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Differential subsidence in the urbanised coastal-deltaic plain of the Netherlands
The urbanised peat-rich coastal-deltaic plain of the Netherlands is severely subsiding due to human-induced phreatic groundwater level lowering, as this causes peat layers to compress and oxidise. To determine the potential susceptibility of this area to future subsidence by peat compression and oxidation, the effects of lowering present-day phreatic groundwater levels were quantitatively evaluated using a subsidence model. Input were a 3D geological subsurface voxel-model, modelled phreatic groundwater levels, and functions for peat compression and oxidation. Phreatic groundwater levels were lowered by 0.25 and 0.5m, and the resulting peat compression and oxidation over periods of 15 and 30 years were determined. The model area comprised the major cities Amsterdam and Rotterdam, and their surrounding agricultural lands. The results revealed that for these scenarios agricultural areas may subside between 0.3 and 0.8m; potential subsidence in Amsterdam and Rotterdam is considerably lower, less than 0.4m. This is due to the presence of several metres thick anthropogenic brought-up soils overlying the peat below the urban areas, which has already compressed the peat to a depth below groundwater level, and thus minimises further compression and oxidation. In agricultural areas peat is often situated near the surface, and is therefore highly compressible and prone to oxidation. The averaged subsidence rates for the scenarios range between 7 and 13mma−1, which is corresponds to present-day rates of subsidence in the peat areas of the Netherlands. These results contrast with the trend of coastal-deltaic subsidence in other deltas, with cities subsiding faster than agricultural areas. This difference is explained by the driver of subsidence: in other deltas, subsidence of urban areas is mainly due to deep aquifer extraction, whereas in the Netherlands subsidence is due to phreatic groundwater level lowering.
Holocene relative mean sea-level changes in the Wadden Sea area, northern Netherlands
Although the Netherlands has a long tradition of sea-level research, no Holocene relative sea-level curve is available for the north of the country. Previous studies hypothesized that the relative sea-level reconstruction for the western Netherlands is also valid for the northern part of the country. However, glacial isostatic adjustment (GIA) models predict a lower and steeper relative sea-level curve because of greater postglacial isostatic subsidence. Long-term data of relative sea-level change are important to inform GIA models and understand postglacial vertical land motion related to the rebound of Fennoscandia and neotectonic activity. We compiled and evaluated a set of basal peat radiocarbon dates to reconstruct the Holocene relative mean sea-level rise in the Dutch Wadden Sea area. For the early Holocene, this reconstruction is lower than the western Netherlands curve. After 6400 cal a BP, the curve for the Wadden Sea is statistically indistinguishable from that for the western Netherlands, a result that conflicts with GIA model results. It remains to be investigated whether the problem lies with the GIA model predictions or with the quality of the available data. Additional basal peat radiocarbon dates from suitable sites should be collected to further resolve this problem.
Quantifying geophysical inversion uncertainty using airborne frequency domain electromagnetic data : applied at the Province of Zeeland, the Netherlands
An accurate understanding of the fresh-saline distribution of groundwater is necessary for effective groundwater management. Airborne electromagnetic (AEM) surveys offer a rapid and cost-effective method with which to map this, offering valuable additional information about the subsurface. To convert AEM data into electric conductivity and ultimately groundwater salinity, an inversion is undertaken. A number of algorithms are available for this purpose; however these are affected by significant uncertainty, owing to inherent non-unique characteristics of this process. The most commonly used inversion codes in hydrogeophysical studies were quantitatively tested using frequency domain AEM and ground data from the Province of Zeeland, the Netherlands. These include UBC1DFM code, and quasi-2D laterally constrained inversions. Following an investigation of inversion parameter settings, data were inverted for four inversion methods and interpolated into 3D volumes. Using geological data and empirical EC and water salinity relationships, each inversion was converted into groundwater EC and split into fresh-brackish-saline regions. For groundwater volume estimates out of a total volume of 2,8 billion m³, a fresh groundwater estimate could differ by as much as 178 million m3, depending on the inversion used. The primary factor here was the choice of model smoothness, which was shown to affect the thickness of the brackish interval. Fresh-brackish-saline interfaces were consistently mapped with an accuracy of ~3m, the brackish being the most accurately resolved. The few layer method was less successful at resolving smoothly varying salinity distributions, but more successful at mapping the brackish interface at greater depth.
Morphological effects of vegetation on the tidal–fluvial transition in Holocene estuaries
Vegetation enhances bank stability and sedimentation to such an extent that it can modify river patterns, but how these processes manifest themselves in full-scale estuarine settings is poorly understood. On the one hand, tidal flats accrete faster in the presence of vegetation, reducing the flood storage and ebb dominance over time. On the other hand flow-focusing effects of a tidal floodplain elevated by mud and vegetation could lead to channel concentration and incision. Here we study isolated and combined effects of mud and tidal marsh vegetation on estuary dimensions. A 2-D hydromorphodynamic estuary model was developed, which was coupled to a vegetation model and used to simulate 100 years of morphological development. Vegetation settlement, growth and mortality were determined by the hydromorphodynamics. Eco-engineering effects of vegetation on the physical system are here limited to hydraulic resistance, which affects erosion and sedimentation pattern through the flow field. We investigated how vegetation, combined with mud, affects the average elevation of tidal flats and controls the system-scale planform. Modelling with vegetation only results in a pattern with the largest vegetation extent in the mixed-energy zone of the estuary, which is generally shallower. Here vegetation can cover more than 50% of the estuary width while it remains below 10%–20% in the outer, tide-dominated zone. This modelled distribution of vegetation along the estuary shows general agreement with trends in natural estuaries observed by aerial image analysis. Without mud, the modelled vegetation has a limited effect on morphology, again peaking in the mixed-energy zone. Numerical modelling with mud only shows that the presence of mud leads to stabilisation and accretion of the intertidal area and a slight infill of the mixed-energy zone. Combined modelling of mud and vegetation leads to mutual enhancement with mud causing new colonisation areas and vegetation stabilising the mud. This occurs in particular in a zone previously described as the bedload convergence zone. While vegetation focusses the flow into the channels such that mud sedimentation in intertidal side channels is prevented on a timescale of decades, the filling of intertidal area and the resulting reduction in tidal prism may cause the infilling of estuaries over centuries.
