Effects of land subsidence on regional groundwater flow in De Stichtse Rijnlanden (The Netherlands)

Abstract

It has long been recognized that the changing climate will influence groundwater resources, but there is no extensive research within the area (IPCC 2001; Holman 2006). The main objective of this MSc Research thesis is to increase the knowledge of how land subsidence influences the regional groundwater flow due to an alteration of the vertical flow resistance of the Holocene top layer. The vertical flow resistance is expected to decrease, which mainly is caused peat oxidation, shrinkage and loss of bouyancy (van der Schans & Houtuessen 2012; Geisler 2014). The research that will be based on computer simulation can help to improve our understanding of how climate change and current water management affects soil subsidence and what the impact is on the regional groundwater flow. Two five year intervals, during 2050 and 2100 are simulated and one worst-case scenario where all the peat in the Holocene top layer is gone. The location of the research is the management area of Hoogheemraadschap de Stichtse Rijnlanden (HDSR). To determine what the effect of peat subsidence will have on the regional groundwater a combination of two computer models and one computer program was used (KNMI 2014). The two models that will be utilized is Phoenix, which is a subsurface model that calculates the new ground surface (van der Schans & Houhuessen 2012) and the regional groundwater model Hydromedah (Borren et al. 2009). The results indicate a strong correlation between land subsidence and lowering of surface water levels. When peat subsides, it generates a lowering of the ground surface. The freeboard is no longer as desired; therefore, the surface water level must be lowered. This is accomplished by pumping surface water out of the system, resulting in a replenishment of groundwater to the river systems (Brunke & Gonser 1997; Sophocleous 2002). By lowering the surface water levels, more peat will be present above the saturated zone and further subsidence will be taking place (Geisler 2014; Price 2003). The negative developing trend of the peat areas will lead to a lowering of the hydraulic head in the regional aquifer, decrease in the vertical flow resistance and an increase in the upward flow. The upward flux from the first aquifer is increasing as peat continues to subside (Schot & Van der Wal 1992). As the global warming continues the rate of peat subsidence is expected to double until 2100 (RECARE 2017). National collaboration is strongly suggested for the purpose of developing drainage systems to reduce the land subsidence.