Proposed Cloudburst Adaptation Measure and Cost-Benefit Analysis : A Case Study in Stockholm

Sammanfattning

Urban climate change projections indicate increasing uncertainty and a higher likelihood of extremecloudbursts (“Torrential rain”), which can elevate flood risk. This risk is amplified by land-use changeand urbanization, as population migration to cities concentrates people and assets in areas that may bevulnerable to flooding. Therefore, assessing flood risk and adapting to a changing climate are essential toreduce the consequences of floods. It is equally important to evaluate the effectiveness of flood-mitigationmeasures before implementation. Conducting a cost-benefit analysis (CBA) provides a sound basis fordetermining whether such investments are justified. This study develops a practical methodology for evaluating cloudburst mitigation in two spaceconstrained study areas in Stockholm: Hammarby-Enskede-Arsta (HEA) and Liljeholmen-Brännkyrka-Vantor (LBV). The approach incorporates 2D hydrodynamic modelling, monetary damage estimation, ¨design and sizing of mitigation measure, and a CBA to quantify avoided damages and assess the potentialfor land development following flood risk reduction. Hydrodynamic modelling was performed usingsynthetic intensity-duration hyetographs with a climate factor of 1.4. The model inputs included a digitalelevation model (DEM), surface roughness files with Manning’s roughness coefficient (M) for differentsurface types, infiltration, and initial and boundary conditions to represent the natural hydrological process.The results of the simulations were time series of depth inundation and maximum depth inundation.Using these outputs from hydrodynamic modelling, monetary estimates of damage were prepared for the100- and 500-year cloudburst return periods (representing events with a 1% and 0.2% annual probability,respectively). A CBA was conducted by comparing the costs due to damages in the scenario without acloudburst tunnel to the construction cost of the cloudburst tunnel, in order to determine whether theflood mitigation measure is economically worthwhile. The results indicate that the high-resolution 2D hydrodynamic model provides site-specific flooddepths and facilitates explicit damage estimates. This enhanced reliability reduces uncertainty in damageassessments and strengthens the case for implementing cloudburst mitigation measures, with the potentialto reduce flood losses. The study also highlights the value of 2D hydrodynamic modeling and conductinga CBA prior to investing in urban stormwater management measures.