The impacts of climate change and urbanisation on drainage in Helsingborg, Sweden: Combined sewer system

Journal of Hydrology (2008) 350, 100– 113

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/jhydrol

The impacts of climate change and urbanisation on drainage in Helsingborg, Sweden: Combined sewer system Annette Semadeni-Davies ¨ran Gustafsson c Lars-Go a b c

a,*

, Claes Hernebring b, Gilbert Svensson b,

Dept. Water Resources Engineering, Lund University, Box 118, 22100 Lund, Sweden DHI Water and Environment, Gothenburg, Sweden ¨xsjo ¨, Sweden DHI Water and Environment, Va

Received 29 June 2006; received in revised form 14 May 2007; accepted 17 May 2007

KEYWORDS Stormwater; Waste water; Combined sewer overflow (CSO); Climate change; Development; Sustainable urban drainage systems (SUDS); Best management practices (BMPS)

Assessment of the potential impact of climate change on water systems has been an essential part of hydrological research over the last couple of decades. However, the notion that such assessments should also include technological, demographic and land use changes is relatively recent. In this study, the potential impacts of climate change and continued urbanisation on waste and stormwater flows in the combined sewer of central Helsingborg, South Sweden, have been assessed using a series of DHI MOUSE simulations run with present conditions as well as two climate change scenarios and three progressive urbanisation storylines. At present, overflows of untreated wastewater following heavy rainfalls are a major source of pollution to the coastal receiving waters and there is a worry that increased rainfall could exacerbate the problem. Sewer flows resulting from different urbanisation storylines were simulated for two 10-year periods corresponding to present (1994–2003) and future climates (nominally 2081–2090). In all, 12 simulations were made. Climate change was simulated by altering a high-resolution rainfall record according to the climate-change signal derived from a regional climate model. Urbanisation was simulated by altering model parameters to reflect current trends in demographics and water management. It was found that city growth and projected increases in precipitation, both together and alone, are set to worsen the current drainage problems.

Summary

* Corresponding author. Present address: National Institute of Water and Atmosphere, Private Bag 99940 Auckland, New Zealand. Tel.: +64 9 3754532; fax: +64 9 3752051. E-mail address: [email protected] (A. Semadeni-Davies). 0022-1694/$ - see front matter ª 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2007.05.028

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Conversely, system renovation and installation of sustainable urban drainage systems (SUDS) has a positive effect on the urban environment in general and can largely allay the adverse impacts of both urbanisation and climate change. ª 2007 Elsevier B.V. All rights reserved.

Background Helsingborg (pop. 123,000), on the south coast of Sweden, is a city undergoing change and is expected to grow over the next few decades. This presents a problem for municipal water managers as the combined sewer system and waste water treatment plant in the central city are already prone to problems such as combined sewer overflows (CSO) and pumping station overflows of untreated waste water to local receiving waters following heavy rainfall. Continued urbanisation could put even more stress on the system leading to further failure. For instance, part of the local development plan over the coming decades is to subdivide land near the city centre, namely the largely rural Lusseba ¨cken catchment, for industry and housing. While source control and a separate pipe network for stormwater are most likely for the Lusseba ¨cken development, the sanitary sewer will probably be joined to the existing central combined sewer which will increase both the hydraulic and nutrient loads in the pipes and at the waste water treatment plant (WWTP). Over the same time frame as city growth, climate change is also expected to have a negative impact on urban drainage in Helsingborg. Of most interest here is the prospect of more frequent and intense rainstorm events by the end of the century. System maintenance, rehabilitation and replacement in the future must take into account the dual effects of both climate change and urbanisation. Hence the main objective of this paper is to assess the relative impacts of both, together and separately, on the combined sewer network in central Helsingborg with respect to inflow volumes at the WWTP, overflow frequency and volume, and nutrient transport. The concern in Helsingborg echoes the calls made by the Intergovernmental Panel on Climate Change (IPCC, 2001) for real-world studies into the impacts on water systems with respect to water resources. Parry (2001) states that quantitative assessment of sensitivity and vulnerability of human systems to climate change is a priority research area. However, there is a lack of both tools and guidelines for climate change impact assessment in hydrology. The impacts of climate change on the hydrology cycle have the potential to affect not only the natural environment but also the human built environment. To date, most investigations have focused on the former with little regard to changes in human behaviour towards climate change including adaptation and mitigation. Failure to account for non-climatic changes such as innovations in system operation or rehabilitation of infrastructure leads to misconceptions because these activities act as a buffer between the bio-physical hydrological effects and impacts on people and their surrounding environment. Furthermore, it implies that society is unable or unwilling to respond to global change whether it be environmental, political, economic or technical. Towns are arguably the most dramatic examples of environmental change as vegetation is removed, soils become

covered by impervious surfaces and streams are replaced by pipes. Towns are characterised by rapid flow responses and high peak flows following even modest rainfalls. Moreover, the urban water cycle is affected by human activities including transport (e.g., water quality) and patterns of domestic and commercial water use (e.g., water supply from outside the catchment boundaries, waste water production, irrigation in parks and gardens). The desire to provide and maintain a pleasant living space with minimal environmental impacts and the legal requirement to protect citizens from floods (European Standard EN 752; Dido ´n, 1995) and other water related nuisance creates a dilemma for city authorities in the face of climate change: how best to manage urban drainage in a sustainable manner in an uncertain and ever-changing world? Urban water management for future conditions is a juggling act. At worst, design and renovation that fail to make use of current changes in technology as well as projected changes in climate could cause health hazards and widespread damage to public and private property. At best, wise and flexible design and construction could not only alleviate the impacts of urbanisation and climate change, but could provide the city with a liquid asset in the form of a well landscaped, open water system for stormwater source control as part of urban bluegreen space creation. For urban areas, climate change impact assessment has hitherto concentrated on flood risk from river systems (e.g., Schreider et al., 2000) or water supply (e.g., Boland, 1997; Liverman and Merideth, 2002) rather than stormand wastewater drainage. Semadeni-Davies (2003, 2004) demonstrated the use of response surfaces to visualise the possible impacts of both climate and non-climatic change on sewer infiltration, and hence water delivery to a wastewater treatment plant in north-central Sweden. An extensive literature review was carried out and no other published examples of impact assessments for urban drainage could be found. The main conclusions of the work were that: the impact of climate on inflows to the WWTP was seasonally variable due to changes in the snowmelt cycle; and renovation to the system with respect to sewer infiltration has a potential to change inflows which is at least as great as climate change. In order to assess the impacts of changes to climate and urban water management in Helsingborg, this study required the creation of both climate and urbanisation scenarios. It must be remembered, however, that futurology is a dangerous game in that a scenario is a picture of a possible future rather than a prediction. IPCC (2000) introduces the idea of social scenarios as storylines where different aspects of society – such as gas emissions, politics, development and environmental awareness – go hand-in-hand. Herein lies the rub, decision makers and the methodologies they use (e.g., the design-storm concept) require certainty yet scenarios are inherently uncertain and require some degree

