Towards an understanding of the limits of extreme event studies on Nature Based Solutions

The European project GreenStorm (https://arceau-idf.fr/en/projects/greenstorm) focuses on nature-based solutions for urban stormwater management (NBS_SW) and addresses the question of their implementation, performance and resilience for current and future climate extremes. It emphasizes the hydrological and thermal benefits of these devices as well as the stress suffered by their vegetation during extreme events in 5 participating European cities. The project proposes to identify effective, resilient designs accepted by the practitioners and citizens, but also the levers to promote their implementation on a city scale and maximize the associated benefits.

A part of the project consists of monitoring and modelling of NBS_SW performance under actual but also future climate extremes. To perform such assessment, the project collaborates with the demonstrator facility SenseCity (https://sense-city.ifsttar.fr/en/), which consists of two 400m² platforms each composed of a ring road and small housing, equipped with sensors. One of these platforms simulates a 10 meter long "canyon" street with 4-meter-high walls and trees on both sides.  This street is also equipped with two NBS_SW for runoff management: storm water trees and a rain garden. The platforms can be covered by a climatic chamber to simulate physically different climate scenarios. The aim of this proposition is to discuss the potential and the limits of real scale climate simulation focused on NBS for storm water management.

Two climate scenarios were elaborated and tested, the reference climate corresponding to an average late summer climate at the location (Paris conurbation) and the extreme climate corresponding to heat waves observed in 2022 at SenseCity.  The scenarios were obtained from statistical analysis of daily cycles of air temperature and humidity at the facility and compared to the climatic projections for 2023-2050 for the strongest CO2 emission scenario (RP8.5) employing 9 different climatic models (from SMHI, IPSL, MP, DMI, CLM, HadGEM, CNRM and KNMI models). Finally, these scenarios were adapted to the technical limits of the climate chamber. The essay was composed of two daily cycles of reference climate followed by three daily cycles of extreme condition to finish with three daily cycles of reference climate before withdrawing of the climate chamber.

The vegetation in the raingarden and of the stormwater trees were daily monitored for leaf pigments and the nitrogen balance index (DUALEX® SCIENTIFIC, Force-A,) and for leaf stomatal conductance and transpiration (LI-COR LI600). The measurements were completed by on-line sap flow (Implexx) for the trees and soil moisture measurements (Campbell) for both equipment.

First results indicate the suitability of conductance and sap flow measurements to follow the climate change and the important effect the applied gradients may have on vegetation.

The presentation will detail the methodology of the climate scenario creation and present, based on the results obtained, the potential and limits of such type of climatic chamber experiments.