SuDS water storage capacity calculator : A decision support tool for the implementation ofSustainable Drainage Systems in Östersund.

Abstract: Heavy precipitation events are expected to increase in intensity and frequency, due to global warming. Sewer systems might overload during heavy rainfall, resulting in floods which potentially affect all municipalities in Sweden. Traditionally, stormwater is seen as pipe-related problem, but a transition towards Sustainable Drainage Systems (SuDS) has started. SuDS aim to reduce the quantity of the runoff from the site, slow down the runoff to allow (in)filtration, and provide treatment of the surface water before discharge. In Sweden, municipalities are responsible for the sewer system and realization and implementation of climate change adaptation measures, like SuDS. Tools and models support the highly complex selection, location, and design of SuDS, by systematically providing the most relevant information that represents the actual drainage system in the best way possible. Furthermore, models are used to predict the behaviour of SuDS, which may form Decision Support Systems (DSS). The highest interest in SuDS modelling and DSS lies in water quantity, however existing models are complex and lack flexibility, transferability, and stakeholder inclusion. The aim of this project is to provide a tool for the Municipality of Östersund that aids in the decision-making and design process for the implementation of SuDS, concerning their capacity to store stormwater. The ‘SuDS water storage capacity calculator’ can be used to test what (combinations of) SuDS are able to store the stormwater of a heavy rainfall event.The following SuDS components were included in the calculator: extensive green roofs, underground infiltration systems, infiltration basins, swales, porous pavements, detention basins, ponds, and wetlands. Secondly, the technical criteria forming the basis of the calculator were identified through a literature review. These criteria were used to calculate precipitation and the water storage capacity of each SuDS. The calculator was then built in Microsoft Powerpoint using Visual Basic for Applications (VBA). Two case studies were selected in Östersund and explored using the calculator. The water storage capacity of each SuDS component was calculated using scenarios in which 25%, 50%, 75%, and 100% of the total available area in each case study was used as input. Finally, four combinations of SuDS were tested concerning their water storage capacity. All calculated water storage capacity was compared to the amount of water falling on the case study areas during a heavy precipitation event that only occurs once every 100 years.The developed calculator can be used to calculate water storage capacity of SuDS and precipitation in a simple way. The tool contains user input and default values, which can still be changed. Furthermore, the calculator allows comparison between the amount of precipitation and water storage capacity. The results of the scenarios show that underground infiltration systems and detention basins have the highest potential to store stormwater, followed by infiltration basins, porous pavements, and ponds or wetlands. The calculator has limited design options, due to its simplification of reality. However, its limits are mostly applicable further in the designing process. The4calculator gives a rough estimate of the potential water storage capacity of a variety of SuDS components. The calculator is a useful tool before the design process has started, by providing an indication of the options that are worthwhile to consider in terms of water storage capacity. Furthermore, opportunities for optimization of the tool were recognized. The water storage capacity resulting from the different scenarios was compared to the precipitation falling on each case study area. Realistically, the amount of precipitation that exceeds the capacity of the sewer system might not fall directly where the SuDS are located. Finally, the calculator allows applications of a wider range of combinations of SuDS components.