The conclusions from studies about the effectiveness of porous groynes are overwhelmingly positive.
The experiments held by the University of Florida laboratory determined that the major influence of permeable wood groynes on the littoral system is the reduction of the longshore current velocity and sediment transport capacity within their reach. The beach profile responses to pile cluster groynes included updrift-downdrift deposition, bar formation, seaward translation and shoreline advance.
Referenced in a case study on the impact of coastal erosion by: Dean, R.G., Mulcahy, S., Poff, M.T., Stephen, M.F. Permeable Wood Groynes: A Case Study and Experiment on Their Impact to the Coastal System. Proceedings from the 2001 National conference on Beach Preservation Technology, pp. 233-248.
There seems to be little to no doubt that the two Makepeace wooden groynes constructed at Bournemouth have resulted in a build-up of the beach in their immediate vicinity. Although erosion would be expected downdrift, it has not been observed.
The shoreline translates parallel to the original shoreline seawards until a balance is reached between onshore and offshore transports.
Referenced in a case study on the impact of coastal erosion by: Raudkivi, Arved J. (1996) Permeable Pile Groynes. Journal of Waterway, Port, Coastal, and Ocean Engineering, Vol. 122, pp. 267-272.