Zoek binnen publicaties
Parametric emulation and inference in computationally expensive integrated urban water quality simulators
Water quality environmental assessment often requires the joint simulation of several subsystems (e.g. wastewater treatment processes, urban drainage and receiving water bodies). The complexity of these integrated catchment models grows fast, leading to potentially over-parameterised and computationally expensive models. The receiving water body physical and biochemical parameters are often a dominant source of uncertainty when simulating dissolved oxygen depletion processes. Thus, the use of system observations to refine prior knowledge (from experts or literature) is usually required. Unfortunately, simulating realworld scale water quality processes results in a significant computational burden, for which the use of sampling intensive applications (e.g. parametric inference) is severely hampered. Data-driven emulation aims at creating an interpolation map between the parametric and output multidimensional spaces of a dynamic simulator, thus providing a fast approximation of the model response. In this study a large-scale integrated urban water quality model is used to simulate dissolved oxygen depletion processes in a sensitive river. A polynomial expansion emulator was proposed to approximate the link between four and eight river physical and biochemical river parameters and the dynamics of river flow and dissolved oxygen concentration during one year (at hourly frequency). The emulator scheme was used to perform a sensitivity analysis and a formal parametric inference using local system observations. The effect of different likelihood assumptions (e.g. heteroscedasticity, normality and autocorrelation) during the inference of dissolved oxygen processes is also discussed. This study shows how the use of datadriven emulators can facilitate the integration of formal uncertainty analysis schemes in the hydrological and water quality modelling community.
The influence of context on the use and added value of Planning Support Systems in workshops : an exploratory case study of climate adaptation planning in Guayaquil, Ecuador
Planning Support Systems (PSS) are a promising tool for involving stakeholders in urban adaptation workshops. Past research has focused on the use and added value of PSS. While earlier studies have widely acknowledged the importance of context in determining the effectiveness of PSS, there has so far been no dedicated study of the influence of context on the use and added value of these tools in real planning workshops. To address this gap, we made an in-depth exploratory case study of a PSS, called the Adaptation Support Tool (AST), used in an adaptation planning workshop in Guayaquil, Ecuador. The workshop used the AST to support collaborative spatial planning for urban water management, at the neighbourhood scale. Interviews, questionnaires, observations and document review were used to investigate the influence of three contextual factors on the use and added value of the AST. The studied contextual factors are: 1) the style of tool use, 2) the phase of planning, and 3) the local project setting. Our findings indicate that the style of tool use and the local project setting were the most important contextual factors in determining the use and added value of the AST during the workshop. Meanwhile, the phase of planning appears to be critical for achieving impacts at the project level. This exploratory case study is a modest first contribution to understanding the influence of context on the use and added value of PSS in practice. Nevertheless, the findings indicate that further exploration of this topic could offer important insights to PSS use in practice.
The rationality of groundwater governance in the Vietnamese Mekong Delta’s coastal zone
This article assesses the rationality of the governance of the Vietnamese coastal zone’s water system. We first specify five assessment criteria, which we apply to a case study. Based on document analysis, stakeholder surveys and in-depth interviews, we found an average score on the criterion that relevant water system knowledge must be available. The scores on the criteria that water usage is systematically monitored, that the legal framework is complied with, that long-term human and wider ecological interests are addressed, and that governance is decentralized appeared to be low. The article concludes with some recommendations to change the governance system.
Seafloor characterization using multibeam echosounder backscatter data : methodology and results in the North Sea
Seafloor characterization using multibeam echosounder (MBES) backscatter data is an active field of research. The observed backscatter curve (OBC) is used in an inversion algorithm with available physics-based models to determine the seafloor geoacoustic parameters. A complication is that the OBC cannot directly be coupled to the modeled backscatter curve (MBC) due to the correction of uncalibrated sonars. Grab samples at reference areas are usually required to estimate the angular calibration curve (ACC) prior to the inversion. We first attempt to estimate the MBES ACC without grab sampling by using the least squares cubic spline approximation method implemented in a dierential evolution optimization algorithm. The geoacoustic parameters are then inverted over the entire area using the OBCs corrected for the estimated ACC. The results indicate that a search for at least three geoacoustic parameters is required, which includes the sediment mean grain size, roughness parameter, and volume scattering parameter. The inverted mean grain sizes are in agreement with grab samples, indicating reliability and stability of the proposed method. Furthermore, the interaction between the geoacoustic parameters and Bayesian acoustic classes is investigated. It is observed that higher backscatter values, and thereby higher acoustic classes, should not only be attributed to (slightly) coarser sediment, especially in a homogeneous sedimentary environment such as the Brown Bank, North Sea. Higher acoustic classes should also be attributed to larger seafloor roughness and volume scattering parameters, which are not likely intrinsic to only sediment characteristics but also to other contributing factors.
