Characterization of Overtopping Waves on Sea Dikes with Gentle and Shallow Foreshores

Abstract

Due to ongoing climate change, overtopping risk is increasing. In order to have effective countermeasures, it is useful to understand overtopping processes in details. In this study overtopping flow on a dike with gentle and shallow foreshores are investigated using a non-hydrostatic wave-flow model, SWASH (an acronym of Simulating WAves till SHore). The SWASH model in 2DV (i.e., flume like configuration) is first validated using the data of long crested wave cases with second order wave generation in the physical model test conducted. After that it is used to produce overtopping flow in different wave conditions and bathymetries. The results indicated that the overtopping risk is better characterized by the time dependent h (overtopping flow depth) and u (overtopping flow velocity) instead of hmax (maximum overtopping flow depth) and umax (maximum overtopping flow velocity), which led to overestimation of the risk. The time dependent u and h are strongly influenced by the dike configuration, namely by the promenade width and the existence of a vertical wall on the promenade: the simulation shows that the vertical wall induces seaward velocity on the dike which might be an extra risk during extreme events.

An Inter-Model Comparison for Wave Interactions with Sea Dikes on Shallow Foreshores

“…A possible cause for this change in behaviour is that thin layered flows with a water depth on the promenade were not captured by DSPH because the water depth was smaller than a couple of particles high. The fact that SW1L/SW8L was also able to achieve a Very Good model performance for the overtopped flow layer thickness on the promenade (Table 3) indicates that a very good wave overtopping accuracy has been demonstrated as well, which is a confirmation of previous validation works on wave overtopping over a dike with a shallow foreshore modelled with SWASH [27,28].…”

“…Another example is SWASH, a non-hydrostatic NLSW equations model, where frequency dispersion is resolved by approximation of the vertical gradient of the non-hydrostatic pressure together with a vertical terrain-following grid in a multi-layer approach [24]. It has been shown to be a very efficient and accurate method for the simulation of wave transformations over a (barred) beach [25], including transfer of wave energy to the LWs [26], and mean overtopping discharge [27] and maximum individual overtopping volume [28] over dikes with a very shallow foreshore. However, SWASH has never been validated before for (overtopped) bore impact loading on vertical walls.…”

Formulating Wave Overtopping at Vertical and Sloping Structures with Shallow Foreshores Using Deep-Water Wave Characteristics

“…This process, known as wave overtopping, can result in damage to critical infrastructure and even loss of life. Despite recent studies which suggest that the flow properties of individual overtopping events are equally important (Altomare et al 2020a;Sandoval and Bruce 2018;Suzuki et al 2020), coastal structures worldwide are typically designed to resist a mean overtopping discharge per meter width of structure, q (m 3 /s or l/s per m)-which is often estimated using empirical formulas.…”

The influence of berms, roughness and oblique waves on wave overtopping at dikes

“…For example, the overtopping flow velocity and layer thickness are used as input in some erosion models (Dean et al, 2010;Hoffmans, 2012;Van Bergeijk et al, 2021) to estimate the cover erosion and stability of earthen dikes. Pedestrian safety during wave overtopping was also assessed using the flow velocity and layer thickness (Endoh and Takahashi, 1995;Bae et al, 2016;Mares-Nasarre et al, 2019;Suzuki et al, 2020). Therefore, flow parameters including the flow velocity and layer thickness are also important for the design and reliability assessment of coastal structures.…”

“…The flow characteristics are key parameters to assess the pedestrian safety when standing on the coastal structures during the overtopping events (Mares-Nasarre et al, 2019). Suzuki et al (2020) suggested that the overtopping risk was better characterised by time dependent flow velocity and layer thickness than maximum flow parameters. Sandoval and Bruce (2018) provided different criterion for human stability on coastal structures.…”