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Hydrological performance of modular-tray green roof systems for increasing the resilience of mega-cities to climate change

Loiola, Claudia, Mary, Wellington, Pimentel da Silva, Luciene
Journal of hydrology 2018
cacti and succulents, cities, climate change, evaporation, evapotranspiration, green roofs, markets, models, rain, runoff, soil water, tropics, watersheds, Brazil
This paper investigates the performance of modular-tray green roof systems on rainwater retention and total runoff delay under heavy rain conditions, typical of countries with a tropical climate. It involved the building of a rain-spray system simulator and worktops to support the experimental units. Three green roof modular-tray systems were studied, M-4L, M-17L and F-17L, with a mix of succulent plant species. The volume and delay of total runoff were sampled for both dry and wet soil-moisture stages for a simulated rainstorm of 155 mm/h and duration of 7 min, which approaches the value of the design rainfall of 10-year return-period and an observed rainfall event that caused flooding in 2011 in the area where the experiments took place. These results were compared to three bare-soil similar experimental units and with a control unit consisting of a corrugated fibre-cement roof sheet. In addition, classic rainfall-runoff parameterization, also important for catchment analysis, such as the rational coefficient method (C) and Curve Number (CN) were also applied. The results showed to be consistent. Overall, module F-17L presented the best performance with regard to both water retention and delay of total runoff. The average percentage of retention, considering all module types, was 58% of the total induced water volume, and the average delay for total runoff was approximately 12 min. In some tests, for the wet soil stage, the bare soil units performed better than the vegetated units. Soil moisture analysis revealed that considering the same period of time, the bare soil units lost more water through evaporation than the vegetated units through evapotranspiration. The average values of C for vegetated units for the wet and dry soil stages were equal to 0.66 and 0.27, respectively. The CN values were high, ranging from 88 to 97. Although fairly new on the market in Rio de Janeiro, modular-tray green roof systems are easy to build and hydrologically effective, mainly in the dry soil stages, performing as well as other extensive green roof systems. The values obtained for C and CN may be useful for the modelling of catchment rainfall and runoff especially for scenarios involving green roofs, as well as for establishing regulatory benchmarks towards more resilient and sustainable cities.