Coastal revetment risk assessment: A case study in UK

Williams A., Ergin A., BALAS C. E.

Proceedings of the International Offshore and Polar Engineering Conference, vol.4, pp.369-374, 1999 (Scopus) identifier


Building civil engineering structures in coastlines that have been designated as special areas because of their natural beauty, is a controversial topic. In the UK, some 33% of the coastline of England and Wales has been delineated as Heritage Coasts, and currently an investment of circa US$ 750,000 for a coastal revetment, is taking place at Colhuw beach, Wales, in the middle of one of the founding (1993/4) Heritage Coasts. Whether such a structure should be built is debatable but if it is, what are the risks associated with structural failure and can one analyze the effect of the structure on conservation? These questions should be crucial in any decision making process. The Reliability-Based risk assessment model (REBAD) was utilized at the rubble mound revetment at Colhuw beach, Wales, to determine the risks associated with structural failure. Deterministic and reliability-based approaches developed for the design of armor layers of rubble-mound revetments are presented. In the reliability-based approach, the design variables are modelled as random variables. The structure's reliability was determined using the limit state equation generated from the Hudson design equation. The exceedance probability of damage levels in the lifetime of the structure (L = 50 years) - a function of the nominal diameter for the mean design wave height of Hd = 3.5 m, indicated that the moderate damage level (15%) in the structure having the mass of the 50% size armour units in gradation as M50 = 7 tones, was exceeded with a high probability of Pf = 49.91% in lifetime. On the other hand, applying the Hudson equation, the current revetment was found to be adequately safe by using the deterministic design approach. However, when design parameter uncertainties were taken into account, e.g. wave characteristics, weathering of the stone, degradation etc., the failure damage level of 25% was exceeded with 31.7% probability within the lifetime of the structure. It is concluded that, reliability-based design should be preferred to conventional methods, since it enables random variations associated with the resistance and load parameters to be taken into account and permits optimization analyses to be performed for several safety levels.