Rainfall runoff estimate

Rational Method: suitable for preliminary peak-flow estimates.

Runoff coefficient, CRainfall intensityCatchment areaUnitsSI (mm/h, ha)US customary (in/h, acres)

Estimated peak flow—Q = C × i × A (with unit conversion)

Pipe flow — Manning equation

Assumes steady, uniform, full circular pipe flow.

Internal diameter (mm)Pipe slope (m/m)Manning roughness, nDisplay unitsm³/s and m/sL/s and m/sft³/s and ft/s

Full-flow capacity—Q = (1/n) A R^(2/3) S^(1/2)

Open channel flow

Rectangular channel using Manning's equation.

Channel bottom width (m)Flow depth (m)Longitudinal slope (m/m)Manning roughness, n

Channel discharge—Q = (1/n) A R^(2/3) S^(1/2); rectangular section

Catchment area converter

Calculate plan area from dimensions or convert an area value.

Length (m)Width (m)Or area to convert (m²)

Rectangular plan area—Area = length × width

Pipe slope and fall

Calculate gradient from invert levels and pipe length.

Upstream invert level (m)Downstream invert level (m)Pipe length (m)

Pipe gradient—Slope = (upstream level − downstream level) / length

Preliminary culvert capacity

Screening estimate using full-flow Manning capacity; not an inlet/outlet control design.

Culvert diameter (mm)Energy/bed slope (m/m)Manning roughness, nNumber of barrels

Preliminary total capacity—Full circular barrel Manning estimate × number of barrels

Engineering note: Results are preliminary estimates, not a substitute for a site-specific drainage design. Verify design rainfall, runoff coefficients, local codes, tailwater, inlet/outlet control, head losses, surcharge, blockage allowance and freeboard with a qualified engineer.