Effective Fetch Length

Calculate the specified effective-fetch convention sum(F·cos²theta)/sum(cos theta) from measured radial fetches; no shoreline GIS or prevailing-wind inference.

Description

Effective fetch combines radial distances around a chosen wind direction.

Effective fetch combines radial distances around a chosen wind direction. Different published weighting conventions exist, so the exact cosine formula matters when comparing results.^[1]

The tool returns cosineWeightedEffectiveFetchM. The computation uses supplied observations and parameters rather than acquiring measurements.

When to use Effective Fetch Length

Use this for the following task: Calculate the specified effective-fetch convention sum(F·cos²theta)/sum(cos theta) from measured radial fetches; no shoreline GIS or prevailing-wind inference. Prepare observations and calibration parameters on the same unit and reporting basis.

How to use Effective Fetch Length and understand its results

Prepare the measured quantities and supplied model parameters. Match the units and align paired lists before calculating.
Enter relativeBearingDegrees, radialFetchesM according to the field table, then run the tool.
Read cosineWeightedEffectiveFetchM. Use the calculation below to interpret the selected mean or the model’s signs, units and comparison flags.
Review the supplied assumptions and calibration before comparing or reporting the result.

Input field	Meaning	Value type and bounds	Requirement
relativeBearingDegrees	Relative bearing degrees	list of number	Required
radialFetchesM	Radial fetches m	list of number	Required

Provide aligned nonnegative radial fetches and bearings strictly inside −90°..90°. Convert degrees to radians and apply the stated numerator and denominator weights.

effectiveFetch=Σ(fetch×cos²(relativeBearing))/Σcos(relativeBearing)

The symbols and boundary conditions refer to the supplied fields and the implementation described above.

Output field	Meaning	Value type
cosineWeightedEffectiveFetchM	Cosine weighted effective fetch m	number

json

{
  "input": {
    "relativeBearingDegrees": [
      -60,
      0,
      60
    ],
    "radialFetchesM": [
      100,
      200,
      100
    ]
  },
  "output": {
    "cosineWeightedEffectiveFetchM": 125
  }
}

Limitations and assumptions

Distances and wind direction must already be measured. No shoreline GIS, prevailing-wind inference or alternate fetch convention is applied.

Numerical inputs must be finite. Input-shape and calculation checks do not establish measurement quality or the validity of a supplied calibration.

Alternative or Complementary methods

Choose Fetch Limited Wave Height when you need its operation: Evaluate a supplied deep-water fetch-limited bulk wave-growth correlation; wind reference height, duration, shallow-water limits and calibration applicability are external. Choose Wedderburn Number for the complementary operation: Evaluate W=g′h²/(u*²L) from caller two-layer density contrast, upper thickness, friction velocity and fetch; no universal upwelling or turnover criterion.

Background reference: Hydrologic Engineering Requirements for Reservoirs: wave analysis.

References

Hydrologic Engineering Requirements for Reservoirs: wave analysis — U.S. Army Corps of Engineers, EM 1110-2-1420

Similar or alternative tools

Fetch Limited Wave Height
Evaluate a supplied deep-water fetch-limited bulk wave-growth correlation; wind reference height, duration, shallow-water limits and calibration applicability are external.
Wedderburn Number
Evaluate W=g′h²/(u*²L) from caller two-layer density contrast, upper thickness, friction velocity and fetch; no universal upwelling or turnover criterion.