don't know about specific buildings or areas, but city storm sewers handle large outflows by being discharged directly to the sea, both sewage and run-off.
ie, this is a backstop that prevents issues in buildings.
http://vancouver.ca/engsvcs/watersew...seperation.htm
Quote:
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During heavy rainfall events, the volume of flow in the system can exceed its capacity and result in a combined sewer overflow (CSO). In the event of a CSO, excess wastewater is discharged untreated directly into local water bodies, being a contributing source of water pollution.
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