The importance of sustainable stormwa- ter management in urban areas A case study on current and future development in Lerum, Sweden Master’s thesis in Infrastructure and Environmental Engineering KARL NORLÉN LINDA OSCARSSON DEPARTMENT OF ARCHITECTURE AND CIVIL ENGINEERING CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2021 www.chalmers.se www.chalmers.se Master’s thesis 2021 The importance of sustainable stormwater management in urban areas A case study on current and future development in Lerum, Sweden KARL NORLÉN LINDA OSCARSSON Department of Architecture and Civil Engineering Division of Water Environment Technology Chalmers University of Technology Gothenburg, Sweden 2021 The importance of sustainable stormwater management in urban areas A case study on current and future development in Lerum, Sweden Master’s Thesis in the Master’s Programme Infrastructure and Environmental En- gineering KARL NORLÉN LINDA OSCARSSON © KARL NORLÉN, LINDA OSCARSSON, 2021. Examensarbete ACEX30 Institutionen för arkitektur och samhällsbyggnadsteknik Chalmers tekniska högskola, 2021. Department of Architecture and Civil Engineering Division of Water Environment Technology Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone: + 46 (0)31-772 1000 Cover: Depression in the ground by the school Torpskolan in Lerum, which is subject to large flooding depths in the case of a flash flood event (Authors’ own image). Typeset in LATEX, template by Magnus Gustaver Department of Architecture and Civil Engineering Göteborg, Sweden 2021 IV CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 The importance of sustainable stormwater management in urban areas A case study on current and future development in Lerum, Sweden Master’s thesis in Infrastructure and Environmental Engineering KARL NORLÉN LINDA OSCARSSON Department of Architecture and Civil Engineering Division of Water Environment Technology Chalmers University of Technology Abstract The risks of pluvial flooding in urban areas are expected to increase due to climate change leading to heavier rainfall events in the future. In addition, urbanisation contributes to impermeable surfaces which further increases the risk of flooding. Sustainable stormwater management that mimics nature based solutions is consid- ered a favourable way to treat urban flooding. The use of sustainable stormwater management also enables incorporation of ecological and social services whilst re- ducing the impacts of urban flooding. Infiltration and surface roughness are two key parameters in assessing how efficient these sustainable stormwater solutions are. In- filtration has been established to have a large impact on flooding, while more recent studies have shown that surface roughness impacts runoff to a large extent. This project aims to further investigate the impact of infiltration and surface roughness on sustainable stormwater management. To achieve the aim, a case study is performed in the municipality of Lerum, Sweden. The softwares SCALGO Live and MIKE 21 are used to identify current and future infrastructure at risk for flooding in Lerum. Stormwater solutions to manage floods in the area are then implemented in suitable locations into the software SCALGO Live. The model is then imported into MIKE 21 where the solutions’ properties are altered to correspond to permeable or imper- meable surfaces to study the effect of infiltration and surface roughness. Thereafter simulations are performed to obtain results on flow speeds and water depths to as- sess risk levels stemming from the flooding, as well as investigating the impact of the alterations in infiltration and surface roughness. The results are compared and show that stormwater solutions with high surface roughness lowers flow speed and solutions with high infiltration capacity lowers water depths. Both parameters con- tribute to lower risk levels, which leads to the conclusion that both parameters are important to manage the runoff by affecting separate aspects of a flooding event. Keywords: flood risk levels, infiltration, multifunctionality, pluvial flooding, surface roughness, sustainable stormwater management CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 V Vikten av hållbar dagvattenhantering i urbana områden En fallstudie av nuvarande och framtida exploatering i Lerum, Sverige Examensarbete inom masterprogrammet Infrastruktur och Miljöteknik KARL NORLÉN LINDA OSCARSSON Institutionen för arkitektur och samhällsbyggnadsteknik Avdelningen för Vatten miljö teknik Chalmers tekniska högskola Sammanfattning Risken för pluviala översvämningar i stadsområden förväntas öka på grund av att klimatförändringar leder till ökade regnmängder. Tillsammans med att urbanisering bidrar till hårdgjorda ytor ökar risken ytterligare för översvämning. Hållbar dagvat- tenhantering, vilken efterliknar naturlig dagvattenhantering, anses vara ett förde- laktigt sätt att motverka översvämningar i städer. Användningen av hållbar dagvat- tenhantering innefattar också möjlighet till ekologiska och sociala tjänster samtidigt som effekterna av översvämningar i städer minskar. Infiltration och ytråhet är två viktiga parametrar för att avgöra hur effektiva dessa hållbara dagvattenlösningar är. Infiltration har bekräftats ha stor inverkan på översvämningar, medan nyare studier har visat att ytråheten påverkar ytavrinning i stor utsträckning. Detta pro- jekt syftar till att ytterligare undersöka effekterna av infiltration och ytråhet på hållbar dagvattenhantering. För att uppnå målet genomförs en fallstudie i Lerums kommun, Sverige. Programvarorna SCALGO Live och MIKE 21 används för att identifiera nuvarande och framtida infrastruktur med risk för översvämning i Lerum. Dagvattenlösningar för att hantera översvämningar i området implementeras sedan på lämpliga platser, genom programvaran SCALGO Live. Modellen importeras till MIKE 21 där indatan ändras så att de motsvarar permeabla eller hårdgjorda ytor för att studera effekten av infiltration och ytråhet. Därefter utförs simuleringar för att uppnå resultat för flödeshastigheter och vattendjup. Detta för att bedöma risknivåer av översvämningen, samt undersöka effekterna av förändringar i infiltra- tion och ytråhet. Resultaten jämförs sedan och visar att dagvattenlösningar med hög ytråhet sänker flödeshastigheten och lösningar med hög infiltrationskapacitet sänker vattendjupet. Båda parametrarna bidrar till lägre risknivåer, vilket leder till slutsatsen att båda parametrarna är viktiga för att minska avrinningen genom att påverka separata aspekter av en översvämning. Nyckelord: hållbar dagvattenhantering, infiltration, multifunktionalitet, pluviala översvämningar, ytråhet, översvämningsrisk VI CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 Acknowledgements There are several people that we would like to express our gratitude towards. We are thankful for our thesis supervisor Sebastien Rauch, for providing support and guidance on the formalities throughout this thesis. The same goes for Mia Bondelind, examiner at Chalmers, for providing feedback and thoughts for reflection. A sincere thank you to our supervisors Alva Kalm and David Hirdman at the municipality of Lerum, for excellent feedback, valuable input, access to information, and for giving us a guided tour through the case study area. Thank you to Mikael Lindgren at AFRY for providing guidance and valuable input on our stormwater modelling, and to the entire department of Water West at AFRY for sharing your office space and providing a welcoming work environment. A thank you to the consulting company Tyréns AB for providing us with the model foundation used in this thesis and to Sten Blomgren at DHI for assistance related to the use of MIKE 21. Karl Norlén and Linda Oscarsson, Gothenburg, June 2021 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 VII VIII CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 Contents List of Figures XIII List of Tables XVII Terminology XIX 1 Introduction 1 1.1 Aim and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Background 5 2.1 Urban water systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.1 The hydrological cycle . . . . . . . . . . . . . . . . . . . . . . 6 2.1.2 Impacts of flooding . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Stormwater management . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.1 Multifunctionality . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.2 Infiltration and surface roughness . . . . . . . . . . . . . . . . 9 2.2.3 Stormwater management solutions . . . . . . . . . . . . . . . 10 2.2.3.1 Green roofs . . . . . . . . . . . . . . . . . . . . . . . 10 2.2.3.2 Swales . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2.3.3 Ditches . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2.3.4 Ponds . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2.3.5 Flooding areas and dry ponds . . . . . . . . . . . . . 12 2.2.3.6 Permeable surfaces . . . . . . . . . . . . . . . . . . . 12 2.2.3.7 Stormwater terraces . . . . . . . . . . . . . . . . . . 13 2.3 Precipitation and climate change . . . . . . . . . . . . . . . . . . . . 13 2.3.1 Return times . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3.2 RCP scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.3 Climate factors . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.4 Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.4.1 SCALGO Live . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.4.2 MIKE 21 Flow Model FM . . . . . . . . . . . . . . . . . . . . 16 2.5 Case study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.5.1 Topography and geology . . . . . . . . . . . . . . . . . . . . . 18 2.5.2 Sensitive infrastructure in Lerum . . . . . . . . . . . . . . . . 18 2.5.3 Stormwater management in Lerum . . . . . . . . . . . . . . . 19 2.5.4 Development . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 IX Contents 3 Methodology 23 3.1 Flood vulnerability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.2 Rainfall events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.3 Model setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.3.1 Adjustment of the elevation model in SCALGO Live . . . . . 26 3.3.2 SCALGO Live model setup . . . . . . . . . . . . . . . . . . . 27 3.3.3 Selection and design of stormwater solutions using SCALGO Live . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.3.4 MIKE 21 model setup . . . . . . . . . . . . . . . . . . . . . . 35 3.3.4.1 Infiltration . . . . . . . . . . . . . . . . . . . . . . . 37 3.3.4.2 Surface roughness . . . . . . . . . . . . . . . . . . . . 38 3.3.5 Modelling scenarios in MIKE 21 and SCALGO Live . . . . . . 39 3.4 Sensitivity analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4 Results 43 4.1 Flood vulnerability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.1.1 Initial scenario . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.1.2 Identified flood vulnerability . . . . . . . . . . . . . . . . . . . 44 4.2 Stormwater management solutions . . . . . . . . . . . . . . . . . . . . 46 4.2.1 Implementation and design of schools . . . . . . . . . . . . . . 46 4.2.2 Effect of implementing stormwater solutions . . . . . . . . . . 46 4.2.2.1 Combination of measures . . . . . . . . . . . . . . . 47 4.2.2.2 Results Solution 2 - Pond and ditches by the elemen- tary school . . . . . . . . . . . . . . . . . . . . . . . 48 4.2.2.3 Results Solution 3 - Flooding area in intersection and terraces by Kring Alles Road . . . . . . . . . . . . . 50 4.2.2.4 Results Solution 4 - Ditch Kring Alles Road . . . . . 50 4.2.2.5 Results Solution 5 - Ditch in grass area by Odhners Road . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.2.2.6 Results Solution 6 - Flooding area in grass area by Odhners Road . . . . . . . . . . . . . . . . . . . . . 51 4.2.2.7 Results Solution 7 - Flooding area in the park by Vattenpalatset . . . . . . . . . . . . . . . . . . . . . 52 4.2.2.8 Results Solution 8 - Removal of Vattenpalatset . . . 52 4.2.2.9 Results Solution 9 - Ditch to Säveån . . . . . . . . . 52 4.2.2.10 Results Solution 10 - Downstream solutions at Torp- skolan . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.3 Effect of infiltration and surface roughness . . . . . . . . . . . . . . . 