Land subsidence by peat oxidation leads to enhanced salinization through boils in Dutch polders
Peat oxidation in deep Dutch polders leads -in addition to subsidence- to the development of new saline boils, enhancing the salinization of these polders. This on-going process is studied in detail in the Middelburg-Tempelpolder. The objective of the study was to get more in-depth knowledge about this process and to assess it for the present situation and for future landscapes (after 10, 50, 100 and 500 years).
Groundwater salinity mapping of the Belgian coastal zone to improve local freshwater storage availability
In the European TOPSOIL project, countries around the North Sea are searching for solutions for climate related threats. They explore the possibilities of using the topsoil layer to solve current and future water challenges. The main objective is to improve the climate resilience of the water management of the topsoil and shallow aquifers in the North Sea region. TOPSOIL is supported by the Interreg VB North Sea Region program in line with priority 3 of the program: ‘Sustainable North Sea Region, protecting against climate change and preserving the environment’. The Belgian part of this project, called FRESHEM for GO-FRESH Vlaanderen (‘FREsh Salt groundwater distribution by Helicopter ElectroMagnetic survey for Geohydrological Opportunities FRESH water supply’), focuses on mapping the salinity distribution of groundwater using airborne electromagnetics and aims to look into a number of measures that could increase the availability of freshwater for agriculture in the polder area. Two pilot projects will evaluate the possibilities for freshwater storage and aims to specify what measures can be taken to achieve this. Together with the other water users and water managers, The Flanders Environment Agency wants to prepare a plan for the realization of one or more pilot projects that can improve the availability of freshwater.
Influence of tides, bathymetry, lithology and regional flows on the salinization process in nature area the Rammegors
Nature area Rammegors, which has recently been transformed from a fresh inner-dyke nature area to a salt tidal area. Due to this transformation, salt water is infiltrating in a fresh waterlens. This salinisation process is investigated in more detail by two- and three dimensional models together with measurements in the area. Zeeland project FRESHEM has provided detailed isohaline maps of the area and Deltares is making transient isohaline maps based on measurements made by an ERT-cable which is situated in Rammegors. These data has been and will be used to investigate which factors; bathymetry, lithology, tides or regional groundwater flow, will have the largest impact on the salinization process in Rammegors. This investigation shows that discretization size has an influence on the speed and spatial distribution of salt plumes. Lithology has the largest influence on the salinization process, followed by bathymetry. Spring and neap tides do differ from the normal tides situation only when bathymetry is not taken into account.
Proceedings of the 20th Australasian Fluid Mechanics Conference (Perth, Australia, 5-8 December 2016)
Physical and numerical modelling of wave transformation through a coastal canopy
Coastal canopies formed by aquatic vegetation (e.g. seagrass, mangroves) or corals can be found along many coastlines worldwide and often have a significant effect on the local wave dynamics. Over the past several decades, many studies have greatly increased our understanding on the physical interaction between coastal canopies and waves in the coastal ocean. However, whereas most studies have investigated (bulk) wave dissipation by coastal canopies through empirical formulations, relatively little attention has been paid to the specific instantaneous wave dynamics inside the canopy and how this mechanistically controls wave transformation over canopies. In this study, we extended a state-of-the-art numerical wave model with a canopy flow model to develop a more accurate formulation of the canopy drag force that controls rates of wave dissipation. To validate the model, experiments were carried in a large wave flume with a rigid, high-density model canopy. Model-data comparison using the in-canopy flow velocity and the wave height distribution over the canopy shows that the model is able to capture the essential physics. The results suggest that the canopy flow model increases the accuracy of the estimation of wave dissipation due to canopy drag. Wave attenuation by coastal canopies may be overestimated by wave models without a canopy flow model due to the lack of physics describing canopy flow attenuation.
Development of in/outflow boundary conditions for MPM simulation of uniform and non-uniform open channel flows
This paper describes the development and application of inflow and outflow boundary conditions (BCs) for the material point method (MPM) in order to simulate fluid flow problems. This corresponds to velocity and pressure controlled BCs. Due to the coupled Lagrangian and Eulerian description of the fluid motion in MPM it is necessary to add and remove material points, with appropriate kinematic properties, to and from the computational domain. The newly-developed BCs have been used to simulate uniform open channel flow and the phenomenon of free overfall in open channels, which is transient conditions leading to non-uniform flow due to a sudden bed level drop. It is shown that the numerical results predict well the flow geometry including end depth ratio, pressure distribution and accelerations, therefore the velocities and displacements.