A design storm is a selected rainfall event or pattern used as an input for project analysis or design.
Definition
A design storm is a defined rainfall amount or pattern selected to evaluate a project under stated criteria. It may specify duration, intensity, frequency, and how rainfall varies over time. Agencies, codes, clients, and project requirements can prescribe different storm inputs depending on location, facility, and purpose. Firms use the selected storm in hydrologic calculations or models, and document its source and assumptions so results can be reviewed. The storm input affects estimated runoff and can influence drainage facility sizing, but it does not describe exactly when a storm will occur or guarantee a particular outcome. A design storm is an analytical scenario, not an observed event, a weather forecast, or the drainage system itself. Confirming which criterion applies is part of defining the project basis.
How owners use it
- Set rainfall inputs for runoff analysis under project or agency criteria.
- For a simple uniform rainfall assumption, 2 inches over 3 acres represents 2 × 3 = 6 acre-inches of rainfall volume before runoff losses.
- Compare modeled drainage response under different stated storm patterns when the scope calls for scenarios.
- Record the storm source, duration, and distribution in calculations and model documentation.
