Source: https://guthlabs.ai/docs/instruments/open-channel-flow-manning-s-equation/mechanism/

instrument mechanism / verified-clean

# Open-Channel Flow — Manning's Equation mechanism

The inspectable method record for Open-Channel Flow — Manning's Equation, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

Forward Manning calculations pass the standard formula checks under the stated unit assumptions. Verdict is NEEDS_REVIEW because the specification is ambiguous about the exact US coefficient and pipe regime, and it claims inverse normal-depth output without defining the iterative algorithm or convergence tolerance.

- The forward formulas are standard: V=(1/n)R^(2/3)S^(1/2), Q=VA, and R=A/P.

- The inverse normal-depth capability is underspecified: the target quantity (usually Q), channel geometry function, iterative method, initial guess/bracketing, stopping criterion, and convergence tolerance are not defined.

- The wording 'pipes' is ambiguous: Manning's open-channel equation applies to a partially full pipe using the wetted area and wetted perimeter; a full pressurized pipe requires a different hydraulic model.

- The US coefficient 1.49 is an accepted rounded form, but the specification should state whether US customary calculations use 1.49 or the more precise 1.486.

## Units and conventions

- SI case: lengths in m, A in m^2, P and R in m, n dimensionless, S dimensionless slope expressed as m/m, V in m/s, and Q in m^3/s; the coefficient is 1.

- US customary case: lengths in ft, A in ft^2, P and R in ft, n dimensionless, S dimensionless ft/ft, V in ft/s, and Q in ft^3/s; the coefficient 1.49 is used.

- A slope entered as a percentage must be divided by 100 before using S; for example, 0.1% means S=0.001.

- For each channel, A and P must be the wetted cross-sectional area and wetted perimeter at the stated flow depth, excluding the free surface from P.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "system": "SI",
    "shape": "rectangular",
    "width_m": 3,
    "depth_m": 1,
    "mannings_n": 0.025,
    "slope": 0.001
  },
  "expected_outputs": {
    "hydraulic_area_m2": 3,
    "wetted_perimeter_m": 5,
    "hydraulic_radius_m": 0.6,
    "velocity_m_per_s": 0.899831,
    "discharge_m3_per_s": 2.69949
  },
  "working": "A=b y=3*1=3 m^2. P=b+2y=3+2(1)=5 m. R=A/P=3/5=0.6 m. V=(1/0.025)(0.6)^(2/3)(0.001)^(1/2). (0.6)^(2/3)=0.711378661 and sqrt(0.001)=0.0316227766, so V=40*0.711378661*0.0316227766=0.8998307 m/s, or 0.899831 m/s. Q=VA=0.8998307*3=2.699492 m^3/s, or 2.69949 m^3/s."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The specification has no stated validation for n=0 or negative slope. n=0 causes division by zero, while a signed negative slope makes the square-root argument invalid; both can produce undefined results unless rejected or assigned an explicit convention.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/open-channel-flow-manning-s-equation/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/open-channel-flow-manning-s-equation/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/open-channel-flow-manning-s-equation/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche6/tranche6-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`