Introducing evaporating water channels in urban canyons: Assessment of ventilation and thermal effects through Large-Eddy Simulation

Cities are particularly vulnerable to extreme weather events such as heat waves or high summer temperatures, which greatly impact public health and well-being. Climate change is expected to intensify these events and blue Nature-Based Solutions (NBS), such as water channels, have the potential to effectively mitigate temperatures and regulate the urban microclimate, but their impact on urban canyon ventilation and thermal comfort is not well understood yet. This study employs Large-Eddy Simulations (LES) with a water evaporation and heat exchange model to investigate how water channels influence airflow dynamics and temperature in a simplified urban canyon. Two configurations with a warm and cold water channel in a mixing convection regime for a Richardson number of $Ri=3.4$, have been studied and compared to the neutral case without channel. Results show that the introduction of a water channel increases the average in-canyon turbulent kinetic energy, where addthe warm channel case showed a 61% increase compared to the cold channel case. Warm channels increase vertical mixing and buoyancy-driven turbulence, enhancing mass and momentum exchange with the atmosphere without a substantial increase in-canyon temperature. In both cases, the estimation of human comfort indices shows that the presence of water channels does not deteriorate the thermal comfort within the canyon. These findings suggest that water channels can effectively mitigate high temperatures in urban canyons, lowering the average temperature in the canyon by 1.6% (0.3%) in cold (warm) case compared to neutral ones. This study provides further evidence of the potential of blue NBS to enhance urban climate resilience.