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 deposited 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, such as informing the public about the risk or restricting access to the flooded area, should be implemented.

Automated summary

Urban cloudburst management may involve temporarily storing flood water in green recreational areas, but this can expose visitors to sewage and increase the risk of diarrhoeal disease. This study presents a unique approach to estimate the risk of diarrhoeal disease after urban flooding by modeling the transport of sewage into flooded parks and estimating the transfer of pathogens to grass and from grass to hand. The estimated probability of illness varied from 0.03% to 17%, and if deemed unacceptable, changes to cloudburst plans or interventions such as public risk awareness or restricted access to flooded areas should be considered.