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Uncertainty in complex three-dimensional sediment transport models : equifinality in a model application of the Ems Estuary, the Netherlands
Estuarine suspended sediment transport models are typically calibrated against suspended sediment concentration data. These data typically cover a limited range of the actual suspended sediment concentration dynamics, constrained in either time or space. As a result of these data limitations, the available data can be reproduced with complex 3D transport models through multiple sets of model calibration parameters. These various model parameter sets influence the relative importance of transport processes such as settling, deposition, erosion, or mixing. As a result, multiple model parameter sets may reproduce sediment dynamics in tidal channels (where most data is typically collected) with the same degree of accuracy but simulate notably different sediment concentration patterns elsewhere (e.g. on the tidal flats). Different combinations of model input parameters leading to the same result are known as equifinality. The effect of equifinality on predictive model capabilities is investigated with a complex threedimensional sediment transport model of a turbid estuary which is subject to several human interventions. The effect of two human interventions (offshore disposal of dredged sediment and restoration of the tidal channel profile) was numerically examined with several equifinal model settings. The computed effect of these two human interventions was relatively weakly influenced by the model settings, strengthening confidence in the numerical model predictions.
Comparing robust decision-making and dynamic adaptive policy pathways for model-based decision support under deep uncertainty
A variety of model-based approaches for supporting decision-making under deep uncertainty have been suggested, but they are rarely compared and contrasted. In this paper, we compare Robust Decision-Making with Dynamic Adaptive Policy Pathways. We apply both to a hypothetical case inspired by a river reach in the Rhine Delta of the Netherlands, and compare them with respect to the required tooling, the resulting decision relevant insights, and the resulting plans. The results indicate that the two approaches are complementary. Robust Decision-Making offers insights into conditions under which problems occur, and makes trade-offs transparent. The Dynamic Adaptive Policy Pathways approach emphasizes dynamic adaptation over time, and thus offers a natural way for handling the vulnerabilities identified through Robust Decision-Making. The application also makes clear that the analytical process of Robust Decision-Making is path-dependent and open ended: an analyst has to make many choices, for which Robust Decision-Making offers no direct guidance.
Functional properties of soils formed from biochemical ripening of dredged sediments : subsidence mitigation in delta areas
In delta areas, dense networks of canals have been developed through time and have to be periodically dredged. Lowering the groundwater level in delta areas deepens the aerobic zone, leading to the oxidation of organic matter and possibly to land subsidence. The use of the dredged sediments on land can be a solution to mitigate land subsidence in delta areas.Five types of dredged sediments with different organic matter content and particle size distribution were dewatered for 7 days and then submitted to biochemical ripening during 141 days on a laboratorial scale with constant temperature and relative humidity. The functional properties analysed were the type and content of organic matter, pH, total C, N, P and S, dry bulk density, water retention capacity, aggregate stability and load-bearing capacity. After biochemical ripening, there was no significant loss in the mass of organic matter but there was an increase in the fraction of stable organic compounds, observed by an increase in oxygen-bearing compounds and a decrease in hydrocarbons during biochemical ripening. The pH was not affected by biochemical ripening, and the total C, N, P and S concentrations are high and therefore the dredged sediments can improve the quality of the land. Most volume lost during dewatering and biochemical ripening can be attributed to the loss of water. The water retention capacity of the dredged sediments changed with biochemical ripening. The soils formed from biochemical ripening have very stable aggregates, and its load-bearing capacity is enough to sustain cattle and tractors. Most volume lost during dewatering and biochemical ripening can be attributed to the loss of water and not organic matter. Therefore, the studied dredged sediments have potential to mitigate land subsidence in delta areas when spread on land.
