Subsidence — dewatering depth 1.5 meters

Subsidence in South Holland has a very strong connection with water management. As a result, it is not possible to make realistic predictions of the subsidence over longer periods. Instead, soil subsidence sensitivity has been mapped. For this, it has been calculated how much peat oxidation and clink would theoretically occur over a period of 100 years, with a fixed dewatering depth of 1 m and maintaining the current elevation in the first aquifer package. Three classes were then made of the calculation results: ‘not relevant’, ‘relevant’ and ‘special focus area’. The classes reflect variations in the structure of the subsurface. Where sand is mainly found, subsidence is not relevant. In clay and peat areas subsidence plays an important role. Clay areas and areas with a relatively thin peat layer fall into the ‘relevant’ class. The “special focus area” covers the ten percent of the provincial land area with the greatest subsidence sensitivity. These are mainly areas with a thick peat layer. The map for soil subsidence shows that a large proportion of peat meadows, from the topic of soil subsidence, require special attention in design and water management. This is especially true for those areas where peat is directly on ground level and is not covered by clay (e.g. parts of Central Delfland, Alblasserwaard, Krimpenerwaard, the area between Gouda and Boskoop and around the Nieuwkoopse Plassen). In the dryhouses around Zoetermeer, the peat has disappeared due to dyeing and soil subsidence is less relevant. Also along the Oude Rijn, on the South Holland islands and in a wide zone along the North Sea coast, with the exception of the lowlands between the beach walls, subsidence is not relevant. After this, three scenarios of subsidence are shown at a fixed dewatering depth of 0.5, 1.0 and 1.5 m. From the scenario with a fixed dewatering depth of 1.0 m, the soil subsistence map is distilled. The three scenarios provide insight into the dependence on soil subsidence for fed water management. It can be seen, for example, that at a 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 dewatering depth of 0.5 m. At the time of zooming in on a specific area or demand, it should always be examined whether further information is available.