53 4.4 Results of sensitivity analysis . . . . . . . . . . . . . . . . . . . . . . 57 5 Discussion 59 5.1 Discussion of results . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 5.2 Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 5.2.1 Discussion of implementation of solutions . . . . . . . . . . . . 63 5.3 Discussion of multifunctionality in proposed solutions . . . . . . . . . 65 5.3.1 Ecological aspects . . . . . . . . . . . . . . . . . . . . . . . . . 65 5.3.2 Societal aspects . . . . . . . . . . . . . . . . . . . . . . . . . . 66 X CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 Contents 6 Conclusion 69 6.1 Future studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Reference list 71 A Map of preschool I B Municipally owned land III C Culverts at Torpskolan V D Map of elementary school IX E Result charts from MIKE 21 XI CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 XI Contents XII CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 List of Figures 2.1 Schematic figure of urban and natural hydrograph, adapted from (Marsalek et al., 2014). . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Identified critical infrastructure where flooding should be avoided. Map is created by the authors with information obtained from Lant- mäteriet and Lerums kommun (2019), Svenska Kyrkan (n.d.) and Google (n.d.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.3 The watershed in the case study area in Lerum. . . . . . . . . . . . . 21 3.1 The red circles show the areas where stormwater solutions are imple- mented and the areas’ respective labels. The reference labels will be used throughout the report to refer to the locations of the solutions. . 27 3.2 Solution 1. Figure (a) depicts the elementary school when imple- mented into SCALGO Live according to the design in the detailed development plan. Figure (b) shows how the school is altered to avoid flooding in the direct proximity, and the implemented solutions connected to the schoolyard. The stormwater solutions are marked in black. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.3 Solution 3. Figure (a) depicts the terraces and dry pond as seen in SCALGO Live. The yellow arrow in the bottom right corner indi- cates where the photograph in Figure (b) is taken. Figure (b) is a photograph taken to illustrate the area where the terraces can be implemented. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.4 Figure (a) depicts the area by Odhners Road as it is today. Figure (b) depicts how the area would look if the serpentine ditch is imple- mented. The black circle surrounding the serpentine shape depicts the removal of the buildings shown in Figure (a). Figure (c) depicts the implementation of a flooding area instead of a serpentine ditch. . 32 3.5 Figure (a) depicts the flooding area by Vattenpalatset and the pé- tanque court in the upper right corner. Figure (b) shows a photo of the area. Vattenpalatset is located to the left and the pétanque court is outside the picture to the right. . . . . . . . . . . . . . . . . . . . . 33 3.6 The available flooding area when Vattenpalatset is removed and used as a retention area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 XIII List of Figures 3.7 Subfigure (a) shows the path for the ditch leading to Säveån along the road Häradsvägen. Also shown to the left in subfigure (a) are culverts to the west of Torpskolan. The circle at the northern part of the ditches depicts the removal of the current bike rack with a roof. Subfigure (b) shows the ditch following the parking lot instead of the road. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.8 Solution 10. Subfigure (a) depicts the SCALGO Live model with the ditch as the solid black line at the bottom left corner of Torpskolan. The flooded area is to the right of the ditch, and the creek is to the left. The dotted lines are culverts which ultimately connect to Säveån at the bottom of the figure. Subfigure (b) shows a photo of the location where the ditch can be implemented and the bridge would cross the ditch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.9 The infiltration rates for the solutions in a permeable scenario, located by Kring Alles Road. . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 4.1 Figures (a) and (b) show the flood and flow conditions before any alterations have been done to the area. Figure (a) shows the flood depth on the three colour scale, and large flow paths with bold blue lines. The topography is also shown, with red colours being higher ground and blue lower. Figure (b) shows the magnitude of the flow speed using a scale where red indicates a flow speed above 3.5 m/s. . 44 4.2 Combination of high flow speed locations, sensitive infrastructure and the investigated watershed. The figure shows three preexisting preschools, one elementary school and a grocery store to be within the watershed. Also located in the watershed is the new elementary school in Norra Hallsås and the new preschool by Kring Alles Road. . 45 4.3 Measuring points for flow speed and depth, marked with stars. . . . . 47 4.4 Subfigure (a) shows the solutions included in System A, and subfigure (b) shows the solutions included in System B. Ditches do not retain any water and are thus not marked as flooded. The light blue circles show the differences in the systems. . . . . . . . . . . . . . . . . . . . 48 4.5 The elementary school with the altered design where the top building is moved. Also shown are the stormwater solutions in the direct proximity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.6 A profile showing the water depth of the 7 terraces and the second, larger, pond by the playground shown on the far left in the figure for a rain volume of 100 mm. . . . . . . . . . . . . . . . . . . . . . . . . 50 4.7 Flow speed results from MIKE 21 for the ditch by Kring Alles Road. 51 4.8 Flow speed results in the reference points for lower infiltration and lower surface roughness parameters. . . . . . . . . . . . . . . . . . . . 54 4.9 The maximum flow speeds in Gatekullen Road. . . . . . . . . . . . . 55 4.10 Flow speeds at Gatekullen Road from 11:00 to 13:00. . . . . . . . . . 56 4.11 The maximum flow speeds for the points located in Kring Alles Road. 56 XIV CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 List of Figures 4.12 The infiltrated volume at the park by Vattenpalatset for Solutions 1 (Edited schools) and System A with permeable and impermeable surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.13 Line chart showing the sensitivity analysis performed in MIKE 21 on surface roughness and infiltration for System A with impermeable surfaces. The evaluated location is the highest flow reference point. . 58 4.14 Line chart showing the sensitivity analysis performed in MIKE 21 on surface roughness and infiltration for System A with permeable surfaces. The evaluated location is the highest flow reference point. . 58 A.1 Suggestion by the architect studio AL Studio for location and design of the preschool and the associated parking lot at Kring Alles Road. . II B.1 Map of the study area in Lerum with land owned by the municipality marked in yellow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV C.1 The dotted green line shows the initial culvert implemented in SCALGO Live diverting water past the creek on the western side of Torpskolan. VI C.2 The orange area shows the flooding by Torpskolan if there are no culverts diverting the water to the creek. . . . . . . . . . . . . . . . . VII C.3 The flooding with two culverts marked as black dotted lines. The flood by the school is significantly smaller. . . . . . . . . . . . . . . . VIII D.1 Location and design of the elementary school in Norra Hallsås. . . . . IX E.1 Chart over the maximum speeds in the reference points in Figure 4.3. XII E.2 Chart over the depth at the time of the maximum speeds in the reference points in Figure 4.3. . . . . . . . . . . . . . . . . . . . . . . XIII E.3 Chart over the maximum depth in the reference points in Figure 4.3. XIV E.4 Chart over the flow speeds in the reference points in Figure 4.3 at the time of the maximum depth. . . . . . . . . . . . . . . . . . . . . . . . XV CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 XV List of Figures XVI CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 List of Tables 3.1 Calculated precipitation volumes. . . . . . . . . . . . . . . . . . . . . 25 3.2 The stormwater solutions included in System A and System B. The differences between the systems are marked in italics. . . . . . . . . . 40 3.3 Infiltration rate and surface roughness represented by infiltration val- ues and Manning’s M for each edited surface. The table is divided into the original values set by Tyréns AB, the scenario with mainly impermeable surfaces and the scenario with mainly permeable surfaces. 41 4.1 Results on flooding from the various stormwater solutions. The table shows how the total water volume by Torpskolan varies in SCALGO Live for the different solutions and rain events. . . . . . . . . . . . . . 53 E.1 Easting and northing coordinates for the reference points. . . . . . . . XI CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 XVII List of Tables XVIII CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 Terminology Blue-green infrastructure Infrastructure with a variety of solutions incorporating vegetation and water to mimic the ways water is managed in nature. Evapotranspiration The total amount of water leaving soil, bodies of water, and vegetation through vapour into the air. Hydrological cycle The total sum of all water moving from the land and ocean into the atmosphere and back in the form of precipitation. Hyetograph A graphical representation of the distribution of rainfall intensity over time. Impermeable surface A surface without the ability to infiltrate water, such as roofs and roads. MIKE 21 Hydrodynamic module found in the software MIKE Powered by DHI, used to model stormwater. Permeable surface A surface with the ability to infiltrate water, such as grass covered areas. Pluvial flood Heavy rainfall too large in volume for the local drainage system to handle. Return time Describes the probability of a specified rain event to occur within a given time span. Runoff Runoff = precipitation - evapotranspiration - infiltration - change in storage. SCALGO Live Software developed by SCALGO, used to model stormwater. Stormwater Water flowing as runoff on an impermeable surface. Stormwater management Treatment, facilitation, and transportation of stormwa- ter. Sustainable stormwater management The use of stormwater solutions that mimics natural stormwater management. Watershed Area of precipitation that contributes to a downstream point. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 XIX XX CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 1 Introduction Flooding has become more prominent globally over the past 20 years and is an increasing challenge (Jha et al., 2012). Large flows of stormwater in urban settings can result in flooded areas and have large impacts on society. The combination of increased precipitation and an increase in permeable surfaces due to urbanisation leads to challenges in managing the stormwater to avoid major floods. Climate change is one of the leading factors of increased pluvial flooding in urban areas, due to the increase in precipitation. Precipitation affects urban spaces more than natural settings due to its permeable surfaces and complexity of the water systems. Stormwater in an urban setting is often subject to transportation of contaminants (Niemczynowicz, 1999). The contaminants present can be of organic or inorganic matter. Common contaminants studied in stormwater are nutrients, organic matter and heavy metals (Fletcher et al., 2013). Other contaminants found to be present in the stormwater system are synthetic compounds and chemicals such as pesticides or hormones. The composition of the pollutants present in the stormwater varies depending on location (Müller et al., 2020). There are various practices to manage stormwater in urban areas. There is the option of collecting the stormwater in drainage systems and diverting it from the surface to a recipient (Zhang et al., 2017). This practice is not preferred and instead the use of stormwater management solutions that better mimic nature is preferred to be implemented. The terms used for these stormwater management practices differs between countries and are for example called; sustainable stormwater man- agement (Stahre, 2004), LID - Low Impact Development, SUDS - Sustainable Urban Drainage Systems, blue-green infrastructure or solely green infrastructure (Chan et al., 2018; Liao et al., 2017). The main objective for using these practices is to mimic nature’s management of stormwater and have additional benefits from the stormwater solutions. In this report, the term sustainable stormwater management is used. The possibility of implementing flooding areas as multifunctional surfaces is an addi- tional benefit of sustainable stormwater solutions. The multifunctional surfaces have other uses than stormwater retention during dry periods. Another benefit of using open, blue-green or only green solutions is ecosystem services. Ecosystem services stem from the nature and are resources and services utilised by people (Millennium CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 1 Ecosystem Assessment, 2005). The services are of varying forms and can be reg- ulating, provisioning, supporting and cultural (Millennium Ecosystem Assessment, 2005; Naturvårdsverket, 2018). An example of a regulating ecosystem service is the lowered temperature that vegetated areas provide in an urban setting. Sustainable stormwater solutions that include vegetation often contribute to stormwa- ter management by infiltrating the initial volume of the rain event and lowering the peak flow (Chan et al., 2018; Svenskt Vatten, 2011b, 2016; S. Wang & Wang, 2018; J. L. Yang & Zhang, 2011). This suggests that infiltration has a large role in stormwater management and flood prevention, while some recent research found that infiltration has a minor impact and that surface roughness has a larger impact on the flood and flow speed (Herrmann, 2019). Y. Yang et al. (2015) identified a lack of studies on the effect of surface roughness on urban stormwater management and concluded in their research that surface roughness impacts peak runoff. Urban development increases, and with it an increase in impermeable surfaces, all while there is an increased risk of flooding due to climate change (Liao et al., 2017). Infiltration is an extensively researched factor in stormwater management while sur- face roughness is not researched to the same extent (Y. Yang et al., 2015). Therefore an aim to investigate the impact of these parameters is formed. 1.1 Aim and objectives Aim The aim of this thesis is to investigate the importance of using sustainable stormwa- ter management in urban areas. The study is performed through the assessment of the impact of infiltration and surface roughness for proposed solutions in a develop- ment project in Lerum, Sweden. Objectives The objectives set to achieve the aim are listed below. • Identify areas sensitive to flooding. • Identify vital infrastructure. • Identify locations with high risk levels for people in case of flooding. • Propose sustainable stormwater management solutions in suitable locations. • Consider multifunctionality in the proposed stormwater solutions. • Study the effect of infiltration and surface roughness in proposed stormwater solutions. 2 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 1.2 Limitations • The only type of flooding considered in the project is pluvial flooding. • Anything outside of the study area in Lerum is not considered in the evalua- tion. • The proposed solutions are only implemented in areas owned by the munici- pality. • The study does not consider the location of existing subsurface infrastructure • The report does not focus on remediation aspects of stormwater, but rather only on flow and flood. • The report does not consider which stakeholders are included in the process, and does not include economical aspects of various solutions. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 3 4 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 2 Background This chapter provides insight on the urban water cycle and how rainfall can affect urban areas. It also gives some examples of sustainable stormwater management solutions as well as information regarding climate change and the impact it can have on future rain events. Some information regarding modelling and its use in the topic is presented, and last is a description of the case study area in the municipality of Lerum. 2.1 Urban water systems Water in urban areas differs from how water behaves in nature. In the natural hy- drological setting, the ground consists of a natural ground cover and the processes included in the hydrological cycle after a rain event is infiltration, both deep and shallow, evapotranspiration and runoff (USEPA, 2003). Stormwater is water flowing as runoff on an impermeable surface (Mansell & Rollet, 2006). The precipitation can be present in a variety of amounts and forms such as rain, snow or hail. Evap- otranspiration is the act of transpiration and evaporation of water by plants, and infiltration is seen as the part of the precipitated water seeping down to the ground- water. The change in storage can refer to storage in the groundwater, or in various storage above ground. Examples of storage above ground are depressions able to hold water, plants holding water temporarily and water attaching to the surfaces of various pavements. According to Mansell and Rollet (2006), the change in storage evens out over periods of time and is therefore negligible in the long term. In an urban setting, as compared to the natural setting, many components connected to water infrastructure are present such as raw water, water treatment, distribution network and collection networks (Marsalek et al., 2014). Sewers for stormwater and sanitary purposes in urban areas can be of either a combined or separate sys- tem (Fletcher et al., 2013). In a separate system the wastewater is transported to the wastewater treatment plant in separate pipes from the stormwater (Field & Struzeski, 1972). The wastewater then goes to a wastewater treatment plant, and the collected stormwater goes directly to a recipient, or to a stormwater treatment facility. In a combined system, the stormwater is conveyed through the same pipes as the wastewater. Thus during dryer periods, when the capacity is sufficient, both types of water are treated at a wastewater treatment plant. However, in the event CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 5 of heavier rainfall the addition of stormwater exceeds the pipes’ capacity and a CSO (Combined System Overflow) can occur. A CSO entails a release of the combined stormwater and wastewater directly into a recipient, without foregoing treatment. The use of underground drainage systems for stormwater entails a limited capac- ity and the reduction of underground drainage is part of the aim for a sustainable stormwater management (Bohman et al., 2020). Flooding occurs when large amounts of water fail to infiltrate to the ground, usually in an area that is dry most of the time (Jha et al., 2012). The flood can originate from raised water level from either the sea or a nearby river, heavy precipitation, or large amounts of snowmelt. The main source of flooding in urban areas is caused by heavy rain over the course of a short time span, usually referred to as a flash flood (Gruntfest & Handmer, 2001). A flood scenario where only rain is taken into consideration is called pluvial flood, and is defined as heavy rainfall that is too large in volume for the local drainage system to handle (Lin et al., 2021). The main issue with urban flooding lies in the water’s inability to infiltrate properly, resulting in increased amounts of water gathering in a certain area (Zhang et al., 2017). A large contributor to this comes from the many impermeable surfaces in a city, caused by the rapid urbanisation (Xu et al., 2013). Flash floods and pluvial floods are floods of greater magnitude and often only problems in urban areas due to the area often being a living space or the area having a certain value to the residents, and any flash flood related events will only be more devastating as a direct consequence of human activities (Gruntfest & Handmer, 2001; Lin et al., 2021). 2.1.1 The hydrological cycle The urban hydrological cycle differs from the natural hydrological cycle in terms of runoff, evapotranspiration and infiltration (Niemczynowicz, 1999). The difference affects the fate of precipitation falling on an urban area. The results of increased permeability are an increased runoff and lower infiltration due to the lack of perme- able surfaces. Peak flows increase and the additional impermeable surface also leads to higher speeds of the flow. Studies have proven that an increase in impermeable surfaces also lead to additional runoff volumes (Chormanski et al., 2008; Olivera & Defee, 2007; Yao et al., 2016). Additionally, as mentioned by Niemczynowicz (1999), a change in the surface of a small area of a city can have large effects on a downstream location. The impact on runoff due to an addition of impermeable surfaces is that the flow increases and the peak flow occurs earlier in the rain event (Wen et al., 2014). This is illustrated with the hydrograph in Figure 2.1 where the green line shows the natural runoff of an area, and the red line shows the urban, impermeable response, both for the same rain event. The goal of implementing sustainable stormwater management is to achieve a curve similar to the green one. According to the United States Environmental Protection Agency (USEPA, 2003) the runoff in a natural setting is approximately 10%, and in an urban area runoff can be up to 55% of the downfall depending on how much of the area has an 6 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 Figure 2.1: Schematic figure of urban and natural hydrograph, adapted from (Marsalek et al., 2014). impermeable cover. The increase in runoff is due to a loss of evapotranspiration in urban areas, as well as lower infiltration. All of these effects together with the continuing densification of cities put great strain on the existing stormwater drainage system (Wihlborg et al., 2019). 2.1.2 Impacts of flooding Stormwater management for flash floods is based on what the consequences might be in the case of a heavy rainfall (MSB, 2017). Buildings and infrastructure can be damaged which has economical consequences. Floods can also impact the func- tionality of a society by affecting vital infrastructure, which means that a loss of this infrastructure’s function can lead to severe consequences in society (Håkanson et al., 2019). The functionality or facilities can also be lost by inaccessibility, caused by flooded roads and access points. A flood map shows the areas at risk of flooding, however the critical infrastructure at risk should also be identified and put in refer- ence to the flooded areas (MSB, 2017). While buildings and infrastructure functions are most sensitive to the water depth of the flooded areas, more factors need to be taken into consideration when it comes to protecting people from flooding. MSB (2017) has created an equation for calculating the risk level for people regarding flood events. The equation includes both water level and flow speed, with both higher flow speed and deeper water leading to higher risks. The equation can be seen below in equation 2.1 and is based on calculations for flood hazards made by CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 7 Defra (2006), which takes into consideration multiple other factors as well. Flood risk level = (V + C) × D (2.1) • C - constant coefficient of 0.5 • V - maximum velocity [m/s] • D - water depth [m] The equation results in various limits presented in MSB (2017), varying from <0.75 posing no risk, the intermediate levels of risk for some (0.75-1.25) and risk for most people (1.25-2.5) up to >2.50 posing a risk for everyone. These results can vary on an individual level, where a higher water level is more dangerous to some while others are more susceptible to higher flow speeds. 