Computational modelling of secondary flow on unstructured grids
For the modelling of river dynamics secondary flow, the helical motion which occurs perpendicular to the primary flow direction, is essential. Recently, modelling on unstructured grids is rapidly becoming more accessible for river engineers. Unstructured modelling allows greater flexibility in setting up a model,where complex boundaries can be more accurately captured than when using a structured curvilinear grid. In previous numerical studies the parameterization of the secondary flow is generally computed on structured grids. In the presence of non-uniformity of unstructured grids, the calculation of the secondary flow undergoes into spatial inaccuracies because of difficulties in the reconstruction of the velocity gradients with sufficient accuracy. In this study, a new method based on least-square reconstruction technique is used to compute the velocity gradients on uniform and non-uniform triangular grids. This method leads to sufficiently high spatial accuracy which allows the computation of the secondary flow intensity with sufficient accuracy. This technique is validated with theoretical and experimental studies conducted by Kalkwijk and Booij (1986). Good agreement is found between the computed secondary flow intensity and the past theoretical and experimental studies. Moreover, in order to study the effect of the spiral flow on the bed morphology, we simulated morphology in a curved bend and found that phenomenologically the pattern from previous Delft3D implementation for structured grids is well reproduced. Further validation with experiments is currently being performed.
Modeling of spatial lag in bed-load transport processes and its effect on dune morphology
In the present study, two bed-load transport models are introduced in an existing idealized dune model. These allow for the modeling of the spatial lag between the sediment transport rate and bed shear stress along dune surfaces. This lag is an important factor in determining transitions between bedform regimes. Results of the original dune model (using an equilibrium transport formula) are compared with (1) a new model version that directly models spatial lag with a relaxation equation and (2) a new model version including pick-up and deposition processes. Both bed-load models use mean particle step length as an important parameter, which is varied to assess which value is appropriate for the dune regime. Laboratory experiments are simulated with the model. This shows that the results are best with the pick-up and deposition model version, combined with a step length of 25 times the particle diameter. It is furthermore shown that in principle the model is also able to wash out fully grown dunes, by increasing the step length parameter.
Processes affecting the spatial distribution of seagrass meadow sedimentary material on Yao Yai Island, Thailand
Many islands throughout SE Asia are experiencing rapid development and land-cover conversion that potentially threaten sensitive coastal ecosystems, such as seagrasses, through increased loading of sediment and nutrients originating from disturbed catchments draining to the sea. To evaluate this threat for one such island in Southern Thailand (Yao Yai), we perform sediment source tracing via end-member mixing analysis using stable isotopes in organic matter to explore sediment loading in a seagrass meadow. The analysis indicates that sedimentary material in the meadow originates mostly from ocean-associated sources (∼62% from seagrass detritus, seston, and ocean sediments). Terrestrial material comprises ∼19% of the organic material found in the seagrass meadow, with another 20% originating from an adjacent mangrove forest. Approximately one-fourth of the seagrass meadow material (24%) is detritus that has been (re)deposited internally. The high contribution of terrestrial-derived organic matter deposited near the river mouth demonstrates that substantial quantities of sediment are being transferred from upslope erosion sources into the seagrass meadow. However, only a small amount of this material is deposited throughout the entire bay because much of the terrestrial- and mangrove-derived sediment is transferred to the open ocean via channels that are periodically dredged to allow boat access to two small inland harbours. This positive affect of dredging has not received very much attention in existing literature. River water flowing to the channels during falling tide delivers sediment to these efficient pathways, where much of it bypasses the seagrass meadow at periods of time when sediment deposition would normally be the greatest. There is growing concern that ongoing land-cover changes and planned urbanization related to tourism and agriculture on the island may boost sediment/nutrients above a critical threshold, beyond that revealed in our baseline survey. Our tracer-based sediment source approach did not corroborate our observations of substantial erosion and land degradation in the upper catchment—but this could be a result of sediment flushing through the dredged channels. We encourage others to combine such methods with sediment budgeting approaches to triangulate results for consistency. Finally, from an ecological perspective, the high presence of seagrass detritus we found in bay sediments suggests seagrass is potentially a key source of nutrients for the meadow itself, as well as other connected ecosystems.
