Session C8 - Infiltration: evaluation and monitoring

Many systems have been developed to treat and detain stormwater runoff. However, few are capable of significantly reducing runoff volumes, i.e. retaining runoff. In this study, we developed a permeable kerb and channel, designed to infiltrate large volumes of road runoff into road verges, to irrigate street trees. In Melbourne, Australia, we constructed a 20-m section of permeable kerb and channel servicing a structural soil infiltration trench planted with trees and monitored hydrological performance over one year. Despite slow exfiltration rates of the in-situ sandy clay soil (<1.5 mm hr-1), runoff retention was 81% and peak flow rate reduction was 79% for events which generated outflow. Soil moisture increased around the infiltration trench, indicating exfiltration as the major loss of runoff. The kerb-maintained infiltration rates >30,000 mm h-1, which increased to 54,000 mm h-1 after pressure washing, indicating clogging occurred. Infiltration rates would need to decrease by 99% before the kerb and channel could no longer infiltrate the highest observed rainfall intensity for the location. The permeable kerb and channel-maintained stability and stiffness, indicating no significant settling or movement due to infiltration of runoff. We plan to continue to monitor the experiment to determine maintenance requirements. Overall, the permeable kerb and channel represent a promising solution for infiltrating large volumes of runoff in urban areas.

Nature-based Solutions (NbS), such as Sustainable Drainage Systems (SuDS), are increasingly promoted to manage urban stormwater and reduce flooding linked to climate change. SuDS enhance on-site infiltration, alleviating pressure on sewer networks and supporting the development of “sponge cities” that safely manage rainwater in densely urbanized areas. The Metropolitan City of Milan (3.25 million inhabitants spread across an area of 1575 km², with a population density of 2064 inhabitants per km²) has implemented 90 SuDS interventions across 32 municipalities under Italy’s National Recovery and Resilience Plan (PNRR). These systems mitigate urban flooding and can act as distributed recharge for shallow aquifers, which — though unsuitable for drinking — can supply water for urban irrigation, functioning as an Alternative Water Resource (AWR). To validate this AWR pathway, a bioretention area in Solaro was equipped with soil-moisture sensors and a rain gauge to monitor infiltration dynamics. Two years of data were analyzed to validate sensor performance and compare observed behavior with theoretical design expectations. A simplified proxy was then developed to quantify actual infiltration from sensor and rainfall data. This proxy demonstrates the feasibility of using soil-moisture sensors as low-cost tools for integrated water management and therefore the urban flooding hazard, enabling performance evaluation, maintenance alerts, and emergency irrigation during droughts.

Climate change significantly affects the hydrological cycle, increasing flood risks in urbanized areas. Nature-based stormwater management approaches, such as Low Impact Development (LID), aim to mitigate these impacts by promoting infiltration, notably using swales. Swales not only reduce peak flows and flood risks but also contribute to aquifer recharge. Characterizing infiltration flows and the recharge potential of aquifers requires monitoring soil water dynamics in the unsaturated zone, a process complicated by hydrogeological heterogeneity. Electrical Resistivity Tomography (ERT) provides a non-invasive method to monitor these variations, offering spatially and temporally resolved insights into soil moisture, saturation, and hydraulic properties. By capturing the heterogeneity of the vadose zone, ERT facilitates the understanding of infiltration dynamics and supports the estimation of water content and conductivity in situ. In this study, we developed a coupled hydro-geophysical approach integrating ERT and HYDRUS-2D simulations to estimate spatiotemporal variations in soil water content under field conditions. The model was validated against field measurements, achieving a correlation coefficient of R² > 0.8 for volumetric water content. Two scenarios, representing a grassed area and a swale, were analysed to assess water transfers to the subsurface. The results enable the quantification of water variations within the loamy horizon and the recharge potential of the Senonian–Turonian aquifer. They also highlight the influence of specific weathering facies on flow dynamics.

This study investigates the impact of a large-scale urban development project located in a shallow groundwater (GW) and low permeability context based on numerical modelling. The analysis is conducted by comparing water balance and groundwater levels simulated prior to the project with those obtained after successively accounting for i) land-use modifications, ii) introduction of new pipes and dewatering systems at building foundations and iii) implementation of sustainable drainage systems allowing infiltration (i-SUDS). Land use modifications produced contrasting effects on GW recharge and levels, with potential rise over former agricultural plots due to drains removal and reduction of transpiration via soil-sealing. The introduction of additional subsurface infrastructures markedly reduced groundwater levels, but the impact varied strongly across the site. I-SUDS provided only marginal runoff control but strongly influenced GW levels, with rises extending well-beyond their immediate vicinity. The study highlights the necessity and challenges of anticipating the combined impact of i-SUDS and urbanization in contexts where interactions with GW are to be expected.