About the Analysis of Sedimentation Dynamics in an Estuarine Waterway and its Implications on Sediment Management Concepts

Maintenance dredging is due to sediment deposits on the river bed. Important to notice is the existing time lag between dredging and the time of sedimentation, which depends on various operational constraints, e.g. scheduling of the sounding campaigns or availability of dredging equipment. Therefore, numbers from dredging statistics do not represent actual sedimentation events. Looking at annual statistics, this time lag might play a rather subordinate role.

However, the analysis of short-term dynamics requires additional data like those obtained from multi-beam echo sounder (MBES) systems. In waterways and harbors MBES data is widely available and continuously recorded to detect missing depths. But MBES data also allow an inspection of morphology characteristics and sedimentation dynamics. The latter one requires difference models of the river bed which can be obtained from two consecutive MBES data sets.

The Elbe estuary, connecting the Port of Hamburg (third largest in Europe) with the North Sea, was chosen as case study to analyze short-term sedimentation dynamics over a multi-year period from 2013 to 2018. Along the waterway and within the port area most of the fine-silty dredged material, which amounts to several million tons per year, originate from a few hotspots for dredging covering a total area of about 20 km². MBES data is available in a high temporal resolution of up to two-weekly campaigns; the spatial resolution is 1*1 m².

This paper is presenting the method developed to automatically process a series of MBES data sets with respect to sedimentation dynamics. A particular challenge was to take into account the frequent dredging activities by using available information on dredging volumes and location of dredging in order to correct the sedimentation rates, which have been previously calculated from difference models.

The results show how sedimentation patterns and rates evolved over time with a particular focus on periods with intensified dynamics. For some hotspots it was found that the sedimentation dynamics is strongly driven by natural forces like storm surges which can remobilize large amounts of sediments on adjacent wadden areas. In other hotspots there are clear indications for an anthropogenic influence on the actual sedimentation rates, which could be linked to disposal activities or an intensification of sediment cycles.

The knowledge about estuarine-wide sedimentation rates is therefore an elementary component for the extension of the system understanding and the further optimization of sediment management and maintenance dredging concepts.