Improving the land subsidence model Phoenix

Abstract

Land subsidence is a problem in the Dutch peat lands. These lands subside when low ground-water tables are maintained, and result in an increased safety risk, more complex water man-agement and CO2 production. In order to predict this subsidence Hoogheemraadschap De Stichtse Rijnlanden and the province of Utrecht created the land subsidence model Phoenix. The problem was that this model was not complete. Two improvements that could be made to this model were identified. The first improvement was adding regional differences in land subsi-dence due to for example differences in water management and soil characteristics. An attempt was made to implement these regional differences by identifying historical land and surface wa-ter subsidence, and how they relate. It was found that the average land subsidence in the study area over the past 40 years has been around 7,7 mm/y, with outliers up to 60 mm/y in Zegveld. The surface water levels on average have subsided less rapid resulting in a decrease in free-board over this same time period. Although regional differences were observed, they cannot be implemented responsibly as they were explained by errors in input data. The second improve-ment was the implementation of the increase in land subsidence due to temperature changes caused by climate change. The impact of climate change, according to scenario W+ of the KNMI, on land subsidence causes the peat soils to subside up to twice as fast in 2200 com-pared to 2000, having the same freeboard and soil characteristics. This climate change impact was added to Phoenix and modelled up to 2200. This run was then compared to a base run. The result was that soils with 30 cm clay had the largest additional land subsidence up to 2,4m. The largest percentage difference in land subsidence was observed in soils with 60 cm clay, which may subside up to an additional 270%.