Shortly after the start of the experiment, it was observed that ‘sand boils’ were present outside the test strip. On the left side of the test section (i.e. southwards), water and sand was emerging from small (mice?) holes near the levee toe area. At 11:58, the test resumed for the last time, until 12:05. The total pumping time was only 67 minutes before final damage. The field hypothesis was that sand eroded from under the cover layer (i.e. sand from the levee core is transported, even partially outside the test section) created a mass deficit and destabilized the cover layer (undermining), leading to failure (sudden collapse)

Failure of the lower levee slope.

- The presence of a tree and surface root system enhances erosion of the soil around the tree. However progress was slow without quickly escalating to any (critical) damage. - The small cliff on the lower part of the slope a couple meters in front of the tree, created by sheep did not give rise to direct damage. - The presence of burrows (mice, mole) is believed to have provoked the flow of water under the clay cover, through the inner sand core. This fairly quickly led to outflow of sediment near the toe of the levee (notable adjacent to the test section). The resulting mass deficit lead to ground subsidence and finally a collapse of the top layer into the created void. - Dewatering of the upwind sediments was observed during which collapse of the sand and tearing of the overlying clay cover occurred, effectively resulting in upward moving headcut erosion.

Factual data report + Annexes: https://stowanl-my.sharepoint.com/:f:/g/personal/llhpp_stowa_nl/EvdAZOO-zylAqEWS8-SEiC4BQ5xCAuFXklsysJXnkcVYaQ?e=XDeXq8

An integration report will be added later (February 2023) as knowledge document.

The integration report discusses insights gained from both the steady overflow and wave overtopping tests.

under development