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of crystal-ball gazing. For this reason, the future drainage system is simulated here with two climate scenarios and three urbanisation storylines. Despite pointing out how sensitive urban drainage in Helsingborg is to change, the wide range of outcomes possible makes this kind of study of limited value for planning and policy purposes (see Jones, 2000). Nonetheless, it can identify the magnitude and direction of the possible impacts.

Modelling approach The combined sewer system was simulated for two 10-year periods corresponding to present (1994–2003) and future conditions (nominally 2081–2090) using the Danish Hydrological Institute (DHI) MOUSE (MOdel of Urban SEwers) model. As MOUSE has been used world wide for a range of climate conditions and sewer systems, it can be considered robust for climate change impact assessment. Fig. 1 gives a schematic overview of the modelling strategy. Climate change is simulated for present and future conditions by adjusting the existing high-resolution rainfall series collected by the municipality according to climate change anomalies determined from the output of a regional climate model. Changes in water management and urbanisation are simulated by changing model parameters such as the connected drainage area and the ratio of impervious to permeable surfaces. The Helsingborg MOUSE model was set up in 1994 as part of a study into the impact of stormwater and sewer infiltration on inflows to the WWTP and is used routinely by the municipality for applications ranging from planning to realtime control. The model set-up used in this paper is fully described by Hernebring et al. (2002). The combined and separate sanitary sewer systems have been fully simulated within MOUSE HD/AD and the model is run as part of the operation of the WWTP. Inputs are high-resolution rainfall and monthly potential evaporation. As snow is only a minor part of the annual water cycle, temperature is not an input. Storm quick-flow into pipes via inlets is related to the area

Land-use

Population

Technology

SUDS

Renovation

Drainage system

Urbanisation

Urban climate

Scenarios for Helsingborg

MOUSE model of combined sewer system

Downscaling + Disaggregation

Regional climate (RCAO)

GCM (HadAM3H )

OUTPUT Waste water inflows Overflow (CSO) NH4 transport

Figure 1 Impact assessment for the combined sewer system in Helsingborg. Climate change is joined by storylines for urban drainage and city development.

covered by impervious surfaces. Permeable surfaces are said to contribute to sewer infiltration or slow flow to the pipes. Sewer and pumping station overflows occur when storage in the system is full in much the same way as a linear reservoir model. Surface runoff and pipe flow for the town centre is simulated using the hydrodynamic module (HD) and sewer infiltration using the surface hydrological model MOUSE RDII (Rainfall Dependant Inflow and Infiltration). Nutrient loads (NH4 is used here as a tracer) flowing through the sanitary sewer are modelled using the advection dispersion module (AD); decay in the sewer is not included.

Helsingborg Location and climate ¨ resund Sound in the south of SweHelsingborg lies on the O den and is within commuting distance of Malmo ¨ and Copenhagen to the south-east. The city faces the Danish city of Helsingør, best known as the setting of Shakespeare’s Hamlet. The location means that Helsingborg is an important historical regional centre with a fortress (now a park), rail hub, ferry terminal and port. The dockyards, culminating at the railway station/ferry terminal are directly opposite the central business district. Within walking distance to the north is a long sandy beach popular with bathers which makes the occurrence of CSO a potential health hazard. The region is cool temperate with fairly mild winters. In south Sweden the most intense rainfalls are associated with summer convective rainstorms while autumn rainfall is dominated by long-duration frontal rain (e.g., Linderson, 2002). Snow is a minor part of the water balance, and is generally confined to December to February. According to monthly climate normals from the Swedish Meteorological and Hydrological Institute (SMHI, 1961–1990, weather station 6203) there is a summer precipitation maximum followed closely by autumn. The municipality installed eight new rain gauges (0.2 mm tipping bucket) in 1991. Site descriptions and data handling methods are discussed in a report to the municipality prepared by DHI (2003). Of the gauges, two are located near the town centre (and can be considered representative of rainfall in the sewer drainage area) and have operated continuously with few maintenance problems. The rainfall data from the two have been collated into 1-min rain blocks to provide the high-resolution rainfall data used in this study. The gauges are roughly 1.5 km apart and are located at the municipal technical department about 1 km to the east of the wastewater treatment plant and Bendzgatan.

The combined system Like most Swedish cities, central Helsingborg, the oldest part of the city, has a combined sewer system (Fig. 2). The WWTP is located at a dockyard industrial area just to the south of the city centre. The catchment area connected to the WWTP is approximately 50 km2. The sewer system is partly combined with 170 ha impervious area connected to the sewers, mainly in the central part of the city. There are 15 overflow sites (i.e., 13 weir discharges, these are so-called weir CSOs, and two off-shore pipes each leading from a pumping station). Overflow via the CSO weirs and

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100mm 100 Internal CSO

WWTP WWTP

Storage tanks with CSO weirs Pumping station off-shore pipes

Internal CSO Pumping stations (with and without overflow weirs) 2 km

Figure 2 Overview of the combined sewer system in central Helsingborg. The inset shows the system geometry at the wastewater treatment plant, note the internal CSO loop with a storage tank and pump station.

