Development of a New Framework on the Operationalization of Green Roofs as a Nature-Based Solution for Flood Management in an Urban Catchment by considering the Impact of Landcover and Climate Changes

The world is becoming more urbanized with the passage of time. As a result, the cities are facing pressure to provide shelter, employment, and business opportunities for the urban population. Almost 50% of humanity currently resides in urban areas. Urbanization helps to provide opportunities for growth and economic development. However, the urbanized areas are under tremendous pressure due to the growth of the population, climate change and landcover changes. The UN has estimated that 5 billion population will be residing in urban areas by 2030. The negative impact of the rising population in urban areas is that it leads to an increase in urbanization, reduction in vegetation and increase in flood risk. Climate change has also resulted in increased frequency and intensity of extreme rainfall events. The traditional drainage systems such as dams, levees, retaining walls etc. are designed for controlling flooding. However, these methods of flood control have been known to have increased the long-term proneness to flood risk in urban cities. In response to this, the adoption of nature-based solutions (NBS) is increasingly recognized for flood management. This thesis studies the assessment of the impact of climate change and landcover change on the annual maximum flood (AMF) in an urban river basin in Dublin, Ireland. Hydrological modelling has been performed to address the problem of flood risk for water resources management. The potential impact of climate change on an urban river catchment is quantified using a physically-based hydrological model called Soil Water Assessment Tool (SWAT). The study area considered is the Dodder River basin located in the southern Dublin, capital city of Ireland. Climate projections from three Regional Climate Models and two Representative Concentration Pathways (RCP 4.5 and RCP 8.5) were used to evaluate the impact of flooding corresponding to different climate change scenarios. AMF is generated by combining the bias-corrected climate projections with the calibrated and validated SWAT model to understand the projected changes in flood patterns for years 2021-2100. Correspondingly, the impact of landcover change on hydrological runoff simulation is observed using SWAT. Results predicted up to 12% and 16% increase in flood quantiles corresponding to 50 & 100 year return periods. NBS are explored to understand the effectiveness of flood management. Performance of green roofs as NBS is explored through a simulation study as well as a real-world deployment study in Dublin. Strategic deployment of green roofs in highly urbanized areas has provided environmental benefits, as it does not require a large space for its installation. Modular green roofs have been deployed which have IoT scales associated with them for measuring the effective reduction in runoff. Hydrometeorological variables (rainfall, temperature, relative humidity, and wind speed) were compared to the amount of runoff reduction by means of a regression-based relationship. The observed runoff reduction from a modular green roof and that estimated based on the developed regression relationship yielded an R2 value of 0.874. This study can help in the identification of locations where the installation of green roofs can help mitigate floods at a city scale. A coupled SWAT and Low Impact Development model was developed by selecting a hypothetical scenario where 160 buildings were converted to green roofs and their combined effect in controlling floods in Dublin was simulated. This revealed the extent of the area being inundated, as well as the flood depth obtained with and without deployment of the green roof at a catchment scale. The integrated model has also explored the extent of flood inundation at a catchment scale based on historical data as well as future climate projections in the Dodder River basin. A reduction of flooded areas upto 16.7% and 0.109m flooding depth for 100 year return period has been expected due to green roofs.