Breakwater Crest Calculator

Input Data

Use the form below to estimate breakwater crest elevation from wave, slope, and overtopping parameters.

White theme • Single main column • Responsive input grid

Significant wave height, Hs (m) Design wave height at the structure toe or nearshore section.

Mean wave period, Tm (s) Used to estimate deepwater wavelength and surf similarity.

Still water level, SWL (m) Reference water level above your site datum.

Seaward slope ratio, 1V : H Enter the horizontal part only. Example: 2 for 1V:2H.

Roughness factor, γf Lower values often represent rougher armour layers.

Berm factor, γb Use 1.00 when no berm correction is needed.

Wave angle factor, γβ Oblique wave attack often reduces effective run-up.

Allowable overtopping, q (l/s/m) Smaller allowable discharge usually requires a higher crest.

Crest width, Bc (m) A wider crest can modestly reduce splash reach.

Settlement allowance (m) Covers expected long-term crest lowering after construction.

Safety margin (m) Extra conservative allowance for uncertainties.

Water level allowance (m) Storm surge, tide allowance, or future sea level margin.

Example Data Table

This sample row uses the calculator defaults and shows a ready-made reference case.

Hs (m)	Tm (s)	SWL (m)	1V:H	γf	γb	γβ	q (l/s/m)	Bc (m)	Settlement (m)	Safety (m)	Water Allowance (m)	Estimated Crest Elevation (m)
4.20	8.50	3.00	1:2.00	0.55	1.00	0.95	10.00	6.00	0.25	0.30	0.60	7.661

Formulas Used

This calculator is intended for preliminary screening and comparison. Final coastal structure design should always be checked by a qualified coastal engineer using project-specific guidance.

Deepwater wavelength: L0 = gT² / 2π
Wave steepness: Hs / L0
Seaward slope tangent: tan α = 1 / H for a slope of 1V:H
Surf similarity: ξ = tan α / √(Hs / L0)
Combined coefficient: γ = γf × γb × γβ
Two percent run-up: R2% = γ × Hs × 1.75ξ for lower ξ, or R2% = γ × Hs × (4.3 − 1.6/√ξ) for higher ξ
Run-up freeboard: Rc,runup = 0.55 × R2% × kB, where kB is a simple crest-width adjustment
Overtopping freeboard: a screening expression links smaller allowable overtopping to a higher required freeboard
Adopted freeboard: Rc = max(Rc,runup, Rc,overtopping)
Design crest elevation: Crest = SWL + water allowance + Rc + settlement + safety margin

How to Use This Calculator

Enter the design wave height and representative wave period.
Set the still water level relative to your project datum.
Enter the seaward slope as the horizontal part of a 1V:H ratio.
Add roughness, berm, and obliquity factors that match the concept design.
Choose an allowable overtopping rate based on use and risk tolerance.
Add crest width, settlement allowance, safety margin, and water level allowance.
Press Calculate Crest Level to show the result above the form.
Review the graph, output table, and exported files for comparison work.

FAQs

1. What does the breakwater crest elevation represent?

It is the design top level of the breakwater relative to your datum. It combines water level, freeboard, settlement allowance, and safety margin into one screening estimate.

2. Why is wave run-up included?

Wave run-up estimates how high water can climb the seaward face. A crest that is too low may experience more splash, overtopping, and operational disruption during storms.

3. Why does allowable overtopping change the answer?

A lower overtopping limit means you are asking the structure to pass less water over the crest. That usually requires a higher freeboard and therefore a higher crest elevation.

4. What is the roughness factor, γf?

It adjusts run-up for armour type and surface roughness. Rougher, more energy-dissipating faces often use lower values, while smoother surfaces tend to produce higher run-up.

5. Why is crest width part of the screening?

A wider crest can slightly reduce splash reach and improve practical resilience. This calculator uses a small screening adjustment, not a detailed hydraulic transmission analysis.

6. Is this suitable for final construction drawings?

No. It is best for concept development, option comparison, and early checking. Final design should use site-specific bathymetry, wave transformation, overtopping guidance, and expert review.

7. What units should I use?

Use metres for levels, heights, and widths, seconds for wave period, and litres per second per metre for allowable overtopping discharge.

8. Can this be used for all breakwater types?

It is most appropriate as a preliminary screening tool for sloped coastal structures. Vertical walls, composite systems, and unusual geometries need more specific design methods.