the off-shore pipes are fundamentally different in that the ¨ resund Sound rather than into latter leads water to the O the harbour. For this reason, pumping overflows through the off-shore pipes is the preferred first option and weir discharge only occurs if there is too much water for the pumping stations to handle. The off-shore pipes are located at the north harbour and the WWTP and each pumping station has a primary and three secondary pumps. The flow capacity of the pumps varies between 0.12 and 0.2 m3/s, respectively. The pumps are turned on successively as needs be, thus all eight of the pumps will only be in action during the most severe rainstorms. In addition to the weirs and off-shore pipes, the system has two internal CSOs – one which is a connection between two parallel sections of the sewer system and the other where excess stormwater is pumped back into the system once the flow in the sewer subsides – neither represents a loss to the combined system. The inset in Fig. 2 shows the system near the WWTP; one of the internal CSOs, with a loop to a storage tank and pumping station, can be clearly seen. On average, overflows occur three or four times per year and are associated with heavy convective rainfall events in late summer and autumn. Despite the low percentage overflow volume in terms of the total flow to the WWTP (around 0.4%), nutrients from waste water are a major threat to the water quality of the harbour, and, as Helsingborg is known for its safe bathing, overflows are a human health hazard. Indeed, toxic algal blooms have occurred on occasion along the coast between Helsingborg and Malmo ¨ to the south, albeit, largely due to fertilizer in agricultural runoff. The MOUSE model parameters which describe the current water and nutrient flows in the combined sewer are listed below. • Permeable surfaces – area contributing to sewer infiltration (ha), 2914. • Impervious surfaces – area contributing to direct flow via drains (ha), 164.

• Specific water use (l/person/day), 250. • Specific pollution – ammonia, NH4 (g/person/day) 9.85. • Population served, 122,895.

Scenarios for change Climate change It is generally accepted that the global climate will warm during this century (IPCC, 1996, 2001). However, the magnitude of that change is uncertain. Climate change projections are made using general circulation models (GCMs) which simulate large scale atmospheric processes using equations that describe the flow of energy and momentum and the conservation of mass and water vapour. Early simulations were often based on a doubling of CO2 from pre-industrial levels over the next century, but the current standard is the IPCC SRES set of gas emission scenarios (IPCC, 2000). In all there are some 40 scenarios grouped into four families: B1 (low), B2 (medium/low), A1 (medium/high) and A2 (high). These allow for different possible futures according to different world views. The gas emission scenarios can be seen as a starting point for constructing social change storylines allowing a link between society and climate within impact assessments (e.g., UKCIP, 2001). Although all GCMs describe basically the same physical processes, climate changes simulated by different GCMs can be contradictory and there is no easy way of deciding on the most reliable simulation. The wide range of possible climate change projections was illustrated for Europe including Sweden by the ACACIA intercomparison of GCM simulations (Hulme and Carter, 2000). Each GCM simulation incorporates errors which is often referred to as a cascade or explosion of uncertainty where the errors are compounded at every step (Jones, 2000). A state-of-theart review of modelling methods can be found in Giorgi (2005) which discusses the effects of initialisation

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RCAO – the regional climate model The basis for the climate scenarios is the RCAO model developed by the Swedish Meteorological and Hydrological Institute (SMHI) Rossby Centre (Ra ¨isa ¨nen et al., 2004). RCAO is a coupled atmospheric and oceanographic regional climate model which uses a dynamical method to nest regional scale (49 km) climate forecasts within a larger GCM grid. The model provides continuous data for the period 2071–2100; while calculations are made with 36-min time-steps, the outputs are summed to save disc space. Thus, the rainfall blocks are at present 6-hourly. In recognition of the wide range of projections relating to gas emission and GCM

Table 1

choice, RCAO has been forced using the Hadley Centre HadAM3H (RH) and Max Planck Institute ECHAM4/OPYC3 (RE) GCM simulations run with the A2 and B2 IPCC SRES gas emission scenarios (IPCC, 2000). There is also a RCAO control run for each GCM which simulates present conditions (1961– 1990). Achberger et al. (2003) found that the RCA1 model, a predecessor of RCAO, was unable to capture the spatial variation of precipitation in southern Sweden which has a observable scale of 20–35 km. The RCAO model has a different formulation of atmospheric and land surface interaction, but there is still a fundamental difference in spatial scale which could not be overcome here. For this project, the grid cell closest to Helsingborg was chosen for scenario development. A summary of the filtered rainfall statistics for present conditions is given in Table 1. RCAO is known to overestimate the frequency of events with low to moderate rainfall intensities in its simulations of northern Europe (Ra ¨isa ¨nen et al., 2004), hence rainfalls with intensities less than 0.4 mm/6 were removed prior to comparison. As the RE control simulation was unable to capture the observed annual volume or seasonal distribution of rainfall, these scenarios were discarded. The RH model is able to capture seasonal variations in monthly rainfall totals for Helsingborg (Fig. 3) but captures neither the number of raindays (over-estimation) nor the distribution of 6-hourly rainfall intensities. Note that the difference between the SMHI normals (1961–1991) and monthly values derived from the high-resolution data (1994–2003) is in the same order as for the RH simulation (the difference between observations is most likely due to location). Despite the ability to simulate monthly precipitation reasonably well, RH overesti-

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Average Monthly Precipitation (mm)

conditions, gas emission scenario, non-linear (e.g., feedbacks, threshold behaviours and circulation regimes) and stochastic (e.g., cumulative convection) processes on climate models. He further states that the stochastic component of natural and anthropogenic forcings, the lack of knowledge about the initial state of the climate system and the non-linearities and stochastic elements in the behaviour of climate make detailed climate prediction for the 21st century a virtual impossibility. The ideal would be to assign probabilities to the various underlying assumptions within climate models, and this seems to be a new thrust in climate change research (Giorgi, 2005). In the interim, the accepted wisdom is to carry out impact assessment for a range of climate scenarios (e.g., Pittock, 1993). Another challenge when building climate scenarios for impact assessment of water systems is the discordant scales between hydrological and climate models. The former must parameterise upwards while the latter must parameterise downwards if land surface and atmospheric processes are to meet in models. Hostetler (1994) commented on this problem over a decade ago and the subsequent interest for simulating realistic precipitation statistics on hydrologically relevant scales has been great. Regional climate models go some way to downscale and disaggregate climate data from GCMs. However, these models add another layer of uncertainty as climate variability becomes more pronounced at the regional scale. Urban areas, which are prone to localised flooding and poor water quality, are particularly problematic as drainage processes operate on spatial and temporal scales of metres and minutes (Schilling, 1991) and require still further downscaling. Thus construction of climate scenarios suitable for urban applications which capture local rainfall patterns was paramount in this project. Generally, intense convective rainstorms have shorter duration and lower frequency than low-intensity rainfalls (e.g., fronts), the latter determine background parameters such as soil moisture while the former are responsible for peak flows (and CSO).

