|Spatial variations in polder subsidence rates in Zeeland, Netherlands
Bonatz, H. (2020). Spatial variations in polder subsidence rates in Zeeland, Netherlands. BSc Thesis. NIOZ Royal Netherlands Institute for Sea Research: Yerseke. 51 pp.
The Dutch coastal plain is a highly populated area, facing an increasing threat due to relative sea level rise (RSLR). The province of Zeeland in the South-West of the Netherlands deals with this threat since its habitation 1000 years ago. The continuous reclamation and drainage of land led to a self-preserving process of subsidence that increases the risk of flooding. The impacts of past cultivation and embankment practices show the need for a new approach of coastal management to guarantee the safety of land and people. Recently, ecosystem-based engineering has been receiving a lot of attention worldwide and could be an effective protection strategy for Zeeland’s coast. One concept in the field of ecosystem engineering is the so-called double-dike solution. It allows a salt marsh to develop in the outmost polder by breaching the primary dike. To assess the feasibility of the double-dike system, many natural processes need to be examined, as they respond to the construction of a double-dike. An important component for the assessment is the subsidence rate of a polder. Because area-wide measurements over longer timescales are not available, this study asks for spatial variations in polder subsidence rates in Zeeland, viewing the elevation change of a polder that occurred since its establishment. The past land elevation is estimated under the assumption that the elevation approximated the mean high-water level (MHWL) in the year of embankment. Thus, a MHW reconstruction for the period 1100 to 1980 is performed in the study. Information on subsidence rates can serve as parameters in various models for the assessment of ecosystems as coastal protection measures. Results show an average subsidence rate of 2.3 mm per year. After the exclusion of polders with insufficient data (data coverage), and obvious human interaction (urban and industrial areas), the range of rates still varies between -4.6 and 30.1 mm per year. Extreme subsidence rates can be attributed to human interventions such as soil extraction or the artificial raising of land. Further the study’s findings indicate that subsidence rates vary between the polders, most likely because of their different soil properties (organic matter). However, to prove this, further analysis needs to be performed. In conclusion, the results show the need for more sustainable coastal protection strategies, and how vital the subsidence rate is to assess the feasibility of ecosystem-based coastal engineering, such as the double-dike system, at specific locations.