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SANTOSS sand transport model: Implementing and testing within the morphological model UNIBEST-TC

Nomden, Harm Gerrit (2011) SANTOSS sand transport model: Implementing and testing within the morphological model UNIBEST-TC.

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Abstract:Due to the large (and increasing) amount of activities in coastal areas, predictions of the short-term and long-term morphological developments are becoming more and more important to ensure safety, navigation, recreation and ecology. To make these morphological predictions different modelling systems are developed including several sand transport formulations. Recently, a new sand transport model was released by Ribberink et al. (2010), as a result of the research project SANTOSS (SANd Transport in Oscillatory flows in the Sheet-flow regime),. The model describes the sand transport within the wave boundary layer under (1) non-breaking waves with different shapes; (2) waves combined with currents; (3) for a large range of sand grain sizes; and (4) for both the ripple and sheet flow regime. The model is calibrated on detailed experiments in oscillatory flow tunnels and wave flumes and it explicitly accounts for unsteady (phase lag) and wave non-linearity effects (skewed and asymmetric waves) and for additional processes under real waves (e.g. boundary layer streaming, Lagrangian effects and vertical orbital velocities. Based on experiment results the SANTOSS sand transport model seems to predict the measured transport rates better in comparison with other transport models. The goal of this research is to explore the applicability and behaviour of the SANTOSS transport model within a morphological model. This has been done by implementing the SANTOSS model within the morphological model UNIBEST-TC and comparing the results of a sensitivity analysis and two test cases with measurements and the results from the TRANSPOR2004 (TR2004) transport model, which was already implemented (Van Rijn, 2007a, 2007b). The SANTOSS model was implemented successfully in the cross-shore profile model UNIBEST-TC. Some small adjustments to the SANTOSS code were necessary to make it more robust. Additionally, special attention is paid to the generation of representative orbital velocity time series which show both velocity skewness and acceleration skewness. Therefore, different theories are analyzed, tested and combined. Because the SANTOSS model does not cover the transport above the wave boundary layer, the current-related suspended load transport at higher levels is computed using the TR2004 formulations. The sensitivity analysis focused on (1) predicted net transport rates and (2) the influence of several processes on the transport. It showed that the SANTOSS model reacts almost in the same way as the bed load and wave-related suspended load of TR2004 together. With the only difference that transport rates predicted by the SANTOSS model are lower over the whole range. In the ripple regime the transport rates predicted by SANTOSS are reduced to zero or become even slightly negative, mainly due to phase lag effects. The TR2004 model predictions are also almost reduced to zero when a phase lag factor is applied to the wave-related suspended load. In the sheet flow regime the increasing undertow velocity near the bed (due to partially breaking waves) and enhanced suspended sediment generate a high offshore directed current-related suspended load, which becomes dominant for both transport models. The influence of different transport and hydrodynamic processes within the two transport models, like phase lag, acceleration and surface wave effects (only for the SANTOSS model) are analysed. Also influences of breaking waves and the use of different orbital velocity theories are explored for both models. Main conclusions are:  In general, the two transport models react in the same way on changes in input or on exclusion of a certain process; 25 January 2011, final report SANTOSS Sand Transport Model: Implementing and testing within UNIBEST-TC  Phase lag effects and surface wave effects (especially vertical orbital velocities) are of importance in the ripple regime. In the higher sheet flow regime the relative influence is low, because it is totally dominated here by the current-related suspended load.  Acceleration effects are taken into account in a different way by the two transport models and has much more influence on the TR2004 predictions;  The level from which the superimposed mean current velocity is extracted as input for the bed load transport is of large importance. The two transport models use a different level. This was shown to have a significant influence on the predicted transport rate. To assess the performance of the two transport models on predicting morphological evolution, two test cases are used: LIP IID test 1B (erosive conditions, sand bar development) and test 1C (accretive conditions, onshore sand bar migration). For test 1B, modeled hydrodynamics agree in general well with measurements, but modeled concentrations of suspended sediment are overestimated offshore of the bar. Both transport models show a weak sand bar development and too much offshore bar migration (SANTOSS slightly more than TR2004). For test 1C there is some differences in modeled and measured hydrodynamics: velocity skewness of the orbital velocities and the undertow velocity in the trough onshore of the sand bar. This also explains the bad performance in morphological predictions by both transport models: small offshore bar migration instead of clear onshore migration. In this research, especially the transport model formulations and the influence of several processes on the final net transport rates have been analysed. Based on the knowledge from the analyses, several recommendations are proposed for further research, changes to the transport formulations and necessary measurements to validate the influence of several processes.
Item Type:Essay (Master)
Clients:
Deltares
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/60220
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