Gilles Erkens

Holocene floodplain sediment storage and hillslope erosion within the Rhine catchment

The response of fluvial systems to land use and climate change varies depending on catchment size. While forcing-response mechanisms of small catchments are reasonably well understood, the response of larger drainage basins is less clear. In particular, the impact of land use and climate change on the Rhine system is poorly understood because of the catchment size (185 000 km2) and the long history of human cultivation, which started approximately 7500 years ago. A sediment budget is calculated to specify the amount of alluvial sediment that was deposited during the Holocene and to estimate long-term soil erosion rates. The results suggest that 59±14 X 109 t of Holocene alluvial sediment is stored in the non-alpine part of the Rhine catchment (South and Central Germany, Eastern France, The Netherlands). About 50% of Holocene alluvial sediment is deposited along the trunk valley and the delta (Upper Rhine, Lower Rhine, coastal plain), while the rest is stored along the tributary valleys. The floodplain sediment storage corresponds to a mean erosion rate of 0.55±0.16 t/ha per yr (38.5±10.7 mm/kyr) across the Rhine catchment outside the Alps. This Holocene-averaged estimate amounts for sediments that were delivered to the channel network and is at the lower limit of erosion rates from other studies of different methodology.

Architecture of the Holocene Rhine-Meuse delta (the Netherlands) – A result of changing external controls

The Holocene Rhine-Meuse delta is formed under the influence of sea-level rise, tectonics, and variations in discharge and sediment supply. This paper aims to determine the relative importance of these external controls to improve our understanding of the evolution of the Rhine-Meuse fluvio-deltaic system. To do this, the geological and lithological composition of the fluvio-deltaic wedge has to be known in detail, both in space and time. This study presents five cross-valley sections in the Holocene Rhine-Meuse delta, based on almost 2000 shallow borings. Over 130 14C dates provide detailed time control and are used to draw time lines in the sections. Distinct spatio-temporal trends in the composition of the Holocene fluvio-deltaic wedge were found. In the upstream delta, the Holocene succession is characterised by stacked channel belts encased in clastic flood basin deposits through which several palaeo-A-horizon levels are traceable. In a downstream direction, the fluvio-deltaic wedge thickens from 3 to 7 m. The Holocene succession in the downstream cross sections formed from ~8000 cal yr BP onwards and is characterised by single channel belts encased in organic flood basin deposits. The main part of the organic beds accumulated between 6000 and 3000 cal yr BP. After 3000 cal yr BP, clastic deposition dominated throughout the delta, indicating an increase in the area of clastic sedimentation. The Holocene fluvio-deltaic wedge is subdivided into three segments based on the relative importance of eustatic sea-level rise, subsidence, and upstream controls (discharge and sediment supply). Before 5000 cal yr BP, eustatic sea-level rise controlled the build-up of the wedge. After eustatic sealevel rise ceased, subsidence was dominant from 5000 to 3000 cal yr BP. From 3000 cal yr BP onwards, increased sediment supply and discharge from the hinterland controlled the formation of the fluvio-deltaic wedge. A significant part of the present-day Rhine-Meuse fluvio-deltaic wedge aggraded after eustatic sea-level rise ceased. We therefore conclude that external controls other than eustatic sea-level rise were also of major importance for the formation of the fluvio-deltaic wedge. Because this is probably true for other aggrading fluvial systems at continental margins as well, all external controls should be addressed to when interpreting (ancient) fluvio-deltaic successions.

Salt-marsh erosion associated with hurricane landfall in southern New England in the fifteenth and seventeenth centuries.

Lithostratigraphic and radiocarbon data from the inland section of Pattagansett River Marsh, Connecticut, show that this sheltered part of the salt marsh underwent significant erosion twice during the past 600 yr, each time followed by rapid and complete infilling of the eroded space with tidal mud and low marsh and high marsh peat. We argue that the erosion cannot be attributed to increases in tidal prism or to lateral migration of tidal channels. The ±2σ age range (A.D. 1390–1470) for the first low marsh growth in the older regressive sequence agrees well with the age range (A.D. 1400–1440) for a hurricane deposit 60 km to the east. The younger regressive sequence is dated with the greatest probability to the period A.D. 1640–1670, i.e., shortly after the hurricanes of A.D. 1635 and 1638. Our conclusion that the most likely cause of the erosion was hurricane activity is relevant to paleostorm research and the study of marsh sensitivity to and recovery from storm erosion.

Impacts of 25 years of groundwater extraction on subsidence in the Mekong delta, Vietnam

Many major river deltas in the world are subsiding and consequently become increasingly vulnerable to flooding and storm surges, salinization and permanent inundation. For the Mekong Delta, annual subsidence rates up to several centimetres have been reported. Excessive groundwater extraction is suggested as the main driver. As groundwater levels drop, subsidence is induced through aquifer compaction. Over the past 25 years, groundwater exploitation has increased dramatically, transforming the delta from an almost undisturbed hydrogeological state to a situation with increasing aquifer depletion. Yet the exact contribution of groundwater exploitation to subsidence in the Mekong delta has remained unknown. In this study we deployed a delta-wide modelling approach, comprising a 3D hydrogeological model with an integrated subsidence module. This provides a quantitative spatially-explicit assessment of groundwater extraction-induced subsidence for the entire Mekong delta since the start of widespread overexploitation of the groundwater reserves. We find that subsidence related to groundwater extraction has gradually increased in the past decades with highest sinking rates at present. During the past 25 years, the delta sank on average ∼18 cm as a consequence of groundwater withdrawal. Current average subsidence rates due to groundwater extraction in our best estimate model amount to 1.1 cm yr−1, with areas subsiding over 2.5 cm yr−1, outpacing global sea level rise almost by an order of magnitude. Given the increasing trends in groundwater demand in the delta, the current rates are likely to increase in the near future.

