From apex to shoreline: fluvio-deltaic architecture for the Holocene Rhine–Meuse delta, the Netherlands

Despite extensive research on alluvial architecture, there is still a pressing need for data from modern fluvio-deltaic environments. Previous research in the fluvial-dominated proximal and central Rhine–Meuse delta (the Netherlands) has yielded clear spatial trends in alluvial architecture. In this paper, we include the backwater length to establish architectural trends from apex to shoreline. Channel-belt sand body width / thickness ratios and interconnectedness were determined, and the proportions of fluvial channel-belt deposits, fluvial overbank deposits, organics and intertidal deposits were calculated for the complete fluvio-deltaic wedge based on high-resolution geological cross sections. It was found that the average width / thickness ratio of channel-belt sand bodies in the proximal delta is 5 times higher than in the distal delta. Other down-valley trends include an 80 % decrease in the channel deposit proportion (CDP) and a near-constant proportion of overbank deposits. Additionally, interconnectedness in the proximal delta is 3 times higher than in the distal delta. Based on the Rhine–Meuse dataset, we propose a linear empirical function to model the spatial variability of CDP. It is argued that this relationship is driven by four key factors: channel lateral-migration rate, channel-belt longevity, creation of accommodation space and inherited floodplain width. Additionally, it is established that the sensitivity of CDP to changes in the ratio between channel-belt sand body width and floodplain width (normalized channel-belt sand body width) varies spatially and is greatest in the central and distal delta. Furthermore, the proportion of fluvial channel-belt sands is generally an appropriate proxy for the total sand content of fluvio-deltaic successions, although its suitability as a total sand indicator rapidly fades in the distal delta. Characteristics of the backwater zone of the Rhine–Meuse delta are (1) sand body width / thickness ratios that are lower as a consequence of channel narrowing (not deepening), (2) a rapid increase and then a drop in the organic proportion, (3) an increase in the total sand proportion towards the shoreline, and (4) a drop in the connectedness ratio. For this paper, unique high-resolution quantitative data and spatial trends of the alluvial architecture are presented for an entire delta, providing data that can be used to further improve existing fluvial stratigraphy models.

Linking levee-building processes with channel avulsion: Geomorphic analysis for assessing avulsion frequency and style

A natural levee is a typical wedge-shaped deposit adjacent to a river channel. Given its location and distinctive features, the levee can serve as a key to revealing depositional processes of the coupled channel to floodplain system preserved in the rock record. Levee-floodplain topographic evolution is also closely linked to river avulsion processes which can spell a catastrophic flood. Nonetheless, the levee geometry and its aggradation pattern on the floodplain have not been fully incorporated in the study of avulsion. Here, we present a levee-building model using an advection settling of suspended sediment to reproduce the evolution of a fluvial levee over floods and to examine the effects of boundary conditions on levee geometry and grain-size trend. We further investigate river avulsion frequencies and styles (i.e., local vs. regional avulsion) associated with the grain-size distribution of supplied sediment and the overflow velocity into the floodplain, which control the levee geometry and especially the aggradation rate at the levee crest. In the modelling results, the levee develops 1) a concave-up profile, 2) exponentially decreasing grain size in the deposit, and 3) a relatively steeper shape for coarser sediment supply. The subsequent scaling analysis supports that the input grain size and levee profile slope are positively correlated with the avulsion frequency, whereas the overflow velocity is inversely proportional to the avulsion frequency. In connection with the avulsion styles and levee geometry, we suggest that steeper levee slopes tend to promote more local avulsions protecting abandoned channels from topographic healing, but gentler slopes of the levee are likely to lead to regional avulsions as abandoned channels with gentler levees are more vulnerable to the removal of topographic memory. The insights drawn from the current modelling work may thus have potential implications for reconstructing paleoenvironments in regard to river sediment transport and flood processes via levee deposits. Based on the roles of a levee on the avulsion frequency and style, the flood hazards triggered by river avulsions as well as the alluvial architecture in sedimentary records can be better assessed.

From apex to shoreline: fluvio-deltaic architecture for the Holocene Rhine-Meuse delta, The Netherlands

Despite extensive research on alluvial architecture, there is still a pressing need for data from modern fluvio-deltaic environments. Previous research in the fluvial-dominated proximal and central Rhine-Meuse delta (The Netherlands) has yielded clear spatial trends in alluvial architecture. In this paper, we include the backwater length to establish architectural trends from apex to shoreline. Channel-belt sand body width/thickness ratios and interconnectedness were determined and the proportions of fluvial channel-belt deposits, fluvial overbank deposits, organics and intertidal deposits were calculated for the complete fluvio-deltaic wedge, based on high-resolution geological cross-sections. It was found that the average width/thickness ratio of channel-belt sand bodies in the proximal delta is five times higher than in the distal delta. Other down-valley trends include an 80 %-decrease of the channel deposit proportion (CDP) and a near-constant proportion of overbank deposits. Additionally, interconnectedness in the proximal delta is three times higher than in the distal delta. Based on the Rhine-Meuse dataset, the authors propose a linear empirical function to model the spatial variability of CDP. It is argued that this relationship is driven by four key factors that change along stream: channel lateral-migration rate, channel-belt longevity, creation of accommodation space and inherited flood-plain width. Additionally, it is established that the sensitivity of CDP to changes in the ratio between channel-belt sand body width and flood-plain with, (normalised channel-belt sand body width) varies spatially and is greatest in the central and distal delta. Also, the proportion of fluvial channel-belt sands is generally an appropriate proxy for the total sand content of fluvio-deltaic successions, albeit that its suitability as a total-sand indicator rapidly fades in the distal delta. With this paper, unique high-resolution quantitative data and spatial trends on the alluvial architecture are available for an entire delta, hereby providing a dataset that can be used to further improve existing fluvial stratigraphy models.

Describing fluvial systems: linking processes to deposits and stratigraphy

The period since the 1960s witnessed significant progress in our ability to decipher the clastic rock record from a wide range of sedimentary environments, and spanning many spatio-temporal scales, from millimetric to that of the sedimentary basin, and involving processes acting on timescales of seconds to millions of years. This review assesses advances in four areas of fluvial sedimentology: the nature of alluvial dunes, the role of fine-grained suspended sediment, the linking of facies models and channel planform, and the reconstruction of drainage networks within ancient sedimentary successions. The synthesis reveals that we require new thinking and research to: (1) address the range of stratification produced by dunes and their palaeohydraulic implications; (2) evolve new bedform phase diagrams capable of incorporating the reality that many fluids transport fine-grained sediment, both in flow and within the bed, which may significantly modify the bedform morphology and phase space when compared with existing bedform stability diagrams; (3) develop new alluvial facies models in which planform channel pattern is not the fundamental discriminant; and (4) re-establish consideration of process mechanics as the heart of developing ideas and debates concerning fluvial deposit preservation and alluvial architecture.