Estimation of flood risk and cost-effective mitigations : A case study in Tierp

Abstract: Climate change is predicted to alter the rainfall patterns in the future, and extreme rain events with large rainfall volumes will become more frequent and intense which increases the flood risk. A clear trend can be seen, where more and more people decide to relocate from rural to urban areas. The concentration of people, infrastructure, businesses and social services in urban areas makes them particularly vulnerable to floods due to the large economic consequences that ensue. Analyzing the future flood risk is therefore of high importance in order to adapt to the changing climate and lower the consequences of future floods. Estimating the flood risk of an area is complicated and usually abstract, especially as i) the definition and understanding of risk varies a lot, ii) there is a large shortage of consistent data and iii) decision management plans are made several years ahead even though it’s hard to predict how cities will evolve. In a hydro-economical flood risk analysis, the risk is expressed in monetary terms, here in terms of Expected Annual Damage (EAD). In order to calculate the true risk of an area, EAD, for any given year there is a need of calculating all consequences for “all” events. With today’s technical limitations it is not possible to compute all possible events in an effective way, which makes the practice both time-consuming and expensive, therefore usually only a several events are considered. Among others, this paper aim to give a better understanding of how this several events should be picked to still get a good estimation. A flood risk assessment of the city Tierp, Sweden, was performed as part of the master thesis for use as decision support. The flood hazard was mapped through hydrodynamic modelling using the 2D-modelling program MIKE21 developed by DHI. The model simulated the future flood extent in the year 2100 by using precipitation corresponding to the return periods 1, 2, 5, 10, 20, 30, 50, 100, 200 and 500 years, with a climate factor of 1.25. The economic consequences were evaluated in the DHI program City Adaptation Decision Support System (CADSS) where the flood maps were overlaid with assets of different categories with assigned damage costs. The program allowed calculation of the flood risk in terms of an expected annual damage (EAD), and the choice and combination of return periods when calculating the EAD was assessed to see how it affects the outcome. Furthermore, sustainable drainage systems (SuDS) were implemented in the hydrodynamic model and a hydro-economic analysis was performed through a cost-benefit analysis to find the optimum design return period of the structures. The flood risk assessment showed that Tierp would face an EAD of 4 304 181 SEK in the year 2100 if climate change predictions proved accurate. The accuracy of the EAD calculation was found to increase with the number of included return periods, where the inclusion of lower return periods was seen to have a larger impact on the outcome compared to higher return periods. The hydro-economic optimization of mitigation structures concluded that the optimum design return period is 50 years, and the benefit of implementing SuDS of larger dimensions is minimal. However, more dimensions need to be included in the optimization to validate this result.  Flood risk assessments have a large potential for being used as decision support in Sweden, but lack of national damage cost data makes the result uncertain and difficult to validate. More research would also be required to better understand the relation between floods and the damage they cause for Swedish conditions.