Soil subsidence in South Holland is strongly related to water management. As a result, it is not possible to make realistic predictions of soil subsidence over longer periods. Instead, the subsidence sensitivity has been mapped. To this end, it was calculated how much peat oxidation and settlement would theoretically occur over a period of 100 years, with a fixed drainage depth of 1 m and the current hydraulic head in the first aquifer being maintained. Three classes were then made of the calculation results: 'not relevant', 'relevant' and 'special area of interest'. The classes reflect variations in the structure of the subsoil. Where mainly sand occurs, subsidence is not relevant. Subsidence does play an important role in clay and peat areas. Clay areas and areas with a relatively thin layer of peat fall into the 'relevant' category. The 'special focus area' comprises the ten percent of the provincial land area with the greatest subsidence sensitivity. This mainly concerns areas with a thick layer of peat. The map for subsidence sensitivity shows that a large part of the peat meadow areas require special attention in design and water management based on the theme of subsidence. This applies especially to those areas where the peat occurs directly at ground level and is not covered by clay (including parts of Midden-Delfland, Alblasserwaard, Krimpenerwaard, the area between Gouda and Boskoop and around the Nieuwkoopse Plassen). In the polders around Zoetermeer, the peat has disappeared due to peat and subsidence is less relevant. Subsidence is also irrelevant along the Oude Rijn, on the islands of South Holland and in a broad zone along the North Sea coast, with the exception of the low areas between the beach barriers. Three scenarios are shown below for soil subsidence at a fixed dewatering depth of 0.5, 1.0, resp. 1.5 m. The soil subsidence sensitivity map has been distilled from the scenario with a fixed dewatering depth of 1.0 m. The three scenarios provide insight into the dependence on subsidence for water management. For example, it can be seen that at a drainage depth of 1.5 m, the area with a subsidence of more than 10 mm/year is more than twice as large as at a drainage depth of 0.5 m. area or question is zoomed in, it must always be checked whether further information is available.
