Source: https://guthlabs.ai/docs/instruments/rational-method-peak-runoff-calculator/mechanism/

instrument mechanism / verified-clean

# Rational Method Peak Runoff Calculator mechanism

The inspectable method record for Rational Method Peak Runoff Calculator, kept separate from implementation and qualification.

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

## Model scope

The arithmetic and core Rational Method are valid under the stated unit assumptions. Verdict is NEEDS_REVIEW because the specification does not identify the local IDF source, return period, duration/time-of-concentration linkage, runoff-coefficient convention, interpolation policy, or jurisdiction-specific area limit. The honest label should say 'simplified Rational Method estimate for small, relatively homogeneous catchments' rather than treating 200 acres as a universal boundary.

- Q=CiA is the standard Rational Method relationship, but the specification omits the unit-conversion constant. In SI units, Q(m^3/s)=C·i(mm/h)·A(ha)/360; in US customary units, Q(ft^3/s)=1.008333333·C·i(in/h)·A(acres).

- Rainfall intensity i must correspond to the catchment time of concentration and the selected return period; accepting arbitrary IDF intensity without those constraints can produce an invalid peak-flow estimate.

- C must be a dimensionless, bounded runoff coefficient, normally 0<=C<=1, selected for the actual surface, soil, slope, and antecedent conditions. The specification does not define how mixed surfaces are area-weighted.

- The stated '<200 acre' applicability limit is not universal. Rational Method limits vary by jurisdiction and project standard; some standards use much smaller limits or permit larger areas only with additional checks.

- The specification calls for runoff-coefficient and local-IDF tables but does not identify their editions, geographic applicability, return period, duration, or interpolation rules.

## Units and conventions

- Each vector explicitly states the intensity, area, and flow units; the production instrument must require an unambiguous unit system.

- C is dimensionless.

- For SI calculations, i is in mm/h, A is in hectares, and Q is in m^3/s, using the exact dimensional factor 1/360.

- For US customary calculations, i is in inches per hour, A is in acres, and Q is in ft^3/s, using 1 acre-inch per hour = 1.008333333 ft^3/s.

- IDF intensity values are assumed uniform over the catchment and applicable to a duration equal to the time of concentration.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "C": 0.9,
    "surface_type": "impervious roof",
    "i": 100,
    "i_units": "mm/h",
    "A": 2,
    "A_units": "ha",
    "output_units": "m^3/s"
  },
  "expected_outputs": {
    "Q": 0.5,
    "Q_units": "m^3/s"
  },
  "reference_data": {
    "runoff_coefficient": {
      "surface_type": "impervious roof",
      "C": 0.9,
      "source_convention": "widely tabulated standard value (ASCE/municipal Rational Method tables)"
    },
    "rainfall_intensity": {
      "duration": "local IDF duration equal to time of concentration",
      "i_mm_per_h": 100,
      "source_convention": "user-supplied local IDF data; geographic source and return period must be specified"
    },
    "si_conversion_factor": {
      "value": 360,
      "meaning": "Q(m^3/s)=C*i(mm/h)*A(ha)/360",
      "source_convention": "SI dimensional conversion from hectare and millimetre per hour"
    }
  },
  "working": "Q = C*i*A/360 = 0.90*100*2/360 = 180/360 = 0.500 m^3/s. The C=0.90 value is used exactly as the tabulated impervious-roof entry."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The specification has no unit-conversion or sign-convention definition, does not explicitly enforce the strict less-than-200-acre scope boundary, and permits extreme inputs that overflow ordinary arithmetic. It also has no true mathematical formula branch; only an applicability switch can be tested.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/rational-method-peak-runoff-calculator/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/rational-method-peak-runoff-calculator/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/rational-method-peak-runoff-calculator/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-tranche7/tranche7-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`