3D characterization of Holocene peat in the Netherlands: Implications for coastal-deltaic subsidence

Abstract

Human-induced subsidence threatens many coastal-deltaic plains, due to the amplifying effects it has on sea-level rise and flood risk. In the coastal-deltaic plain of the Netherlands, subsidence is primarily caused by the compression and oxidation of Holocene peat. The understanding of subsidence in the Netherlands and the capacity to model and predict it for future management scenarios greatly benefit from methods that enable physical property mapping of subsurface peat layers, which are examined in this thesis. The objectives were to 1) reconstruct provisioned accommodation space during the Holocene build-up of the coastal-deltaic plain, 2) develop new approaches to reconstruct and quantify the amount of subsidence that peat experienced due to compression, 3) characterize in-situ physical properties of peat with various degrees of compression, 4) upscale field and laboratory measurements to 3D maps, and 5) use this to predict future subsidence for scenarios of groundwater-level lowering. Approximately 94 km3 of accommodation space was created in the back-barrier wetland of the coastal-deltaic plain. 54 km3 (57 %) of this volume was provisioned prior to the formation of a back-barrier peat layer, and 40 km3 (43 %) during its formation. In urbanized areas this peat layer is overlain with maximal 8 m of overburden and subsided up to 6 m during by compression. In agricultural areas, where overburden is less thick, it subsided between <1 - 3 m. In total, 12 % of the peat layer its volume is lost due to human-induced peat compression. This thesis shows that CPT is a promising method to determine the compression of peat. When peat fibers are progressively packed tighter together, higher resistances are provided by the peat beds to the penetrating cone. Net cone resistance increases by 0.3 MPa, when compression of surficial buried peat reduces their thickness by about 40 % and void ratio decreases from 10 to 6. A 3D map showed that Holocene peat in the Netherlands consist of 1.5 km3 of organic matter, 0.4 km3 of sediments, and 13.1 km3 of voids. If this peat oxidizes, the area will lose 14.6 km3, thereby emitting 2.0 Gton of CO2. The back-barrier peat contains most organic matter, with volumes ranging between 5000 and 8000 m3/hectare. Future subsidence predictionsreveal that cities are less susceptible to subsidence than agricultural lands. This is caused by thick anthropogenic brought-up soils in urbanized areas that already has compressed the underlying peat; further compression is restricted due to low void ratios, and oxidation is minimized as peat is pushed below groundwater levels. This thesis demonstrates that spatial differences exist in past, current, and future subsidence in the coastal-deltaic plain of the Netherlands. It shows that agricultural lands are more prone to subsidence than urbanized areas. Methodologies were presented to 3D map physical peat properties. CPT is a promising tool for mapping properties that are necessary to determine the potential of peat to future compression. 3D mapping is essential to assess the potential of the coastal-deltaic plain to future subsidence by peat compression and oxidation.