Cyclone-Driven Sediment Loads on the Lower Mekong River: Does the Monsoonal Flood Pulse Matter?

Abstract:

Understanding the relationships between environmental change and rates of sediment transport in the Asian mega-rivers that dominate the flux of terrestrial sediment to the oceans remains a major challenge. Many of these large river systems are strongly affected by the monsoon, with most displaying a clear monsoonal flood pulse, but in many cases additional variability is induced by the effects of tropical storms. In this paper we quantify the extent to which flood events induced by the monsoon versus flood events induced by tropical storms are effective in driving sediment flux through one of the world’s largest rivers, the Lower Mekong. We first derive robust sediment rating curves at study sites in Laos, Thailand and Cambodia in order to predict suspended flux as a function of flow discharge. These derived sediment rating curves are then used to evaluate mean annual sediment flux at each site for the period 1981-2005. Specifically, we employ a hydrological model to estimate daily flow discharge values for scenarios with and without the effects of tropical storms. This is achieved by forcing the hydrological model with daily precipitation values that account for (i) rainfall totals that account for both monsoonal precipitation and precipitation associated with cyclones, and (ii) precipitation anomalies associated only with the passage of observed tropical storms. The difference in cumulative sediment transport estimated by combining the two flow discharge scenarios with the constructed sediment rating curves allows the contribution of tropical storms to the Lower Mekong’s suspended sediment transport regime to be isolated. We show that in specific areas (Cambodia) of the Lower Mekong basin, the suspended sediment flux is in fact dominantly forced by tropical cyclones. Our findings have significant implications for predicting future sediment loads to the Mekong’s delta, one of the world’s most vulnerable delta systems, and specifically for contextualizing the relative significance of future changes in monsoon intensity versus shifts in the location of cyclone tracks.