Choose the rainfall intensity deliberately.
The intensity should match the project location, return period, and duration selected for the catchment response. A convenient number without that basis is not a defensible design input.
Runoff coefficient is a model choice.
C compresses losses and land-cover response into one value. Mixed catchments, changing soil, slope, storage, and future development may require subdivision or a different hydrologic method.
What still needs judgement
- Time of concentration, IDF source, climate allowance, and catchment subdivision.
- Routing, detention, surcharge, tailwater, blockage, and downstream control.
- Inlet spacing, pipe capacity, overland flow paths, and safe exceedance routes.