Storm Sewer Design: Relevance of Time of Concentration

In the context of designing storm sewers, the 'time of concentration' is a crucial parameter. Let's break down why it's important and how it relates to the given options.

Understanding Time of Concentration

The time of concentration ($t_c$) is defined as the time required for runoff from the hydraulically most distant part of a catchment area to reach the point under consideration (in this case, the inlet or design point of the storm sewer). It includes both overland flow time and time of flow within the drainage channels or pipes.

Connecting Time of Concentration to Rainfall Intensity

The primary relevance of the time of concentration in storm sewer design is its direct influence on determining the design rainfall intensity. Hydrologic design methods often use Intensity-Duration-Frequency (IDF) curves, which relate the average intensity of rainfall to its duration for a specific return period.

• The duration component of the IDF curve is directly linked to the time of concentration ($t_c$) of the catchment area draining into the sewer.
• Typically, the rainfall duration used for design is assumed to be equal to or slightly longer than the time of concentration.
• Generally, for a given return period, rainfall intensity decreases as the duration increases. Therefore, accurately calculating $t_c$ is essential to select the appropriate rainfall intensity value from the IDF curve. A shorter $t_c$ implies a longer duration needs to be considered for the rainfall event, leading to potentially lower average intensity, but the total volume might be higher. Conversely, a longer $t_c$ means a shorter duration rainfall event is critical. The relationship is inverse: as $t_c$ increases, the rainfall duration considered increases, and thus the corresponding rainfall intensity usually decreases. The correct selection ensures the sewer is sized adequately for the expected runoff.

Hence, the time of concentration is fundamentally relevant to determine the design rainfall intensity.

Analysis of Other Options

• Velocity in the sewer: While the flow rate (influenced by rainfall) affects sewer velocity, the time of concentration itself isn't the primary determinant. Sewer velocity is more directly calculated using formulas like Manning's equation, based on pipe dimensions, slope, and roughness.
• Time of travel: The time of travel is a component *within* the time of concentration, specifically the time taken for flow to travel through the pipes or channels. The time of concentration is the *total* time from the furthest point, encompassing both overland flow and travel time.
• Area served by the sewer: The catchment area is used *to calculate* the time of concentration, not the other way around. The area is a prerequisite input, while the time of concentration is a calculated parameter influencing the design inputs.

Conclusion

Based on the analysis, the 'time of concentration' is most directly relevant for determining the design rainfall intensity used in storm sewer calculations.