Rainwater runoff quantity and quality performance from a greenroof: The effects of short-term events

This paper describes the stormwater retention potential and runoff water quality of a lightweight aggregates (LWA)-based greenroof in Estonia. Three rainfall events and snow cover melting were measured. The investigated extensive greenroof was also compared with the modified bituminous membrane roof. The studied greenroof effectively retained light rain—the retention for 2.1 mm rainfall was 85.7%. In the case of a heavy rainstorm (12.1 mm), the greenroof can delay the runoff for up to half an hour, but cannot fully retain it—the runoff volume was the same as that of the reference roof. The observation of snow cover melting showed that there are two meltings of a greenroof: the melting of the snow cover and the melting of the frozen water in the substrate layer. Snow cover melted fast, but the greenroof nevertheless prolonged the runoff to a longer timescale than that of the reference roof. The quality of the runoff water varies depending on the character of the runoff and the pollutants accumulated on the roof. When rain and runoff were moderate, values of COD, BOD7, and concentrations of total N and total P were higher on the bituminous roof. In samples taken during a heavy rainstorm, the components were less concentrated, as the rain washed more phosphates and nitrates off the greenroof. In snow melting water, the concentrations of all components were greater on the greenroof. In addition, the greenroof runoff always contained more sulphates and Ca–Mg salt because of their presence in the LWA-material.