Inputs
This estimator assumes a sinusoidal tide and basin exchange through a channel.
Formula used
The calculator treats the tide as a sinusoidal water level change and estimates the exchange flow between a basin and the sea through a channel.
H is tide range, ω = 2π/T, Ab is basin surface area, Ac = width × depth, C is contraction coefficient, and F is a friction loss fraction. Always verify with local tidal stream atlases.
How to use this calculator
- Enter the local tide range and tidal period from charts or predictions.
- Set a slack reference, then enter time since that slack moment.
- Provide basin area and channel dimensions for the flow section.
- Apply coefficients to reflect contraction and friction at the site.
- Press calculate, review speed, direction, and discharge outputs.
- Export CSV or PDF for work packs, logs, and permits.
Example data table
| Scenario | Range (m) | Period (h) | Time since slack (h) | Ab (m²) | W (m) | D (m) | C | F | Speed (m/s) | Direction |
|---|---|---|---|---|---|---|---|---|---|---|
| Harbor inlet | 3.2 | 12.42 | 1.50 | 1,200,000 | 180 | 12 | 0.85 | 0.15 | 0.73 | Flood |
| Narrow channel | 2.6 | 12.42 | 2.00 | 900,000 | 90 | 10 | 0.80 | 0.20 | 1.10 | Ebb |
| Wide estuary | 4.0 | 12.42 | 3.00 | 2,500,000 | 300 | 14 | 0.90 | 0.10 | 0.88 | Flood |
Example outputs are illustrative. Use measured currents to calibrate coefficients for your site.
1) Why tidal current estimates matter on construction sites
Marine works often fail on logistics, not engineering. A 0.5–1.5 m/s stream can rotate suspended loads, increase tag‑line demand, and push barges off line during alignment. Currents also affect turbidity curtains, cofferdam leakage paths, and diver effort. This calculator provides a consistent, auditable estimate to support lift plans, tow windows, and shift briefings.
2) Inputs that control the result the most
The strongest drivers are tide range H, basin surface area Ab, and the channel cross‑section Ac (width × depth). Doubling H doubles peak speed, while doubling Ac halves it. Because Ac is often the most uncertain input, measure width and depth at the working stage and avoid mixing charted depth with a different datum.
3) Period selection and common values
A semidiurnal cycle is commonly approximated with T = 12.42 hours. If your site is mixed or diurnal, use the dominant period from local predictions. The model also reports a practical rule: peak current tends to occur about T/4 hours after slack water, which supports crew scheduling when only slack times are published.
4) Coefficients for contraction and friction
Real channels are not ideal. A contraction coefficient C typically falls between 0.60 and 1.00, representing losses at bends, gates, piers, or shoals. The friction loss fraction F is commonly 0.05–0.25 for planning. If measured peak currents are available, adjust C and F so the predicted peak matches within about 10–20%.
5) Interpreting speed, direction, and discharge
Speed is shown in m/s and knots (1 m/s ≈ 1.94 kn). Direction is a simple flood/ebb label based on the selected slack reference. Discharge Q is reported in m³/s, useful for silt management, dredge plume expectations, and intake/outfall checks. Treat the sign of Q as a convention tied to the selected phase.
6) Using the series table for work windows
When enabled, the series table steps the cycle at 5–120 minutes (default 30). For lifts, diving, or piling, identify continuous periods where speed stays below your site limit and direction is stable. Finer steps (10–15 min) are helpful for short critical lifts, while 30–60 min steps suit daily planning. Export the table to attach with method statements and toolbox talks.
7) Practical limits for operations
Many marine tasks become difficult above about 1.0–1.5 m/s, but your threshold should come from equipment manuals, mooring analyses, and permit conditions. Use the optional safety limit to flag exceedance and document why an activity was paused or rescheduled. Consider separate limits for tow, lift, and dive phases.
8) Quality assurance and responsible use
This estimator assumes sinusoidal tide and uniform section flow, which is a simplification. Verify with local tidal stream atlases, ADCP measurements, and port authority guidance. After the first week of monitoring, update Ab and section dimensions if the effective exchange area differs from drawings. Record your final coefficients in the project QA file so future shifts reproduce the same assumptions.
FAQs
1) Does this replace a tidal stream atlas?
No. It is a planning estimator for speed and direction trends. Always compare with official tidal stream atlases, port authority notes, and measured currents before high‑risk marine operations.
2) What tide range should I enter?
Use the predicted or observed peak‑to‑trough range for the relevant day. If you only have amplitudes, enter twice the amplitude. For spring tides, the range is typically larger than neaps.
3) How do I choose basin surface area?
Use the approximate surface area of water exchanging through the inlet connected to your work zone. For complex lagoons or estuaries, segment the basin and start with the portion that drives the inlet flow.
4) What do contraction coefficient and friction loss represent?
They reduce ideal velocity to reflect real‑world losses. C accounts for constrictions and geometry effects; F represents overall friction/energy loss. Calibrate both using a few measured peak currents.
5) Why is peak current around T/4 after slack?
With a sinusoidal tide, the water level change rate is zero at slack and maximum a quarter cycle later. This model uses that relationship to estimate when speed reaches its maximum magnitude.
6) My site has wind‑driven or river flow. Can I include it?
This tool focuses on tidal exchange only. For strong river discharge or wind setup, treat the result as a tidal component and add measured background flow separately in your marine operations plan.
7) Why do I sometimes see “Slack” direction?
Near the slack reference time, the computed speed approaches zero, so direction becomes indeterminate. Increase the time since slack slightly, or use the series table to see the direction transition smoothly.