The effects of large-scale green roof implementation on the rainfall-runoff in a tropical urbanized subcatchment - A Singapore case study
by Jim van Spengen
The hydrological effects of urbanization affect the rainfall-runoff regime of many cities in the world. While traditional stormwater drainage systems are often able to effectively serve the function of flood control, they increase downstream peak flows and do not provide a habitat to support a healthy aquatic ecosystem. In order to improve this situation, water managers introduced the concept of Low Impact Development. The main purpose of Low Impact Development is to mimic predevelopment site hydrology as much as possible, by reducing the volume and peak rate of flow, controlling the water quality and promoting the recharge of stormwater with decentralized, on-site detention. Green roofs are an interesting Low Impact Development measure with great large-scale implementation potential in existing urban areas as well as in areas with new housing development, because roofs account for 20-50% of the total urban land cover. This research was initiated by a lack of knowledge related to the quantitative hydrological effects of green roofs in the tropics. Singapore was chosen as a case study for this research. The main goals of the research are 1) to analyze the effects of green roofs on the rainfall-runoff in Singapore and 2) to determine the quantitative hydrological contribution of large-scale green roof implementation to sustainable stormwater drainage systems in Singapore.
Methodology
To achieve these goals a research approach is used, which combines green roof experiment measurements with green roof model simulations. First, an approach to measure and simulate the rainfall-runoff processes that occur on 1 m2 experiment platforms has been set up. Runoff data from an extensive green roof platform, with a 12 cm soil media, and a reference roof were analyzed for 66 rainfall events in four periods of one month. Retention, peak discharge reduction, detention and base flow variables and performance indicators were used to quantify the hydrological effects of green roofs. An unsaturated zone model for the simulation of green roof rainfall-runoff was conceptualized and specified in the physically based HYDRUS-1D simulation software, which numerically solves the Richards equation for water flow in variably saturated porous media. A 9.4 ha subcatchment was used as a case study in order to quantify the hydrological effects of large-scale green roof implementation on the rainfall-runoff in Singapore. Based on a conceptualized representation of the Sunset Way subcatchment, a hydraulic routing model was build in the SOBEK software package. As in other available commercial hydraulic models, an explicit green roof modelling option is not available in the current SOBEK software. Therefore, this thesis presents a methodology that couples pre-processed green roof runoff from the HYDRUS-1D model to the SOBEK model for an accurate simulation of the scaled-up green roof effects in the case study area.
Results
Analysis of experiment measurements shows that the green roof platform retained 39% (234 mm) more rainfall than a reference roof and reduces the peak discharge with 64% compared to the reference roof. The average time to peak of the green roof hydrograph appeared 34 minutes after the reference roof peak and green roof platforms provided an average additional base flow runoff of 0.41 liters/m2 . However, average experiment performance values do not give practical implications whether green roofs are an effective stormwater management solution. Individual event analysis shows that green roofs do not simply absorb water and slowly release it over a period of time. Instead, retention is the primary function of green roofs: rainfall is retained until the maximum storage capacity of the soil media. Detention and peak discharge reduction is provided by green roofs until the moment of soil saturation. After this moment, green roof peak detention is limited to 1-2 minutes and runoff intensities closely follow the rainfall and reference roof intensities. Base flow conservation by extensive green roofs is not provided for rainfall events that are nearly or totally retained. If green roofs get saturated during the rainfall event, minor base flow conservation was observed for up to 3 hours after the last rainfall.
Several laboratory and field experiments were performed in order to provide a physically based initial set of parameters for the HYDRUS-1D model. Four unique optimized parameter vectors for the hydraulic functions of van Genuchten were determined with inverse modelling of transient flow experiments. It was shown that runoff at the bottom of a green roof can be described with a seepage face boundary. Runoff starts or stops when the soil water content at the bottom respectively exceeds or drops below a threshold value of 0.61. The physical background of the model provided a better understanding of the hydrological processes and the soil physics in a green roof media. Simulation results of twelve rainfall-runoff events in September, 2009 (calibration period) and December, 2009 (validation period) show that the HYDRUS-1D model gives an accurate representation of the measured green roof runoff characteristics on event scale. A limitation of the model is the over prediction of the green roof runoff when events quickly follow-up.
Scenario simulations in the Sunset Way subcatchment in Singapore show that the combined hydrological effects of large-scale roof greening on the rainfall-runoff in the main outflow canal of the area are limited. First, runoff analysis in the case study area under extreme meteorological conditions showed that extensive green roofs provide a negligible peak reduction and detention under Singapore’s current design storm with a T=5 years return period. However, unilateral design requirements bias this result. Still, extensive green roofs would not have prevented flooding under the actual meteorological conditions of the November 19 flood storm. Second, negligible improvements in the reduction of non-natural water level variations were observed when the green roof scenario simulation results were compared to the base case scenario simulation results under the actual meteorological conditions of September 2009. Peak flow reduction is conditionally significant, but green roofs are particularly not suitable to provide a base flow, which can replace the natural function of groundwater during dry spells. A factor that reduces the effectiveness of green roofs at subcatchment-scale compared to experiment results, is the limited building coverage.
Conclusions and recommendations
Result analysis of this research shows that a standalone large-scale implementation of 12 cm extensive green roofs does not significantly contribute to the quantitative hydrological goals of Singapore’s ABC Waters Programme. The hydrological contribution of green roofs to sustainable stormwater drainage systems in Singapore can be enhanced when the design fundamentals and implementation strategy are reconsidered and adapted to the proposed hydrological effects and site specific requirements. The presented approach which is based on model coupling of a physically based runoff generation model to an existing routing model created theoretical and practical spin-offs for the development of green roofs, as one of the LID measures that can contribute to sustainable stormwater drainage system development. Hence, the promotion and international exchange of the obtained knowledge, ideas and recommendations can contribute to the development of sustainable stormwater drainage systems in other tropical cities and the rest of the world.