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Abstract: Avulsion, the abandonment of all or part of a channel belt in favor of a new course, is controlled by both autogenic (intrabasinal) and allogenic (extrabasinal) processes. However, their relative roles are not known well. The Rhine–Meuse delta is without doubt the best studied delta regarding Holocene avulsion processes. Based on studies of this delta, an overview is given of (1) allogenic and autogenic processes influencing different aspects of avulsion, (2) the relative importance of these processes in time and space, and (3) observations that can be used to differentiate autogenic and allogenic causes of avulsion. The following avulsion parameters are quantified, and their allogenic and/or autogenic nature are discussed: period of activity of channel belts (period between beginning and end of overbank sedimentation by the river channel), interavulsion period (time period between successive avulsions of a channel belt), avulsion frequency (number of avulsions per time interval in a given area), and avulsion location. In the Rhine–Meuse delta, the period of activity of the Holocene channel belts varies considerably, but shows no significant trend over time, whereas extrabasinal factors (climate, base-level, and tectonics) changed. This suggests that this parameter is mainly autogenic controlled. The average interavulsion period increased from 8000 cal yr BP to 2800 cal yr BP, and decreased since then. Significant fluctuations occurred on a shorter time scale. A maximum variability in the interavulsion period occurred between 3200 and 1800 cal yr BP. This was a time when large channel belts gradually came into existence. During this period the avulsion frequency also reached a maximum. The long-term increasing trend in interavulsion period is related to the decreasing rate of sea level rise. The decreasing interavulsion period since 2800 cal yr BP can be explained by increased discharge and sedimentation, resulting in an increase in the number of avulsions. Seven avulsion sequences (as predicted by the Mackey and Bridge [Mackey, S.D., Bridge, J.S., 1995. Three-dimensional model of alluvial stratigraphy: theory and application. Journal of Sedimentary Research B65 (1), pp. 7–31.] model) may be recorded in the Holocene dataset of the Rhine–Meuse, suggesting a periodicity of ∼500–600 yr. Each avulsion sequence coincides with a peak in the avulsion frequency. The periodicity of ∼500 yr in the avulsion frequency is proposed to be autogenic. However, there is also a long-term trend in the avulsion frequency that can be related to sea level rise, and (after 3000 14C yr BP) to climate change and human influence. Avulsion locations are not randomly distributed over the delta, and are related to the allogenic factors: relative sea level rise, local tectonics, and changes in discharge and sediment load. [Copyright &y& Elsevier]

An increasing number of people lives in coastal zones with a subsurface consisting of heterogenic soft-soil sequences. Many of these sequences contain substantial amounts of peat. While population growth and urbanization continues in coastal zones, they are threatened by global sea-level rise and land subsidence. Peat compaction and oxidation, caused by loading and drainage, are important contributors to land subsidence, and hence relative sea-level rise, in peat-rich coastal zones. Especially built-up areas, having densely-spaced urban assets, are heavily impacted by land subsidence, in terms of livelihoods and damage-related costs. Yet, built-up areas have been largely avoided in peat compaction and oxidation field studies. Consequently, essential information on the relative contributions of both processes to total subsidence and underlying mechanisms, which is required for developing effective land use planning strategies, is lacking. Therefore, we quantified subsidence due to peat compaction and oxidation in built-up areas in the Rhine-Meuse delta, The Netherlands, using lithological borehole data and measurements of dry bulk density, organic matter, and CO 2 respiration. We reconstructed subsidence over the last 1000 years of up to ~4 m, and recent subsidence rates of up to ~140 mm·yr −1 averaged over an 11-year time span. The amount and rate of subsidence due to peat compaction and oxidation is variable in time and space, depending on the Holocene sequence composition, overburden thickness, loading time, organic-matter content, and groundwater-table depth. In our study area, the potential for future subsidence due to peat compaction and oxidation is substantial, especially where the peat layer occurs at shallow depth and is relatively uncompacted. We expect this is the case for many peat-rich coastal zones worldwide. We propose to use subsurface-based spatial planning, using specific subsurface information mentioned above, to inform land use planners about the most optimal building sites in organo-clastic coastal zones. [ABSTRACT FROM AUTHOR]

This paper presents the current state of knowledge on the evolution and depositional history of the River Rhine in the southern part of the North Sea basin during the upper Middle and Late Pleistocene, and its response to climate change, sea-level oscillation and glacio-isostasy. The study focuses on the development of the Eemian interglacial lower-delta in the central Netherlands and its relation to records of climate and sea-level rise, and uses the Saalian and Weichselian pre- and postdating periods to place its development in context. The Rhine fluvial system fills the gradually subsiding North Sea basin, but its development has strongly been affected by the Saalian glaciation and its remaining topography. Ice-pushed ridges originating off the limit of maximum glaciation basically divided the central Netherlands into two sedimentary depocentres: a central depocentre within the former ice-limit, and a southern depocentre south of it. The sedimentary record of the central depocentre, including an incised-valley fill, shows a 20–40 m thick stacked sequence consisting of three units. The incised-valley fill consists of a Late Saalian to early Eemian age lower fluvial unit and a Weichselian age upper fluvial unit, both composed of coarse-grained channel deposits. Sandwiched in-between is a 5–15 m thick record composed of fine-grained fluvial and estuarine (tidal) floodbasin and shallow-marine deposits. It is of Eemian interglacial and Early Weichselian age, and comprises transgressive and highstand deposits that show the drowning of a fluvial system. Inland parts transformed from fluvial to deltaic and estuarine environments, and the most downstream parts transformed to a shallow-marine embayment. Preservation of these units occurred, despite considerable sea-level fall and climate-controlled erosion taking place in the last-glacial. Preservation potential was increased by the fact that the Rhine system avulsed away to the southern depocentre, halfway the Weichselian Pleniglacial. Consequently, the infill of the southern depocentre is of an entire different nature, and last-interglacial transgressive or highstand units are hardly preserved. Because of glaciation and resulting depocentre configuration, the Netherlands in NW Europe thus offers a very good opportunity to study the transgressive interglacial lower-deltaic records and falling-stage preservation thereof – both key elements for understanding sedimentary development over full 100-ky glacial-interglacial cycles of climate and base-level change. [ABSTRACT FROM AUTHOR]

Accurate data on the relative elevation of coastal plains and deltas to local sea level is crucial to assess the impacts of relative sea-level rise in coastal areas. However, as many of world's coastlines and major deltas are located in data-sparse regions, high accuracy elevation data is often unavailable, forcing researchers to rely on 'best available' space-borne elevation data for sea-level rise impact assessments. However, due to lacking local validation data, the quality of these assessments often remains unknown. Here we address this important issue using a new, elevation dataset of the Vietnamese Mekong delta, world's third largest delta. We found that the Mekong delta appears to be elevated ~1.5-2 m lower above local mean sea level than previously (erroneously) concluded based on global space-borne elevation models. This revelation has far-reaching implications as the delta's 18 million inhabitants are far more prone to progressive land subsidence and future sea level rise than previously anticipated. Moreover, our results demonstrate the possible major uncertainties in assumed relative elevation of coastal plains and deltas throughout the world, potentially leading to large errors in sea-level rise impact assessments. [ABSTRACT FROM AUTHOR]