The influence of a green roof configuration’s moisture balance on hydrological performance

Climate change and urbanization has increased the risk of pluvial flooding. Sustainable Drainage Systems (SuDS) aim to control a greater proportion of rainwater at source. Green roofs can partly offset the loss of urban terrestrial landscape and provide additional capacity for the retention and detention of rainwater. The objective of this research was to improve the understanding of the physical controls that affect a green roof’s hydrological response. Experimental studies were undertaken to monitor the performance of nine different extensive (80 mm substrate) green roof configurations.
A four-year record of rainfall, runoff, climate and moisture content data has been analysed for a field research site in Sheffield. Nine test beds incorporated three substrates with different moisture retention characteristics and three different vegetation treatments (Sedum, Meadow Flower and non-vegetated). Consistent differences were observed. The effects of vegetation and substrate were most evident for rainfall events where the depth exceeded 10 mm: mean per-event retention varied between beds from 14% to 70%. Retention was highest from the configuration with the highest moisture storage capacity (Sedum-vegetated HLS) and lowest from the configuration with the lowest moisture storage capacity (non-vegetated LECA). The difference between rainfall depth and available moisture capacity provided a highly credible indication of runoff. Evapotranspiration (ET) regenerates the moisture retention capacity. Experimental studies in a climate-controlled chamber enabled the monitoring of ET from the nine configurations across continuous dry periods of up to 28 diurnal cycles in spring and summer. ET rates were variably influenced by climate, vegetation treatment, soil and residual moisture content.
A conceptual hydrological flux model was developed to allow both long term continuous simulation of runoff and drought risk and per-event responses to design storms. The model includes a function that links ET rates to residual moisture content, and is validated against observed runoff data. Detention was characterised via the calibration of a reservoir-routing model that linked net rainfall to the measured runoff response. The parameter values identified here – when combined with the retention model component – provide a generic mechanism for predicting the runoff response to a time-series or design rainfall for any unmonitored system with comparable components, permitting comparison against regulatory requirements.