The Polder2C's project aims to gain an improved insight into the effects of climate change on the required flood protection of the hinterland. The knowledge, information and data that are collected is a valuable basis for future choices and applications in the field of safety against flooding.

The first overflow and overtopping tests of Polder2C’s, executed in the winter season of 2019/2020 were aimed at acquiring new insight in the occurrence and effects of these failure mechanisms and their influence on dike strength. During these tests, mechanisms were identified that exhibited symptoms related to piping. Sand transporting water flows were observed on the inner slope of the dike, emerging from holes in the cover layer and in some cases resulting in erosion of the sand core of the levee.

On the location of Patrik’s-cliff on the Scheldt-levee (Belgian part of the Hedwige-Propserpolder) during one of the field experiment a large subsidence took place. The damage that occurred erode the clay cover layer and eroded part of the sand core. This damage was repaired with rock bags as part of a Polder2C’s emergency exercise. The rock bags were a to be validated repair measure for these kinds of damages.

During subsequent Polder2C’s exercises in the field waterflow was planned to hit the rock bags to test their erosion-resistance. During these exercises, performed on the 30th and 31st of March 2021 waterflow was initially diverted past the rock bags. However, even whilst the water was being diverted, water flow could be heard and seen taking place underneath the rock bags. The water flow started to erode sand and subsidence of the rock bags took place. Water flows were also detected visually through the clay cover layer on the location of Patrik’s cliff.

Questions about the origin and flow paths came to mind: what was observed on site? How did the water flow through the cover layer? Could this water be the cause of sand eroding from the inside of the levee? It was then decided by STOWA and Rijkswaterstaat that further fact finding research to clarify the observations made needed to take place on short notice.