Drainage design links hydrology—how much runoff is generated—to hydraulics—how that water is conveyed, stored and discharged.

Catchments

Define catchment boundaries, flow paths, slopes, land cover and drainage connections. Poor catchment definition undermines later calculations.

Design rainfall

Use the project jurisdiction's accepted rainfall data, return periods and climate allowances. Do not transfer design storms between locations without justification.

Runoff

Runoff methods estimate flow or volume from rainfall and catchment characteristics. Method suitability depends on scale, data and local guidance.

Storm sewers

Pipe design considers inflow, diameter, slope, levels, capacity, surcharge, structures, cover, clashes and downstream conditions.

Open channels

Check depth, velocity, freeboard, erosion and transitions using an appropriate hydraulic method.

Detention

Storage can attenuate peak discharge. A credible design shows inflow, storage, outlet assumptions and downstream criteria.

Modeling

Models can improve system-level analysis, but inputs, boundary conditions and validation matter more than software screenshots.

Design checks

Check levels, hydraulic grade, velocities, capacity, constructability, maintenance access and overflow/failure routes.

Portfolio evidence

Create a small catchment, calculate runoff, size a network or detention feature, and document assumptions and verification.

Sources