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

摘要

Quantity and quality of sediment deposition in complex floodplains areaffected by many processes that are typically highly spatially and temporallyvariable and hard to quantify exactly. The main processes in this context aresuspended sediment transport dynamics in rivers, floodplain channelinteractions, and internal floodplain processes. In consequence, any pointmeasurement of sedimentation in floodplains contains a high degree ofuncertainty, both stemming from measurement errors and from the lack ofrepresentativeness for a larger area. However, up to now, uncertaintyanalyses have not been performed as part of publications on floodplainsedimentation data. Therefore the present work illustrates a field samplingstrategy aiming at the monitoring of floodplain deposition and spatialvariability on a large scale and at the quantification of uncertaintiesassociated to sediment deposition data. The study was performed in the MekongDelta, being an example for a large and complex floodplain with a high degreeof anthropogenic disturbances. We present a procedure for the quantificationof the uncertainty associated to the data, based on the design of themonitoring campaign, sampling procedures, and floodplain characteristics.Sediment traps were distributed strategically over the floodplain in clustersof three mat traps representing one monitoring point. The uncertaintyoriginating from collection of the traps from still ponding water isquantified by lab experiments. The overall uncertainty of the depositionsamples and the associated nutrient content is quantified in a Monte Carlosimulation and illustrated by uncertainty bounds. For the study area theresults 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. Nocorrelations in the spatial distribution of sedimentation in thefloodplains could be found. This is caused by the highly complexchannel and dike system and the high number of hydraulic structures. Also, nodifferences in deposition between floodplain compartments protected with highand low dikes could be detected. However, it can be shown that within singlefloodplain compartments the spatial deposition variability depends on thedike levels and operation and location of hydraulic structures.