Storm and network
The clogging factor derates every inlet's rated capacity. 25% is a common design allowance for grated inlets on a maintained network; raise it where leaf litter or windblown sand is a known problem.
One row per inlet: ID, contributing area (ha), runoff coefficient C, rated inlet capacity (L/s).
Network summary
- Inlets over capacity
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- Inlets ≥ 80% loaded
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- Total peak runoff
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- Total derated capacity
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- Network utilisation
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- Contributing area
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Inlet loading
Peak inflow against derated capacity, worst firstCatch basin results
| Inlet ID | Area ha | C | Peak Q L/s | Rated L/s | Derated L/s | Utilisation | Status | Action |
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Method and limits
Peak flow is the Rational Method in metric form: Q = C · I · A / 360, giving Q in m³/s for I in mm/hr and A in hectares. Each inlet's rated capacity is derated by the clogging factor, and utilisation is peak inflow divided by derated capacity.
The Rational Method assumes rainfall intensity is uniform over the catchment and constant for at least the time of concentration, and that the runoff coefficient is constant through the storm. It is accepted practice for small urban catchments — conventionally under about 80 hectares — and becomes unreliable on larger or mixed-response catchments, where a unit hydrograph or a full 1D/2D model is required. It gives a peak flow only, not a hydrograph, so it cannot size storage.
Rainfall intensity must come from your own IDF curve for the stated return period and a duration equal to the time of concentration. The return period selector here is recorded in the export for traceability but does not itself change the intensity — enter the intensity your IDF curve gives. Inlet rated capacities should come from the manufacturer's grate curve at the actual approach gutter depth and road gradient, not from a nominal figure.
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