Iribarren number

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In fluid dynamics, the Iribarren number or Iribarren parameter – also known as the surf similarity parameter and breaker parameter – is a dimensionless parameter used to model several effects of (breaking) surface gravity waves on beaches and coastal structures. The parameter is named after the Spanish engineer Ramón Iribarren Cavanillas (1900–1967),[1] who introduced it to describe the occurrence of wave breaking on sloping beaches.[2]

For instance, the Iribarren number is used to describe breaking wave types on beaches; or wave run-up on – and reflection by – beaches, breakwaters and dikes.[3][4][5]


The Iribarren number – often denoted as Ir or ξ – is defined as:[4]

\xi = \frac{\tan \alpha}{\sqrt{H/L_0}},   with   L_0 = \frac{g}{2\pi}\, T^2,

where ξ is the Iribarren number, α is the bed slope, H is the wave height, L0 is the deep-water wavelength, T is the period and g is the gravitational acceleration. Depending on the application, different definitions of H and T are used, for example: for periodic waves the wave height H0 at deep water or the breaking wave height Hb at the edge of the surf zone. Or, for random waves, the significant wave height Hs at a certain location.

Breaker types[edit]

Breaker types.
Further information: breaking wave

The type of breaking wave – spilling, plunging, collapsing or surging – depends on the Iribarren number. According to Battjes (1974), for periodic waves propagating on a plane beach, two possible choices for the Iribarren number are:

\xi_0 = \frac{\tan \alpha}{\sqrt{H_0 / L_0}}   or   \xi_b = \frac{\tan \alpha}{\sqrt{H_b / L_0}},

where H0 is the offshore wave height in deep water, and Hb is the value of the wave height at the break point (where the waves start to break). Then the breaker types dependence on the Iribarren number (either ξ 0 or ξ b) is approximately:[3]

breaker type ξ 0–range ξ b–range
surging or collapsing ξ0 > 3.3 ξb > 2.0
plunging 0.5 < ξ0 < 3.3 0.4 < ξb < 2.0
spilling ξ0 < 0.5 ξb < 0.4




  • Battjes, J.A. (1974), "Surf similarity", Proceedings 14th International Conference on Coastal Engineering: 466–480, doi:10.9753/icce.v14 
  • Bruun, P., ed. (1984), Design and construction of mounds for breakwaters and coastal protection, Developments in geotechnical engineering 37, Elsevier, pp. xi & 39, ISBN 0-444-42391-5 
  • Goda, Yoshimi (2010), Random seas and design of maritime structures (3rd ed.), World Scientific, p. 213, ISBN 9814282405 
  • Holthuijsen, L.H. (2007), Waves in oceanic and coastal waters, Cambridge University Press, p. 242, ISBN 1139462520 
  • Iribarren, C.R.; Norales, C. (1949), "Protection des ports", Proceedings XVIIth International Navigation Congress, Section II, Communication 4, Lisbon, pp. 31–80