This chapter discusses and assesses processes underlying the loss of functionality of (elements) of urban drainage systems. The chapter does not pretend to cover all literature nor do the authors claim to cover all processes involved. As the body of literature experiences a rapid growth, the authors want to provide the reader a comprehensive overview of the literature to-date to be used as an entry for further exploration. Several processes are described and the possibility to model them in a deterministic manner is discussed. The last sections provide a brief introduction to the use of statistical models based on condition classes (as is applied in practice) along with a brief overview of advancements made in machine learning applications.

Time-lapse ground-penetrating radar (GPR) surveys, combined with automated infiltration experiments, provide a non-invasive approach for investigating the distribution of infiltrated water within the soil medium and creating three-dimensional images of the wetting bulb. This study developed and validated an experimental protocol aimed at quantifying and visualizing water distribution fluxes in layered soils under both unsaturated and saturated conditions. The 3D images of the wetting bulb significantly enhanced the interpretation of infiltration data, enabling a detailed analysis of water movement through the layered system. We used the infiltrometer data and the Beerkan Estimation of Soil Transfer parameters (BEST) method to determine soil capacitive indicators and evaluate the physical quality of the upper soil layer. The field survey involved conducting time-lapse GPR surveys alongside infiltration experiments between GPR repetitions. These experiments included both tension and ponding tests, designed to sequentially activate the soil matrix and the full pore network. The results showed that the soil under study exhibited significant soil aeration and macroporosity (represented by AC and p_MAC), while indicators related to microporosity (such as PAWC and RFC) were notably low. The RFC value of 0.55 m³ m⁻³ indicated the soil’s limited capacity to retain water relative to its total pore volume. The PAWC value of 0.10 m³ m⁻³ indicated a scarcity of micropores ranging from 0.2 to 30 μm in diameter, which typically hold water accessible to plant roots within the total porosity. The saturated soil hydraulic conductivity, K_s, values ranged from 192.2 to 1031.0 mm h⁻¹, with a mean of 424.4 mm h⁻¹, which was 7.9 times higher than the corresponding unsaturated hydraulic conductivity measured at a pressure head of h = −30 mm (K₋₃₀). The results indicated that the upper soil layer supports root proliferation and effectively drains excess water to the underlying limestone layer. However, this layer has limited capacity to store and supply water to plant roots and acts as a restrictive barrier, promoting non-uniform downward water movement, as revealed by the 3D GPR images. The observed difference in hydraulic conductivity between the two layers suggests that surface ponding and overland flow are generated through a saturation excess mechanism. Water percolating through the soil can accumulate above the limestone layer, creating a shallow perched water table. During extreme rainfall events, this water table may rise, leading to the complete saturation of the soil profile.

Urban areas are increasingly under pressure due to rapid expansion and extreme rainfall events driven by climate change. Rather than relying solely on pipes to regulate stormwater, new management systems promote direct infiltration of water and pollutants into the soil. Soils fulfill multiple functions, including water infiltration and pollutant filtration. Urban soils, however, are often highly heterogeneous; the presence of macropores can accelerate water flow while reducing pollutant retention. Yet, infiltration and filtration are typically assessed using surface measurements and soil‐destructive methods, which fail to capture the complexity of water and pollutant transport pathways in the subsurface. In this study, we present a prototype of a large‐ring infiltrometer coupled with a ground‐penetrating radar (GPR) to simultaneously monitor water and nanotracers movement in the subsurface. This provides a more comprehensive characterization of both infiltration and filtration functions. Superparamagnetic iron oxide nanoparticles (SPIONs) were chosen as nanotracers due to their nontoxicity, small size, and advantageous electromagnetic properties. Although distinguishing between subsurface zones imbibed with water and those containing SPIONs was challenging, GPR imaging revealed distinct spatial patterns and temporal dynamics even under identical infiltration conditions. Comparative analyses with water and solute transport models further elucidated the underlying subsurface processes. Typologies of infiltration and filtration patterns, linked to soil heterogeneity, are presented. Our results emphasize the need for a more comprehensive characterization of infiltration and filtration functions to improve flood risk assessment and evaluations of groundwater pollution threats.

Stormwater management zones must enable water to infiltrate easily, primarily due to macropores, but little is known about the transport of pollutants through these macropores. Some coupled methods using, for example, dyes, disc, or ring infiltrometers were developed to give insight on the respective contributions of the macropores versus the matrix to the bulk infiltration. However, these methods do not visualize where and how water infiltrates. Besides, no information is given on the solute transfer in soils, whereas this issue is crucial regarding the quality of soils and groundwater. One of the goals of the national program INFILTRON (https://infiltron.org) project granted by the French national research agency is to develop superparamagnetic iron oxide nanoparticles (SPIONs) which mimic both pollutant and bacteria flow behavior in soils (Raimbault et al., 2021) and are detectable by ground-penetrating radar (GPR). Within its framework (Lassabatere et al., 2020), we aim to show how nano-tracers can help detect preferential treatment flows (lithological heterogeneity, root system) and quantify or qualify the pollutant transfer in heterogeneous soils. A specific device was designed and presented for the concomitant monitoring of water infiltration and nano-tracer injection. This specific infiltrometer involves two water supply reservoirs and a ring diameter of 50 centimeters. This device maintains a constant depth of water (10 cm) above the soil and delivers the water infiltration into the soil. Two rules posed on the reservoirs allow monitoring the water drop and computation of the cumulative infiltration. Fifty volumes of SPIONs solutions (5 mL of 3.35g/l SPIONs solution) were injected into the ring to maintain a constant concentration in the ring. GPR monitors the bulb of infiltrated water and SPIONs. GPR data is treated with ReflexW (©Sandmeier geophysical research) and RockWorks (RockWare®) software. Combining this specific prototype with the use of GPR for the detection of water and the SPIONs gives insight into the processes of infiltration and SPIONs transfer and localization in the soil. These data allow us to understand and model pollutant transfer into the vadose zone.Raimbault, J., Peyneau, P.-E., Courtier-Murias, D., Bigot, T., Gil Roca, J., Béchet, B., and Lassabatère, L.: Investigating the impact of exit effects on solute transport in macropored porous media, 2020, 1–20, https://doi.org/10.5194/hess-2020-494, 2020Lassabatere, L., De Giacomoni, A.-C., Angulo-Jaramillo, R., Lipeme Kouyi, G., Martini, M., Louis, C., Peyneau, P.-E., Rodriguez-Nava, V., Cournoyer, B., Aigle, A., and others: INFILTRON package for assessing infiltration & filtration functions of urban soils, in: EGU General Assembly Conference Abstracts, 11269, 2020. https://doi.org/10.5194/egusphere-egu2020-11269