2.2 Stormwater management Stormwater management is the prevention of floods by minimising the excess water flowing on impermeable surfaces. Because of the rampant increase of urbanisation, cities need to incorporate more sustainable solutions to lower the percentage of im- permeable surfaces as these contribute to high stormwater flows (Xu et al., 2013). These sustainable solutions incorporate a variety of vegetation and methods inspired by the ways water is handled in nature (L. Liu et al., 2019). Wright (1996) states that by using this as a starting point, stormwater can be seen as a resource rather than an issue. According to European Environment Agency (n.d.) and Svenskt Vat- ten (2016), sustainable stormwater management aims to mimic nature’s stormwater management, meaning not using subsurface stormwater solutions. Stormwater solu- tions with open surfaces are also more sustainable than subsurface ones. Examples of these open surface solutions are green roofs, rain gardens, swales and ponds. The goal of sustainable stormwater management is to delay and decrease the amount and flow of stormwater in an urban area. Additionally, L. Wang et al. (2001) mentions how detention ponds are efficient in reducing urban flooding caused by urbanisation. 2.2.1 Multifunctionality Stormwater solutions can have other purposes, in addition to delaying water. This is especially important to consider when designing solutions for large rains with a low probability to occur frequently as the multifunctionality provides a use during dry weather (Keyvanfar et al., 2021). Using these multifunctional aspects, sustainable stormwater management can be of an ecosystem service nature and also have social value (Mell, 2009). Urban areas have been described to have a strong symbiosis between both social and ecological systems (Pickett et al., 2011). Therefore it is important to take into consideration the aspect of the multifunctional properties when designing a solution, to ensure usability of the space when not fulfilling the water delaying purpose (Keyvanfar et al., 2021). 8 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 Some benefits to sustainable stormwater management are environmentally related, such as; energy saving by having green roofs (Pfoser et al., 2014), increased biodiver- sity (Ghofrani et al., 2017), reduction of pollutants in the air (Berardi et al., 2014), and flood mitigation (Cristiano Id et al., 2021). Other benefits are social values, such as offering places for people to gather for recreational purposes and improving the scenic quality (Nurmi et al., 2016). Large stormwater retention facilities are suitable for implementing social areas. An example of a facility is Enghaveparken in Copenhagen, which is a large park that also serves as a stormwater retention pond (Tredje Natur Architects, n.d.). Eng- haveparken’s plans show various examples of how the space can be utilised during dry periods, including having sporting areas, flowerbeds, park benches and more. The various activities that can be performed during the dry periods can be beneficial both in social aspects and with respect to ecosystem services. Ecosystem services is a concept describing how ecosystems impact human wellbeing and are often presented in four different categories: provisioning, regulating, cul- tural, and supporting (Alcamo et al., 2005; Prudencio & Null, 2018). These services stem from nature and they highlight humans’ dependency on it. Examples of services include food production, providing oxygen, binding of pollutants, providing shelter, reducing stress, and noise reduction. Urbanisation removes many of these services and they have to subsequently be replaced by other means, often man made. Using sustainable stormwater management is a way cities can incorporate more ecosystem services into a city. Some examples given by Elmqvist et al. (2015) are increased biodiversity, health benefits, and the inclusion of many cultural services. A body of water or a green area in an urban area could also attract animals that normally do not live in the city (Mottaghi et al., 2020). Vegetation has proven to be beneficial to human wellbeing (Nghiem et al., 2021). During the start of the COVID-19 pandemic in 2020, a study was conducted in Chengdu, China to assess the wellbeing of its residents (Xie et al., 2020). The survey indicated that the residents felt a lack of social interaction and their health status being negatively affected by the restrictions in place. According to Xie et al. (2020), a visit to a local urban park would be beneficial to a person’s wellbeing, both in terms of social needs and overall health, even if only once a week. Xie et al. (2020) also states that most of the correspondents preferred to visit a park close to their homes to minimise travelling. 2.2.2 Infiltration and surface roughness Infiltration is the act of water entering the soil via gravitational forces (Kirkham, 2014). The amount of water able to infiltrate in a soil depends on a number of factors, ranging from type of soil, porosity, vegetation, and land use (Blackburn, 1975). Coarse soil such as gravel or sand has a greater infiltration capacity than finer soils such as clay or silt (MSB, 2017). Sand can infiltrate up to 100 mm/h whilst clay can only reach a few mm/h. In urban areas the soil is often replaced by an impermeable cover as described in section 2.1, which makes infiltration in CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 9 these areas difficult and in turn generates excess amounts of runoff. During heavy rain events, a large amount of water can enter the soil and make it over-saturated, making it more vulnerable to erosion (MSB, 2017). Extreme rains generally occur in the summer in Sweden, making the soil more susceptible to infiltration due to dry weather before the rain event (Olsson et al., 2017). Infiltration takes part in stormwater management by infiltrating the water from smaller rain events (Stahre, 2004). Therefore, in the case of a heavier rain event, the initial volume can be managed and infiltrated by the porous media, until the soil is saturated and the remaining rain flows on the surface as runoff (Wagener et al., 2007). The type of soil also has an effect on the infiltration capacity (J. Bai et al., 2019). Surface roughness is a measurement describing the unevenness of a surface. In many cases it is described with Manning’s number, M, (MSB, 2014) and has a vital role when calculating the surface runoff volume. Manning’s M, is in turn based on an empirically derived Manning’s roughness coefficient, n. Values of n ranges from 0.01-0.013 [s/m1/3] for asphalt and 0.39-0.63 [s/m1/3] for grass (Engman, 1986). The higher the value of n the more uneven the surface is, in turn making Manning’s M small as a result of their inverse relationship, see equation 2.2. M = 1/n [m1/3/s] (2.2) Consequently, surfaces with high values of Manning’s M, such as smooth asphalt, can convey water more efficiently than coarser surfaces (Lau & Afshar, 2013). This in turn leads to more stormwater runoff with a higher flow and flow speeds from areas with large amounts of impermeable surfaces that usually have a high Manning’s M. If the surface on the other hand is for example a grass covered area it is likely that both surface roughness and infiltration values are high. 2.2.3 Stormwater management solutions There are various solutions for managing stormwater and avoiding flooding. In the following sections, structures for stormwater management are described. The stormwater structures have two main objectives; retention of stormwater to delay the water, and to divert it from objects at risk from being flooded (Stahre, 2004). 2.2.3.1 Green roofs Green roofs consist of a small layer of vegetation placed on the asphalt board of the roof (Parizotto & Lamberts, 2011). The layer of vegetation delays water and enables evapotranspiration. According to the US Department of Energy (2004) green roofs are able to retain up to 75% of annual precipitation. The ability to retain precipita- tion in an individual rain event depends on the size of the rain, where small rains can be retained up 100% producing zero runoff from the roof, while large rains create more runoff and smaller volumes infiltrate (VanWoert et al., 2005). Some bene- 10 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 fits with green roofs are isolation, increased lifespan of roofs (Teemusk & Mander, 2009), improved air quality (Rowe, 2011), and noise reduction (Van Renterghem & Botteldooren, 2011). Other benefits are their ability to be implemented to existing buildings in addition to planned buildings (Stahre, 2004). However, using green roofs increases the weight of the roof significantly and if implemented on an already built roof, the building has to be able to manage the added weight (Stahre, 2004). Stahre (2004) emphasises the inclination of the roof, since it has to be minimal to allow the construction of green roofs. 2.2.3.2 Swales Swales are large patches of grass with a slight inclination with the objective of collecting, diverting, and allowing runoff to infiltrate into the soil (Stahre, 2004). Because of its simplistic and linear design, swales are often a preferred choice of stormwater management alongside roadways (Davis et al., 2011). Swales are also preferred in areas with limited space (Yu et al., 2013). At the lowest point a well is often located to collect excess water to the stormwater network (Stahre, 2004). Stahre (2004) further explains to avoid erosion of the swale it is important that the steepness of the downward slope does not exceed 2 degrees inclination. Several studies have shown that swales performance can vary, reducing peak runoff rates from anywhere between 4% to 87% (Deletic & Fletcher, 2006; Rujner et al., 2018). Rujner et al. (2018) further describes the varying performance to possibly be re- lated to initial soil moisture and infiltration capabilities. The height and density of the grass plays a role in the performance (Deletic & Fletcher, 2006), as does the characteristics of the soil (Rujner et al., 2016). Despite its variance in performance, swales have proven to be efficient in reducing local urban flooding and flash floods (Shafique et al., 2018). 2.2.3.3 Ditches A ditch’s main function is to convey water (Sustainable Technologies Evaluation Program, 2019). Ditches, by comparison to swales, are often both steeper and deeper and are naturally vegetated (J.F. Sabourin and Associates Inc., 2000). Some advantages of ditches, with or without culverts, are their ability to increase the concentration time of runoff leading to better design flows, some amount of filtration for ditches with grass bottoms, and lowering the risk of water gathering on the road during intense storms. A ditch can be formed to a serpentine shape using berms to prolong the flow route (Ontario Ministry of Environment, 2003). The same formation can be achieved by adding berms to a pond or dry pond. According to Larm and Blecken (2019) the slope perpendicular to the flow path of a ditch should not exceed 1:3, or roughly 18 degrees inclination. Drawbacks related to the construction of ditches are potential vegetation and the excavated soil. Trees and their roots might be in the way of excavation, and the excavated soil has to be attended to (Q. Bai et al., 2021). The ditch could also be subject to soil erosion from large volumes of flowing water. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 11 2.2.3.4 Ponds Stormwater ponds are man made structures with the main purpose to collect runoff thus reducing peak runoff and risks of flooding, as well as providing recreational and aesthetic services (Tixier et al., 2011). The ponds contain water permanently (Stahre, 2004) and also provide a degree of biodiversity, since the areas where they are integrated are often dominated by urban landscapes (Céréghino et al., 2014). Some studies have now identified that due to being so closely located to roads, the ponds accumulate pollutants and can become harmful to wildlife. (Meland et al., 2020). The design of the pond should prevent accidents by for example having fences around larger ponds (Stahre, 2004). For smaller and more shallow ponds it can be sufficient to enclose the pond using vegetation to ensure safety. The preferred design standards for slopes on the edges of the pond is to not exceed 1:7, or roughly 8 degrees inclination, from the permanent surface, but it can be steeper if necessary (Ontario Ministry of Environment, 2003). 