Final answer:
Bypass structures like weirs and bypass channels are necessary to prevent floods when there is a significant risk of flooding and its associated damage. They work by containing excess water and controlling its downstream flow, and their benefits often outweigh their costs.
Step-by-step explanation:
Bypass structures are necessary for actively preventing floods when there is a significant risk of water overfilling its natural or artificial boundaries and causing damage to surrounding areas. For instance, a weir built on the Humber River in Ontario is an example of such a structure designed to prevent catastrophic flooding by containing excess water and controlling its flow downstream.
In addition to constructed structures, natural solutions such as mangrove forests have proven to be highly effective in flood mitigation. A decrease in natural forest area has been correlated with an increase in flood frequency and duration, highlighting the importance of vegetation in hydrological regulation.