. The calculated results show that the maximum value of shear stress distribution near the riverbed in the different cross sections of the surveyed channel occurs in a 70-degree cross section and occurs near the outer bank. From the beginning of a 40-degree curved channel section, the maximum value of the shear stress occurs near the outer bank at the end of the channel.

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