Simulation, quantification and comparison of in-channel and floodplain sediment processes in a lowland area - A case study of the Upper Stör catchment in northern Germany

Erosion and sedimentation processes within the channel and on the floodplain are indicative of catchment hydrodynamic procedures, as well as the associated nutrient and contaminant transportation. In this paper, we linked the SWAT model with the hydraulic HEC-RAS model to set up a sediment model for 10 river sections in the Upper Stör catchment and simulated the sediment processes from 2001 to 2010. Based on the HEC-RAS output, quantification and comparison of channelized and floodplain sediment processes were conducted. The results indicate that (1) with an average sedimentation depth of 2.85 cm, the deposition process dominated the Upper Stör catchment at the decadal time scale, and the land use/cover condition resulted in differences in sedimentation amounts between different sub-catchments, and (2) the mean deposition rates were 1.75 g/m2/d in the channel and 1.69 g/m2/d on the floodplain, and the floodplain deposition accounted for only 1% of the total sedimentation amount. This observation was mainly caused by the stream power distribution in the channel and on the floodplain. (3) The granularity of the channelized sedimentation was determined by the altitude of the river section, while the granularity of the floodplain sedimentation was positively correlated with the stream power of the flood. The D50s of the channelized and floodplain sediments were 0.92 mm and 0.16 mm, respectively, while the D90 of the sediment was 4.2 mm in the channel and 0.32 mm on the floodplain. Despite the higher uncertainty of the sediment simulation, the results yielded by the combination of the HEC-RAS and SWAT models are comparable to the traditional radioactive dating, sediment trapping or combined model methods used in similar and nearby catchments.