What is this notice I get about hydrant flushing?

Hydrant flushing or main flushing is performed twice per year in the Spring and Fall usually during the months of April and May and October and November.  Starting at the treatment plant sections of main are flushed by the opening of designated hydrant/s. This creates a higher velocity of flow creating a “scouring” effect to the inside of the pipe until clean water is achieved. This process is repeated working out towards the extremities of the system.