Preface: Land subsidence processes

Both natural and anthropogenic land subsidence are global phenomena caused by a variety of factors, many of which are related to hydrogeologic processes. Common natural subsidence processes include consolidation related to sediment loading, tectonics, volcanism, and dissolution of relatively soluble carbonate and evaporite minerals. Some natural subsidence processes are directly influenced by human activities related to land and water use and by climatic variability. The development of water resources to support human habitation and cultivation for agriculture typically results in the use and diversion of available surface-water supplies and a reliance on groundwater supplies. These practices can alter the natural hydrologic system in ways that amplify natural subsidence processes or create new anthropogenic subsidence. For example, engineered diversion of runoff can focus recharge in areas susceptible to mineral dissolution which can lead to sinkholes or other collapse features in the karst landscape, or engineered drainage of wetlands or saturated organic soils can cause oxidation and consolidation of the soils. Anthropogenic groundwater abstraction from susceptible (generally, unconsolidated alluvial, fluvial and lacustrine sediments) aquifer systems to support water use principally for agriculture, municipal-industrial and energy development typically can lead to local and regional groundwater storage depletion and accompanying aquifersystem compaction and land subsidence related to increases in effective stresses caused by groundwater-level declines. Note that the terms ‘compaction’, commonly used by geologists, and ‘consolidation’, commonly used in soil mechanics, are used interchangeably in this preface and theme issue. Climate variation in terms of global warming, whether natural or anthropogenic, can indirectly cause glacial isostatic adjustments (uplift and subsidence) of the Earth’s crust related to melting of ice sheets, or can thaw permafrost with subsequent loss of ice volume and drainage of shallow groundwater leading to mechanical and even oxidation mediated subsidence. Climate variation may result in either reductions (droughts) or enhancements (wet periods) of precipitation, surface runoff and groundwater recharge. These reductions can cause subsidence owing to lowered groundwater levels contributing to aquifersystem compaction and to oxidation and consolidation of organic soils; enhancements can cause subsidence owing to increased dissolution of karst minerals and reduced mechanical support for pre-existing karst features. The 14 articles (13 papers and one essay) constituting this theme issue address several of the subsidence processes where human use of natural resources and climate variability have combined to create critical anthropogenic land subsidence problems: oxidation and consolidation of organic soils (three articles), dissolution and collapse of carbonate and evaporite rocks (karst) (three articles), thawing permafrost (thermokarst) (one article) and aquifer-system compaction (seven articles). Each of these subsidence processes is intricately related to the Published in the theme issue BLand Subsidence Processes^

A new soil mechanics approach to quantify and predict land subsidence by peat compression

Land subsidence threatens many coastal areas. Quantifying current and predicting future subsidence are essential to sustain the viability of these areas with respect to rising sea levels. Despite its scale and severity, methods to quantify subsidence are scarce. In peat-rich subsidence hot spots, subsidence is often caused by peat compression. We introduce the standard Cone Penetration Test (CPT) as a technique to quantify subsidence due to compression of peat. In a test in the Holland coastal plain, the Netherlands, we found a strong relationship between thickness reduction of peat and cone resistance, due to an increase in peat stiffness after compression. We use these results to quantify subsidence of peat in subsiding areas of Sacramento-San Joaquin delta and Kalimantan, and found values corresponding with previously made observations. These results open the door for CPT as a new method to document past and predict future subsidence due to peat compression over large areas. ©2016. American Geophysical Union. All Rights Reserved.

The relative contribution of peat compaction and oxidation to subsidence in built-up areas in the Rhine-Meuse delta, The Netherlands

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 CO2 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.

Sinking deltas: trapped in a dual lock-in of technology and institutions

ABSTRACT In delta areas, flood protection structures and large-scale land reclamation are preferential water management strategies to cultivate soft delta soils. Over the past decades, river embankments, upstream dams, land reclamation, and groundwater use have intensified, and increasingly contribute to subsidence. In addition, the influence of institutions implementing these strategies has strengthened as they have acquired technical skills, knowledge, and vast financial resources. Sinking deltas are therefore trapped in a dual lock-in as dominating technology and institutions act as constraints to moving into a more long-term sustainable direction. Nine factors for the lock-in are introduced and illustrated for delta regions in Asia, Europe, and the US. To gain a better understanding of what researchers and practitioners can do to address the dual lock-in, a practical case is presented of Gouda, a Dutch subsiding city in search of more sustainable strategies and institutions. The paper ends with three steps to change the configuration of a dual lock-in: (1) getting to know the lock-in; (2) temporarily bypassing it; and (3) constituting a new, more sustainable lock-in. These steps should be further investigated in action-oriented research programmes with local experts, and targeted to policy processes and human behaviour in the sinking deltas.