The Swedish Meteorological and Hydrological Institute projects that Sweden will experience significant climate change, with temperatures increasing by 3-5°C by 2080. This temperature rise is anticipated to bring significant changes in precipitation patterns, particularly an increase in autumn, winter, and spring rainfall, which will in turn elevate runoff volumes and rates. These shifts necessitate an improvement of stormwater management practices, particularly in urban environments where ongoing trends in urbanization, combined with the increased frequency of short-term extreme rainfall events, are likely to cause an increase in the number of flood events. Managing stormwater effectively in urban areas is crucial to achieving sustainable city objectives. One innovative approach for managing stormwater is the concept of dynamic storage in sponge-like porous bodies (SPBs). The SPB solution could be integrated with existing methods; from pipes to green roofs and small-scale infiltration practices to enhance stormwater retention. The primary research focus is on developing a theoretical model to evaluate the performance of SPBs under different conditions.

In the first paper, a pre-existing model for water uptake in a SPB up-flow storage device is further developed by refining its analysis of water absorption from impermeable to partially permeable surfaces, including scenarios involving water uptake from the saturated soil and the surrounding flood. The research investigates various conditions, such as the impact of having an impervious bottom surface with or without precipitation and examining the effects of permeable substrate under similar circumstances. A mathematical model is derived using mass conservation, Darcy’s law, and a sharp wetting front modelling approach. Results for water uptake height, flood depth, and wetting front are numerically computed and analytically resolved, where possible, with critical values identified. Parametric studies reveal increased water absorption with soil infiltration when precipitation is included, as compared to when there are no precipitation events. The model is optimized for various rainfall and soil permeability values based on Swedish rainfall data.

The second paper explores the added mass phenomenon during the initial stages of capillary flow. This understanding is important, as it improves the accuracy of modelling water absorption by porous materials governed by capillary flow, particularly in the context of SPBs. A novel approach is proposed for determining the added mass coefficient, also applicable to general scenarios involving capillary rise. Traditionally, added mass is calculated by analysing changes in kinetic energy, typically using a hemispherical cap control volume situated beneath the entrance of the tube, where the velocity profile is considered radial outside the cap and uniform within it. The added mass coefficient is computed by assuming potential flow downstream the entrance to the tube for three distinct scenarios. In the first scenario, involving the immersion of a cylindrical tube into an infinite water reservoir, the exact value of the coefficient is determined to be ​  . For the subsequent cases, which involve coaxial cylindrical structures with finite reservoir depth, both analytical derivations and numerical plots of the coefficients are presented.

The third paper revisits the first study and enhances it by incorporating the nonlinear Richards equation to account for water flow into unsaturated soils. This modification makes the model significantly more realistic by simulating water movement under various external conditions. The theoretical analysis explores the absorption process and the head pressure change. The first phase involves absorption by the soil alone, occurring when the pressure is lower than the capillary head pressure -h_c. In the second phase, both the soil and the SPB contribute to absorption within the pressure range [-h_c,0]. When full saturation is achieved, the flood is starting to form on the surface. The initial phase permits a comprehensive analytical solution, while the subsequent phase requires a blend of semi-analytical approach and numerical computations. Solutions are plotted, and the efficiency is evaluated by comparing flood formation times in the presence and absence of the SPB. Future research will focus on experimental validation of SPBs as well as further theoretical improvements to assess and enhance their effectiveness in stormwater management in urban environments affected by climate change.