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Modeling sand transport under breaking waves

Schnitzler, Bram (2015) Modeling sand transport under breaking waves.

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Abstract:Morphological models are often used to predict the effect of interventions on coasts. A sediment transport formula within such morphological models developed by among others the University of Twente is the SANTOSS model (van der A et al., 2013). The SANTOSS model calculates the near-bed sediment transport for regular non-breaking waves. These morphological models are simplifications of the reality and require constant improvement. To improve the predictive capability of existing sediment transport formulations for breaking-wave conditions, the University of Twente recently conducted measurements in the CIEM wave flume in Barcelona (van der Zanden et al., 2015). The objective of this master thesis is to improve the prediction of sediment transport under breaking waves with the SANTOSS formula within the 3D hydrodynamic and morphodynamic software package Delft3D by adding wave breaking effects to the SANTOSS formula. This required firstly a calibration of a Delft3D model based on the measured hydrodynamics during the CIEM wave flume experiment. Secondly, wave breaking effects were included to the model in order to improve modeled sediment transport rates for this experiment. Finally, the improved model with breaking effects was applied within Delft3D and validated using a separate data set (LIP Experiments, Reniers & Roelvink, 1995). During the calibration of the Delft3D model it seemed that Delft3D had troubles modeling regular waves. Delft3D uses a parameterization for the dissipation due to wave breaking which is developed for irregular breaking waves. This parameterization has been adapted to a parameterization which is suitable to model the dissipation of regular breaking waves. Therefore Delft3D was well suitable to reproduce the measured wave heights. The measured set-up/down was also modeled quite well with the Delft3D model. However, Delft3D was not able to reproduce the measured net currents properly. The measured net currents (undertow) were underestimated at the offshore side of the breaker bar and overestimated at the top of the breaker bar. These errors are most likely due to poor representation of the modeled the Stokes mass flux due to waves or due to rollers. To model sediment transport using the SANTOSS model, intra-wave velocities are required. Since Delft3D is a wave-averaged model the parameterization from Ruessink, Ramaekers, & Van Rijn (2012) is used to predict the intra-wave velocities. This model seemed not very suitable for this test-case. The parameterization method of Ruessink et al. (2012) is developed for field conditions, this is probably the reason of the underestimation of especially the peak orbital velocities and the acceleration skewness. After finishing the calibration of the Delf3D model the different measured and modeled sediment transports components (wave- and current related bed-load and suspended transport) have been assessed. For both suspended and bed-load sediment transport discrepancies between model outcomes and measurements appear. In the breaking region, modeled reference concentrations and mixing coefficients are lower than values extracted from the measurements. This leads to an underestimation of suspended sediment concentrations, and in combination with the undertow current which magnitude was also underestimated, leads to a substantial underestimation of offshore-directed suspended sediment transport. Pre- and post-breaking, modeled concentrations are higher than measured and suspended transport is overestimated by the model. In terms of near-bed transport modeled with the SANTOSS formula, the transport in the post-breaking region is predicted quite accurately. However, the onshore directed sediment transport in the shoaling/breaking region is underestimated. The underpredictions of both offshore-directed suspended load and onshore-directed bed-load cancel each other out to some extent. This causes a proper prediction of the direction of total sediment transport, although the magnitude of the total sediment transport is underestimated. The SANTOSS near-bed transport formula performs approximately similar as the default transport formula within Delft3D (van Rijn, 2007a). Since Delft3D seems not able to accurately reproduce the hydrodynamics and morphodynamics of this test-case, the SANTOSS model was run stand-alone with measured and Delft3D-modeled hydrodynamic input to check the effect of errors in the modeled hydrodynamics on the sediment transport. It seemed that small errors in the modeled hydrodynamics cause various errors in the SANTOSS model (crest/trough periods, phase lag, Shields parameter, etc.) which add substantial errors in the net transport rates. When looking at the modeled sediment transport it is better predicted for some locations using the modeled hydrodynamics. This is mainly due to an overestimation of the crest periods, due to an underestimation of the net currents. The near-bed sediment transport is therefore predicted better using the modeled hydrodynamics for some locations since there is more onshore directed sediment transport due to an overestimation of the crest period. Slope effects have been added to the SANTOSS model and the near-bed velocity reference height has been changed to see if the slope effect or changing the near-bed velocity reference height has remarkable effects. Adding slope effects improved the predicted sediment transport a little bit. Changing the near-bed velocity reference height to the height of the maximum overshoot velocity worsened the prediction of the sediment transport; since the peak orbital velocities at the maximum overshoot velocity height is larger than at the standard height of the lowest velocity measurement device (11 cm above the bed). The errors on the modeled hydrodynamics have a substantial effect on the predicted sediment transport rates; therefore wave breaking effects on near-bed sediment transport were mainly tested in the stand-alone SANTOSS model. Three wave breaking effects have been tested during this study. The formulation for the wave Reynolds stress has been adapted to a formulation that also accounts for energy dissipation of rollers. The other wave breaking effects are adding turbulence to the root mean square orbital velocity (Reniers, Roelvink, & Thornton, 2004) and adding turbulence to the Shields parameter (Reniers et al., 2013) It seems that adding turbulence to the Shields parameter during the crest period (Ting & Kirby, 1995) with a calibration factor for the importance of the turbulence (Ribas, de Swart, Calvete, & Falqués, 2011; Van Thiel De Vries, 2009) seems to work well for this test case, this is also a physically representative formulation. Computations with morphological updating have been done to check whether Delft3D predicts the locations of the breaker bar at the right location. Due to an underestimation of the offshore directed sediment transport and due to an underestimation of the onshore directed bed-load transport in front of the breaker bar Delft3D has trouble with predicting dimensions of the breaker bar. The breaker bar is higher and shorter (steeper slopes) compared to the breaker bar developed during the experiment. The wave breaking effect added to the SANTOSS model has been tested on another experimental campaign in the CIEM wave flume in Barcelona with a nearly flat bottom (Ribberink et al., 2014) and on the LIP1B and LIP1C case (Reniers & Roelvink, 1995). The breaker bar developed during modeling of the first measurement campaign occurs more onshore than measured due to an underestimated of the offshore directed current-related suspension transport. The sediment transport predictions for the LIP cases are improved using the SANTOSS formula including wave breaking effects. The prediction of the sediment transport is especially improved at the locations where most (irregular) waves break. In the start of the flume of the LIP cases, the prediction of the sediment transport became worse. This is due to presence of modeled roller energy in the start of the flume due to irregular breaking waves.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/67726
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