Preliminary peak-runoff and circular storm-drain sizing/checking with a canonical SI engine, explicit project criteria, and transparent engineering assumptions.
Enter the design conditions and run the check.
The result will show peak flow, hydraulic diameter, selected-pipe capacity, rule checks, assumptions and source traceability.
Export a self-contained TOOL-001 project package with the original input, archived result, engine/input digest, Rule versions and Source versions. Reopened projects are never silently migrated.
This methodology explains what the frozen Gate-2 engine calculates, what each input means, how to interpret the result, and where the tool must not be used as a substitute for project-specific engineering judgement.
The engine combines the runoff coefficient, design rainfall intensity and catchment area to calculate a preliminary peak discharge. Internally the calculation is performed in canonical SI units.
The rainfall intensity must come from the project-approved design storm method. TOOL-001 does not create an IDF curve or silently choose a return period.
Tc is carried as an explicit project input because the selected rainfall duration and intensity should be consistent with the catchment response time and the governing design method.
TOOL-001 preserves Tc for method traceability but does not calculate Tc or automatically select rainfall intensity. Tc is not directly used in the frozen peak-flow arithmetic; that arithmetic uses C, design rainfall intensity and catchment area.
For a circular conduit, the frozen engine uses the full-flow Manning relationship to estimate hydraulic capacity from pipe diameter, slope and Manning roughness coefficient.
The same frozen relationship is inverted to estimate the minimum hydraulic diameter required for the calculated design flow.
Sizing answers “what hydraulic diameter is required by these inputs?”. Selected-pipe checking answers “how does this entered diameter perform?” and reports capacity, full-flow velocity, utilization and rule-check status.
Values marked as project criteria must come from project or authority requirements.
SI: hectares · US: acres
Use the contributing drainage area for the scenario being checked.
Dimensionless and explicitly entered.
Select from the project-approved method based on surface, land use and design assumptions.
SI: mm/h · US: in/h
Obtain from the approved rainfall/IDF source for the relevant return period and duration.
Unit: minutes
Use the project-approved Tc result. It provides context for design storm duration and rainfall intensity selection.
Unit: m/m or ft/ft
Use the hydraulic grade/pipe slope applicable to the preliminary full-flow check.
Dimensionless roughness coefficient.
Choose an appropriate value for the pipe material and project design basis.
Optional.
Leave blank for sizing only. Enter a real candidate diameter to run the selected-pipe capacity and velocity checks.
Minimum velocity, maximum velocity and maximum utilization are explicit inputs.
The tool intentionally does not hide universal design limits.
Read the hydraulic result together with assumptions, warnings and rule checks.
Preliminary Rational Method peak discharge for the entered catchment and design rainfall intensity.
The full-flow circular hydraulic diameter implied by the calculated peak flow, slope and Manning n. It is not a standard commercial pipe-size recommendation by itself.
Calculated full-flow Manning capacity for the entered candidate diameter.
Full-flow velocity associated with the calculated selected-pipe capacity.
Design peak flow divided by selected-pipe capacity. Compare it with the explicit maximum utilization criterion entered for the project.
Transparent comparisons against user/project criteria. A PASS is not an automatic jurisdictional compliance statement.
Use TOOL-001 as preliminary engineering support within the approved project method.
This example is aligned with the frozen Gate-2 golden fixture.
Interpretation: under the entered preliminary full-flow assumptions, the 1.20 m candidate pipe has calculated hydraulic capacity above the design peak flow. The engineering decision still depends on project criteria, local requirements and whether the Rational/Manning assumptions are appropriate.
Gate 4A reuses the frozen Source Registry and does not introduce untraceable formula sources.
FHWA HEC-22, Urban Drainage Design Manual — Rational Method peak discharge relationship and design-duration context.
Applicability: Preliminary peak-runoff calculation where Rational Method assumptions are accepted and design rainfall intensity is selected for an appropriate duration informed by catchment time of concentration.
FHWA hydraulic design guidance — Manning equation Q=(1/n)AR^(2/3)S^(1/2); for a full circular conduit R=D/4.
Applicability: Steady uniform full-flow preliminary hydraulic capacity check.
WaterEngineerLab PRODUCT_ENGINEERING_SPEC_V1.0.md — TOOL-001 frozen functional boundary.
Applicability: WaterEngineerLab TOOL-001 V1.0 only.
Practical questions for preliminary stormwater drainage design and review.
It calculates preliminary peak runoff with the Rational Method, estimates the minimum hydraulic diameter for a full circular pipe using Manning hydraulics, and checks an explicitly selected pipe against user-supplied velocity and capacity-utilization criteria.
Use a project-approved design rainfall intensity for the relevant return period and storm duration. The duration should be selected consistently with the catchment time of concentration and the governing project or authority method. This tool does not derive an IDF value automatically.
Time of concentration provides the hydrologic context for selecting a design rainfall duration and intensity. TOOL-001 accepts Tc as an explicit input but does not replace a separate Tc calculation or local design procedure. Tc is preserved for traceability and is not directly multiplied into the frozen peak-flow arithmetic, which uses C, design rainfall intensity and catchment area.
Sizing returns the minimum hydraulic diameter implied by the design flow, slope and Manning n. A selected-pipe check evaluates a user-entered diameter and reports its full-flow capacity, velocity, utilization and configured rule-check results.
No. PASS means the result satisfies the explicit project criteria entered into the tool. It is not an automatic code-compliance determination, permit approval or final engineering design.
Do not rely on it directly where the governing method requires dynamic routing, significant storage, complex interconnected drainage networks, unusual hydrograph behavior, or other conditions outside the project-approved Rational Method assumptions.
The Gate-2 engine uses a steady, uniform, full-flow circular-conduit Manning formulation with a positive slope and a user-supplied roughness coefficient. It is a preliminary hydraulic capacity model, not a transient or network-routing model.
Those thresholds are explicit project criteria used for rule checks. WaterEngineerLab does not inject a hidden universal velocity limit because acceptable values depend on material, jurisdiction, design practice and project requirements.
It is the design peak flow divided by the selected pipe full-flow capacity. A value of 1.00 means the calculated design flow equals the calculated full-flow capacity under the entered slope and Manning n.
Yes. The UI converts display/input values while the authoritative calculation path remains canonical SI. Repeated unit switching is designed to preserve the underlying engineering values to high precision.