Final answer:
Fresh water must be provided to take the place of wastewater as it flows through a drainage system.
Step-by-step explanation:
As wastewater flows through a drainage system, it needs to be replaced with fresh water in order to ensure the continued flow and functioning of the system. This is because wastewater contains various contaminants and pollutants that can clog or damage the system if not properly treated and removed.
By providing fresh water, the drainage system can effectively carry away the wastewater to a treatment plant where it can be processed and treated before being released back into the environment or reused for other purposes.