2.2.3.5 Flooding areas and dry ponds Flooding areas, also known as dry ponds, are often a large, lowered grassy area de- signed to be flooded in the event of a large storm, whilst being dry during times of no precipitation (Sinclair et al., 2020). After a storm event the water is either diverted through a ditch or left to infiltrate into the soil where usually a drainage pipe is located (Stockholm Vatten och Avfall, 2017). During dry periods when there is no water collected the area can be utilised as a park or playground (Shammaa et al., 2002). In recent times, during the COVID-19 pandemic, both Xie et al. (2020) and Jenkins (2020) put emphasis on urban parks and other large outdoor open spaces, describing how they have been crucial to maintaining a good mental wellbeing with humans. The area should only have an inclination of a few degrees to ensure mainte- nance can be performed (Stockholm Vatten och Avfall, 2017). According to Ontario Ministry of Environment (2003) guidelines, the side slope in a dry pond should not be steeper than 1:4, or 14 degrees inclination. Furthermore, the area should re- ceive equal maintenance compared to a park in a similar area (Stahre, 2004). It is important to make sure that the area is fully drained to prevent a constant water table, which could affect the multifunctional purpose of the solution. These types of areas generally require a large area to be useful (Stockholm Vatten och Avfall, 2017). Another factor to take into consideration is related to soil compression when draining groundwater, as the settlements could have a negative impact on nearby buildings (Shen et al., 2006). 2.2.3.6 Permeable surfaces Permeable surfaces consist of a permeable layer allowing water to infiltrate into the soil beneath (Eisenberg et al., 2015). These can either be natural such as grassy areas, or man made such as permeable asphalt or gravel with a storage tank placed beneath it (Lewis et al., 2019). From there the water can either be diverted to the natural soil layers or drained (Stahre, 2004). Solutions consisting of permeable surfaces have proven to lessen the impact of runoff entering the drainage system, 12 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 and therefore lowering the risk of flooding (Lewis et al., 2019). As such they are often placed where there is a great amount of impermeable areas, such as parking lots and driveways. Additional benefits of this solution include recharging of the groundwater table, improved water quality, and reduction of infrastructure costs related to drainage systems (Eisenberg et al., 2015). One common disadvantage related to permeable surfaces arises when they are clogged and no longer can operate sufficiently (Bean et al., 2007). If these are not cleared and maintained regularly the surface can be completely clogged, in turn making the surface unable to perform and in need to be replaced entirely (Scholz & Grabowiecki, 2007). 2.2.3.7 Stormwater terraces A series of terraces located on different height levels along a path can be used to collect and slow down water. The design is similar to dry flooding areas connected in a series, making them flood subsequently. According to research made by J. Bai et al. (2019), terraces are effective in reducing runoff rates when comparing the amount of vegetated areas of a terrace system to a simple slope. The terracing also increases infiltration rate compared to a constant slope since infiltration rates decrease with increased inclination (Morbidelli et al., 2018). 2.3 Precipitation and climate change Climate change affects multiple aspects regarding flooding and precipitation. Some of the effects stemming from climate change are; an increase in global temperature, increased global sea levels, and an increased number of heavy rain events (Wolff et al., 2020). From The Fourth Assessment Report from the IPCC (the Intergovernmental Panel on Climate Change), it is clear that the biggest contributor to the global rise in temperature stems from the increase in greenhouse gas emissions, which in turn can lead to altered precipitation patterns (Solomon & Intergovernmental Panel on Climate Change, 2007). According to SMHI (Swedish Meteorological and Hydrological Institute), the amount of precipitation in Sweden has increased since 1980, and will by the end of this century have increased between 20-60% (SMHI, 2020a). There are a number of potential consequences with climate change related to stormwa- ter. One potential consequence is an increased risk of extreme rain events and flash- floods. Furthermore, an increase in precipitation will increase the amount of water coming from the increased number of impermeable areas (Alamdari et al., 2017). All of the events listed will put great strain on society unless adequate preparations are made before they occur. 2.3.1 Return times Return times are used to describe the intensity of a rain event. SMHI has defined a cloudburst as a rain event of at least 50 mm in an hour, or a minimum of 1 mm per minute (SMHI, 2017). The definition by SMHI is very specific, and another way CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 13 of describing the size of rain events is return times. Using return times, the rain event is classified by the magnitude of rain which falls on a surface for a period of time (Svenskt Vatten, 2016). If the same volume falls during one longer and one shorter time period, the shorter time period will have a higher intensity and thus be classified as a higher return time, meaning it is a more rare event with a possibility of more extreme consequences. The intensity and return times are a way of describing how likely it is that an event will occur each year. For example a rain with a 100-year return time has a 1% chance of occurring each year, independently of when the last 100-year rain event occurred (Svenskt Vatten, 2016). On average, a 100-year rain would occur 10 times in 1,000 years. Rain events of different magnitudes have various chances of occurring each year. For example a 5-year rain has a 67% chance of occurring in 5 years, and 100% in 50 years, while a rain of a 1,000-year magnitude has less than 1% chance of occurring in a 5-year period, and 10% in 1,000 years. These figures of return times are what societal functions are designed to withstand in the case of a flood event, now and in the future. 2.3.2 RCP scenarios Representative Concentration Pathways (RCP) are scenarios describing how the ef- fect of global warming will impact the world in the future. These were projected by IPCC in 2014 and are updated versions of their former projections called Special Report on Emissions Scenarios (IPCC, 2019). The scenarios are based on the possi- ble radiative forcing value reached by the year 2100, these being RCP2.6, RCP4.5, RCP6.0 and RCP8.5. All scenarios are possible outcomes, however which path will become reality depends on how much greenhouse gases will be emitted in the future (Moss & Intergovernmental Panel on Climate Change., 2008). For the lowest sce- nario, RCP2.6, the highest emissions of carbon dioxide are comparable to present day values and will have peaked around the year 2020 (SMHI, 2018). For RCP4.5 it is predicted that the emissions will peak close to 2040, and for RCP6.0 it will peak in 2060. RCP8.5, which is the highest scenario, predicts the carbon dioxide emissions are three times higher than today by the year 2100, with the world having little to no environmental action plans whilst using fossil fuels to great extents. The RCP-scenarios are the foundation to the climate adaptation of stormwater manage- ment. 2.3.3 Climate factors To account for climate change when selecting design precipitation, a climate factor is added to the selected rain (SMHI, 2020b). The climate factor is based on the RCP- scenarios and reflects the possible increase in precipitation at the end of the century, based on the followed scenario as discussed in section 2.3.2. The climate factors reflect an increase, in percent, of the precipitation. According to SMHI (2020b), a trend in precipitation points towards an increase of 30-40% by the year 2100, if the worst case scenario of RCP8.5 is followed. The 30-40% increase corresponds to an addition of a climate factor of 1.3 or 1.4 for design precipitation. The addition of 14 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 1.4 as climate factor is recommended by Länsstyrelserna (2018). However, Svenskt Vatten recommends the use of climate factors 1.25 for rain shorter than 60 minutes, and 1.2 for rain events lasting longer than an hour (Svenskt Vatten, 2016). The same recommendation is given by SMHI for events that might occur in the coming 50 years, but at the end of the century it is more likely that the factors 1.3-1.4 will be accurate (Olsson et al., 2017). This means that the selection of a higher climate factor gives a higher safety for a longer period of time when designing solutions to manage flash floods. 2.4 Modelling Stormwater modelling is used to predict and simulate scenarios related to stormwa- ter, such as flooding or elevated sea levels. The earliest versions of modelling in this area derive from the US in the 1970s and were created by government agencies (Zoppou, 2001). Today the range of softwares has expanded and cover a variety of areas and provide varying degrees of difficulty and depth, while they have various advantages and disadvantages (Viklander et al., 2019). When choosing a software for a project it is important to consider what kind of parameters are studied and what results should be obtained. Another important factor to consider is the complexity of the model, where the complexity should correspond to the studied phenomena (Rauch et al., 2002). A more complex model might not be the most suitable, since it could leave room for more errors. Modelling can assist in predicting where water will gather in the case of a large rain event when the drainage system is unable to operate efficiently. This can in turn aid in the decision of where to place measures to counter the risk of flooding (Svenskt Vatten, 2016). Certain parameters are able to be studied using modelling as well. The impact of infiltration has been studied previously using softwares such as SWMM (Lee, 2011) and MIKE 21 (Gunnarsson, 2015). SCALGO Live can be used to study flow paths and water volumes in an easy manner (Eriksson & Wilkås, 2018). Surface roughness has been studied as well, however not to the same extent (Z. Liu et al., 2018). As described in section 2.3, the ever changing nature of stormwater related events can be difficult to prepare for. Therefore, use of modelling softwares for predicting these events have become increasingly important. In this project, two softwares are used to assess the impact of the proposed solutions, these being SCALGO Live and MIKE 21. These are described in more detail below. SCALGO Live is used to assess proposed stormwater solutions in a fast and effective manner, while MIKE 21 is used to assess the hydrodynamics as well as impacts of infiltration and surface roughness. 2.4.1 SCALGO Live SCALGO Live is a web based software with high resolution terrain data for flooding simulations developed by SCALGO (SCALGO, n.d.-a). The software allows the CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 15 user to analyse the terrain data with different tools, such as Flash Flood Scenario and Sea-Level Rise. The flash flood scenario, used in this report, simulates flooded areas and flow paths caused by extreme rain events (SCALGO, 2021a). When using the flash flood scenario the user can manually set the amount of rain to be analysed and the software then visually shows how the land would be affected by the conditions set. Other functions of the software include altering the terrain, flood pathways, and flooded areas (SCALGO, n.d.