Experimental investigation and numerical analysis of a green wall system

Stormwater management has emerged as one of the main issues, especially in urban areas. When cities expand, urbanization increases with the result of more impermeable areas. In this way, there is a modification of some pre-development hydrological cycle's functions like infiltration and evapotranspiration with direct consequences in increasing pluvial urban flood phenomena.

In addition, the increase in extreme events due to climate change makes the traditional urban drainage systems inadequate to manage stormwater, and this increases the cities' vulnerability to pluvial urban flooding.

Recently, to mitigate the cited issues, the scientific community has concentrated on a series of "green" facilities known as Nature-based Solutions (NBS). Among NBS the most popular are green systems such as Green Roofs and Walls.

The aim of this work is to propose a comprehensive approach to assess the hydrological/hydraulic benefit of a GW cascade system using experimental investigation on the soil substrate coupled with a physically based approach applying the HYDRUS-1D model.

The GW investigated in this work consists of a cascade of five boxes. Each box contains a seepage face layer (to permit the water flux from one box to the other); a drainage layer of 5 cm composed of natural gravel material; a highly permeable geotextile to avoid fine particle migration into the underneath layer; a substrate of 10 cm; a surface layer with vegetation. The mixture of the soil substrate is made up of 40% mediterranean soil, 40% compost, and 20% glass sand while the drainage layer consists of fine gravel. To assess the hydraulic properties of the soil substrate the evaporation method simplified by Schindler has been performed using the HYPROP device applying the unimodal van Genuchten-Mualem model.

To carry out the modeling analysis with the HYDRUS-1D model, a one-year data set (2022) gathered from a meteorological station located in Cosenza was taken into consideration.

The potential effect of the GW cascade on a building has shown promising results in reducing runoff volume. It should be noted, however, that this study considers only the rainfall falling on the first box of the GW cascade as the first precipitation input. Further investigation on inflow coming from an adjacent roof surface should improve the knowledge about the GW cascade runoff reduction.