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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.
Experiences in developing and applying decision support systems for strategic flood risk management
Decision support systems (DSSs) have been developed for many years. Their main goal was originally to support decision-makers. Reviews show, however, that the actual use of most DSSs did fall short of these ambitions. The goal of DSSs has gradually shifted to enhancing stakeholder engagement for better informed and societally supported decisions. In this paper, we present and discuss a DSS which has been extensively used by a large and diverse group of stakeholders. This so-called Planning Kit Room for the River did successfully support the development of alternative strategies for flood risk management of the riverine area in the Netherlands. Based on these strategies a major investment programme was developed, which has been implemented recently, within time and budget. In the paper, we explore the key factors for its successful application. Based on this analysis and our experience with both interactive planning processes and DSSs, we propose a typology of DSSs for strategic planning of flood risk management. We outline three types of DSSs with different set-ups and associated users, including a DSS based on an integrated model system. The paper concludes with a discussion of the main characteristics and the potential and limitations of these types of DSSs from both a user and a hydro-informatics perspective.
Assessment of critical infrastructure resilience to flooding using a response curve approach
Following a flood the functioning of critical infrastructure (CI), such as power and transportation networks, plays an important role in recovery and the resilience of the city. Previous research investigated resilience indicators, however, there is no method in the literature to quantify the resilience of CI to flooding specifically or to quantify the effect of measures. This new method to quantify CI resilience to flooding proposes an expected annual disruption (EADIS) metric and curve of disruption versus likelihood. The units used for the EADIS metric for disruption are in terms of people affected over time (person days). Using flood modelling outputs, spatial infrastructure, and population data as inputs, this metric is used to benchmark CI resilience to flooding and test the improvement with resilience enhancing measures. These measures are focused on the resilience aspects robustness, redundancy and flexibility. Relative improvements in resilience were quantified for a case study area in Toronto, Canada and it was found that redundancy, flexibility, and robustness measures resulted in 44, 30, and 48% reductions in EADIS respectively. While there are limitations, results suggest that this method can effectively quantify CI resilience to flooding and quantify relative improvements with resilience enhancing measures for cities.