Sedimentation monitoring including uncertainty analysis in complex floodplains: a case study in the Mekong Delta

Abstract

Abstract. Quantity and quality of sediment deposition in complex floodplains are affected by many processes that are typically highly spatially and temporally variable and hard to quantify exactly. The main processes in this context are suspended sediment transport dynamics in rivers, floodplain channel interactions, and internal floodplain processes. In consequence, any point measurement of sedimentation in floodplains contains a high degree of uncertainty, both stemming from measurement errors and from the lack of representativeness for a larger area. However, up to now, uncertainty analyses have not been performed as part of publications on floodplain sedimentation data. Therefore the present work illustrates a field sampling strategy aiming at the monitoring of floodplain deposition and spatial variability on a large scale and at the quantification of uncertainties associated to sediment deposition data. The study was performed in the Mekong Delta, being an example for a large and complex floodplain with a high degree of anthropogenic disturbances. We present a procedure for the quantification of the uncertainty associated to the data, based on the design of the monitoring campaign, sampling procedures, and floodplain characteristics. Sediment traps were distributed strategically over the floodplain in clusters of three mat traps representing one monitoring point. The uncertainty originating from collection of the traps from still ponding water is quantified by lab experiments. The overall uncertainty of the deposition samples and the associated nutrient content is quantified in a Monte Carlo simulation and illustrated by uncertainty bounds. For the study area the results show a very high variability of the annual floodplain deposition (2.2–60 kg m−2) with uncertainty bounds ranging from −61 to +129% relative to overall mean deposition of 11.4 kg m−2. No correlations in the spatial distribution of sedimentation in the floodplains could be found. This is caused by the highly complex channel and dike system and the high number of hydraulic structures. Also, no differences in deposition between floodplain compartments protected with high and low dikes could be detected. However, it can be shown that within single floodplain compartments the spatial deposition variability depends on the dike levels and operation and location of hydraulic structures.