Redifining infiltration drywell design

Abstract

This research is aimed at creating a generic design method for infiltration drywells on sandy soils by acquiring knowledge on the functioning of these wells. Infiltration drywells are vertical infiltration pipes that are installed above the groundwater table and through which stormwater is drained to the subsurface. Infiltration drywells can have a prominent place in urban water management since they mimic processes that occur under natural conditions. In urban areas the hydrological cycle is altered due to impermeable surfaces. Utilization of infiltration reduces the effects of the alteration on the hydrological cycle. Hereby, reducing the risk on urban flooding and surface water contamination. Furthermore, urban heat stress is reduced by enabling more drought resilient vegetation through groundwater replenishment. Especially considering that climate change will result in more extreme weather conditions, infiltration facilities can aid in creating more resilient urban areas. Until now, there are no design rules for these wells which hinders the implementation in urban areas that are fit for infiltration facilities. In this research a design method for sandy soils in the Netherlands is created and set forth. The theoretical and practical performance of infiltration drywells is analysed by conducting experiments with Hydrus 3-dimensional geohydrological model simulations, as well as in the field and laboratory. In the field falling head tests were performed with existing infiltration drywells to determine the functioning while soil samples were analysed in the laboratory to determine the hydraulic conductivity. The model simulations also exist of falling head tests and are compared to the experiments in practice. It was found that the most important parameters on functioning of infiltration drywells are the soil hydraulic conductivity and well dimensions. When comparing the simulated falling head tests to field tests and laboratory tests at the same location, discrepancies were discovered. This can be clarified by simplifications that were made like homogeneity and isotropy of the soil in the model. Furthermore, the absence of wall resistance of the well in the model and the method that was used for the calculation of hydraulic conductivity using in practice falling head test data could be the cause. To this end the generic design method is based on the Hydrus 3D model. This design method consists of empirical contour plots that give the necessary number of wells based on multiple input parameters, including a design storm of 21 mm in 10 min, which has a statistical return period of 25 years in the Netherlands. Due to discrepancies in the research, the design method is used to test the viability of a plan to implement infiltration drywells. Afterwards, a detailed design procedure is still necessary. Overall, the research resulted in a generic design method and shows the advantages of using infiltration drywells, which could be an essential part of urban water management in the Netherlands in the future.