Inputs are arranged in 3 columns on large screens, 2 on tablets, and 1 on mobile.
- Collect breaker height, period, and approach angle for the work window.
- Estimate surf zone width from surveys, imagery, or wave models.
- Set friction factor based on bed roughness and turbulence.
- Choose blend for planning, or a single method for sensitivity checks.
- Press Calculate and export CSV/PDF for records and briefings.
The calculator estimates wave-driven alongshore forcing and converts it into a longshore current. It then derives a compact circulation strength and a loop-time estimate.
Sample scenarios to sanity-check expected outputs.
| # | Hb (m) | T (s) | alpha (deg) | W (m) | V (m/s) | Rating |
|---|---|---|---|---|---|---|
| 1 | 1.20 | 7.0 | 8 | 90 | 0.3542 | Low |
| 2 | 1.80 | 9.0 | 12 | 140 | 0.6277 | Moderate |
| 3 | 0.90 | 6.0 | 5 | 70 | 0.2113 | Low |
| 4 | 2.20 | 10.0 | 15 | 160 | 0.9720 | Moderate |
| 5 | 1.50 | 8.0 | -10 | 120 | -0.5244 | Moderate |
1) Why nearshore circulation matters on active sites
Nearshore currents can move floating booms, barges, divers, and debris along the shoreline within minutes. A 0.50 m/s longshore flow travels 30 m per minute and 1.8 km per hour, which can quickly shift exclusion zones and increase vessel workload. Planning with site-specific inputs reduces rework and safety exposure.
2) Field inputs and typical engineering ranges
Breaker height (Hb) commonly ranges from 0.5–2.5 m on many project coasts, while peak period (T) often sits between 5–12 s. Surf-zone width (W) can vary from 50–200 m depending on bathymetry and tide. A practical friction factor (Cf) is frequently 0.01–0.03, and seawater density (rho) is typically 1025 kg/m³.
3) Understanding breaker geometry and direction
The breaker angle (alpha) drives the sign of the alongshore current. Positive angles produce positive alongshore direction in this tool, and negative angles reverse it. Keeping alpha within ±30° is common for many coasts, but the calculator accepts wider values for scenario testing.
4) Method A: slope–period scaling
Method A emphasizes beach slope (m = tan(beta)) and wave period. Because it scales with sin(2alpha), it peaks near 45° and goes to zero at 0°. The tuning factor C1 lets teams align results to ADCP or drifter observations, improving repeatability for recurring work packages.
5) Method B: stress–friction balance
Method B uses wave energy density (E = 1/8 rho g Hb²) and converts it to an alongshore stress term (Sxy). It then balances forcing with bed friction over surf width W, producing velocities that respond strongly to Hb and Cf. Lower Cf or narrower W generally increases predicted speed for the same wave forcing.
6) Interpreting velocity and a simple operational rating
The calculator reports a Low/Moderate/High rating based on absolute speed. As a planning rule-of-thumb, 0.00–0.49 m/s is often manageable for nearshore operations with controls, 0.50–0.99 m/s may require tighter vessel standby and stronger moorings, and ≥1.00 m/s can justify rescheduling or enhanced barriers.
7) Circulation strength and loop timing for work zones
Circulation strength is summarized as Gamma ≈ V·L (m²/s). Larger L produces larger circulation potential even when V is modest. Loop time ≈ 2(L + W)/|V| indicates how quickly material can recirculate through the nearshore cell. Use these outputs when placing silt curtains, turbidity sensors, and spill-response assets.
8) Documentation, reporting, and good practice
Export CSV for daily logs and quick spreadsheet checks, and export PDF for permits, method statements, and toolbox talks. For best results, keep inputs consistent (same tide stage, same measurement source) and run sensitivity checks by varying Cf and W within realistic bounds. This calculator supports planning; final decisions should consider local observations.
1) Which computation mode should I choose?
Use Blend for planning and reporting. Choose Method A for slope-driven sensitivity checks. Choose Method B when you trust Hb, W, and Cf and want forcing–friction behavior.
2) What does a negative alpha mean?
Negative alpha flips current direction in the outputs. It represents waves approaching from the opposite side of shore-normal, producing alongshore transport in the reverse direction.
3) How do I estimate surf-zone width W on site?
Use bathymetry plus tide level, UAV imagery, or visual breaker line mapping. Measure the distance from shoreline to the outermost persistent breaking line during the work window.
4) What friction factor Cf should I apply?
Start with 0.01–0.03. Use higher values for rough beds and heavy turbulence. If you have current measurements, adjust Cf (or C1) until predicted speeds match observations.
5) Why is breaker index gamma included?
Gamma links breaker height to breaker depth (hb = Hb/gamma). It affects celerity and power estimates used in the derived values. A common default is 0.78 for spilling breakers.
6) Can I use this for enclosed harbors?
It is best for open-coast surf zones where wave breaking dominates circulation. In enclosed basins, wind setup, tidal jets, and geometry can dominate, so treat results as a rough screen only.
7) Why does the drift distance change with exposure time?
Drift is computed as V multiplied by exposure duration. It helps teams estimate how far floating equipment, debris, or dye tracers may move alongshore during the planned activity period.