Alluvial architecture of the human-influenced river Rhine, The Netherlands

Abstract

The alluvial architecture of the embanked Rhine distributaries in The Netherlands is largely controlled by human interference with fluvial processes. Embankment, straightening, and stabilization of the rivers have influenced both in-channel and overbank sedimentation. Embanked river floodplain deposits are lithologically and sedimentologically different from natural (not human-influenced) floodplain deposits. After the embankments between 1050 and 1350 AD, the dominant process remained lateral within-channel accretion. However, the morphology of the point bars changed: in the natural situation, the bars had a convex shape (as seen from the channel), whereas in the embanked situation, they are concave (as seen from the channel). The latter is due to the development of bars along the downstream end of a previous formed bar. Deposits associated with lateral within-channel accretion are typically very fine to medium or coarse sand. The infill of residual channels is characterized by the absence of peat. After the normalization of the river channel to a fixed width (1850 AD), concave bank erosion, and development of scroll bars were constricted, and overbank deposition became the dominant embanked floodplain-forming process. Thick layers of sandy and clayey deposits were deposited on the embanked floodplain. Splay-like sand lenses are present in the overbank deposits. In case of dyke failures, a deep scour-hole was formed. Typical deposits associated with dyke-breaches are sandy dyke-breach deposits and scour-hole fills consisting of silty to sandy clay within the embanked floodplain and gyttja within the polder.