The sedimentary dynamics in natural and human-influenced delta channel belts

Abstract

This study investigates the increased anthropogenic influence on the within-channel belt sedimentary dynamics in the Rhine delta. To make this investigation, the sedimentary dynamics within the life-cycle of a single channel belt were reconstructed for three key periods of increasing human impact, which are the pristine delta, the period after embankment and the recent situation after channelization. Reconstruction of sedimentary dynamics requires reliable dating of sediments. Hence, as inherent objective, an OSL-dating protocol was developed to improve dating of young fluvial deposits. This protocol was based on the single-aliquot regenerative dose procedure, in which parameters were chosen to isolate the fast OSL component, eliminate an ultrafast OSL component, avoid thermal transfer, and select the lower part of the equivalent dose distribution. Based on internal and external controls, it was concluded that there was no evidence for large systematic offsets in the OSL ages. In the remainder of the thesis, this technique and other chronological methods were combined with lithogenetic data to estimate volumetric erosion and deposition rates from each period. Hence, the results were integrated with ‘building blocks’ from previous studies, providing quantitative insight in the natural delta dynamics, and how these have been affected the increased human influence. In a natural channel, bank retreat rates were initially low due to the high resistance to erosion of the floodbasin material. When the channel incised into the sandy Pleistocene subsurface, these rates rapidly increased, reaching constant values when the channel-belt-reworking stage starts. Trapping of sandy channel sediments occurred in the channel-belt-building stage – when floodbasin material was eroded and replaced by sand – but varied greatly among different channels, depending on length, duration of activity, and location within the delta. It was most efficient in the central delta, where the presence of the sandy Pleistocene sediments in the banks enabled sufficiently high bank retreat rates, and where a thick-enough cover of floodbasin deposits existed to become replaced by sand. Trapping of overbank deposits has been less variable because they were largely deposited far away from the erosive action of the channel. After embankment of the rivers, no new channels were formed anymore. The existing channels remained in their reworking stage, in which erosion and deposition rates were relatively high, but where net trapping of channel sediment was nil. Overbank deposition became limited to a relatively small embanked floodplain, where overbank sediments became part of the reworking cycle. After normalization reworking stopped, and sand is now only deposited as overbank material on a limited area near the channel edge during major flood events. Overbank deposition continued, however, along with rising floodplain elevation, overbank deposition rates have decreased. Insight into the changing sedimentary dynamics is essential for adequate river and floodplain management in the Rhine delta, but also in other deltas in the world. It helps understanding to what extent earlier human interference and engineering measures have changed the natural sedimentary dynamics. Moreover, this insight is of crucial importance to provide references and benchmarks, and to predict direct and long-term morphodynamic effects of the measures.