Graduation of Jan-Willem van Lente

29 mei 2019 14:00 - Locatie: Room 3.98, Faculty of Civil Engineering and Geosciences - Door: Webredactie

"Using flotsam lines for improving predictions of hydraulic loads on coastal dikes" | Professor of graduation: Prof. Dr. Ir. M. Kok, supervisors: Ir. R. Slomp (Rijkswaterstaat), Dr. Ir. V. Vuik (TU Delft & HKV), Dr. Ir G.Ph van Vledder (TU Delft & Van Vledder Consulting).

Flotsam lines may arise on coastal dikes after a storm surge. These lines (Figure 1) are generated when material due to debris accumulation on the foreshore has been transported onto the dike surface by wave run-up. Hydraulic engineers who used flotsam line measurements, interpreted a flotsam line as a line which is marking the highest wave run-up. In the past, flotsam lines were used to assess the safety of coastal dikes along the Wadden Sea. Currently, models (e.g. SWAN) fulfil this function and this may result in loosing the value of flotsam lines. This study aims to rediscover the value of flotsam lines.

Figure 1: Two flotsam lines along the dike are clearly visible after two high waters during the storm of 5/6 December 2013. [Photo: Regional water authority Hunze en Aa's]


The objective of this study was to shed light on the value of flotsam lines. Using the spatial information obtained from flotsam levels it is investigated if flotsam levels can be used to assess the quality of models (e.g. SWAN). This was done by calculating flotsam lines using a sequence of models and wave run-up formulas and then compare to measured flotsam lines. The results showed that the calculated lines obtained by the applied set of models and formulas show the same features as the measured flotsam lines. This means that the calculated flotsam levels can serve as an useful indicator of local wave conditions, when applying the used set of models and formulas in this study. A large database of detailed flotsam lines with corresponding storm conditions (e.g. water level, wave conditions, wind, etc) can provide valuable information to low costs. This database can be used to detect spatial irregularities which give more insight in certain processes during storm conditions. Finally, observations of a flotsam line in storm conditions show that a flotsam line is marking the maximum wave run-up in case of a thin layer of flotsam.