Land subsidence in drained peat areas of the Province of Friesland, The Netherlands

The Holocene pear areas in the Province of Friesland consisting of peat and clay deposits are used extensively for agriculture and are drained by maintaining a low surface water level in polder ditches. Considerable rates of land subsidence (1 to 12mm/year) result from processes such as soil shrinkage, consolidation and peat oxidation, the last named being the major contributor. In the Province of Friesland studies of the future effects of continuous long-term land subsidence have been made as part of the hydrological masterplan. For these studies the Geological Survey of The Netherlands (RGD) has compiled maps showing the estimated subsidence over a 50-year period for three scenarios of freeboard (difference in elevation between surface water level and the ground surface). Data sources consist of recent ground surface levels measured for the construction of utility lines, to be used for the transport of water, gas or sewage, earlier topographical maps, historical surface water level records and soil profiles. Maximum estimates for rates of subsidence for peat with and without clay cover have been established by evaluation of the data with subsequent subsidence process analysis for various situations of drainage, duration of drainage, soil and hydrological conditions. Three maps showing the estimated land subsidence over a 50-year period for freeboards of 0.4, 0.8 and 1.2 m have been compiled using a two-dimensional Geographical Information System (2D-GIS), coupled with the RGD database, and have been presented in 1 km(2) grids. A comprehensive map of estimated land subsidence is being compiled by the Province, taking into account the actual freeboards at each location. The presented study forms a new approach to the estimation of future land subsidence in peat areas on a regional scale.