SMHI normals High resolution

80

RCAO

60 40 20 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3 Simulated RCAORH control run (1961–1990) compared to observed mean monthly precipitation (SMHI normals 1961–1990 and high-resolution data 1994–2003).

Precipitation summary for present climate conditions Observations

Annual total (mm) Raindays (days/year) Max. rainfall (mm/6 h)

RCAO control runs

SMHI daily (1961–1990)

Municipal tipping bucket (1994–2003)

HadCM2 (1961–1990)

ECHAM (1961–1990)

737 – –

641 84 64 (September 1994)

780 95 23 (August)

916 111 27 (August)

The impacts of climate change and urbanisation on drainage in Helsingborg, Sweden

Mean monthly precipitation (mm)

120 100 80 60 40 20 0

b

14

Average number rain days

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

12 10 8 6 4 2

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6-hourly intensity for the simulated rainstorms is consistently around a third of that observed. To illustrate, the T = 12 year 6-h rainstorm for the RH simulation is only 20 mm compared to 64 mm for the observed maximum rainfall for the 12 years that observations were available. Fig. 4a–d give a summary of the precipitation simulations made by RH. Both the A2 and B2 runs suggest increased annual precipitation, although there is a decrease in summer. The total number of raindays (24 h periods with rain) shows fairly similar pattern with an increase in winter and decrease in late summer. However, there is a marked change in the storm rainfall and the number of storm events (defined as rainfalls over 10 mm/6 h, see Semadeni-Davies et al., 2005). Storminess increases in early summer and autumn but drops in July and August – the months which currently have the most rainstorms. Intense rainfall is currently associated with convection cells which occur as a result of warming over sea which suggests a shift in seasonality possibly due to a shift in the mechanisms that drive rainfall. The pattern of increased storm rainfall and number of storm events roughly coincide, thus, the increase in monthly storm rainfall volumes (Fig. 4c) is probably due to more frequent rainstorms (Fig. 4d) rather than higher rainfall intensities.

0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean monthly storm precipitation (mm)

c

18 16 14 12 10 8 6 4 2 0

d

0.35

Average number storm days

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0.3 0.25 0.2 0.15 0.1 0.05 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 4 Change in precipitation patterns simulated by RH: (a) mean monthly precipitation; (b) raindays; (c) monthly rainstorm precipitation; (d) stormy days. Dark grey = control run (1961–1990), medium grey = A2 high gas emission scenario (2071–2100), light grey = B2 medium gas emission scenario (2071–2100).

mates the number of low-intensity rainfalls at the expense of rainstorm intensity which is underestimated. When ranked and normalised for the length of the data set (i.e., 12 years of observation vs. 30 years simulation), the

Delta-change Several disaggregation methods have been reported in the literature with a range of complexities and underlying assumptions. A recent comparison of urban applications can be found in Hingray and Ben Haha (2005), they show that deterministic and simple stochastic methods perform poorly while scale-based methods (fractals) were only marginally better. Cowpertwait et al. (2004) had more success with a stochastic weather generator to extend historical rainfall records. However, both studies started with hourly observed rainfall records rather than 6-hourly and neither intended that the methods be used to disaggregate future rainfall scenarios for impact assessments. For this project, a commercially available stochastic weather generator was trailed to disaggregate the RH 6hourly climate data into 5-min intervals. However, the results were unreliable due to both the inability of RH to capture the intensity of rainstorms in particular and as the weather generator was developed in the UK for British conditions and could not readily be adapted to south Sweden. Given the uncertainties involved and in the absence of other viable methods, it was decided to use the simpler deltachange method. Delta-change is commonly used to transfer the signal of climate models to hydrological models by manipulating observed input data (e.g., Andre ´asson et al., 2004). The method compares present and future climate simulations from the climate model to determine monthly change anomalies which are then applied to perturb observed data from current or historical records. Thus for precipitation, the delta-change anomaly is the percentage increase or decrease in average monthly or seasonal precipitation and observations are scaled up or down accordingly. There are two main assumptions: a. that progressive GCM runs simulate relative changes in climate rather than absolute changes; and b. that there is no change in the number of precipitation events (i.e., raindays). The second assumption is problematic as the RH model clearly shows an increase in storm

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frequency over parts of Sweden including Helsingborg – as was noted above. The major advantage of delta-change is its simplicity, that is, no manipulation of the climate model output data is required. As an existing data set forms the basis of the transformation, the impact of climate change on individual rainstorms and the response of the hydrological system to those rainstorms can be compared. In the strictest sense, the method should only be applied to observations with similar spatial and temporal resolutions to avoid over-generalising trends seen in the climate model. However, in practice, delta-change has been used to adjust higher resolution observations for climate change in order to carry out hydrological impact assessments. The most documented example is Hay et al. (2000) which used both the deltachange method and a statistical downscaling model to create regional climate scenarios with daily time-steps for three alpine catchments from monthly GCM data. Indeed, the method was the main future scenario generation technique recommended by the US Global Change Research Program (National Assessment of the Potential Consequences of Climate Variability and Change, http://www.usgcrp. gov/usgcrp/nacc/default.htm). In the case of Helsingborg, both the A2 and B2 RH scenarios predict decreased summer rainfall, particularly for lowintensity events. However, there seems to be increased early summer and autumn storminess. Thus, two sets of monthly delta-change anomalies were created called storm and drizzle (Table 2). Semadeni-Davies et al. (2005) looked at observed rainfall distributions and found that 10 mm/6 h seems to be a natural threshold for rainstorms. Indeed, the curves for observed (1994–2003) and simulated RH (1961– 1990) rainfall intensity against return period converge at this point. An event of this intensity has a return period of around four months and, under current conditions in the drainage system, could be expected to result in a sewer overflow. To apply the delta-change anomalies, the high-resolution data was first aggregated into 6-hourly time-steps to identify rainstorms and drizzle using the same threshold. The tipping bucket readings were tagged and then adjusted

Table 2

accordingly. As the 6-hourly rainfall maximums have approximately the same timing as the high-resolution rainfall peaks, it is reasonable to assume that the method captures all high intensity storm events. While the delta-change method is not ideal, particularly in the assumption that storm frequency is stable, it does offer a workable solution to the problem of discordant temporal scale. Caution must be exercised for some months, such as November, which has few rainstorms at present but is likely to have more storm events in the future: the assumption that the number of raindays does not change means that the method could overestimate the increase in storm intensity. Similarly, delta-change values less than one could point to fewer rainstorms rather than reduced intensity. Delta-change transformations were also applied to monthly potential evapotranspiration.