3D numerical simulation of backward erosion piping tube experiments
Backward erosion piping (BEP) is an internal erosion mechanism, driven by the detachment of particles at the pipe tip and erosion of the particles along the pipe walls and bottom. This erosion mechanism was extensively studied in (Robbins et al., 2018), who describe horizontal tube experiments in which the head loss along the erosion pipe was measured during pipe formation. This study investigates the ability of numerical models to predict the head loss in the erosion pipe in a 3D situation by simulating the tube experiments using DgFlow (Van Esch et al., 2013), a finite element (FE) program in which the groundwater flow is coupled to the pipe flow using line elements, and using the 3D finite element program described in Robbins and Griffiths (2018), in which backward erosion is simulated by changing soil elements to pipe elements.
Analysis of pipe progression during backward erosion piping in the presence of a coarse sand barrier
The coarse sand barrier (CSB) is considered as a promising measure to prevent failure of embankments due to backward erosion piping. In this method, a trench consisting of coarse sand is placed below the blanket layer at the downstream side of the embankment, across the possible path of the pipe to prevent development of a pipe. A pipe can progress upstream until it encounters the CSB, which prevents further progression of the pipe unless a significantly higher head drop (compared to the case without CSB) is applied. This results in a much higher safety level for the levee. The increased strength is due to the barrier’s higher resistance against erosion, and the relatively high hydraulic conductivity contrast between the barrier and the background material leading to a reduction of the hydraulic load in the barrier. The feasibility of this method has been investigated in a three-phase experimental programme at Deltares consisting of small-, medium- and large-scale experiments, confirming this method as a highly effective piping inhibiting measure. This contribution presents the results of one phase of this research, the medium-scale experiments, during which several laboratory experiments were conducted. In this paper, the piping process and observations on the pipe progression in presence of a CSB are presented and analyzed to get a better insight in the principle of pipe progression with respect to different barrier materials.
Hydraulic losses through sand boils : measurements and theory
The results of this study illustrate that the head loss across a sand boil (and associated cover layer) is a function of the flow velocity through the boil and the properties (diameter and density) of the sand grains being transported. A simple hydraulic theory was found to favourably predict the trends in the head loss profiles observed in the field. By coupling the hydraulic theory for sand boil head losses to numerical models for predicting BEP, the risk of embankment failure due to BEP can be more accurately assessed.
Large-scale test of a coarse sand barrier as a measure against backward erosion piping
A novel remediation technique against backward erosion piping is being investigated in a multi-scale experimental program. The coarse sand barrier (CSB) is a trench containing densified coarse sand that is placed below an embankment dam or levee in order to prevent the upstream progression of a pipe. The effectiveness of the measure is based on the larger resistance of the densified coarse sand in the barrier against piping erosion, and on the low hydraulic load in the barrier resulting from the conductivity contrast between barrier and background material. This method was investigated in laboratory experiments on a small-scale (aquifer depth 0.10 m) and a medium-scale (aquifer depth 0.40 m) and was found promising. In order to increase the confidence in the potential of the measure for application in the field, two experiments at a larger scale (aquifer depth 3 m) were conducted. This contribution presents the analysis of the piping process of the first large-scale experiment based on measurements during the test and excavation of the sample after the test.
Towards multi-objective optimization of large-scale fluvial landscaping measures
Adapting densely populated deltas to the combined impacts of climate change and socioeconomic developments presents a major challenge for their sustainable development in the 21st century. Decisions for the adaptations require an overview of cost and benefits and the number of stakeholders involved, which can be used in stakeholder discussions. Therefore, we quantified the trade-offs of common measures to compensate for an increase in discharge and sea level rise on the basis of relevant, but inexhaustive, quantitative variables. We modeled the largest delta distributary of the Rhine River with adaptation scenarios driven by (1) the choice of seven measures, (2) the areas owned by the two largest stakeholders (LS) versus all stakeholders (AS) based on a priori stakeholder preferences, and (3) the ecological or hydraulic design principle. We evaluated measures by their efficiency in flood hazard reduction, potential biodiversity, number of stakeholders as a proxy for governance complexity, and measure implementation cost. We found that only floodplain lowering over the whole study area can offset the altered hydrodynamic boundary conditions; for all other measures, additional dike raising is required. LS areas comprise low hanging fruits for water level lowering due to the governance simplicity and hydraulic efficiency. Natural management of meadows (AS), after roughness smoothing and floodplain lowering, represents the optimum combination between potential biodiversity and flood hazard lowering, as it combines a high potential biodiversity with a relatively low hydrodynamic roughness. With this concept, we step up to a multidisciplinary, quantitative multi-parametric, and multiobjective optimization and support the negotiations among stakeholders in the decision-making process.
EWG-IE 27th annual meeting - European Working Group on Internal Erosion in embankment dams, levees and dikes, and their foundations (Vancouver, Canada, 18-21 June 2019) : book of abstracts