Assessing breaching risk in coastal gravel barriers

Abstract

This paper presents a method to assess the breaching risk to coastal barriers under a no-active-intervention (NAI) scenario. The approach categorises breaches as small high-level breaches, which operate periodically; to larger permanent tidal inlets with water flowing in/out on each tide. Assessing the risk of breaching involves four stages that consider the most probable location of a breach; the possible breach dimensions; the permanence of the breach; and the timeline of breach formation. The most probable location for a breach to form is assessed by considering the past and future shoreline behaviour, the susceptibility of beach profiles to overwashing, the current defence standard and probability of defence failure. The potential permanence and size of the breach is evaluated by considering a stability rating based on the ratio of the tidal prism to total annual littoral drift. The potential cross-sectional area of a breach is calculated by considering the relationship between the cross-sectional area and tidal prism. The results from this analysis are brought together in a written description of breach formation over time. The gravel barrier between Aldeburgh and Orford Ness on the Suffolk coast in the UK is used as a case study. The approach described within this paper is generic and can be used to assess the breaching risk for other vulnerable coastal barrier features under an NAI scenario.