Urbanisation Urban areas are arguably the most modified of human environments and urbanisation affects all parts of the hydrological cycle, usually to the detriment of local water resources. Removal of vegetation, increased imperviousness (i.e., roofs, roads, footpaths, etc.), short flow paths to drain inlets and efficient drainage via buried pipes means that urban hydrographs are characterised by high flow peaks and fast response to even minor precipitation events. Towns also tend to have reduced evapotranspiration and ground water recharge which can lead to other problems like subsidence. The question in this project is the role that urban water policies, management practices and drainage structures will play in future urban drainage under changed climate conditions. Obviously, these practices are linked to the values, economy and technology of society as a whole. One of the most comprehensive studies to determine socio-economic storylines for climate change impact assessment has been undertaken as part of the UK Climate Impacts Programme (UKCIP, 2001; Berkhout et al., 2002; Shackley and Deanwood, 2002). A variety of stakeholders ranging from researchers and policy makers to industry

Monthly delta-change anomalies for precipitation and potential evaporation Rainfall

Potential Evapotranspiration

RCAO Had B2

January February March April May June July August September October November December

RCAO Had A2

Drizzle

Storm

Drizzle

Storm

1.10 1.28 1.04 1.11 1.14 0.87 0.64 0.69 1.00 1.04 1.15 1.25

1.00 0.95 1.00 2.24 1.87 2.30 0.61 0.81 0.97 2.54 2.88 4.17

1.40 1.36 1.21 0.95 1.31 0.85 0.68 0.55 0.60 1.11 1.13 1.28

1.00 0.67 1.00 0.32 3.74 1.44 0.71 0.80 1.42 5.55 1.27 2.02

RCAO Had B2

RCAO Had A2

1.19 1.16 1.17 1.14 1.16 1.04 1.09 1.20 1.17 1.26 1.29 1.24

1.59 1.40 1.38 1.26 1.22 1.17 1.21 1.32 1.33 1.26 1.29 1.32

The impacts of climate change and urbanisation on drainage in Helsingborg, Sweden and NGOs were consulted to create four storylines based on differing world views. Each storyline has interlinked impacts on, among others, greenhouse gas emissions, education, trade, industry, agriculture, water and biodiversity. The storylines are national enterprise, world markets, global sustainability and local stewardship and are roughly equivalent to the IPCC SRES A2, A1, B1 and B2 gas emission scenarios, respectively. The experiences of the UKCIP were used when deciding on factors important for the creation of storylines for Helsingborg. Trends in urbanisation The world is currently undergoing a period of rapid urbanisation largely due to population growth and rural to urban immigration. However, in much of Europe population growth is slowing and demographics point to an aging society; thus urbanisation has been largely driven by the trend towards smaller households. This can be seen in Sweden where people are remaining single longer. In Sweden, there was a trend of urban expansion due to government initiatives such as the Million Homes Programme (Miljonprogrammet) between 1960 and 1975. Today, where new subdivisions are constructed, brown field sites and spaces between existing town areas are often used in preference to expansion. Over the last decade there has also been gentrification of city centres (Engstro ¨m and Legeby, 2001). Increased urbanisation in some towns, particularly in southern Sweden, can partly be attributed to the high number of immigrants. Overlying urbanisation is the current trend towards sustainable cities. At an international level, ensuring a sustainable environment is goal seven of the UN Millenium Project (e.g., UN, 2005). Sweden has embraced this ideal, Hammarby Sjo ¨stad in Stockholm and Augustenborg in Malmo ¨ are good examples. The former was established on a port/industrial brown-field site in 1995. New apartment blocks for 20,000 residents have been built or are planned in the near future, these buildings are separated by green spaces and there is integrated energy, waste and water management. Clean stormwater is used directly in green spaces as either open water or for irrigation while roadrunoff is treated before it flows to the sea. In contrast, Augustenborg is an existing inner city suburb established in the 1950s that has been renovated as part of the Ecostaden (eco-city) urban renewal project under the auspices of Malmo ¨ public works and municipal housing authorities. The project is committed to waste reduction, reuse and recycling, energy efficiency and local disposal of stormwater. Augustenborg is discussed further below as the new stormwater system is the model for the Helsingborg urban drainage storylines. Trends in urban water management: day-lighting of combined sewers Drainage system maintenance, rehabilitation and renovation is to be expected over the course of the century, but innovation is ongoing and it remains to be seen what strategies will be adopted in the future. The strategies currently employed in sustainable urban drainage systems (SUDS; also called best management practices, BMPS) are often in the same direction as climate change adaptation behaviour. Whilst SUDS are currently installed for reasons other than

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climate change, such new drainage systems may become imperative to reduce the impacts of any possible increase in precipitation, particularly with respect to flooding. Actors demanding changes to urban drainage for flood risk management are likely to include local residents, service providers, developers and insurers. These groups can exert considerable pressure on urban water managers due to the legal requirements of municipalities to plan for flooding and to compensate for damage if that planning is inadequate (Dido ´n, 1995). However, the position of municipalities with respect to climate change is unclear. Sweden has an historical legacy of combined sewer systems for waste- and stormwater in inner cities which were built between the mid 19th and 20th centuries in a period of the rapid urbanisation following the industrial revolution. At that time, domestic waste was disposed of in the streets and there was no distinction between waste- and stormwater. This polluted water was neither collected nor treated. The ethos behind combined systems was a need to remove water as quickly as possible from towns. Combined sewers were designed to cope with a certain level of flow, too little and solid waste may not be transported causing blockages, too much and the system overloads. Combined sewer overflows which remove excess water from the sewer are an integral part of the combined sewer system. These are storage structures with a main ‘‘dry weather’’ outlet and an overflow weir leading to a secondary ‘‘wet weather’’ outlet. During rainstorms, not all the water is able to flow via the main outlet and the water level rises behind the weir. For low-intensity rainstorms, the water is stored temporarily and is able to eventually flow via the main outlet. For high intensity rainstorms, the water overtops the weir and flows via the secondary outlet to local receiving waters. As towns have grown (i.e., more impervious areas and connections to the pipe network), the result has been heightened flood risk and overflows. Pumping stations have similar overflows which are activated during rainstorms. At present, wholesale replacement of combined sewers is not feasible due to the cost and logistics of re-construction. The best that can be hoped for in the near future is renovation or stormwater disconnection similar to Augustenborg (see below). However, as combined systems age, replacement of some sections may become necessary, especially where there is excessive sewer infiltration of groundwater (this is equivalent to baseflow and can greatly increase hydraulic loads to the WWTP in the long-term). From the 1950s, separate pipe networks for waste- and stormwater have become standard. In the last decade, there has been a paradigm shift towards urban water recovery and re-use where the primary objective is to limit the impacts of urbanisation on the wider water system. SUDS for stormwater control and treatment are becoming commonplace in newer housing developments the world over (US EPA, 1991; Urbonas and Stahre, 1993) including in Sweden. While more common in new subdivisions, SUDS have also been installed in inner city areas. A prime Swedish example is the urban renewal project at Augustenborg in central Malmo ¨ that was discussed above. The area, some 50 km south of Helsingborg, is the model for the drainage storylines developed for this study. The stormwater was disconnected in 2001 in favour of a new open water system consisting of a variety of SUDS for source control which

