Longshore Current Calculator for Coastal Construction

Tip: Enter wave conditions and either breaker depth or breaker index. Submit to compute longshore current and generate export files.

Inputs

Use realistic breaker-zone values for better results.

Scenario name

Units

Wave height at breaking, Hb (m)

Wave period, T (s)

Breaker angle, α (degrees)

Angle between wave crest and shoreline normal.

Breaker depth mode

Breaker depth, hb (m)

Breaker index, γ (dimensionless)

Common starting value: 0.78.

Friction coefficient, Cf

Typical range: 0.01 to 0.03.

Calibration factor, K

Use to match local measurements if available.

Safety factor (design)

Applies to velocity for conservative planning.

Water density, ρ (kg/m³)

Assumptions / notes (optional)

Add this run to the session log table

Clear form

Example data table

Sample scenarios for planning and method statements.

Scenario	Units	Hb	hb	T (s)	Angle (°)	Cf	K	V	Design V
Beach Nourishment	SI	1	1.28	7	12	0.016	1	0.73	0.8
Cofferdam Setup	SI	1.5	1.92	9	18	0.014	1.05	1.25	1.38
Jetty Repair	IMP	4	5.1	8	10	0.018	0.95	2.1	2.3

These are illustrative; compute site-specific values from measured conditions.

Session log

Your last 20 calculations in this browser session.

Time	Scenario	Units	Hb	hb	T	Angle	Cf	K	V	Design V
No runs yet. Submit the form to create a log entry.

Export log to CSV Export log to PDF

Formula used

This tool estimates a breaker-zone longshore current from alongshore wave energy flux and a friction balance.

Wave energy density: E = (1/8) ρ g H_b²
Group speed at breaking: C_g ≈ √(g h_b) (shallow-water approximation)
Alongshore flux: P_ls = E · C_g · (sin 2α)/2
Longshore current: V = K · P_ls / (ρ h_b C_f)
Design value: V_design = V · SF

Notes: K is a calibration factor; C_f represents bottom friction. For detailed design, calibrate against field measurements and confirm assumptions with coastal specialists.

How to use this calculator

Select your preferred unit system and name the scenario.
Enter breaker wave height (H_b), period (T), and breaker angle (α).
Choose a depth method: enter h_b or compute it using γ.
Set friction C_f and calibration K to match site conditions.
Press Compute to view results above the form.
Use the session log to compare runs and track assumptions.
Export CSV or PDF for sharing and documentation.

Professional guidance

1) Why longshore current matters on site

Alongshore currents influence turbidity curtains, barge positioning, dredge discharge, and safe diver windows. Even a modest current of 0.5 m/s can pull floating booms off alignment and increase rework. This calculator provides a fast screening value for daily coordination.

2) Recommended input ranges for checks

For many nearshore projects, breaker wave height H_b often falls between 0.5–2.5 m (1.6–8.2 ft) during workable conditions. Period T is commonly 5–14 s. Breaker angles α around 5–25° are typical; larger angles should be verified with local observations.

3) Collecting field data efficiently

Use a consistent source for wave conditions (buoy, nearshore model, or site staff estimates) and record the timestamp. When possible, confirm α from shoreline video frames or drone imagery. Keep a simple daily log with tides, wind, and any measured current from drogues or ADCP snapshots.

4) Breaker depth options and breaker index

If you know breaker depth h_b, enter it directly. Otherwise, compute h_b from the breaker index γ using h_b = H_b/γ. A common starting point is γ ≈ 0.78, while 0.70–0.85 covers many sandy beaches. Unusual γ values should be documented.

5) Friction and calibration for realistic outputs

The friction coefficient C_f typically ranges 0.01–0.03 depending on bed roughness and surf-zone turbulence. The calibration factor K lets you tune results to site measurements; begin with K=1.0, then adjust using at least several paired observations to avoid overfitting.

6) Interpreting velocity and design velocity

The calculator returns V (screening speed) and V_design = V·SF for conservative planning. Many construction controls become difficult above 0.8–1.2 m/s (2.6–3.9 ft/s), depending on equipment. Treat the design value as a coordination trigger, not a replacement for specifications.

7) Example planning scenario with numbers

Assume H_b=1.2 m, γ=0.78 (h_b≈1.54 m), T=8 s, α=15°, C_f=0.015, and K=1.0. You may obtain V near 1 m/s. With SF=1.10, the design value supports a “tight controls” day with added monitoring.

8) Documentation, QA, and communication

Include scenario names, assumptions, and export files in method statements and daily reports. Compare multiple runs using the session log to show sensitivity to α, C_f, and γ. When the tool flags atypical inputs, attach supporting notes (photos, tide table, buoy data) for auditability.

Frequently Asked Questions

1) Does this replace detailed coastal modeling?

No. It is a screening tool for planning and daily coordination. Use calibrated models, surveys, and project specifications for final design decisions.

2) Which wave height should I enter?

Use an estimate of breaker wave height near the site. If you only have offshore significant height, convert using local transformation guidance or a nearshore forecast, then document the assumption.

3) What if I do not know breaker depth?

Select the breaker index option and start with γ=0.78. If your beach is steep, barred, or armored, refine γ using observations and record why the value changed.

4) How should I pick the friction coefficient?

Start with C_f=0.015 for sandy beds. Increase toward 0.02–0.03 for rougher beds, rock, or strong turbulence. If you have measured currents, adjust K instead of forcing C_f.

5) Why does angle α matter so much?

The alongshore forcing scales with sin(2α). Small changes around 10–25° can noticeably change P_ls and V. Confirm shoreline orientation and keep angle measurement consistent.

6) What is a reasonable safety factor?

For planning, SF between 1.05 and 1.25 is common. Choose a value aligned with your risk tolerance, equipment limits, and the consequences of loss of control.

7) How do I share results with the team?

Run the scenario, then export CSV for spreadsheets or PDF for briefings. Include the scenario name and notes so supervisors can trace assumptions and compare conditions across shifts.