Session D1 - Changes in SCM hydraulic performance over time

Blue-green infrastructure (BGI) systems have more inherent variability of performance than traditional stormwater infrastructure (e.g., storm sewers) due to environmental factors, such as plant health, soil structure, and temperature, under the same rainfall patterns. However, this variability is not expressed when expressing the expected performance of a BGI within the engineering community, or the larger community of users. This work seeks to quantify the “fuzziness” of BGI performance through a series of laboratory experiments that isolate environmental variables, such that it is understood where in the BGI the greatest variability may occur as well as provide a data-driven approach to public communication on the benefits and risks of BGI. Preliminary results show that there is variation in the BGI’s response to an event of the same size, intensity and duration with the same antecedent dry time due to construction variability (variations between the columns for each trial repetition) and experimental variability (variations between the same column for all trial repetitions).  This variability was observed through quantifying the differences in the ponding depth, overflow depth, and percolated volume.  These findings signify the importance of further determining the variability of BGIs under varying conditions to enhance understanding of BGI performance and to develop a methodology for further quantifying this variability.

Two experimental, adjacent, and lined rain gardens have been continuously monitored in Paris since 2021. The aim is to better understand their hydrological functioning and to study the role of a storage layer at the bottom, which is only activated in garden 2 (the rest being identical in both gardens). A comparative analysis of variations in soil water storage and hydrological balances indicates contrasting behaviours between the two gardens over the five-year period. In garden 1, the loamy soil dried out significantly during the summer months, creating preferential flows that fostered drainage at the expense of retention and evapotranspiration. Conversely, in garden 2, the existence of a significant capillary fringe allowed for the maintenance of uniform and high moisture levels in the silt, with profiles that were nearly saturated in winter; drainage was slowed and evapotranspiration was activated at its potential rate. Metrological findings are then highlighted, and recommendations for the optimal design of this type of garden are finally presented.

Since 2021, the urban community "Valence Romans Agglo" has adopted a sustainable and integrated rainwater management strategy based on infiltration at source and reducing impermeability. To demonstrate the benefits of permeable surfaces, an experimental rainwater infiltration car park was created in 2022, featuring 15 types of permeable parking spaces designed to be observed in real-life conditions. Of the 725 m² developed, 340 m² consist of permeable surfaces, and the entire site manages stormwater exclusively through surface infiltration, without any underground drainage system. The permeable surfaces allow normal rainfall to infiltrate, while sunken green spaces absorb exceptional rainfall events, up to a 100-year return period. Annual permeability tests show overall stability in infiltration performance across all parking spaces, with a notable improvement on vegetated surfaces. Finally, summer temperature measurements indicate that vegetated parking spaces are on average 5°C cooler than mineral ones, confirming their role in mitigating urban heat islands.