Determining drought-induced subsidence in urban areas

Abstract

Drought and subsidence are two out of several water-related urban climate adaptation challenges many cities in the Netherlands currently face. Drought is expected to increase in frequency and extent due to climate change. Therefore, drought is likely to further pressurize (subsiding) urban areas in the coming decades. Although the impact of drought on soft soils and hence subsidence is described in academic literature, in-practice analyzes are limited. Given the expectation of increased drought impacts, as well as the current urge for new housing developments to combat housing scarcity, this research focuses on in-practice drought impacts on subsidence in two soft-soil urban areas in the Netherlands. This research’s objective is to gain insight into drought-induced subsidence in soft-soil urban areas in the Netherlands, in order to better understand drought impacts on subsidence rates in future housing developments on comparable soils. With a better understanding more appropriate site preparation strategies can be applied. The objective is endeavoured by an in-practice analysis of two study areas: a 90s neighbourhood in Diemen and a recently finished urban area in Kampen. Firstly, variations in relative surface levels, observed by InSAR (2015-2019), are compared for dry and wet periods. Afterwards, this subsidence data is separated based upon local characteristics (soil structure, pavement type, vegetation percentage and vicinity of surface water) to analyze their influence on drought-induced subsidence. Lastly, an expert questionnaire is conducted on suggestions for both mitigation and adaptation strategies. Subsidence is not a linear process in time: surface level movements vary due to the soil’s wetting and drying. The extent of soil compaction in dry periods is found to increase with intenser droughts. The extent of soil swell in wet periods is approximately similar for varying wetness extents. This research found that the severe drought of 2018 thereby caused a soil compaction to such extent it could not be balanced with subsequent winter swell, and hence resulted in approximately 1 to 1.5 millimeter of drought-induced subsidence. Drought-induced subsidence is thus a net result of the (change in) seasonal surface level fluctuation. This net result is assumed to be consequence of extensive groundwater level drops, although the exact share of processes and subsidence mechanisms could not be estimated. Moreover, surface level movements are found to be prone to a lag and difference in duration (hysteresis) in comparison to the start and duration of dry and wet periods. Various local characteristics are found to influence drought-induced subsidence. In general more compressible soils show slightly larger surface level movement between dry and wet periods. The (sand) cover thickness is found to be influential on which subsidence mechanisms are triggered during a drought: a cover thickness resulting in groundwater levels to drop to present clay/ peat layers causes shrinkage and/ or peat oxidation additional to clinch. Unpaved surfaces are found to fluctuate more extensively than paved surfaces, but this does not necessarily result in more irreversible subsidence. Furthermore, abundant vegetation might result in extra irreversible subsidence due to its extensive water usage in dense urban areas. Lastly, in the analyzed soft-soil areas the surface waters seem to influence groundwater levels and hence surface level movements only on short distance. Suggestions on feasible enhancing site preparation strategies are given based upon research results and experts’ opinions. The suggested mitigation strategies consist of two approaches in order to hamper the variations in effective stresses and hence minimize the seasonal surface level fluctuation. The first approach focuses on preventing extensive groundwater level variations: increasing storage and infiltration of water, reversed drainage, building crawl-space free and choosing vegetation types based on their water usage. Additionally, it is suggested to apply a sufficient cover thickness (at raises) if soft soil layers are near the surface. This prevents that future extensive groundwater drops within these layers, and thereby limits (seasonal) shrinkage and/ or peat oxidation. The second approach focuses on reducing the top soil’s weight, via lightweight materials or self-carrying constructions. Adapting to drought-induced subsidence starts with measuring/ monitoring surface level movements in order to analyze spatial and temporal trends. Additionally, drought is to be considered to greater extent in subsidence modelling in order to improve subsidence estimations. This can be done by applying variable or lower groundwater levels in estimations of the (change in) effective stresses, at calculations of consolidation or creep. Lastly, urban utility management should focus on long-term costs via e.g. Life Cycle Analysis, and on overlapping maintenance cycles of surface level raising and e.g. sewer pipe replacements. The most important conclusion derived on urban drought-induced subsidence is that despite individual drought impact on subsidence is limited to 1 to 1.5 millimeters, its seasonal occurrence continuously affects surface levels. Moreover, due to climate change drought is expected to increasingly impact surface levels in Dutch soft-soil urban areas in the coming decades. The suggested strategies mainly hamper variations in soil stresses, via fluctuating groundwater levels, and hence drought impacts on soft soils. These strategies help to limit future soil movements and hence result in more climate adaptable soft-soil urban areas. The focus of this research is on qualitative analyses of in-practice data such that significant processes have been disclosed, rather than statistically verifying the results. Consequently, the research initiates further specific studies on statistical verification of drought-induced subsidence, and moreover topics on its mechanisms; its spatial and temporal variation; its measurement and modelling; the influences of local characteristics hereupon; and the effectiveness of the suggested enhanced site preparation strategies.