Method and limits

SCS (now NRCS) Curve Number method: S = 25400/CN − 254 mm, Iₐ = 0.2S, and Q = (P − Iₐ)² / (P − Iₐ + S) for P > Iₐ, zero otherwise. Runoff volume is Q × disturbed area. The basin is sized to the full runoff volume of the design storm plus 20% for sediment accumulation, and the footprint assumes a truncated pyramid with the entered side slopes.

The Curve Number is the whole calculation. Moving CN from 80 to 90 on the same storm roughly doubles the runoff volume, and bare graded earth on site can behave anywhere between the two depending on compaction and crusting. The table above shows that sensitivity deliberately. Pick CN from the NRCS tables for your actual soil group and surface state, and re-check it after the site has been trafficked.

The standard Iₐ = 0.2S ratio is known to over-predict initial losses on disturbed urban and construction surfaces; a ratio of 0.05S with a re-fitted S is the better-supported alternative in current literature and would give a larger basin. The method gives a volume, not a hydrograph, so it cannot size an outlet structure or compute drawdown time — and it says nothing about sediment removal efficiency, which depends on basin geometry, residence time and particle size. Local regulator rules on basin sizing override all of this.