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include swales, rain-gardens, porous pavement in car parks, greenroofs, open water channels and ponds. Prior to daylighting, there were nuisance surface flows and basements were regularly inundated by wastewater following surges in the combined sewer system. These events have now ceased and CSO has been reduced by an estimated 75% (Villarreal et al., 2004). Also of interest here are advances in sanitary engineering and low-water use appliances. Dual flush toilets are now standard in Sweden, these reduce urine flushes to 3 l/flush from the ordinary 6 or 9 l/flush. Other advances include the development of low-water use appliances (e.g., washing machines which use 40 l instead of the more usual 60 l). Villarreal and Dixon (2005) looked at water use at an apartment complex in the Swedish city of Norrko ¨ping, they estimate that of the 190 l/day each person currently uses on average, 40 l is for toilet flushing and 30 l for laundry – thus with low-water use washing machines and low-flush toilets, each person could reduce their water use to around 150 l/day. Installation of urine separation toilets and on-site or in-sewer treatment would also reduce nutrient and water loads reaching wastewater treatment plants. However, their implementation is limited by installation costs, and a lack of storage space for waste, municipal services (e.g., waste removal and processing) and social acceptance. Storylines for Helsingborg: the central combined sewer system The drainage system storylines progress from no-change to an ideal where new technologies and changes in human behaviour work to conserve water and improve water quality. The storylines are not strictly comparable to IPCC SRES or UKCIP storylines described above, rather, they have aspects of these based on local plans and trends. At opposite ends, Storyline 2 (growth but limited innovation) is most akin the national enterprise/A2 storylines, whereas Storyline 4 (growth met with sustainable development) is an ideal in the same vein as the global sustainability/B1 storylines. The storylines are: • Storyline 1 – current situation. This storyline is the business-as-usual control run for comparison in which nothing but climate is changed. The storyline represents a society that is unable to respond to change whether it be environmental, political, technological, economic or social. • Storyline 2 – projection of current situation with increased population. This storyline assumes that the city size increases but that there is no change to the type of urban drainage system installed from the status quo. The growth is largely due to the subdivision of Lusseba ¨cken catchment which is currently dominated by arable

Table 3

land with some urbanisation at the lower reaches. Wastewater from the planned new subdivision will likely be added directly to the combined system. Thus the population served by the combined sewer system increases by 50% but water supply metering will slightly lower per capita water use. Suburban areas in Sweden currently have around 30% impervious surfaces, which will be applied to the subdivided area. The subdivision will have a separate system and neither stormwater nor sewer infiltration from Lusseba ¨cken are added to the combined system. • Storyline 3 – stormwater disconnection in the central city. This storyline builds on the population growth and urbanisation in storyline 2. CSO limiting systems are installed so that 75% of the impervious surfaces are disconnected from the combined sewer system. The precedent for this storyline is the introduction of source control at Augustenborg in central Malmo ¨. For wastewater, sewer infiltration in the combined and separate wastewater systems will be reduced by 25% using remediation or pipe replacement. • Storyline 4 – water demand reduction This storyline represents an ideal situation. Urine separation toilets in 20% of new households (i.e., the Lusseba ¨cken subdivision) will reduce both the hydraulic and nutrient loads in the sewer system. It is estimated that urine separation in new homes could reduce the specific load of nitrogen by some 25%. Low-water use appliances and dual-flush toilets would reduce water use to around 180 l/day per person. Despite the population increase following subdivision in Lusseba ¨cken catchment, the geometry of the combined sewer system was not changed. The resulting MOUSE parameters are given in Table 3. Note that the net effect of disconnecting stormwater from the system is a reduction of impervious surfaces. The reduction of sewer infiltration is modelled as a reduction of permeable surfaces, this conceptually lowers simulated groundwater recharge and therefore the water table.

Results with discussion In recognition that CSOs and pumping station overflows have both acute and chronic impacts on receiving waters, the results are given for the entire 10-year period and for separate events. For current conditions, the WWTP receives on average 19.33 million cubic metres of inflow per year, 11.24 million (58%) of which is wastewater (black and grey water). The remaining flow is stormwater from inlets (0.98 mil-

Parameters within the MOUSE model for simulation of the combined sewer system

Permeable surfaces (ha) Impervious surfaces (ha) Population Specific water use (l/person/day) Specific pollution (NH4 g/person/day)

Storyline 1

Storyline 2

Storyline 3

Storyline 4

2914 164 122,895 250 9.85

2914 164 182,927 234 9.85

2185 41 182,927 234 9.85

2185 41 182,927 180 8.37

The impacts of climate change and urbanisation on drainage in Helsingborg, Sweden 3.0 2.5

8

3

VOLUME (x10 m )

lion m3) and sewer infiltration from groundwater (7.12 million m3). Overflows occur as a result of high intensity rainstorms, usually during the summer months, and total around 0.38% of the total flow. Over the 10-year simulation period, an estimated 2610 kg of ammonium (NH4) was re¨ resund Sound via overflows. leased to the harbour and O

