Samenvatting

Animal burrows create discontinuities on the body of earthen levees that under certain circumstances can threaten their structural integrity through hydraulic alteration and surface erosion.
Although it is a virtually established fact that discontinuities can undermine the structural integrity of levees, there are limited studies that clarify when animal activity can be considered dangerous and which are the most effective approaches for dealing with it. Interaction with levee management agencies in the network of Polder2C’s has shown that there is much tacit knowledge on the topic among levee guards in the Netherlands, Belgium, France and the UK, which has only been marginally reported so far. It is also of interest that the Dutch Government approved the inclusion of measures against harmful animal activity in flood defences more than a decade ago (Tweede Kamer 2010), but to this end, an integrated national management framework is still pending. This highlights the fact that formal knowledge on the topic is limited and fragmented.

In order to enable the development of a rational framework for dealing with animal activity on levees, a coordinated effort is required for the review and reporting of existing knowledge, identification of knowledge gaps and development of approaches to determine the level of risk incurred by various types of animal activities on levees and the effectiveness of reduction actions. This report presents activities that took place within Polder2C’s and could support the development of such a rational framework.

After the first winter of experimentation in the Living Lab Hedwige-Prosperpolder, evidence was produced about possible adverse impact of animal- and vegetation-induced discontinuities on levees (Holscher & Zomer, 2021; Koelewijn et al. 2021). Overflow experiments at locations of anomalies caused by grazing sheep, trees and fox holes led to severe erosion of the structure within a considerably short time (see also Figure 1.2a). In June 2021 a mole burrow system was detected in a levee section that had been damaged during overflow experiments and where an emergency repair application with rock bags had been subsequently applied. In order to explore the geometry and extent of the system, the mole tunnels were grouted with fast drying concrete. Upon excavation of the concrete an extensive burrow system was revealed with exit points close to the toe and close to the crest of the section (see also b).

In preparation of the second winter of experimentation, a cross-work package team was created to identify knowledge gaps in the management of animal burrows on levees and suggest research activities that could potentially fill in identified gaps. This led to a longlist of possible activities, a number of which were eventually executed in the period September 2021 till February 2022. The final selection of activities was limited by the contractor’s work and other large-scale experiments and exercises. Priority was given to activities that had no conflicts with the abovementioned operations and for which there was available equipment and manpower within the involved organizations.

This report presents the work that was performed within Polder2C’s on the management of harmful animal activity on levees in a chronological order. It starts with a description of observations in the first year of the project, and a subsequent study that led to the development of a knowledge agenda in the period February till June 2021. It continues with a description of activities that took place in a more coordinated manner in the second year of the project and it ends with a summary of key findings.