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

Abstract

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 can lead to severe damages. While in recent years the hydraulic processes have been the focus of investigations, currently no validated rock sizing method existents for this special load case. To investigate this problem a prototype experiment with two optimized groyne designs was conducted. Associated monitoring recorded wave heights, flow velocities and structural changes over the course of 4 years. This paper conducts an analysis of this accumulated data. Relationships of primary wave heights and overtopping flow conditions and thus to rock movements are discussed. Similarities to weir flow and overflowing of riprap embankments are exploited by applying existing analytical and empirical formulae. This leads to good stone size predictions as validated against the observed riprap erosion data. From 28 observed erosive events, 23 could are predicted correctly. Ultimately, a rock sizing method for rock groynes against primary ship wave loads is proposed.