Holocene Rhine delta evolution: resolving larger flooding events amidst gradual trends

Abstract

Flooding, overbank deposition and channel network change in the lower Rhine has traditionally been studied from multiple perspectives. Mapping flood deposits has typically traced overbanks from proximal to distal settings, distinguishing packages formed over longer periods of flood sedimentation rather than resolving individual events. Their distribution resembles avulsions that successively changed the network. In understanding delta evolution and in morphodynamic behaviour of deltaic rivers, a major challenge was to be able to reconstruct and numerically simulate channel initiation and abandonment, through stages of bifurcation functioning. Whether avulsion principally relates to major flood events or whether it should be understood as a process requiring the passage of multiple flood peaks to succeed or fail is a critical question, that was difficult to attack with traditionally collected data.

Our current research is on resolving marked Holocene flooding events. We have updated our sea level history (Hijma & Cohen, 2010) and the dating of channel belts and the associated avulsion history (Stouthamer et al., 2011), in interaction with the Netherlands’ professional archaeological user community. We have quantified the sediment budget received by the delta over the Holocene (Erkens, 2009) and progressive changes in grain size therein (Erkens et al., 2012). In interaction, we are numerically modelling and sedimentologically reconstructing the evolution of sediment and water division of bifurcations for Late Holocene cases (Kleinhans et al., 2011; Toonen et al., 2012). We are making an inventory of the large floods as recorded in sedimentary archives such as residual channels (Minderhoud et al., 2011; Toonen et al., 2012) to improve flood frequency-magnitude analysis.

The resolved events are superimposed on gross trends known from earlier studies, and clarify control interplays in the evolution of the delta, in the transgressive and in the high stand stage. Furthermore, the event-minded approach is benefitting the integration of ‘process-based’ and ‘mapping-based’ lines of research in the delta, especially where the pacing of channel abandonment and the maturing of new channels is considered.