Effect of Variations in Water Level and Wave Steepness on the Robustness of Wave Overtopping Estimation

Abstract

The wave overtopping discharge at coastal defense structures is directly linked to the freeboard height. By means of physical modelling, experiments on wave overtopping volumes at sloped coastal structures are customarily determined for constant water levels and static wave steepness conditions (e.g., specific wave spectrum). These experiments are the basis for the formulation of empirically derived and widely acknowledged wave overtopping estimations for practical design purposes. By analysis and laboratory reproduction of typical features from exemplarily regarded real storm surge time series in German coastal waters, the role of non-stationary water level and wave steepness were analyzed and adjusted in experiments. The robustness of wave overtopping estimation formulae (i.e., the capabilities and limitations of such a static projection of dynamic boundary conditions) are outlined. Therefore, the classic static approach is contrasted with data stemming from tests in which both water level and wave steepness were dynamically altered in representative arrangements. The analysis reveals that mean overtopping discharges for simple sloping structures in an almost deep water environment could be robustly estimated for dynamic water level changes by means of the present design formulae. In contrast, the role of dynamic changes of the wave steepness led to a substantial discrepancy of overtopping volumes by a factor of two. This finding opens new discussion on methodology and criteria design of coastal protection infrastructure under dynamic exposure to storm surges and in lieu of alterations stemming from projected sea level rise.

The effect of variations in water level on wave overtopping discharge over a dike: An experimental model study

“…The measured non-dimensional average overtopping discharge Q mea for VWL tests are plotted over the non-dimensional relative freeboards R cavg /H m0 and R cpeak /H m0 (see Figure 10). Note, that in these case the freeboard at the start of the test and the average one as suggested by Kerpen et al (2020) were used to calculate the non-dimensional freeboard. However, the data are divided in synthetic and non-synthetic dataset and as shown in Figure 10 the average overtopping discharges are strictly in line with each other.…”

“…In literature, based on the authors' knowledge, only two experimental campaigns have investigated the role of water level variation: on armour stability (Van Gent et al, 2016 and on wave overtopping (Kerpen et al, 2020). Marzeddu et al (2020) were used to assess the damage response of a rock armoured dike (slope 1:3) due to the variations in water level during a simulated storm.…”

“…In the work of Kerpen et al (2020) hydraulic model tests were conducted in a wave flume over a 1:6 smooth slope and the overtopping wave volumes were measured. The sea water level was adjusted during the tests based on the measurements of three of the most recent and severe storms in the North sea and following three different approaches:(1) linear increase over the entire test duration, (2) linear decrease and (3) linear increase for the first half followed by a linear decrease in the second half.…”

Selected Papers from the Future Paths and Needs in Wave Modelling Workshop

“…However, there are cases where this is not a reasonable choice. Kerpen et al [9] have developed a procedure where they divide the time axis into segments with different water levels and different wave steepnesses. The results turned out to be important especially when varying the steepness.…”

“…As an outcome of the International Workshop "Future needs and challenges in wave modelling", held by SINTEF in Trondheim, Norway, 21-22 October 2019, this Special Issue includes 10 full Journal scientific papers [1][2][3][4][5][6][7][8][9][10] and two Technical notes [11,12], which have been prepared based on a selection of the Workshop presentations. All publications were peer-reviewed according to Journal standards by international experts.…”