Time dependent groundwater flow under river embankments

Abstract

The Dutch Meuse - Rhine Delta mostly consists of Holocene clay and peat layers, overlying a Pleistocene sand substratum. The lowlands are below MSL, are protected against inundation by riverdikes and are mechanically drained. Time dependent variations of the river levels provoke time and distance dependent piezometric level responses in the Pleistocene sand layers. During storm surges, high piezometric pressures may reduce the bearing capacity of the embankment's foundation. Predictions of the maximum piezometric pressures are needed for safe and economic design of the dikes, taking into account the unsteadiness of groundwater flow during design storm surge. An analytical method allows to describe the piezometric response for both a tidal and step (surge) input. This method has been applied for the geo-hydrological conditions of the Meuse-Rhine delta: a pervious aquifer (Pleistocene stratum) overlain by an aquitard (Holocene layers) with time dependent leakage. The model accounts for the possible presence of a silt or mud layer between the river and the aquifer. If the calculated pressure in the sand somewhere exceeds the weight of the Holocene layers, pressure redistribution occurs and an area will be uplifted. The model can be adapted to this non-linearity and then allow to evaluate the time dependent length of the uplifted area and new piezometric levels. The model parameters can be obtained from measurements made during normal tides.