Reference Data

Drainage and stormwater design depends on a handful of empirical parameters that rarely appear in a single place: the roughness of a corrugated pipe, the curve number for a clay soil under turf, the runoff coefficient for an asphalt parking lot, the inside diameter of a 24-inch reinforced concrete pipe. This reference library gathers those numbers into one set of searchable tables so you can size a channel, estimate peak flow, or check a pipe's capacity without hunting through a dozen design manuals. Every value here feeds directly into the equations behind our calculators, so the inputs you read are the inputs the tools use.

The collection covers five core data sets. Manning's n roughness coefficients support open-channel and full-flow pipe calculations, spanning closed conduits, lined and excavated channels, and natural streams. SCS/NRCS curve numbers drive the runoff method used for small watersheds, organized by land use, cover condition, and Hydrologic Soil Group. Rational Method runoff coefficients (C) map surface and land-use types to the fraction of rainfall that becomes runoff, including the frequency adjustment factors applied for longer return periods. Standard pipe dimensions list inside diameters, wall thicknesses, and hydraulic properties for RCP, CMP, PVC, HDPE, elliptical, and box sections. And soil properties tie USDA texture classes to infiltration behavior and Green-Ampt parameters for more detailed hydrologic modeling.

Each table cites its origin so you can trace a number back to its standard. Sources include Chow's Open-Channel Hydraulics, NRCS TR-55 and the National Engineering Handbook, FHWA's HEC-22 and HDS-4, ASCE Manual of Practice 77, and the relevant ASTM pipe specifications. These are the same references practicing civil and stormwater engineers rely on for design submittals and plan review. That said, local codes, jurisdictional design manuals, and project-specific conditions can override generic tabulated values. Treat this library as a fast, well-sourced starting point, and always confirm controlling parameters against the standard that governs your project before stamping a design.