Inputs
Example data table
| Section | Units | b | y | z | d | n | S |
|---|---|---|---|---|---|---|---|
| Rectangular | SI | 2 | 0.6 | — | — | 0.013 | 0.001 |
| Trapezoidal | SI | 1.5 | 0.8 | 1.5 | — | 0.025 | 0.0008 |
| Triangular | SI | — | 0.7 | 2.0 | — | 0.03 | 0.0012 |
| Circular (full) | US | — | — | — | 3 | 0.013 | 0.002 |
| Trapezoidal | US | 6 | 2 | 2.0 | — | 0.035 | 0.0015 |
Formula used
The calculator uses Manning’s equation for uniform, steady open‑channel flow:
Q = (K / n) · A · R^(2/3) · S^(1/2)
- Q = discharge (flow rate)
- K = 1.0 (SI) or 1.49 (US Customary)
- n = Manning roughness
- A = flow area
- R = hydraulic radius = A / P
- P = wetted perimeter
- S = energy slope
Additional checks: V = Q / A, hydraulic depth D = A / T, and Froude number Fr = V / √(gD).
Boundary shear is estimated as τ = γ · R · S.
How to use this calculator
- Select units and choose the cross‑section that matches your reach.
- Enter Manning n (use a preset or your field‑verified value).
- Provide slope S and geometric dimensions (b, y, z, or d).
- Optionally add a design allowance and a target flow for slope back‑calculation.
- Press Calculate to view results, then export CSV or PDF.
Design context for RAS‑style flow checks
Flow capacity checks support culvert approaches, temporary diversions, lined drains, and channelized outfalls. Consistent discharge estimates help coordinate grading, erosion controls, and inlet/outlet protection. This calculator focuses on uniform, steady conditions that are commonly reviewed during preliminary design.
Inputs that control capacity the most
Roughness, slope, and hydraulic radius dominate the result. Small changes in Manning n can shift discharge materially, especially in shallow, rough channels. Slope represents energy grade losses; use reach‑average values and avoid mixing local drops with long runs. Geometry affects both area and wetted perimeter, which together define hydraulic radius.
Interpreting velocity, regime, and shear
Velocity helps screen for lining needs and erosion risk. The Froude number indicates whether flow is generally tranquil (subcritical) or rapid (supercritical). Supercritical flow may require transitions, energy dissipation, or increased freeboard. Boundary shear provides a quick stress indicator; compare with lining guidance or soil resistance to decide whether armoring is needed.
Cross‑section selection and practical constraints
Rectangular sections suit lined drains and box channels. Trapezoids fit earthworks and roadway ditches because side slopes are constructible and stable. Triangular sections represent shallow roadside gutters. Full circular sections are useful for gravity conduits when flowing full under pressure‑like conditions. Always confirm that the assumed depth and section match the controlling condition at the design flow.
Example dataset for documentation workflows
Example (SI): Trapezoidal ditch with b=1.5 m, y=0.8 m, z=1.5, n=0.025, S=0.0008. Record the resulting Q, V, Fr, and τ in your design notes and export the report for review. When revising geometry, keep the same roughness assumptions to maintain comparability.
FAQs
1) What does “RAS flow” represent here?
It represents discharge capacity checks using standard hydraulic relationships often used in channel modeling workflows. This tool calculates uniform-flow discharge from geometry, roughness, and slope.
2) Which Manning n value should I use?
Use a value that matches lining, vegetation, and surface irregularity. Presets are starting points only. If field conditions differ, adjust n and document the basis in the Notes field.
3) Is the slope the same as bed grade?
Not always. Manning’s equation uses energy slope. In many steady, uniform reaches it is close to bed slope, but localized drops, transitions, and losses can make them different.
4) How is the trapezoid side slope entered?
Enter z as horizontal-to-vertical (H:V). For example, z=2 means the side runs 2 units horizontal for every 1 unit vertical. This affects both area and wetted perimeter.
5) What should I do if Fr is greater than 1?
Review transitions, check for hydraulic jumps, and consider energy dissipation or lining upgrades. Supercritical conditions can increase erosion risk and reduce freeboard tolerance.
6) Does the circular option handle partially full pipes?
This version assumes the conduit is flowing full. For partially full flow, the wetted perimeter and area depend on depth, and a partially full routine would be needed for best accuracy.
7) What goes into the exports?
CSV and PDF exports include your computed values and any Notes you enter. Use them to attach calculation summaries to submittals, RFIs, or internal design check packages.