Investigation, assessment and remediation of levee transitions

Abstract

Transitions between levees (earthen flood embankments) and other hard structures are areas of weakness within an overall system of flood defences and can be found at any location, on or within a flood defence where there is a change in flood defence structure or of protection revetment (slope or crest, road, etc.), in an internal cross section and in the construction or foundation materials. The transition typically promotes the processes of internal and external erosion. If these processes are undetected, or left unchecked, they can ultimately lead to flood defence failure and breach. Historical floods indicate that most levee failures occur around transitions as illustrated by well-known cases such as the levee failures in New Orleans during hurricane Katrina, and by the findings from reviews of levee performance during UK flood events since 2007. That transitions often form weak points within a defence system is recognised internationally, and agencies and organisations – particularly within the Netherlands, France and USA – have directed effort to understand how transitions affect overall defence performance and how these impacts should be addressed within an overall risk based approach to their management. Some of this work is reflected in the International Levee Handbook. However, current guidance in England and Wales on the visual inspection of flood defence asset does not explicitly account for the potential effects of transitions on defence performance. Likewise, current methods of assessing defence performance also fail to directly account for the increases in loading and reductions in strength arising at interfaces. Guidance on the design detail of transition zones is also limited to that for new defences provided by the International Levee Handbook. The Environment Agency in the UK therefore teamed up with a group of international experts to conduct research to consider the presence of interfaces during flood defence condition assessment, quantify the effects of interfaces on flood defence performance (as expressed by fragility curves), and produce design guidance for retrofitting existing interfaces. This paper describes the outcomes of the research.