Hydrological Performance of Green Roofs in Mediterranean Climates: A Review and Evaluation of Patterns

Joana Silva,Teresa A Paço,Cristina M Silva,Vítor Sousa

doi:10.3390/w13182600

Abstract

The capacity of green roofs to intercept rainfall, and consequently store and slow runoff resulting in a reduction in flood risk, is one of their main advantages. In this review, previous research related to the influence of green roofs on the hydrological cycle is examined with a special focus on studies for Mediterranean climate conditions (Csa and Csb according to the Köppen–Geiger climate classification). This climate is characterized by short and intense rainfall occurrences which, along with the increased area of impervious surface on Mediterranean regions, intensify the risk of flooding, particularly in the cities. The analysis covers the variables rainfall retention (R, %), runoff delay (RD, min or h), peak delay (PD, min or h), peak attenuation (PA, %), and runoff coefficient (RC, −), in relation to physical features of the green roof such as layers, substrate depth, slope, and vegetation, as well as, weather conditions, such as monthly temperature and monthly precipitation. Following a statistical analysis, some patterns for the average rainfall retention (%) were found in the published literature for green roofs under Mediterranean climate conditions—namely, that the most significant variables are related to the substrate depth, the existence of certain layers (root barrier, drainage layer), the origin of the vegetation, the types of green roofs (extensive, semi-intensive, intensive), and the precipitation and temperature of the location. Moreover, a multivariate analysis was conducted using multiple linear regression to identify the set of green roof features and weather conditions that best explain the rainfall retention (%), taking into consideration not only the studies under Mediterranean conditions but all climates, and a similar pattern emerged. Recommendations for future research include addressing the effect of physical features and weather conditions on the other variables (RD, PD, PA, RC) since, although present in some studies, they still do not provide enough information to reach clear conclusions.

Highlights

By 2018, 55.3% of the world population lived in cities, and by 2030 it is predicted that globally 60% of people will live in urban settlements [1]
This review focuses on the variables of rainfall retention, runoff coefficient, runoff delay, peak delay, and peak attenuation because they are the most studied, and all of them are crucial for understanding the rainfall–runoff relationship of green roofs
We studied each one of the physical features and weather conditions variables separately, and for the categorical variables, we used the one-way analysis of variance (ANOVA), which is an extension of independent two-samples t-test for comparing means in a situation where there are more than two groups

Summary

Introduction

By 2018, 55.3% of the world population lived in cities, and by 2030 it is predicted that globally 60% of people will live in urban settlements [1] This growing urbanization influences the natural water cycle because it increases the proportion of impermeable surface, and there is more surface runoff and a reduction in infiltration [2]. The complex geography of the Mediterranean region, which includes a nearly enclosed sea with high sea surface temperature during summer and fall, surrounded by urbanized littorals and mountains where various rivers originate, has a crucial role in steering airflow. This is the case of the Mediterranean. Green roofs can have the ability to mitigate flood risks in compact cities [4]

Methods

Results

Conclusion