Breakwater design using multivariate probability

JBP and JBA coastal engineers have been working on the first coastal defence structure to be designed using a detailed multivariate probability approach.


The new detached breakwater and beach nourishment scheme has been developed on behalf of the Denbighshire County Council in Rhyl, North Wales, UK, to protect against wave overtopping, coastal flooding and erosion.  

The Heffernan and Tawn method for multivariate probability was used for the designs to calculate the liklihood of extreme wave, wind, tide and surge conditions occuring simultaneously.  A 10,000 year event dataset was then developed, allowing four defence options to be conditdered in terms of their required size to meet design criteria.  

Multivariate design events were simulated through wave transformation, boussinesq, overtopping and inundation models to allow the structures benefit:cost ratio to be estimated.  This provided greater certainty that the breakwater offered the most economic approach against other options, which included a seawall, a terminal groyne and a beach nourishment scheme.

The use of the new statistical aproach gave a greater certainty that design parametres woudl be effective in reducing the communities coastal risk.  They were used to meet a dual overtopping standard; providing safe access to pedestrians during a 1 in 1-year event whilst limiting overtopping to just 10 l/s/m during a 1 in 200-year event.  All criteria have included climate change allowances, ensuring rising sea levels and larger nearshore wave conditions were included over the structures design life. 

Images from top: Joint probability assessment; multivariate probability assessment; subset of multivariate wave tracks; final breakwater concept design.