Hydrological modelling of vertical green-screen nature-based solutions using HYDRUS-2D

Nature-based solutions (NbS) that combine vertical greening with stormwater management are increasingly deployed in dense urban environments; however, key hydrological processes, including storage, overflow pathways, and evapotranspiration, remain poorly quantified. Compared to conventional horizontal NbS, vertical systems are subject to distinct hydrological constraints related to boundary conditions, flow patterns, and geometry, yet appropriate process-based modelling approaches remain underdeveloped.

This study presents a physically based numerical framework for the conceptual representation and analysis of the hydrological behaviour of a freestanding green-screen nature-based solution using the HYDRUS-2D/3D software. The investigated NbS consists of a vertically oriented mineral wool wall that receives roof runoff at its top and is positioned above a stepped, open-bottom planter box with vegetation, hydraulically connected to the underlying native soil. The system is designed to temporarily store incoming roof runoff within the vertical wall and vegetated planter, with stored water gradually depleted through evapotranspiration and infiltration to the underlying soil.

A representative two-dimensional cross-section is used to simulate variably saturated flow, water storage, evapotranspiration, and infiltration processes within the system. Roof runoff is represented as a time-variable inflow applied at the upper boundary of the vertical wall. Atmospheric boundary conditions are imposed on exposed vertical and horizontal surfaces to represent evaporation from the wall and evapotranspiration from the vegetated planter. To address the challenge of vertical evaporation, atmospheric forcing is spatially varied along the wall to account for differences in solar exposure. Hydraulic continuity is assumed between the open-bottom planter and the underlying soil, allowing infiltration into the subsurface.

Event-based simulations are used to investigate system responses under different rainfall conditions, including wet and dry extremes, evaluate the restoration of retention capacity between successive storm events, and assess and optimise key design parameters such as wall height, planter geometry, and hydraulic properties of system materials with respect to stormwater retention and system recovery. Particular attention is given to the role of spatially variable vertical evaporation from the wall, and evapotranspiration from the planter, in controlling system recovery and overall stormwater retention performance.

The proposed HYDRUS-2D conceptualisation provides a quantitative tool for evaluating and optimising vertical green-screen NbS and supports their integration into quantitative urban stormwater management and climate adaptation strategies.

This work is carried out within the framework of the GreenStorm project, funded under the Driving Urban Transitions to a Sustainable Future (DUT) Call 2022.