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Preserving Medieval Farm Mounds in a Large Stormwater Retention Area

The Netherlands has denoted large areas as stormwater retention areas. These areas function as temporary storage locations for stormwater when rivers cannot cope with the amount of water. A large area, the Onlanden — 2,500 hectares — was developed as such a storage area between 2008 and 2013. This peat area contains up to 300 medieval housing areas. These ‘peatmounds’, named after the current visible small mound, were explicitly mentioned as a preservation goal in the project. The preservation of the peatmounds during and after the project has been guaranteed by a combination of protective measures, research, and monitoring actions. At first a risk evaluation was performed, followed by a monitoring project focusing on the preservation of the organic part of the mounds. This evaluation showed that the rewetting of the mounds might improve the overall preservation. A total of fifteen monitoring stations were a-selectively distributed over the total area, covering the four main types of land use. The measured parameters at these stations focus on the desiccation/rewetting effects. The second threat, as determined in the risk evaluation, is the growth of deep penetrating plants. These roots might disturb the archaeological profile. Preventive coverage with plastic and up to a metre of soil could prevent root growth into the archaeology. An experimental coverage showed the effectiveness of this technique, but also the risks. This paper focuses on the lessons that can be learned from this six-year project, the monitoring results, and clearly shows the benefits of a combined approach in large-scale projects.