The effect of subsurface heterogeneity on groundwater flow and the hazard of piping underneath dikes

In Holocene deltas, sandy channel belts conduct groundwater flow underneath dikes. During high water, concentrated flow can lead to the formation of pipes, putting dikes at risk. We focus on the impact of subsurface heterogeneity on 3D groundwater flow patterns below dikes by modelling and consider two spatial scales of heterogeneity: (i) the configuration of channel-belt deposits and (ii) the internal architectural elements within these channel belts. We evaluated a comprehensive set of 3D steady-state groundwater models for a section of the Stuivenberg Channel Belt (SCB) in the Dutch Rhine Delta. Over the modelled study area, all potential outlets of groundwater flow (discharge wells) were sampled, taking different subsurface hydro-lithological conditions into account. We show that the emergent groundwater flow to a well is strongly influenced by the imposed boundary conditions and the subsurface heterogeneity: (i) flow converges to the outlet and results in a lowering of the total potential around the well and (ii) concentrates in the channel-belt; (iii) flow is strongly convergent or divergent at the edge of the channel belt due to impaired infiltration and drainage. Groundwater flow rates vary locally and this implies that observed well discharge may underestimate the likelihood of pipe formation.