HY-8
Gov-PublicVerified
FHWA's free culvert analysis program — the reference implementation of HDS-5 culvert hydraulics.
Hydraulics & River SystemsActiveCulvert SizingOpen Channel Flow
7 tools · Collection
The Free Drainage Engineer's Toolkit
A junior drainage engineer with no software budget can still produce agency-grade work: HY-8 for culverts, the FHWA Hydraulic Toolbox for channels and inlets, WinTR-55 for small-watershed hydrology, EPA SWMM for networks and detention, the National Stormwater Calculator for LID screening, and HEC-HMS when the hydrology gets real. Every tool in this collection is free, established and recognised by reviewers.
FHWA Hydraulic Toolbox
Gov-Public
A free bundle of FHWA drainage calculators: channels, weirs, curb & gutter, riprap, detention and more.
Hydraulics & River SystemsStableOpen Channel FlowRational Method
WinTR-55
Gov-Public
NRCS's official implementation of TR-55 small-watershed hydrology: peaks, hydrographs and storage.
Hydrology & Catchment AnalysisStableTR-55SCS Curve Number
EPA SWMM
Gov-PublicVerified
The public-domain engine of urban drainage: dynamic-wave pipe networks, runoff, LID and water quality.
Urban Stormwater & InfrastructureActiveSWMMSewer Modelling
EPA National Stormwater Calculator
Gov-Public
EPA's free tool for estimating site runoff reduction from green infrastructure, with climate scenarios.
Urban Stormwater & InfrastructureStableSuDS & LID DesignStormwater Design
HEC-HMS
FreeVerified
The US Army Corps standard for rainfall-runoff modelling — event and continuous simulation, free to use.
Hydrology & Catchment AnalysisActiveHEC-HMSSCS Curve Number
Delta Hydro Studio
FreemiumVerified
Browser-based hydrology and hydraulics calculators for practising engineers — no install, report-ready output.
Urban Stormwater & InfrastructureActiveRational MethodCulvert Sizing