-b). It should be noted that SCALGO Live only shows the final outcome of a flash-flood or raised sea level and not a continuous event. Additionally, SCALGO Live does not take soil properties into account for its simulations, such as infiltration or surface roughness. This makes the results somewhat overestimated when compared to the real life scenario. As soil properties are not defined in SCALGO Live, buildings and other structures are simply altered elevations with no unique properties to differentiate them from the ground. The terrain model of Sweden used in SCALGO Live is based on data from Lantmä- teriet and consisted, at the time of the project, of a 2x2 meter resolution grid that covers most of Sweden (SCALGO, 2021b). As of April 2021 the resolution has been increased, to have a 1x1 meter grid available (SCALGO, 2021c). In SCALGO Live, water is collected in depressions with a specific volume capacity. When a depression has reached its capacity, water will flow to other depressions downstream. When the total rain amount increases, so does the size of the area that contributes to the lowest located depression (SCALGO, 2021a). 2.4.2 MIKE 21 Flow Model FM MIKE Powered by DHI is a 2D modelling software developed by DHI used to analyse, model, and simulate various water related properties and scenarios (DHI, n.d.-a). The scenarios can be of varying sizes and types; from a flooded parking lot, to a river and up to the ocean and estuaries. The model can include various parameters such as waves, flows, sediments, and precipitation (DHI, n.d.-c). There are a va- riety of options available for different research topics, called modules, where these aforementioned parameters can be specified before simulation. MIKE 21 Flow Model FM, henceforth referred to as MIKE 21, is a 2D tool designed for simulating processes in coastal, oceanic and estuarine settings, which can also be used to model inland flooding (DHI, n.d.-b). Some of the processes available as modules for modelling are oil spill, ecology, and hydrodynamics. The tool is based on a flexible mesh (FM) consisting of linear triangular elements (DHI, 2017). In the tool, the user defines conditions such as; boundary, bathymetry, simulation period, and choice of module. There are six modules available in the tool, the one being used for this thesis is the hydrodynamic module. The hydrodynamic module allows the user to specify a number of input features related to stormwater such as; precipitation, wind conditions, surface roughness, infiltration capacity, and evaporation (DHI, n.d.-b). Furthermore, the module allows 16 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 the user to include different structures and sources of pollutants if desired. Precipitation in MIKE 21 can be defined in three ways; no precipitation, net pre- cipitation, and specified precipitation (DHI, 2017). The first choice includes no precipitation and the second one simply calculates the difference between the pre- cipitation and evaporation. The third option uses a time series to describe the varying precipitation over time in the domain that can either be constant, varying in time but constant in domain, or varying in both time and domain. MIKE 21 uses an input feature called Flood and Dry to visualise flooding (DHI, 2017). The user has to specify two parameters called the drying depth, hdry, and wetting depth, hwet, a value written in meters. A third value, called hflood, is used to define if a cell is flooded. MIKE 21 determines a certain cell in the mesh to be flooded if two conditions are met; the water depth, h, at the cell is lower than hdry on one side of the cell and larger than hflood on the other side, and the sum of the depth lower than hdry and elevation on the other side is greater than 0. If h is greater than hwet in a cell, then the cell is considered wet. A cell is dry if h is less than hdry, and partially dry if h is greater than hdry but lower than hwet. The default values set in MIKE 21 for hwet and hdry are 0.05 m and 0.005 m respectively. The precipitation pattern used for a series that varies in time is called a hyetograph (Svenskt Vatten, 2011a). The hyetograph consists of three main parts; a pre-rain, a rain intensity peak and a post-peak duration. The hyetograph is based on IDF curves, Intensity Duration Frequency, to create a simulated rain that is representa- tive to the modelled area. One such standard design hyetograph is called CDS-rain, which stands for Chicago Design Storm. In the hyetograph for the CDS-rain, the pre-rain is usually shorter than the post-rain, meaning that the peak is not located in the middle. 2.5 Case study area The case study area for this project is located within the urban area of Lerum, the largest urban area in the municipality of Leurm. The municipality of Lerum is located in western Sweden, approximately 20 km northeast of Gothenburg. The municipality has an area of 260 km2, and approximately 43,000 inhabitants (Re- gionfakta, 2021a, 2021b). The urban area of Lerum has an area of 25 km2 (SCB, 2019) and 20,000 inhabitants (Gustafsson, 2020). Henceforth ’Lerum’ will indicate the urban area of Lerum. If the municipality is implied, it will be distinguished. Through Lerum runs the river Säveån, which runs through the lake Aspen in the western part before continuing to Gothenburg. The lake Aspen is adjacent to Lerum and can be a possible flood risk for areas near the lake. The flood risk from the lake is outside the scope of this study, as this project only relates to pluvial flooding. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 17 2.5.1 Topography and geology According to the available soil information at the site, the soil in Lerum consists mainly of postglacial or glacial clay, with some elements of crystalline rock, till/sandy till and postglacial sand (SGU, n.d.-a). Lerum is situated in an area with presence of quick clay, as is the case for some locations in Lerum including the central parts (Håkanson et al., 2019; SGI, 2017). The presence of quick clay can affect the possibility for implementations to manage flash floods. The river Säveån that runs through Lerum, from east to west, is sensitive to climate change driven impacts (SGI, 2017). Parts of the river flowing through Lerum are also sensitive to slope failure which is a consequence of for example flow and flow speed in the river. Säveån can be subject to flooding in some parts of the river, as can be seen by using SCALGO Live with flooding predictions by MSB (2018). The flooding predictions by MSB can be examined for both 100-year and 200-year return times, with an additional climate factor. The water quality status according to VISS (n.d.-b) is good in most aspects, with traffic and stormwater being part of the contribution to pollution of the river. Säveån connects to the lake Aspen, which has a generally good ecological status, with some possible eutrophication (VISS, n.d.-a). 2.5.2 Sensitive infrastructure in Lerum Sensitive infrastructure and facilities that are of societal importance are services that have negative effects on society should they fail (Håkanson et al., 2019). Examples are healthcare facilities, power grid networks and drinking water distribution. Sen- sitive infrastructure and facilities which can be identified using a map are used in this project. The identified facilities are shown in Figure 2.2 and are found through Lantmäteriet and Lerums kommun (2019), Svenska Kyrkan (n.d.) and Google (n.d.). Vital infrastructure included in the figure is healthcare facilities and a fire depart- ment. The facilities identified to be more sensitive to flooding in terms of livability are grocery stores, and risk for people are preschools and senior homes. Elementary schools are also included in the map as young children can be at risk in a flood- ing event, while higher education is not included. As mentioned, other sensitive or vital infrastructure relates to for example IT-services, power supply and drinking water supply (Håkanson et al., 2019). However, these are not included due to the information being classified. Other than protecting facilities for livability, it can also be of interest to investigate how a flood can affect historically and culturally significant locations (MSB, 2015). Significant buildings are listed in the database on built heritage, a database created by the Swedish National Heritage Board (Riksantikvarieämbetet, n.d.). In Lerum, there are several buildings that are included in the list. However, most of the listed buildings are located south of Säveån, and are not affected by the catchment area. Two churches listed in the database are located on the north side of Säveån and are therefore included in the map. 18 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 Figure 2.2: Identified critical infrastructure where flooding should be avoided. Map is created by the authors with information obtained from Lantmäteriet and Lerums kommun (2019), Svenska Kyrkan (n.d.) and Google (n.d.) . 2.5.3 Stormwater management in Lerum There are multiple reports and documents that relate to the subject of stormwa- ter management in Lerum, including, but not limited to; the comprehensive plan (Lerums kommun, 2008), stormwater strategy (Lerums kommun, 2015a), stormwa- ter management handbook (Lerums kommun, 2015b) and a climate adaptation plan (Lerums kommun, 2015c). Adding to this list are the regional and national plans and recommendations that affect stormwater management (Länsstyrelserna, 2018; SMHI & Svenskt Vatten, 2020). The municipality of Lerum has a comprehensive plan, valid from 2008 (Lerums kommun, 2008) with another one under develop- ment. The comprehensive plan describes goals and operations for different parts of the society. In the most recent comprehensive plan, it is mentioned that the goal for stormwater management is to be ecologically sustainable and that management should mainly aim to be local. Included in the stormwater strategy is a goal to prioritise sustainable stormwater management (Lerums kommun, 2015a). In Lerum’s comprehensive plan, it is also mentioned that at the time, in 2008, the sewage system was local and the sewage was treated in Lerum (Lerums kommun, 2008). However the goal for the municipality was to connect to Ryaverket in Gothen- burg. According to the website of Gryyab, the company running Ryaverket, this has been achieved and Lerum’s sewage system is connected to Gothenburg through a large wastewater tunnel (Gryyab AB, n.d.). CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 19 In the stormwater strategy, information is provided on what types of rain should be the design standards. It is stated that rains of the size with 100-year return time should be design standard when the rain risks damaging buildings, and a 400-year rain is design standard for the risk of damaging or affecting important functions in society (Lerums kommun, 2015a). These can for example be healthcare facilities or fire stations. The stormwater strategy brings up a municipal goal for Lerum, which is that the stormwater management should be sustainably adjusted to climate change in the year 2025, with respect to quality, quantity and design (Lerums kommun, 2015a). These parameters are based on the design standards for sustainable stormwater management as stated in the publication P110 by Svenskt Vatten (2016). 