University of Twente Student Theses


Improving Suspended Sediment Transport Models for Breaking Wave Conditions

He, Minfei (2017) Improving Suspended Sediment Transport Models for Breaking Wave Conditions.

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Abstract:In coastal regions, the prediction of erosion or accretion of beaches is a critical issue to society. However, current models like Delft3D do not predict suspended sediment transport under breaking wave conditions very well. Generally, the current-related suspended sediment transport is largely influenced by the sediment mixing coefficient and reference concentration. In this project, the reference concentration model in Delft3D was improved on the basis of measurements in SINBAD wave flume experiment. Potential reference concentration models were firstly stand-alone tested with Matlab. Then, Hsu and Liu(2004)’s adaption for the default model in Delft3D and Van der Zanden, et al.(2017)’s reference concentration model were selected to be implemented into Delft3D environment. In order to ensure an accurate hydrodynamic input for the implemented models, the Delft3D hydrodynamic model of SINBAD wave flume experiment was investigated and re-calibrated on the basis of a sensitivity analysis. According to the hydrodynamic re-calibration, the wave height prediction, undertow prediction and turbulent kinetic energy cannot be well-modelled at the same time. Considering the turbulent kinetic energy cannot be directly calibrated well, the wave height and undertow predictions were prior to be calibrated. Later on, an additional adaption in Delft3D source code, which increases the turbulence injection depth and decreases the near-surface turbulence production, was implemented for improving near-bed turbulent kinetic energy prediction under regular wave conditions. Moreover, the modelled breaking point was shifted shoreward by 2 m in order to improve the mismatch between the maximum predicted near bed turbulent kinetic energy and the measurement, which sacrifices the well-predicted wave height. After the hydrodynamic validation of Delft3D model under the regular wave condition, Hsu & Liu(2004)’s adaption and Van der Zanden, et al.(2017)’s model were tested against SINBAD measurements. Both models improve the reference concentration prediction to a certain extent in the breaking region. In terms of offshore-directed suspended sediment transport, these implemented models give better predictions at the breaker bar, while they underestimates the offshore-directed suspended load transport at the bar trough. In order to test these implemented models under irregular wave conditions, LIP 1B case was selected for the test as it is more similar to SINBAD wave flume experiment due to strong waves and undertow. Both implemented models generally improve, but overestimate the reference concentration in the breaking region. Under this circumstance, the offshore-directed suspended sediment transport is overestimated as well. Furthermore, due to the overestimated near-bed undertow and suspended sediment concentration at a secondary breaker bar, the suspended sediment transport is significantly overestimated.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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