Bed shear stress estimation on an open intertidal flat using in situ measurements
Accurate estimations for the bed shear stress are essential to predict the erosion and deposition processes in estuaries and coasts. This study used high-frequency in situ measurements of water depths and near-bed velocities to estimate bed shear stress on an open intertidal flat in the Yangtze Delta, China. To determine the current-induced bed shear stress (τc) the in situ near-bed velocities were first decomposed from the turbulent velocity into separate wave orbital velocities using two approaches: a moving average (MA) and energy spectrum analysis (ESA). τc was then calculated and evaluated using the log-profile (LP), turbulent kinetic energy (TKE), modified TKE (TKEw), Reynolds stress (RS), and inertial dissipation (ID) methods. Wave-induced bed shear stress (τw) was estimated using classic linear wave theory. The total bed shear stress (τcw) was determined based on the Grant–Madsen wave–current interaction model (WCI). The results demonstrate that when the ratio of significant wave height to water depth (Hs/h) is greater than 0.25, τcw is significantly overestimated because the vertical velocity fluctuations are contaminated by the surface waves generated by high winds. In addition, wind enhances the total bed shear stress as a result of the increases in both τw and τc generated by the greater wave height and reinforcing of vertical turbulence, respectively. From a comparison of these various methods, the TKEw method associated with ESA decomposition was found to be the best approach because: (1) this method generates the highest mean index of agreement; (2) it uses vertical velocities that are less affected by Doppler noise; and (3) it is less sensitive to the near-bed stratification structure and uncertainty in bed location and roughness.
Ecological Key Factor Toxicity, part 5 : background document on effect-based trigger values for environmental water quality
STOWA developed the conceptual framework of the Ecological Key Factors (Ecologische Sleutelfactoren) for the ecological assessment of water quality issues. The key factors describe preconditions for good water quality. The key factors help to structure the available information of water quality and make it possible to pinpoint dominant processes in water system functioning. In a series of five reports the ESF Toxicity is elaborated. This report (part 5) is a background document on effect-based trigger values for the assessment of the ecological risks of (a combination of) chemicals. The aim of these EBTs is that they should provide initial hazard identification of organic micropollutants for the aquatic organisms. The EBTs will be included in a model called “SIMONI” (Smart Integrated MONItoring) that can be used to discriminate between ‘low risk’ sites, where no further analyses are needed, and ‘potential risk’ sites, where an additional risk assessment has to be conducted.
Ecologische Sleutelfactor Toxiciteit, deel 4 : SIMONI procedures voor effectgerichte monitoring
De Ecologische Sleutelfactor Toxiciteit geeft inzicht in de mate waarin toxische stoffen het waterleven bedreigen. Deze sleutelfactor maakt deel uit van een serie van Ecologische Sleutelfactoren die gezamenlijk inzicht geven in de ecologische omstandigheden in het oppervlaktewater. Het verbeteren van die omstandigheden is één van de belangrijkste opgaven bij de uitvoering van de Kaderrichtlijn Water (KRW). In een reeks van vijf rapporten is de ESF Toxiciteit uitgewerkt. Dit bijrapport (deel 4) beschrijft algemene procedures (SIMONI) voor het monitoren van de toxiciteit van stoffen. Deze procedures maken onderdeel uit van het toxicologie spoor dat wordt beschreven in het hoofdrapport. De monitoring wordt uitgevoerd met passieve bemonstering en effectgerichte analyses (bioassays).
Ecologische Sleutelfactor Toxiciteit, deel 3 : technische handleiding en tips ESF-toxiciteit Chemie tool
De Ecologische Sleutelfactor Toxiciteit geeft inzicht in de mate waarin toxische stoffen het waterleven bedreigen. Deze sleutelfactor maakt deel uit van een serie van Ecologische Sleutelfactoren die gezamenlijk inzicht geven in de ecologische omstandigheden in het oppervlaktewater. Het verbeteren van die omstandigheden is één van de belangrijkste opgaven bij de uitvoering van de Kaderrichtlijn Water (KRW). In een reeks van vijf rapporten is de ESF Toxiciteit uitgewerkt. Dit bijrapport (deel 3) is een technische handleiding voor het toepassen van de ontwikkelde rekentool waarmee toxische druk kan worden afgeleid. De handleiding geeft aanwijzingen en tips voor het installeren en gebruiken van de ESF-toxiciteit Chemie rekentool.