Final answer:
Leaders and storm drains connected to a combined sewer must be designed to handle overflow during heavy rainfall, as combined sewers collect all wastewater in one pipe and can lead to pollution due to untreated sewage overflow.
Step-by-step explanation:
Leaders and storm drains, where connected to a combined sewer, shall be designed with considerations for overflow during heavy rain events to prevent the discharge of untreated sewage. A combined sewer system is a type of sewerage system that collects rainwater runoff, domestic sewage, and industrial wastewater into one pipe. Under normal conditions, it transports all wastewater to a sewage treatment plant. However, during intense rainfall or flood conditions, the system is designed with overflow relief structures to prevent sewage backup but as a consequence, some untreated wastewater may be discharged into nearby water bodies. This is a significant source of water pollution and is closely regulated by environmental agencies.