Bluegreengrey (BGG) solution for future climate and flood-resilient urban drainage network by enhancing the natural hydrological cycle

Sammanfattning: A loop of urbanization, increasing impervious surface in urban areas, increasing temperature, rainfall, surface runoff and flooding, and lack of new spaces have been adding challenges to urban drainage systems for decades. The climate change impacts are exposing the existing vulnerable conventional urban drainage systems to more challenges in the future. A new idea of sustainable drainage system named Bluegreengrey (BGG) which was introduced by EDGEs brings a solution to retrofit the existing landscape and drainage system in a more efficient way to detain stormwater in open subbase layer of existing roads and use it for more healthy green lives in urban areas while solving the basement flooding problem. In southern Sweden, downpouring will be frequent due to climate change, which threatens existing drainage systems in urban areas and may lead to more basement flooding. In this study, a district in Trelleborg municipality in the south of Skåne, the southernmost county of Sweden, experiencing repeated basement flooding for a long time was investigated with BGG structures to test their performance to mitigate basement flooding in this area. Incorporating BGG with proper settings of existing LID tools in the model software Mike+ was one of the outcomes of this study. Mike+ is a modeling software introduced by DHI. The hydraulic and physical parameters were chosen as per guidelines for usage of Mike+ and practicing properties of BGG suggested by EDGEs. The porosity of substrate and storage layers and the infiltration capacity of the top surface layers are the dominant factors to define the capacity of BGG structures. The size and distribution of BGG structures, the formation and size of catchments, and the size of rainfall events cause variations in the surface runoff reduction capacity of BGG structures. By storing stormwater and providing it for trees and plants BGG enhances the natural hydrological cycle in urban areas which was concluded from the stormwater distribution results with BGG deployed in the model. The model was run with a 10-year synthetic rainfall event provided by DHI with a climate factor of 1.25 and then BGG structures were deployed along several roads until they can make the basements of all the households claimed flooding, become free from flood. A real rainfall event of 2021 was provided by the Trelleborg municipality. This real rainfall event recorded in 2021 was found to be equivalent to a 50-year rainfall event. The same model with deployed BGG structures was then tested with this real rainfall event to see their performance with a 50-year rainfall event. The results showed the characteristics of BGG structures to enhance the natural hydrologic cycle and at the same time, flood-claimed households remain out of flood risk. In this study, BGG structures reduced the surface runoff produced from impervious surfaces in the case area by up to 96% for individual catchments for 10-year rainfall events. The average surface runoff reduction capacity of BGG structures was 48% for the 10-year rainfall. On average the BGG structures infiltrated 0.08m3 /sec, stored 0.37m3 /sec, and produced 0.11m3 /sec runoff when the inflow in the BGG structures was 0.56m3 /sec. Replacement of 14% of the existing impervious surface with BGG in a catchment can reduce the surface runoff by 50% for 10-year rainfall. The deployed BGG structures showed the same performance for 50-year rainfall and reduced the runoff by 48%. As the surface runoff capacity is subjected to the size of BGG structures, catchments, and rainfall it cannot be summarized with any specific number in case of reduction capacity. BGG structures were found in this study to reduce the discharge rate and flow velocity through the pipes. The discharge rate and flow velocity through the outflow pipe indicate a smoother flow discharge which is good for the downstream recipients and biodiversity as well. iv The impact of source control or disconnecting impervious surface areas of private lands from the drainage network was also tested in this study. As obvious the runoff volume reduced with reduced impervious surface in the catchments.