Morphodynamic modeling using the Telemac finite-element system

Contamination with plastic debris has been recognized as one of today’s major environmental quality problems. Because most of the sources are land based, concerns are increasingly focused on the freshwater and terrestrial environment. Fate and transport models for plastic debris can complement information from measurements and will play an important role in the prospective risk assessment of plastic debris. We review the present knowledge with respect to fate and transport modeling of plastic debris in freshwater catchment areas, focusing especially on nano- and microplastics. Starting with a brief overview of theory and models for nonplastic particles, we discuss plastic-specific properties, processes, and existing mass-balance-, multimedia-, and spatiotemporally explicit fate models. We find that generally many theoretical and conceptual approaches from models developed earlier for other types of (low density) particles apply also to plastic debris. A unique feature of plastic debris, however, is its combination of high persistence, low density, and extremely wide size distribution, ranging from the nanometer to the >cm scale. This causes the system behavior of plastic debris to show a far wider variety than most other materials or chemicals. We provide recommendations for further development of these models and implications and guidance for how fate and transport models can be used in a framework for the tiered risk assessment of plastic debris.

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Modeling the Fate and Transport of Plastic Debris in Freshwaters: Review and Guidance

Numerical morphological modelling of braided rivers is increasingly used to explore controls on river pattern and for applied environmental management. This article reviews and presents a taxonomy of braided river morphodynamic models and discusses the challenges facing model development and use, illustrating these challenges with a case example. The taxonomy is contextualised by an initial discussion of the physical mechanisms of braiding. The taxonomy differentiates between reach-scale and catchment-scale models. Reach-scale models are usually physics-based, which are further divided based upon the mathematical approach used to solve equations (analytical or numerical) and their dimensionality (1D, 2D or 3D). Cellular automata models are one type of numerical model that replace at least some physical processes with expedient rules. A 2D physics-based approach encapsulates sufficient process complexity to provide behavioural predictions. Predictions from catchment-scale landscape evolution models have potential for providing boundary conditions. Future progress in physics-based modelling needs to address three challenges: (i) representation of flow and sediment transport; (ii) temporal and spatial scaling; and (iii) model calibration, sensitivity, uncertainty and validation. The key problem for addressing these is the dearth of laboratory or natural experiment datasets. To show that progress can be made by comparing reach-scale predictions to high-resolution observations, a case study of monitoring and modelling, conducted in the Rees River, New Zealand, is presented. Hydraulic predictions of cellular automata and shallow water equation (Delft3d) models are compared to observed inundation extent. The efficacy of high-resolution, multi-temporal morphological data for assessing 2D physics-based morphodynamic model predictions is also demonstrated.

Numerical Modelling of Braided River Morphodynamics: Review and Future Challenges

Regional sediment resuspension and transport are examined as Hurricane Sandy made landfall on the Mid-Atlantic Bight (MAB) in October 2012. A Teledyne-Webb Slocum glider, equipped with a Nortek Aquadopp current profiler, was deployed on the continental shelf ahead of the storm, and is used to validate sediment transport routines coupled to the Regional Ocean Modeling System (ROMS). The glider was deployed on 25 October, 5 days before Sandy made landfall in southern New Jersey (NJ) and flew along the 40 m isobath south of the Hudson Shelf Valley. We used optical and acoustic backscatter to compare with two modeled size classes along the glider track, 0.1 and 0.4 mm sand, respectively. Observations and modeling revealed full water column resuspension for both size classes for over 24 h during peak waves and currents, with transport oriented along-shelf toward the southwest. Regional model predictions showed over 3 cm of sediment eroded on the northern portion of the NJ shelf where waves and currents were the highest. As the storm passed and winds reversed from onshore to offshore on the southern portion of the domain waves and subsequently orbital velocities necessary for resuspension were reduced leading to over 3 cm of deposition across the entire shelf, just north of Delaware Bay. This study highlights the utility of gliders as a new asset in support of the development and verification of regional sediment resuspension and transport models, particularly during large tropical and extratropical cyclones when in situ data sets are not readily available.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014JC010474

Glider observations and modeling of sediment transport in Hurricane Sandy

[1] This paper addresses an important question of modeling stream dynamics: How may numerical models of braided stream morphodynamics be rigorously and objectively evaluated against a real case study? Using simulations from the Cellular Automaton Evolutionary Slope and River (CAESAR) reduced-complexity model (RCM) of a 33 km reach of a large gravel bed river (the Tagliamento River, Italy), this paper aims to (i) identify a sound strategy for calibration and validation of RCMs, (ii) investigate the effectiveness of multiperformance model assessments, (iii) assess the potential of using CAESAR at mesospatial and mesotemporal scales. The approach used has three main steps: first sensitivity analysis (using a screening method and a variance-based method), then calibration, and finally validation. This approach allowed us to analyze 12 input factors initially and then to focus calibration only on the factors identified as most important. Sensitivity analysis and calibration were performed on a 7.5 km subreach, using a hydrological time series of 20 months, while validation on the whole 33 km study reach over a period of 8 years (2001–2009). CAESAR was able to reproduce the macromorphological changes of the study reach and gave good results as for annual bed load sediment estimates which turned out to be consistent with measurements in other large gravel bed rivers but showed a poorer performance in reproducing the characteristics of the braided channel (e.g., braiding intensity). The approach developed in this study can be effectively applied in other similar RCM contexts, allowing the use of RCMs not only in an explorative manner but also in obtaining quantitative results and scenarios.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/jgrf.20154

Reduced‐complexity modeling of braided rivers: Assessing model performance by sensitivity analysis, calibration, and validation

With the rapid developments in numerical methods for fluid mechanics,computational modeling has become an attractive tool for studying flow/sediment transport and associated pollutant fate processes in such different environments as rivers,lakes,and coastal areas,over the past three decades,a large number of computational hydrodynamic/sediment transport models have been developedThe article traces the devel-opmental stages of current representative(1D,2D,and 3D)models and describes their main features,strengths,and limitations,and is intended as a first guide to readers interested in immersing themselves in modeling and at the same time sets the stage for discussing current limitations and future needsThe article provided model formulation,the spatial and temporal characteristics,the coupling/linkage of the hydrodynamic and sediment components,the model capabilities to handle unsteady flows,bed load and suspended load,sediment exchange processes,type of sediment(cohesive versus cohesion-less),and multi-fractional sediment transportSome examples of the different model applications are summarized,the reader can use these case studies as a reference guide for model setup,calibration,and verificationThe authors also attempt to provide an insight for model users about model choice,problem and way to improve model input and calibrationThe article discussed model limitations such as simulating complex turbulent flows,sediment entrainment,multi-fractional transport rates,the treatment of the dispersion and diffusion coefficients,soil contributions from the bank or the hill slopes and floodplains,coupling between flow and sediment processes,and highlighted the future research needs for improving available hydrodynamic/sediment transport modelsAt last,the two-phase flow approach to formulate models of sediment-flow interaction and other cross-cutting issues are discussed and reviewed

Erm In

Papers

Sediment transport modeling review―current and future developments