Degradation of grass revetments: a comparison of field observations and structured expert judgement

Abstract

Many flood defences in areas with a mild wave climate and lack of strong currents are covered with a grass revetment, typically on top of an erosion-resistant (clay) layer. The grass revetment increases the erosion-resistance due to its dense root structure, and experimental overtopping and wave impact tests have shown that it significantly increases the resistance to wave impact, run-up and overtopping loads. However, grass is a natural material, and especially when exposed to loads from weather, storms and human influences, assessment of its deterioration rate is difficult. Therefore, a Structured Expert Judgement study was carried out in which experts, mostly dike inspectors, were asked to estimate rates of deterioration for different types of grass revetments. The experts were weighted based on their performance on seed questions. In this paper we compare the results to damage observations in the Rhine delta which have been obtained during routine inspections. From this comparison it is found that degradation rates of grass revetments vary greatly for different locations, which is a probable cause of the considerable variance found from the Structured Expert Judgement study. Using an illustrative case study, we show the implications for inspection and maintenance of grass revetments.