Inputs
Formula Used
This calculator estimates the 2% exceedance run-up, then adds allowances to recommend a crest elevation.
- L0 = g·T² / (2π) deep-water wavelength estimate.
- tan(α) = 1 / (slope ratio) for a 1V:H slope.
- ξ = tan(α) / √(Hm0/L0) spectral breaker parameter.
- Ru2%/Hm0 = min(C1·γb·γf·γβ·ξ, γb·γf,surging·γβ·(C2 − C3/√ξ)).
- Crest = SWL + Ru2% + freeboard + SLR + settlement, then rounded.
Use outputs as screening values and confirm with project standards.
How to Use This Calculator
- Enter still water level relative to your project datum.
- Provide design wave height and period at the structure toe.
- Set slope ratio and select roughness and berm factors.
- Add allowances for freeboard, sea level, and settlement.
- Calculate and export results for reports and checks.
Example Data Table
| Scenario | SWL (m) | Hm0 (m) | Tm-1,0 (s) | Slope | γf | β (deg) | Freeboard (m) | Indicative Crest |
|---|---|---|---|---|---|---|---|---|
| Rubble mound, normal waves | 2.50 | 3.00 | 6.00 | 1:2 | 0.55 | 0 | 0.50 | Varies by inputs |
| Steeper slope, oblique waves | 2.30 | 2.50 | 5.50 | 1:1.5 | 0.60 | 30 | 0.40 | Varies by inputs |
| Smoother face, higher margin | 3.10 | 3.50 | 7.00 | 1:2.5 | 0.80 | 10 | 0.80 | Varies by inputs |
Professional Guidance Article
1) Crest Design Objective
Breakwater crest elevation is selected to manage overtopping and protect the sheltered zone. A practical crest recommendation combines the still water level with wave run-up and project allowances. The goal is a crest aligned with performance expectations.
2) Hydrodynamic Inputs That Drive Crest Height
The dominant inputs are still water level, significant wave height Hm0, and a representative wave period. As Hm0 increases, run-up typically rises roughly proportionally. Period influences the deep-water wavelength estimate and therefore the breaker parameter used in the run-up model.
3) Interpreting Ru2% for Design Checks
Ru2% represents the run-up level exceeded by only 2% of waves in a storm record. It is commonly used for conservative crest screening before detailed overtopping studies. The calculator reports Ru2% and the derived freeboard Rc above SWL for clear traceability.
4) Slope Effects and the Iribarren Number
Slope affects run-up through the spectral breaker parameter ξ, which blends geometry with wave steepness. Typical rubble mound slopes range from about 1V:1.5H to 1V:3H, and steeper faces often increase ξ. The tool computes ξ directly so you can validate whether the condition is more breaking or surging dominated.
5) Roughness and Berm Factors in Practice
Surface roughness reduces run-up compared with smooth impermeable slopes. For rock armor, γf is often near 0.5–0.6, while smoother faces approach 1.0. A berm or geometry factor γb can reduce run-up if a pronounced berm interrupts uprush; set γb to 1.0 when no reduction is justified.
6) Wave Obliquity and Directional Reduction
Oblique waves usually reduce the run-up component normal to the structure. The calculator applies a bounded directional factor based on the attack angle β, keeping reductions realistic. Use site wave roses to pick β for the design storm scenario.
7) Allowances for Uncertainty, Settlement, and Future Levels
Engineering crest selection rarely relies on run-up alone. Add freeboard margin for tolerances, modeling uncertainty, and operational risk. Sea level and settlement allowances can be included as explicit inputs, making assumptions visible and helping reviewers see which margins drive the final elevation.
8) Rounding, Constructability, and Reporting
Crest levels are commonly rounded up to 0.05 m or 0.10 m increments to match survey and construction practice. This tool rounds upward to avoid under-design. The CSV and PDF exports provide a compact audit trail of inputs, intermediate parameters, and the rounded crest value for reporting packages and internal QA checks.
FAQs
1) What does Ru2% mean?
Ru2% is the run-up level exceeded by 2% of waves. It is a conservative run-up metric often used for crest screening before detailed overtopping evaluation.
2) Which wave period should I enter?
Use the period consistent with your selected sea state and design method. If you have spectral parameters, enter the representative period used in your run-up guidance for the project.
3) How do I choose the roughness factor γf?
Select γf based on armor type and surface texture. Rock armor commonly uses values around 0.5–0.6, while smooth impermeable faces approach 1.0. Document the source used.
4) Why is there a wave angle input?
Oblique wave attack reduces run-up normal to the structure. Enter β consistent with the design storm direction and local wave climate so the directional reduction is defensible.
5) What is the deterministic mode for?
Deterministic mode applies slightly higher coefficients to provide a more conservative crest screening value. Use it when crest level decisions must remain robust under uncertainty.
6) Should I always add settlement and sea level allowances?
Include them when they are relevant to the asset life and ground conditions. Explicit allowances make the crest basis transparent and help compare present-day and future-case elevations.
7) Does this replace a full coastal design?
No. It is a practical calculator for estimating crest elevations using run-up and allowances. Final design should follow project standards, detailed hydraulic checks, and professional review.