Graduation of Anouk Lako
22 January 2019 13:00 till 14:30 - Location: Lecture Hall E, Faculty of Civil Engineering and Geosciences - By: Webredactie
"Morphodynamic behaviour of coastal hard-soft transitions: a case study of Maasvlakte 2" | Professor of graduation: Prof. Dr. Ir. S.G.J. Aarninkhof, supervisors: Ir. T. Vijverberg (Boskalis), Ir. A. P. Luijendijk (TU Delft - Coastal Engineering, Deltares), Dr. Ir. M. Zijlema (TU Delft - Environmental Fluid Mechanics), Ir. P. Brandenburg (van Oord).
All over the world, coastal protection measures are taken to protect the hinterland from flooding. These protection measures can be soft (e.g. sand nourishments on sandy beaches) or hard (e.g. seadikes or seawalls). Special care has to be taken to design the transition between these hard and soft flood defences, as they are often found to be vulnerable components of a coastal defence system. This study therefore aims to give more insight into the morphological processes around hard-soft transitions, in order to make recommendations on the design and nourishment plans for hard-soft transitions in the future.
To this end, the hard-soft transition of Maasvlakte 2 was studied (see Figure 1) . For the case of Maasvlakte 2 the limited knowledge of hard-soft transitions resulted in a conflict of the assessment method of the flood defence and the nourishment strategy according to the client requirements. Monitoring of the soft flood defence revealed that the northernmost transects of the soft flood defence showed a higher dynamic variability than was allowed, so that the safety requirements were not met during several assessment moments.
The most important factors governing the morphological behaviour were studied using different numerical models (UNIBEST-LT/CL, SWAN1 and XBeach2). It was found that the system is mainly dominated by longshore coastal processes. The highest erosion rates around the hard soft transition are caused by gradients in longshore transport due to northern storms, whereas the area around the hard soft transition can recover by supply of sediment during southwestern storms. Moreover, it was found that the northward directed flood current along the soft flood defence strengthens the supply of sediment by northward wave-induced sediment transport, whereas the southward directed ebb current along the hard flood defence is not saturated with sediment and hence does not contribute to morphological changes. Wave reflections at the breakwater were found to play a limited role in the overall erosion observed around the hard-soft transition.
Since it was found that the system is dominated by longshore processes, it means that wave direction is a governing factor for the morphological behaviour of the system. Whereas the wave height shows a clear trend throughout the year (high, energetic waves in winter; calmer conditions during summer), for the wave direction the trend is less clear, and this makes it hard to predict during which months or seasons the hard-soft transition will be eroded. Though, the northern storms that cause the most severe erosion around the hard soft transition were found to occur often during spring. Furthermore, the combination of coastline orientation and the wave climate of Maasvlakte 2 is favourable, since the large southwestern component of the wave climate frequently allows the system to recover naturally.
For Maasvlakte 2, it is recommended to base the required nourishment volumes on Q3-surveys rather than Q2-surveys, as it was found that erosion of the transition often occurs after the Q2-survey. Moreover, it is recommended to keep nourishing on the shoreface to protect the beach from eroding. For future hard-soft transitions some design optimisations are suggested that take into account the natural dynamic variability of the transition zone, such as constructing a hidden flood defence in the dune or widening the beach over the northernmost part of the soft flood defence.
1 N. Booij, R.C. Ris, L.H. Holthuijsen (1999). A third-generation wave model for coastal regions: 1. model description and validation. Journal of Geophysical Research - Oceans, 104:7649–7666, 1999.
2 D. Roelvink, A. Reniers, A. van Dongeren, J. van Thiel de Vries, R. McCall, J. Lescinski (2009). Modelling storm impacts on beaches, dunes and barrier islands. Elsevier Journal of Coastal Engineering, Volume 56, pages 1133-1152