Graduation of Sjoerd Leenders

26 June 2018 11:00 - Location: Room G, Faculty of Civil Engineering and Geosciences - By: Webmaster Hydraulic Engineering

"Numerical modelling of the migration direction of offshore sand waves using Delft3D" | Professor of graduation: S.G.J. Aarninkhof, supervisors: M. Zijlema (TU Delft), D.J.R. Walstra (TU Delft/ Deltares), Ir. J.J. Schouten (Deltares), Ir. R. Hoekstra (Deltares), B.W. Borsje (University of Twente)

The growth of the offshore wind industry results in intensive usage of the sandy seabed in the North Sea currently and in the coming decades. Large-scale bed forms are present in shallow seas with sandy beds such as the North Sea. The most relevant bed form for engineering and research purposes are sand waves. Due to their dynamic behaviour, sand waves can interact within offshore human developments and together with their dimensions pose a threat; e.g. decrease in navigation depth, exposure of submarine cables, interaction with foundations of offshore wind turbines and destabilizing of bed protections. A thorough understanding of the dynamics can result in less risks for the offshore wind sector and therefore bring down the levelized cost of electricity from offshore wind.

Currently, sand wave field dynamics are investigated by data analyses. The data analyses are based on seabed surveys over preferable more than 10 years and are considered most reliable at the moment. However, these surveys are very costly and/or often not available. Complex numerical models may provide a way to analyse sand wave dynamics cost and time efficiently, however two aspect have to be considered. Not all relevant processes regarding sand wave dynamics are yet are understood. Furthermore due to the large scale of sand wave fields in combination with the fine grid resolution required to model sand waves, large computational efforts form a difficulty for numerical modelling of sand wave fields. Previous numerical studies focused on reproducing the length and height of sand waves. The migration direction is the next step towards the full prediction of sand wave fields and the subject of this research.

Recent data analyses showed migration directions of sand waves on small spatial scale in opposite direction, possibly related to underlying seabed topography. Understanding the governing processes of the migration direction of sand waves including underlying seabed topography is the focus of this research using the numerical process-based model Delft3D. First, the migration direction of offshore sand waves is investigated over a horizontal underlying bottom. In the second part of the study underlying bed forms are included in the model and a comparison with field data is made.