2.5.4 Development The municipality of Lerum is an expanding municipality, and the expansion is likely to lead to an increase in impermeable surfaces in its urban areas (Björkman et al., 2019). The municipality of Lerum is planning to build two new schools in the area. The one furthest in planning is an elementary school in the area Norra Hallsås (Lerums kommun, 2021a), which is the starting point for the catchment area examined in this project. The school is planned to have a capacity of between 800- 1000 students and will also include a school for children with special requirements, as well as a sports hall. The area is currently a forested area with rocky terrain mixed with lower vegetated areas containing water, as well as water in springs. A parking lot will also be constructed together with the school. The road leading up to the area is heavily sloped. This new development has a possibility to affect the flow to downstream areas by having an increased amount of impermeable surfaces. The second planned school is a preschool that will be located at Kring Alles Road in Lerum (Lerums kommun, 2017b). It is still early in its development process. The area where the preschool is planned to be located is sloping steeply towards Kring Alles Road and is mainly covered in grass. The suggested location by the municipality of Lerum for the preschool is in the south-west corner of the currently grass covered area. There are also plans for a parking space in the south-east part. The location and suggestion of the design made by the architects at AL Studio can be seen in Appendix A. The road Kring Alles is part of the course the water takes from Norra Hallsås. To obtain the watershed related to the case study area in Lerum, the watershed connected to Torpskolan is used. Torpskolan then becomes the downstream point of the watershed that is studied. Torpskolan is located in the centre of Lerum, north of the river Säveån. The watershed can be seen in Figure 2.3. The watershed corresponds to an area where high flows and flow speeds have been identified as a challenge when it rains, along with floods by Torpskolan. The area is also subject to developments upstream, downstream and along the way which can affect the stormwater situation (Lerums kommun, 2017a, 2017b, 2021a). Within the examined area in the project is a stormwater flow path, identified by the 20 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 Figure 2.3: The watershed in the case study area in Lerum. municipality and studied in SCALGO Live. The flow path can be seen in Figure 2.3 and starts in an elevated area in the northeast, and ends by the school Torpskolan. The water flows from the elevated area where the planned elementary school in Norra Hallsås will be located, down to Kring Alles Road. Then it follows Kring Alles Road past the location for a new preschool. After the location for the planned preschool there is a natural depression in the roadway, where the water is diverted into a residential area and flows evenly through the area, down to a park. Before the water reaches the park it passes through a developed lot for housing currently occupied by temporary buildings. The park is an open, green field with trees and a social area. When the water exits the park area it flows via a bicycle lane running in between residential houses and a water park, Vattenpalatset. The flow path, along with the mentioned schools, can be seen in Figure 2.3. The downstream point of the water flowing on this course is the school Torpskolan, which is at risk of flooding due to its location and elevation. Torpskolan is a school currently attended by children ages 12-15 (Lerums kommun, n.d.). A current facility in Lerum is the swimming centre Vattenpalatset. It is located just upstream from Torpskolan and has been in use for almost 30 years (Lerums CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 21 kommun, 2020). Due to the long usage time the future of Vattenpalatset is under evaluation (Lindblom, 2020). The location is deemed suitable in this project to be evaluated for a larger stormwater retention system with a multifunctional focus. By the downstream location Torpskolan in the watershed, which can be seen in Fig- ure 2.3, a residential area with apartments is planned to be built (Lerums kommun, 2017a). At the time of the project the final decision on the detailed development plan has yet to be decided on. The relevance of the area for this project is the fact that half of the current parking lot is part of the watershed contributing to the flood at Torpskolan. 22 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 3 Methodology To fulfil the aim and objectives of finding sustainable solutions to the flood man- agement in Lerum and investigate the effects of infiltration and surface roughness on the implemented solutions, various methods are applied. The methods are based on Swedish standards and guidelines, where applicable, as the case study area is located in Sweden. 3.1 Flood vulnerability As mentioned in section 2.1.2, both water depth and flow speed impact different aspects of society such as people or sensitive infrastructure. Therefore it is of interest to identify what areas pose a hazard. This is conducted using flood maps for water depth, and a 2D-model for flow in combination with the previously presented risk level equation 2.1. In this project, SCALGO Live is used to create a flood map showing areas with high flood, and MIKE 21 to show areas with high water flow speed. In addition to the risk for flooding of buildings and for people’s safety, there is also a risk that roads become flooded and emergency vehicles are stalled as a result. In research conducted by Pregnolato et al. (2017), it was concluded that the water depth where most vehicles can still operate safely is 30 cm. However, this depth varies with the size of the vehicles, where smaller cars can be affected by a water depth of 15 cm and larger cars have the possibility to be operated safely in depths up to 45 cm. In planning documents obtained from the City of Gothenburg it is concluded that the guidelines for flooding on roads is a maximum of 20 cm to ensure access for ambulances and police cars (Blomquist, 2015a, 2015b). To identify which facilities are subject to risk of flooding, the map of critical infras- tructure is combined with the flooded areas within the watershed for the case study area. This is to exclude the facilities located outside of the watershed, which can be seen in Figure 2.3, and only take into consideration the ones affected by the water depth in the study area. Flooding can also be a danger to people through water depth or flow speed, as is seen in equation 2.1. Using equation 2.1 and a simulation in MIKE 21, the initial conditions where high flow speed, or a combination of flow speed and water depth is high can be found. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 23 3.2 Rainfall events The flood scenario used for this project is called pluvial flood. To calculate the amount of rain to test the solutions for, and base the design on, the recommenda- tion by SMHI and Svenskt Vatten is to use Dahlström’s formula (2010) (SMHI & Svenskt Vatten, 2020). The formula is a reworked version of a formula developed in (2006), and the science of predicting rain events is an ongoing process. According to SMHI there is currently a process to develop the formula by Dahlström (2010) to an improved formula called Dahlström (2018). However the recommendation by SMHI and Svenskt Vatten is currently to use Dahlström (2010), and the formula can be seen in equation 3.1. iÅ = 190 × 3 √ Åln TR T 0.98 R + 2 (3.1) • iÅ = intensity for a specified return time [mm/h] • Å = return time [months] • TR = rain event duration [min] The return times chosen for investigation are based on the stormwater management guidelines from the municipality of Lerum (Lerums kommun, 2015a). The investi- gated return times are 100 years and 400 years as these return times are the current guidelines of the municipality of Lerum on withstanding floods. According to the demands, facilities should withstand a flood corresponding to a 100-year rain event, and vital infrastructure should withstand a 400-year rain event. For both the 100- and 400-year events, two different durations are investigated to enable comparison between a longer and a shorter rain event. The chosen durations are 10 minutes and 4 hours and are chosen since a shorter duration of maximum a few hours is usually of higher interest in urban areas (MSB, 2017; Svenskt Vatten, 2016). The duration of 10 minutes is of interest since it is predicted that short durations will increase the most in a future climate. The 4 hour duration is chosen as it is still in the range of a short duration, while also differing from the 10 minutes as to ease a comparison to investigate the different effects of precipitation. A climate factor of 1.4 is added to the precipitation volume, as per recommendation by Länsstyrelserna (2018). Neither of the modelling programmes include drainage systems, as is further dis- cussed in sections 3.3.2 and 3.3.4. Therefore, a deduction is made on the entire rain event of an amount corresponding to the rain that is collected in the drainage pipes. The drainage system manages the initial volume in a rain event. Lerum’s stormwater system is designed to manage a 2 year rain with respect to filled pipes, and 10 year return time with respect to overflows onto the surface (Björkman et al., 2019), both for a duration of 10 minutes (A. Kalm, personal communication, February 24, 2021). New pipes should be designed to collect this precipitation, in- cluding a climate factor, but since the concept of an additional climate factor has not always been standard, it cannot be expected that older pipes can manage the 24 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 same amounts. The presence of a drainage system can not always be assumed, as the inlets can be clogged with debris (Svenskt Vatten, 2011b). With the drainage in consideration, the deduction made to account for the stormwater drainage system is a rain with a 2 year return time, and 10 minute duration, resulting in an amount of 8 mm to be deducted. The corresponding intensity is calculated using formula 3.1. SCALGO Live does not account for soil properties in it’s simulations, and thus does not have an infiltration capacity. To account for infiltration in simulations in SCALGO Live, the infiltration capacity is deducted from the total amount of rain to the area, similar to the deduction of the drainage system. In this project, there is no deduction of infiltration capacity, since the capacity varies to a large extent over the case study area (SGU, n.d.-b). The calculated precipitation volumes used in the modelling are presented in Table 3.1. Initially the precipitation intensity for the different rain scenarios are calculated using the formula by Dahlström (2010), formula 3.1, which corresponds to the first row. Certain units are preferred when calculating runoff, and to convert the intensity from [l/s × ha] to [mm/h] a factor of 0.36 is multiplied with the values in the first row to create the values in the second row (Svenskt Vatten, 2016). To account for the climate factor the values are then multiplied by 1.4, as seen in the third row. The fourth row is the amount of precipitation for the selected duration, for example 10 minutes in the first column. Lastly the deduction is made for the drainage system, which then gives the rounded off values that are used to investigate results from the simulations. Table 3.1: Calculated precipitation volumes. 100 years 100 years 400 years 400 years 10 min 4 h 10 min 4 h Intensity [l/s×ha] 488.81 53.45 774.77 83.67 Intensity [mm/h] 175.97 19.24 278.92 30.12 Intensity×Climate factor [mm/h] 146.36 26.94 390.48 42.17 Amount of precipitation [mm] 41.06 107.75 65.08 168.68 Deduction of 8 mm [mm] 33.06 99.75 57.08 160.68 Design precipitation [mm] 33 100 58 160 The figures calculated in Table 3.1 are used in the simulations to investigate how the stormwater solutions perform during different rain events. When designing stormwa- ter solutions it is of interest to consider various rain events as to not have an un- derdimensioned or overdimensioned system. MSB (2014) states that it is of most interest to study short and intense rain events in an urban setting. However, it is also of interest to study a variation in rainfall events and precipitation volumes as the requirements regarding flooding differ between facilities, as discussed in section 3.1, and the various rain events contribute with different properties to the floods and flows. For the modelling in MIKE 21, another rain event scenario is used, and thus not the values presented in Table 3.1. In MIKE 21, a predetermined CDS-rain created CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 25 by Tyréns AB is used (Björkman et al., 2019). The CDS-rain used in this project consists of a rain event with a 100 year return time and an added climate factor of 1.4, as specified by Lerum in the municipal stormwater management guidelines (Lerums kommun, 2015a). The peak rain volume is modelled to last for 10 minutes, and begins about 2 hours into the simulated 6-hour period (Björkman et al., 2019). The precipitation volume used for the 100 year, climate adapted rain event is 118.3 mm, with a deduction made on impermeable surfaces that connect to a drainage system. The deduction is made for a rain with a 2-year return time and the volume deducted is 26.1 mm. This is implemented in MIKE 21 as a precipitation file that is varying in time and domain. 