Wave overtopping
Excessive overtopping is undesirable because it can compromise the integrity of the structure or result in a safety hazard, particularly when the structure is in an area where people, infrastructure or vehicles are present, such as in the case of a dike fronting an esplanade or densely populated area.
Wave overtopping typically transpires during extreme weather events, such as intense storms, which often elevate water levels beyond average due to wind setup.
This phenomenon is inconsequential when water levels and wave heights exhibit correlation; however, it poses difficulties in river systems where these factors are uncorrelated.
Overtopping is typically expressed in litres per second per metre of dike length (L/s/m), as an average value.
The official website of the EurOtop Manual, which is widely used in the design of coastal engineering structures, features a number of visualisations of wave overtopping.
For example, seawalls (which are typically vertical, or near-vertical, as opposed to sloping breakwaters or revetments), are often situated behind natural beaches.
This overtopping doesn't occur continuously; rather, it's a sporadic event that takes place when particularly high waves within a storm impact the structure.
[3][11] The extent of wave overtopping is quantified by the volume of water that overflows onto the adjacent land.
[11] Much research into overtopping has been carried out, ranging from laboratory experiments to full-scale testing and the use of simulators.
[12][13][14][15][16][17][18] In 1971, Jurjen Battjes developed a theoretically accurate equation for determining the average overtopping.
[19][20] However, the formula's complexity, involving error functions, has limited its widespread adoption in practical applications.
[21][22] Since the governing overtopping is the boundary condition, this means that the use of such elements allows for a slightly lower flood barrier.
However, significant overtopping typically results in a horizontal flow across the crest, similar to what happens with dikes.
[29] An understanding wave overtopping involves a combination of empirical data, physical modelling, and numerical simulations to predict and mitigate its impacts on coastal structures and safety.
[3] Traditionally, permissible average overtopping discharge has been utilised as a standard for designing coastal structures.
Often, to ensure a more dependable level of safety for pedestrians and vehicles, or to evaluate the stability of the inner slope of a revetment, it is necessary to consider the peak velocity and thickness of the overtopping flow.
It is dependent on a number of factors including the intended use of the dike or coastal structure, and the quality of the revetment.
Tolerable overtopping volumes are site-specific and depend on various factors, including the size and usage of the receiving area, the dimensions and capacity of drainage ditches, damage versus inundation curves, and return period.
For coastal defences safeguarding the lives and well-being of residents, workers, and recreational users, designers and overseeing authorities must also address the direct hazards posed by overtopping.
This necessitates evaluating the level of hazard and its likelihood of occurrence, thereby enabling the development of suitable action plans to mitigate risks associated with overtopping events.
greater than 5m on the outside, a heavy rubble mound revetment on the inside is required for overtopping of 10-30 L/s per metre.
For very good grass cover, without special elements or street furniture such as stairs, sign poles, or fences, 10 L/s per metre is allowed.
[33] Developing a grass cover takes time and requires a suitable substrate, such as lean and reasonably compacted clay.
[34] Research in The Netherlands has found that dikes with a well-compacted and flat clay lining can withstand a limited wave height or limited wave overtopping, such as in the majority of river areas, during the first winter after construction even without a grass cover, for many days without significant damage.
An immature grass cover can be temporarily protected against hydraulic loads with stapled geotextile mats.
The transmission depends only on the wave height on the outer side, the freeboard, and the roughness of the slope.
[36][24] In order to assess the safety and resilience of dikes, as well as the robustness of the grass lining on their crests and landward slopes, a wave overtopping simulator can be employed.
This allows the responsible organisation overseeing the structure to evaluate its capacity to withstand predicted wave overtopping during specific extreme scenarios.
[37] During these tests, the wave overtopping simulator is positioned on the dike's crest and continuously filled with water.
The device features valves at its base that can be opened to release varying volumes of water, thereby simulating a wide range of wave overtopping events.
