For a combined drainage system, what should be the basis for sizing and why?

In a combined drainage system, the appropriate basis for sizing is based on storm drainage design due to higher flow rates. This approach is essential because stormwater events can generate significant and rapid increases in flow, often exceeding the typical flow rates experienced in sanitary systems. The design must account for these peak flow scenarios to ensure the system can handle the maximum expected discharge during rain events without overwhelming the wastewater components.

By prioritizing storm drainage in sizing, the system is engineered to mitigate the risk of backups, overflow, and potential flooding that can arise from extreme weather conditions. Additionally, stormwater typically requires separate considerations in terms of flow rate calculations, making it crucial for the system design to accommodate these higher demands effectively. This ensures that both components of the combined system function together without disruption during peak demand times.