Controlling urban groundwater in delta areas

Abstract

In the light of the European Work Program DRS-11-2015, a Dutch Collision “Solid City on Soft Soil” was formed in order to mitigate disasters brought by subsidence and safeguard cultural heritage assets. As an initial approach, the Dutch Collision planned to explore and implement solutions for this challenge in the historic city center of Gouda and made this project as a paragon for other cities suffering from the similar problem. Multiple parties – governments, scientific institutes, private parties, and civilians – are interested and involved in this project. The inner city of Gouda is an independent polder built on the Holland-Utrecht peat. The combination of subsidence and climate change increases the overall vulnerability, and a sustainable and efficient urban water management plays a significant role when it comes to the solutions. As the starting point for the Gouda project, this research focuses on the investigation of the fluctuation characteristic of groundwater under different influential factors (primarily precipitation, evapotranspiration, and sewage water) in two contiguous drainage areas – Nieuwe Haven and Centrum, where the impacts of climate change and land subsidence had been identified enormously negative. In order to have a comprehensive understanding about the groundwater flow pattern in the research area, a new groundwater observation network was designed and implemented on 20th of May, 2016 to collect sufficient groundwater level data. The network took into consideration of the influential factors on groundwater, susceptible areas to groundwater fluctuation, and accessibility for subsequent validation and maintenance. Once the new observation network started to measure the groundwater levels properly, a field experiment was implemented to investigate the leaky extent of the back-stowed sewer system as well as the response of the groundwater to the variation of the sewage water levels. Both the time series and cross-section analysis among groundwater levels, sewage water levels, precipitations, and evapotranspiration were carried through. At the end, a preliminary construction and parameter analysis of a 3D finite-difference transient daily groundwater flow model were performed with the help of iMODFLOW v2.6.37. However, further calibration of the model is required to improve the model performance in the future. The current results illustrate that the leaking extent of the back-stowed sewer system depends on the year of the construction and the pipe material. Generally speaking, the reactions of groundwater levels to the changes of the sewage water levels are up to the leaky extent of sewer pipes and the distance to the leaky system; groundwater in the vicinity of vegetation displays a similar variation pattern with the potential evaporation within a day; and groundwater in the unpaved and riparian areas are more sensitive to the precipitation. The whole project is still in progress, and certain aspects of current investigation require further improvements. Additional recommendations related to data collection, model calibration, and investigation in other areas of the inner city of Gouda were given at the end of this thesis. Although there is no “one-size-fits-all” solution, due to the fact that seldom research has focused on the urban groundwater management related to land subsidence so far, the methods, procedures, experience and lessons accumulated in this research can still be considered as a guide or assistance for other areas or cities which are willing to make a change.