When does temporary storage run out?
Fictional combined system: background wastewater 50 litres/second; outgoing capacity 150 litres/second. Rain adds a constant flow for one event. Initially empty spare storage holds excess inflow; after it fills, the model counts further excess as overflow.
Excess rate = max(0, background + rain − outgoing capacity). Excess volume = excess rate × duration in seconds. Overflow = max(0, excess volume − spare storage).
This is a constant-flow accounting model, not a sewer design tool or forecast. It omits hydraulic gradients, travel time, variable storms, pumps, backwater, distributed storage, actual overflow regulators, water quality and post-storm draining. Real permits and local alerts cannot be inferred from these numbers.