Automated implementation of the rational method using Python and volume verification in Civil 3D for an Urban Drainage Design in Houston (Phase I of a Two-Phase Study on Urban Drainage Modeling)

This study introduces a generalized Python automation framework for the Rational Method, designed to generate complete peak-runoff and detention calculations for small urban drainage systems. The script incorporates all hydrologic components of the Rational Method, including time of concentration, rainfall intensity from editable IDF tables, pre- and post-development runoff, allowable release rates, detention storage, and automatic restrictor-orifice evaluation. The tool was validated using a fully documented 2022 drainage design project in Houston, and the automated results matched the original engineer-approved calculations exactly, confirming both accuracy and repeatability. To complement the computational workflow, the same project site was reconstructed in Autodesk Civil 3D using real grading, subbasins, catch basins, and the underground detention system. Detention volume was computed using surface-to-surface volumetric analysis, providing a geometry-based estimate that reflects actual site elevations. A comparison between Civil 3D volumetric results and the Rational Method output illustrates how precise grading and surface modeling influence detention capacity. This integrated methodology, combining automated hydrologic calculation with three-dimensional CAD reconstruction, establishes a robust foundation for subsequent hydraulic validation, which will be performed in Storm and Sanitary Analysis (SSA) in a companion study.