2.0 1.5 1.0

Inflow to the WWTP

0.5

Fig. 5 compares the relative inflows of stormwater and sewer infiltration to the WWTP. MOUSE models surface water contributions to the combined sewer conceptually where stormwater is analogous to quickflow and sewer infiltration to baseflow. While sewer infiltration over the 10-year period is seven times greater than stormwater for the unchanged sewer system (storylines 1 and 2), irrespective of climate change, it is the latter which has the most impact on overflows and uneven water delivery at the WWTP. The main impact of sewer infiltration is to dilute waste water. The ratio between the two sources of surface water increases to over 20 for the storylines 3 and 4 largely due to the disconnection of stormwater from the sewer system. Fig. 6 shows change in inflow. Climate change could cause an increase in inflows due largely to sewer infiltration. With no system changes (storyline 1), total inflow to the WWTP, for instance, will increase by 10% for the A2 climate scenario. In terms of the drainage storylines, there are two clear groupings which relate to the degree of sewer renovation and stormwater disconnection. Storylines 1 and 2 (city growth) show an increase in total inflow with climate change whereas storylines 3 (source control, reduced sewer infiltration) and 4 (ideal) show that the relative impacts of both climate change and urbanisation can be more than met by implementing current trends in urban water man-

Present Future B2

3

0.12 0.1

8

0.08

Future A2

0.06

2

Present 3

4

Figure 6 Total inflow to the WWTP over the 10-year simulation period for present and future climate scenarios and urbanisation storylines.

agement. For storyline 2, climate change and urbanisation could increase total inflow at the WWTP by up to 33% compared with present volumes. With no climate change, storyline 3 would result in a 10% increase in total inflow compared to storyline 1, but a switch to low-water use technologies (storyline 4) would lead to a decrease of the same amount despite the increased population size. Fig. 7 shows the ratio of wastewater to total inflow for the 10-period. There is a slight decrease (5% for the A2 climate scenario) with climate change largely due to increased sewer infiltration. All the socio-economic change storylines show increased long-term wastewater ratios due to population growth. Obviously, the dilution and high hydraulic loads are greatest following discrete rainfall events (not shown). At present, most peak flows are in summer, although there are some periods with continuous high flows in late winter and spring. A comparison of inflows for storyline 1 shows that increased storminess with climate change leads to a slightly raised number of high flows in early summer and autumn – but the peak volumes are roughly the same as now. However, the incidence of flow events and continuous high flows increase in autumn and winter during wet years. By contrast, the number of high flows for storylines 3 and 4 is greatly reduced for all the climate simulations, thus although there is still wastewater dilution, flow is evened out.

0.02

1.0

1 and 2

3 and 4

1 0.8

8

VOLUME (x10 m3 )

1

0.04

0

b

Future A2 Future B2

0.0

0.6 0.4

Waste water / Total inflow

VOLUME (x10 m )

a

109

0.8 0.6 0.4 0.2 Future A2

0.2

Future B2

0.0

0

1 and 2

3 and 4 Urbanisation storyline

Figure 5 Surface water volumes flowing to the WWTP for the 10-year simulation period given changed climate and drainage characteristics: (a) stormwater; (b) sewer infiltration.

1

2

Present 3

4

Figure 7 Ratio of wastewater to total inflow over the 10-year simulation period for present and future climate scenarios and urbanisation storylines.

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Overflow volumes The fact that overflow of untreated wastewater is a non-linear process can be seen in the greater relative decreases in overflow volumes of the progressive storylines (Fig. 8) with respect to both sewer infiltration and total inflow . This reflects the decrease in stormwater inflow due to the reduction of impervious surfaces contributing to inflow after rain events. Pumping station overflows via the off-shore pipes, which are the first overflow option, occur more often than discharges at CSO weirs. The total overflow for the 10-year period is twice as great for the pumping stations compared to weir CSOs (488177 m3 vs. 247,737 m3). Pumping station overflows occur around 15 times per year at the north harbour and 1–2 times at the WWTP; weir CSO occurs 3–5 times per year, depending on the weir location. Most overflows occur in summer and autumn, but they can occur in winter and spring during wet years. The wettest year was 2002, this year had the most weir CSOs (up to 10) and pumping station overflows (27 at the north harbour) and the greatest overflow volume. Discharges from the CSO weirs were as almost great in 1994, though the pumping station overflow volumes were much less.

2.5

6

3

VOLUME (x10 m )

2.0 1.5

For the present combined sewer system, the 10-year total volume of weir CSO and pumping station overflows doubles for the B2 climate scenario and almost trebles for the A2 scenario due to increased storminess. Fig. 9 gives a comparison of storyline 1 weir CSO discharge events for the years 2001–2002 calculated with the present and A2 climates. This period covers an average year and the wettest in the study. The pumping station overflows (not shown) have much the same pattern of more frequent overflows and greater volumes, that is, the secondary pumps will be in use more often. Storyline 2 shows increased overflow events for all the simulations. With no climate change, population growth will lead to an increase in total overflow volumes (both weir and pumping station overflows) of 18% due to increased water use (and wastewater in the sewer). This incidence of overflow is much greater for the climate change scenarios; the A2 simulation results in a 318% increase in total overflow volume compared to today. However, renovating the system and implementing source control measures (storylines 3 and 4) all but eliminate these overflows. Climate change causes a modest increase in overflows, but the total volume is less than experienced today. The increase in overflow following climate change can be seen in the discharge records for the individual CSO weirs. For storyline 1, not only are the discharge volumes greater for most of the weir locations, there are more events leading to overflow. Indeed, some locations which rarely have discharge today could have several overflows per year. By implementing new technology, idealised in storyline 4, overflows from some CSO weirs could cease altogether.

1.0

Ammonium transport and loss

0.5 Future A2 0.0

Future B2 1

2

3

Present 4

Figure 8 Total volume of combined sewer and pumping station overflows for the 10-year simulation period for present and future climate scenarios and urbanisation storylines.

8000 7000

A2 climate scenario Present

3

CSO volume (m )

6000 5000 4000 3000 2000 1000 0 Jan Mar May Jul

Sep Nov Jan Mar May Jul

Sep Nov

Figure 9 Total CSO volumes (via weir discharges) for urbanisation storyline 1 and climate scenario A2, base period 2001– 2002.

