A design method for rock groynes exposed to overtop-ping from long-period ship wave loads

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A design method for rock groynes exposed to overtop-ping from long-period ship wave loads

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Seemann, Arne / Melling, Gregor / Jansch, Hanne / Kondziella, Bernhard / TU Delft

In German estuaries ship-induced loads have increased in recent years due to the gradual change of the fleet structure towards bigger vessels. Groynes in particular are vulnerable to long-period primary ship wave loading due to an overtopping phenomenon which at some localities leads to severe damages. While in recent years the hydraulic processes have been the focus of investigations, currently no rock sizing method exists for this special load case. To develop a design method and to better understand the pro-cesses leading to groyne erosion, the data from a previously con-ducted prototype experiment with two groynes is analysed in this paper. The prototype experiment was carried out over several years and was accompanied by a comprehensive monitoring, consisting of wave and current measurements as well as laser scan surveys. Relationships of primary wave heights, overtopping flow con-ditions and rock movements are investigated. Making use of simi-larities to weir flow and overtopped riprap embankments, existing analytical and empirical formulae are applied to this problem and evaluated. Important influencing factors like the ambient water level are discussed. Ultimately, using the Thornton et al. (2014) sta-bility function the majority of observed erosion events could be pre-dicted correctly with only a small number of false detects. On this basis, a workflow for the stone sizing of rock groynes exposed to long-period ship waves is formulated.