3.3 Model setup The modelling starts with elevation edits and simulations in SCALGO Live corre- sponding to the implementation of the schools and the stormwater solutions, and then proceeds to MIKE 21. In the modelling, the solutions will only be placed on land owned by the municipality, where the available land is shown on the map in Appendix B. The larger solutions will be placed in areas identified as suitable for stormwater delay and collection. The locations are identified during a study visit to the case study area. These areas and their labels are shown in Figure 3.1. Areas that can be seen as unsuitable for stormwater management, in addition to privately owned land, are on contaminated soil. The occurrence of contaminated soil is investigated using SCALGO Live and the built-in function to see contaminated soil. The facilities that are future development in the case study area, the elementary school in Norra Hallsås and the preschool by Kring Alles Road, do not exist yet. Therefore they are not automatically included in the elevation models of the mod- elling programmes SCALGO Live and MIKE 21. To investigate how these areas can affect future stormwater flow, the areas are implemented manually in the pro- grammes. 3.3.1 Adjustment of the elevation model in SCALGO Live Before modelling the possible stormwater management solutions, the available ele- vation model in SCALGO Live is compared to maps of the area and information from the study visit. SCALGO Live has some terrain edits already implemented in the interface, which should be controlled before use. An area where the model and reality do not correlate is by the school Torpskolan, which is the downstream point of the case study area. On the west side of the school, a creek runs from the north and down to the river Säveån. In reality, it contains culverts to direct the flow under the road Frödings Allé north of the school, and another culvert under the bike lane south of the school to connect with Säveån. Maps from SCALGO Live showing the area and the culverts can be found in Appendix C. To amend this in the SCALGO Live model, the original model contains a culvert going straight from Frödings Allé to Säveån, which does not correlate with reality. Before implementing the stormwa- 26 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 Figure 3.1: The red circles show the areas where stormwater solutions are implemented and the areas’ respective labels. The reference labels will be used throughout the report to refer to the locations of the solutions. ter solutions, the culvert is altered to represent the two culverts diverting the water to the river Säveån while running through the creek west of the school. 3.3.2 SCALGO Live model setup In this project, simulations are performed in SCALGO Live using the flash-flood tool with a set precipitation based on Table 3.1. A workspace covering the whole area can be seen in Figure 2.3. This is the area that SCALGO Live will take into consideration when calculating the stormwater flow. The locations where the elementary school and preschool will be built are marked in the figure. To understand how the buildings will affect the stormwater, representations of the buildings were created by raising the terrain a minimum of 3 meters above the ground. The shape of the building is based on development plans from Lerum (Lerums kommun, 2021b). As mentioned in the background about SCALGO Live, in section 2.4.1, the grid resolution was CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 27 improved in April of 2021 to be 1x1 m instead of 2x2 m. However this was done after the modelling in SCALGO Live was completed for this project, meaning that the surface elevation edits in SCALGO Live are based on a 2x2 m grid. First, the initial conditions showing what the flooding impacts would be if it occurred today is simulated in SCALGO Live. The simulation of the initial conditions is used to show the conditions of the study area without any modifications, neither stormwater solutions nor the addition of the planned new developments. The next step is to add the planned buildings, the school and preschool, to identify the flooding risks they are exposed to, as well as how the flood and flow is altered because of the development. Both schools are initially implemented directly on the terrain without any alterations of the terrain around the schools. After the schools are implemented into the model, the option of flattening the ground around the elementary school is examined. After the examination, some of the ground around the school is flattened, along with measures to avoid flooding in the proximity of the building as it is deemed of more interest to study the impact of the water downstream. It is also deemed likely that the surface around the school will be flattened in reality, which could divert the water from the school. By the preschool, there is a parking lot which is added according to design plans that can be seen in Appendix A. The parking lot is added to enable investigations on how the surface can be used as a stormwater management solution in addition to serving as a parking lot. Both the elementary school model and the preschool model are based on the available zoning plans at the time of investigation and might therefore not correspond to the final design. Another new development mentioned in section 2.5.4 is the residential buildings in the current parking lot by Torpskolan. Future alterations to the area could affect the stormwater flow and possibly infiltration depending on the design. The buildings in this area are however not added to the SCALGO Live main model. This is since the studied floods are unlikely to be affected by these buildings due to the minor contribution of this downstream parking lot area. The flow speeds are also unlikely to be altered into a higher flow speed since the parking lot is currently an impermeable asphalt surface. 3.3.3 Selection and design of stormwater solutions using SCALGO Live In the following sections, the different stormwater management solutions are sim- ulated in different scenarios using SCALGO Live. The scenarios are based on the solutions mentioned in Chapter 2.2.3 and simulated one by one. Locations of the solutions are presented in Figure 3.1 above. The aim is to find the most appropriate and effective solutions and move on with further analyses. The further analysis in SCALGO Live is an evaluation of the solutions’ performance for the rain events calculated in Table 3.1. Further analyses in MIKE 21 entail an investigation of infiltration and surface roughness on the solutions. The stormwater management 28 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 solutions presented below are simulated in SCALGO Live. The stormwater management solutions are designed following design standards re- lated to safety based on standards set by Larm and Blecken (2019), Stahre (2004) and Ontario Ministry of Environment (2003). Stahre (2004) mentions that one way to include safety aspects is by planting high grass by pond sides, which could also be part of a multifunctional aspect. Vegetation in the solutions can assist in providing ecosystem services (Elmqvist et al., 2015). The solutions and the location of the aforementioned are also based on availability of municipally owned areas. The solutions are solely designed as open surfaced so- lutions, and not as subsurface drainage systems as the open solutions have a larger capacity than a closed subsurface system (Shukri, 2010; Svenskt Vatten, 2016). Furthermore, open stormwater solutions comply with sustainable stormwater man- agement practices (Svenskt Vatten, 2016). Solution 1 - Edited Schools In the original model of the elementary school, based on the detailed development plan which can be seen in Appendix D, there is a risk for large amounts of water to gather by the north side of the building where the sports hall and the rest of the school connects. The proposed layout of the sports hall is changed to allow water to flow past it more effectively and thereby provide a more accurate flow of stormwater downstream. The new, edited design is conceptual for stormwater flow, while in reality there are multiple other aspects to consider which are not addressed in this project. The difference between the design as proposed in the detailed development plan (Lerums kommun, 2021a) and the suggested altered design can be seen in Figure 3.2. The figure also shows the proposed stormwater solutions for the school area, which are further described in section 3.3.3. The change entails moving the sports hall and rotating it 90° and having it connect to the northern corner of the school to avoid a flood by the north- eastern wall. To prevent flooding on the southern side of the school, between the middle and right wing where there is a slight elevation sloping towards the school wall, the ground is flattened to allow water to flow away from the building. No major edits are done to the preschool as the layout is in its initial stages. The area is not a location where a lot of water is collected, but rather flows past so there is no need for flood conveying measures. Therefore the preschool is only added to SCALGO Live as an elevated rectangle according to the plans presented in Appendix A. Solution 2 - Pond and ditches by the elementary school As is mentioned in section 3.3.3 above, there are stormwater management solutions implemented in the school area. The solutions can be seen in the figure above, Figure 3.2, subfigure (b). Starting at the top left hand of the figure, to the west of the school, along the mountain, a swale or smaller ditch with the dimensions 0.25 m deep and 2 m wide is created to avoid flooding from the elevated area to the west. On the northern side of the school, the elevation is also higher than the school ground. Therefore, to avoid flooding on the north side of the school, a small area is lowered directly to CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 29 (a) The original elementary school. (b) The altered elementary school. Figure 3.2: Solution 1. Figure (a) depicts the elementary school when implemented into SCALGO Live according to the design in the detailed development plan. Figure (b) shows how the school is altered to avoid flooding in the direct proximity, and the implemented solutions connected to the schoolyard. The stormwater solutions are marked in black. the west of the sports hall. The water is led to this spot from the north side of the hall before being led further west, away from the school. The design of this lowering should be so that the area can have a multifunctional purpose of gathering water in rain events, while in dry periods it could be used as a playground. Running along the south-east part of the figure is a ditch, with dimensions 1 m deep and 6 m wide. It starts in the north-east corner of the area and runs along the elementary school area boundary down to the south of the school, as can be seen in subfigure (b) in Figure 3.2. A pond, 15 m wide, 50 m long, and 1.7 m at its deepest, is created along the ditch to delay the water collected from the ditch. The water in the pond is conveyed further downstream in the ditch with the same dimensions as the previous when the pond overflows. This ditch is then merged with the water coming from the swale or smaller ditch on the western side and subsequently diverted to the Gatekullen Road. For a pond located near a school, it is important to consider safety aspects such as slope rate and other measures. Examples of safety measures are presented in section 2.2.3.4. Solution 3 - Terraces Kring Alles Road A large dry pond is constructed by the intersection Kring Alles and Richerts Road. It measures 46 m in length, 13 m in width, and is approximately 80 cm deep. Water will gather there