Fetch Length Calculator for Construction Projects

Calculator Inputs

Choose a method, add distances, then calculate effective fetch.

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Project / Area name

Used in exports and print-ready summaries.

Distance units

All radial entries use this unit.

Calculation method

Effective fetch is recommended for irregular shorelines.

Straight-line fetch distance

Used only for the straight-line method.

Exposure factor (0.10–2.00)

Adjusts for partial sheltering or enhanced exposure.

Rounding precision (decimals)

Applied to displayed output and exports.

Fixed radial lengths (angles from wind direction)

Enter distances to shoreline along each bearing. Use zeros to ignore a bearing.

Angles: −45° to +45°

−45° radial

−30° radial

−15° radial

0° radial (aligned)

+15° radial

+30° radial

+45° radial

Custom angle table

One row per line: angle, length. Example: 15, 1200.

Angles are degrees from the wind direction centerline.

Optional wave estimate (planning aid)

Provide wind speed and water depth to estimate waves using fetch-limited growth.

Wind speed

Leave blank to skip wave estimate.

Wind speed units

Use 10-minute average if available.

Average water depth

Depth affects wave growth in shallow water.

Results appear above this form, below the header section.

Example Data Table

Angle (deg)	Radial length (m)	Cosine weight	Weighted length
-45°	600	0.707	424.3
-30°	900	0.866	779.4
-15°	1100	0.966	1062.5
0°	1400	1.000	1400.0
15°	1200	0.966	1159.1
30°	900	0.866	779.4
45°	650	0.707	459.6
Effective fetch (example)			997.7 m

This example shows how cosine weighting reduces off-axis radials.

Formula Used

Straight-line fetch is the direct distance over open water along the wind direction.

Effective fetch uses cosine-weighted radials to account for shoreline shape:

L_e = ( Σ (L_i · cos θ_i) ) / ( Σ cos θ_i )

Adjusted fetch applies an exposure factor to reflect sheltering or openness:

L_adj = L_e · k

Optional wave estimates use fetch-limited growth equations and are intended for planning only.

How to Use This Calculator

Select a method: fixed radials, custom angles, or straight-line.
Choose distance units, then enter your measured distances.
Set an exposure factor if partial sheltering applies.
Optionally enter wind speed and average water depth.
Click Calculate to view results above the form.
Download CSV or PDF to attach in project documentation.

Measurement tip

For effective fetch, measure from the point of interest to the shoreline along bearings within ±45° of the wind direction.

Professional Notes and Practical Data

1) What fetch length represents on sites

Fetch length is the uninterrupted distance wind travels across open water toward your work area. On retention ponds, basins, and temporary lagoons, a longer fetch generally increases surface roughness, wind setup, and the likelihood of wave run-up on slopes.

2) Straight versus effective fetch

A straight fetch is a single centerline measurement. Effective fetch averages multiple bearings using cosine weighting, which reduces the influence of off-axis radials. For irregular shorelines, effective fetch better reflects wind-driven conditions at a specific point than a single maximum distance.

3) Typical ranges and exposure bands

Small excavations often have fetch values below 500 m and behave as sheltered water. Many construction ponds fall between 0.5–2.0 km, where moderate wave development becomes noticeable during strong winds. Large borrow pits and reservoirs can exceed 10 km and should be treated as very exposed.

4) Measuring radials efficiently

Use mapping tools or survey bearings from the point of interest. Measure to the nearest shoreline along bearings within ±45° of the wind direction. Seven radials (−45, −30, −15, 0, +15, +30, +45 degrees) provide a good balance between speed and representation for field checks.

5) Exposure factor guidance (k)

The exposure factor adjusts effective fetch to reflect sheltering. Values around 0.70–0.90 can represent partial wind breaks from stockpiles, tree lines, or temporary hoarding. Use 1.00 for typical open conditions. Values above 1.10 can represent channeling or fully open terrain where gust impacts are amplified.

6) Optional wave estimate inputs

If you enter wind speed and average depth, the calculator provides a planning estimate for significant wave height (Hs) and peak period (Tp). For example, 12 m/s winds over a 2 m deep pond with ~1.4 km adjusted fetch can produce Hs on the order of a few tenths of a meter, depending on depth limitation.

7) Using results in temporary works

Include adjusted fetch in method statements for floating platforms, silt curtains, and liner protection. Pair the output with slope angles and freeboard checks. When fetch increases seasonally due to water level rise, re-run the table and retain the export as a QA record.

8) Quick validation and reporting

Validate entries by checking whether the “straight fetch” reference is close to your longest radial and whether the effective fetch is lower than that maximum, as expected. Export CSV for design notes and PDF for site packs, toolbox briefings, and permit documentation.

FAQs

1) When should I use effective fetch instead of straight fetch?

Use effective fetch when shoreline shape varies by direction or when your point is near a corner or embankment. It better represents wind exposure by averaging multiple bearings with angle weighting.

2) How many radials are enough for reliable results?

Seven radials within ±45° are commonly adequate for site screening. For complex basins, use a custom table with more angles to capture bays, islands, and wind corridors.

3) What if some radials cross land quickly?

Enter the shorter distance to the shoreline along that bearing. If a bearing is not applicable, use zero and it will be ignored in the effective calculation.

4) How do I choose the exposure factor?

Start at 1.00. Reduce toward 0.70–0.90 for meaningful wind breaks, and increase slightly for highly open or channeled conditions. Document your rationale in the notes attached to exports.

5) Are the wave outputs suitable for final design?

No. They are planning estimates for site awareness and temporary works discussion. For final design, use project-specific wind statistics, bathymetry, and an approved coastal or hydraulic methodology.

6) Can I mix distance units within the same run?

Keep one distance unit per run. If you have mixed sources, convert them first, then enter consistent values so the effective and adjusted outputs remain comparable.

7) What should I attach to a report or permit file?

Attach the PDF export, plus a short note describing wind direction used, measurement source, and exposure factor. If the shoreline changes, rerun the calculator and archive both versions.