In terms of environmental impact, the concentration and mass of pollutants released from overflows is of greater importance than overflow volumes per se. Pollutant release at receiving waters results in both short-term shocks following overflows and long-term degradation as pollutants accumulate. In this project, NH4, a nutrient largely originating from urine, was used as a tracer pollutant for wastewater. Pollutant load is the product of flow volume and concentration, and while the general trend is for increased load with increased overflow volumes, overflows do not always result in high losses of NH4 if the wastewater is sufficiently diluted with stormwater. Logically, population increase leads to greater nutrient transport in the sewer irrespective of climate change. Table 4 shows the dry weather concentration and 10-year total load of nitrogen (NH4–N) carried in wastewater for the different storylines. Compared to today’s situation, population growth (storyline 2) has a greater potential impact on nutrient release than climate change (Fig. 10). Moreover, population growth has a greater impact on nutrient release than on overflow volumes. When climate change is added, the total load of nitrogen carried in overflows could almost quadruple. Conversely, the reduction in overflows for storylines 3 and 4 will mean that pollution at the receiving waters will likewise be reduced. Storyline 4 shows the greatest reduction in nitrogen release via overflows due to the installation of urine separation toilets –

The impacts of climate change and urbanisation on drainage in Helsingborg, Sweden Table 4

Estimated ammonium carried in wastewater for the 10-year simulation period

Concentration (mg/l) Load (kg)

Storyline 1

Storyline 2

Storyline 3

Storyline 4

39 4,421,410

42 6,581,190

42 6,581,190

47 5,594,010

12 10

NH4 -N (1000 kg)

111

8 6 4 2

Future A2

0

Future B2

1

2

Present

3

4

Figure 10 Total load to nitrogen released to receiving waters via overflows from the combined sewer for the 10-year simulation period for present and future climate scenarios and urbanisation storylines.

even with the A2 climate change, the loss of nitrogen via overflow would be not even half of today’s value.

Summary and conclusions This study has undertaken an assessment of the relative impacts of climate change and system change on wastewater flows in the city of Helsingborg, south Sweden. The objective was made in accordance with IPCC calls for climate change impact assessments of real-world water systems. Water and pollution flows in the central city combined sewer network was simulated with the MOUSE urban drainage model for several climate change scenarios. The software has been applied all over the world and can be considered robust for climate change impact assessment. Climate change was simulated by altering high-resolution rainfall data while the urbanisation storylines were simulated by changing model parameters. As well as the current conditions (no-change) three storylines for urban drainage were identified which ranged from city growth through to urban renewal. The storylines were based on current trends in Swedish urbanisation. The storylines are roughly equivalent to those determined by the UKCIP (2001) which are in turn related to the IPCC SRES gas emission scenarios (2000). The starting point for the two changed climate scenarios was the RCAO regional climate model developed by the Rossby Centre forced with the Hadley Centre HadAM3H GCM. The RCAO simulation has a control run (1961–1990) as well as two future runs (2071–2100) based on the IPCC SRES B2 and A2 gas emission scenarios, respectively. As RCAO output has a temporal resolution (6 h) which is unsuited to urban applications, the delta-change method was used to transform the existing high-resolution rainfall

series (0.2 mm tipping bucket) according to climate change anomalies identified between the RCAO control and future runs. Thus, the results of the drainage simulations must be treated with caution in that the method assumes that changes in precipitation are due to changed rainstorm intensity rather than frequency. This means, for instance, that the rise in overflow volumes may be overestimated, since at least a part of the increase in storm precipitation very likely comes from an increase in the frequency rather than intensity of storm events (see Fig. 4c and d). As a result, the magnitude of the extremes increases slightly, at the same time as a significantly larger number of precipitation events reach the threshold of 10 mm/6 h. While the use of the delta-change method is far from satisfactory, it is often used for impact assessments and was chosen for this project in the absence of any viable alternatives. Specific findings are: – Without further city development, climate change projected for Helsingborg could exacerbate the current drainage problems (i.e., overflows from the combined sewer) by increasing precipitation and therefore surface runoff. – City growth (storyline 2) could lead to significant increases in overflow volumes and nutrient release from the combined sewer irrespective of climate change. – Climate change could increase sewer infiltration and reduce the ratio of wastewater to total inflow at the WWTP. However, population growth (storyline 2) could lead to lowered inflow dilution by increasing water demand and thus wastewater production. The effects of climate change on dilution can be further mitigated by stormwater disconnection and installation of water saving appliances in households (storyline 4). These have the added benefit of decreasing inflow at the WWTP and reducing flow variability. – Together, city growth (storyline 2) and the A2 climate scenario have the potential to cause the worst drainage problems. Climate change is likely to lead to increased stormwater flows and sewer infiltration thus reducing the capacity of the system. – Combined sewer overflow volumes from the south of the city may double following urbanisation. With urbanisation and climate change, the volumes could increase by 450%. The release of NH4 could have a 10-fold increase making this area the most important overflow location with respect to environmental impact. Such increases may mean that a new overflow system could become necessary. – The use of SUDS and stormwater disconnection from combined sewers (storylines 3 and 4) could reduce the number of overflows to a very low, if not negligible, level for the present and future climate scenarios.

112 – Similarly, storylines 3 and 4 result in very low overflow ¨ resund volumes and therefore nutrient release to the O sound and harbour. Finally, while many researchers have recognised the need for a range of climate change scenarios in order to combat uncertainty within GCMs and regional climate models, the equally important notion of change scenarios or storylines for other environmental changes has hitherto been less well represented in impact assessments. A failure to account for such changes implies a society that is unable to respond to global change whether it be environmental, political or economic and which is devoid of technical innovation. The worldwide trend towards SUDS for instance, not only improves the urban environment through blue-green space creation, it also offers some future protection in the face of both continued urbanisation and climate change and should be recognised as a valuable adaptation to change. By presenting a broad range of possible outcomes for future drainage problems in Helsingborg, each of which is plausible given current trends in city development, this and the accompanying paper on suburban stormwater drainage have demonstrated the need for urbanisation storylines within climate change impact assessments and provided an example of storyline development and use for urban drainage systems.

Acknowledgements This work was jointly funded by the Technical Office at Helsingborg Municipality and FORMAS (Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning). Dr. Semadeni-Davies would like to acknowledge the support of her colleagues at the Department of Water Resources Engineering, Lund University, during her years as a researcher there.

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