Application of hydraulic modelling and quantitative microbial risk assessment (QMRA) for cloudburst management in cities with combined sewer systems

Urban cloudburst management may include the intentional temporary storage of flood water in green recreational areas. In cities with combined sewers, this will expose the population visiting the area to sewage and increase the risk of diarrhoeal disease. We present a unique approach to estimate the risk of diarrhoeal disease after urban flooding. The exposure scenario was: rainwater mixed with sewage flows into a park; sewage with pathogens deposit on the grass; after discharge, a baby plays on the grass and is exposed to the pathogens in the deposited sewage by hand-to-mouth transfer. The work included modelling the transport of sewage into four parks intended to be flooded during future cloudbursts. A flood simulation experiment was conducted to estimate the deposition of pathogens from sewage to grass and transfer from grass to hand. Hand-to-mouth transfer, based on literature values, was used to estimate the ingested dose of pathogens. The probability of illness was estimated by QMRA. The estimated average probability of illness varied between 0.03 and 17%. If the probability of illness is considered unacceptable, the cloudburst plans should be changed, or interventions, e.g. informing the public about the risk or restricting access to the flooded area, should be implemented. HIGHLIGHTS The work presented provides a new method for assessing the probability of diarrhoeal disease after urban flooding of green recreational parks by combining hydraulic modeling, flood simulation experiments and quantitative microbial risk assessment.; Urban flood risk managers can use the method for planning cloudburst solutions and for implementing risk reduction measures after a flood event.; For four evaluated parks, the average probability of illness for a baby playing on the grass after the flood event varied between 0.03 and 17%.; The probability of illness decreased to acceptable levels within a few weeks, as the pathogens from the sewage deposited in the grass died out.