What does the time of concentration for a storm sewer system refer to?

The time of concentration for a storm sewer system is defined as the time it takes for water to travel from the most distant point in the drainage area to the point of interest, typically the design point of the drainage system. This includes the time it takes for water to reach the inlets and flow through the pipes.

The correct choice highlights that time of concentration is calculated as the sum of the inlet times from the uppermost inlet plus the flow time. This is a crucial aspect in stormwater management because it helps in designing adequate drainage systems to handle runoff during storm events, ensuring that they can accommodate peak flows effectively.

Understanding this concept is fundamental in hydraulic design because it determines how quickly water will be introduced to the sewer system, ultimately influencing the sizing and capacity needed for pipes and manholes to prevent overflow or flooding. It reflects not only the travel times but also recognizes that various factors can affect how water moves through the drainage system, including terrain, surface materials, and conditions of the inlet structures.