Failure of Grass Covered Flood Defences with Roads on Top Due to Wave Overtopping: A Probabilistic Assessment Method

Abstract

Abstract:Hard structures, i.e., roads, are commonly found over flood defences, such as dikes, in order to ensure access and connectivity between flood protected areas. Several climate change future scenario studies have concluded that flood defences will be required to withstand more severe storms than the ones used for their original design. Therefore, this paper presents a probabilistic methodology to assess the effect of a road on top of a dike: it gives the failure probability of the grass cover due to wave overtopping over a wide range of design storms. The methodology was developed by building two different dike configurations in computational fluid dynamics Navier-Stokes solution software; one with a road on top and one without a road. Both models were validated with experimental data collected from field-scale experiments. Later, both models were used to produce data sets for training simpler and faster emulators. These emulators were coupled to a simplified erosion model which allowed testing storm scenarios which resulted in local scouring conditioned statistical failure probabilities. From these results it was estimated that the dike with a road has higher probabilities (5 × 10 −5 > P f >1 × 10 −4 ) of failure than a dike without a road (P f < 1 × 10 −6 ) if realistic grass quality spatial distributions were assumed. The coupled emulator-erosion model was able to yield realistic probabilities, given all the uncertainties in the modelling process and it seems to be a promising tool for quantifying grass cover erosion failure.

Modelling Wave Overtopping for Grass Covers and Transitions in Dike Revetments

“…Transitions in cover type affect both the hydraulic load on the cover and the strength of the cover. The change in bed roughness increases the hydraulic load by creating extra turbulence (Bomers et al, 2018). In case of transitions from grass to another cover type, the strength of the grass cover is reduced because the grass cover is unattached on one side.…”

“…Experiments and numerical models have shown that transitions are weak spots along the dike (Steendam, 2014;Bomers et al 2018;Aguilar-López et al 2018). The increase in the hydraulic load and the reducing cover strength makes transitions vulnerable for dike cover erosion.…”

“…Dike cover erosion is dominated by the turbulence-dominated shear stress at the jet front (Aguilar-Lopez et al 2018). Therefore, a detailed FEM model was developed with a sufficiently accurate turbulence model (Bomers et al 2018). However, such detailed models require high computational resources.…”

“…(1) with the cross-dike coordinate x , the relative turbulence intensity r0, the overtopping period T0 and the strength parameter CE. The critical flow velocity is a function of the cross-dike coordinate since it depends on the cover type and the quality cover (Hoffmans, 2012;Bomers et al, 2018).…”

“…The Closure Dike is designed for wave heights of 4 m resulting in overtopping waves with a maximum overtopping volume of 2700 l and a maximum flow velocity of 6.1 m/s (Hoffmans and Jacobsen, 2014). The overtopping period was set to 4 s and the relative turbulence was set to 0.15, which is within the range reported by Bomers et al (2018). The grass cover is of average quality (Hoffmans and Jacobsen, 2014), resulting in a critical flow velocity of 4.5 m/s and a strength parameter of 2•10 6 s/m (Verheij, 1995).…”