ICCE 2016 Cover Image


mound breakwater
roughness factor
armor unit

How to Cite

MEDINA, J. R., & MOLINES, J. (2017). ROUGHNESS FACTOR IN OVERTOPPING ESTIMATION. Coastal Engineering Proceedings, 1(35), structures.7.


The roughness factor (γf) is a key variable to estimate wave overtopping discharge on mound breakwaters. In this study, the γf is re-calibrated using a dataset extracted from the CLASH database. Compared to previous roughness factors calibrated using less restrictive data, overtopping estimators with a few explanatory variables showed variations up to 15% in the 50% percentile of γf. On the contrary, the CLASH neural network overtopping predictor showed insignificant variations in the roughness factor, since it is less sensitive to the variability in the data used for calibration. The confidence interval width of the CLASH neural network was narrow compared to simple explicit overtopping estimators, given that it is less sensitive to the number of data used for calibration. The γf values used to estimate wave overtopping discharge should be carefully calibrated, especially when using simple empirical formulas.


Bruce, T., Van der Meer, J.W., Franco, L., and J.M. Pearson. 2006. A comparison of overtopping performance of different rubble mound breakwater armour. Proceedings 30th International Conference on Coastal Engineering. World Scientific, Vol. 5, 4567-4579.

Bruce, T., Van der Meer, J.W., Franco, L., and J.M. Pearson. 2009. Overtopping performance of different armour units for rubble mound breakwaters. Coastal Engineering 56 (2), 166-179.

Coeveld, E.M., Van Gent, M.R.A., and B. Pozueta. 2005. Neural Network, Manual NN_OVERTOPPING 2.0, CLASH: Workpackage 8, 38 pp.

Deltares. 2017. Overtopping neural network. [on line] Deltares. Available at: [Accesed 25 Jan. 2017].

EurOtop, 2007. Wave Overtopping of Sea Defences and Related Structures: Assessment Manual (EurOtop Manual). Pullen, T., Allsop, N.W.H., Bruce, T., Kortenhaus, A., Schuttrumpf, H., and Van der Meer, J.W. Environment Agency, UK/ENW Expertise Netwerk Waterkeren, NL/KFKI Kuratorium fur Forschung im Kusteningenieurwesen, Germany, 178 pp.

Molines, J., and J.R. Medina. 2015. Calibration of overtopping roughness factors for concrete armor units in non-breaking conditions using CLASH database. Coastal Engineering 96 (2015), 62-70.

Molines, J., and J.R. Medina. 2016. Explicit wave overtopping formula for mound breakwaters with crown walls using CLASH neural network-derived data. Journal of Waterway, Port, Coastal and Ocean Engineering, 142 (3), 04015024, 13 pp.

Pearson, J., Bruce, T., Franco, L., Van der Meer, J., Falzacappa, M., and R. Molino. 2004. Report on additional tests, part B: Standard tests for roughness factors, CLASH WP4 report, University of Edinburgh, UK, 53 pp.

Smolka, E., Zarranz, G., and J.R. Medina. 2009. Estudio Experimental del Rebase de un Dique en Talud de Cubípodos. Libro de las X Jornadas Españolas de Costas y Puertos, Universidad de Cantabria, 803-809 (in Spanish).

Stewart, T.P., Newberry, S.D., Simm, J.D., and J.P. Latham. 2002. The hydraulic performance of tightly packed rock armour layers. Proceedings 28th International Conference on Coastal Engineering, World Scientific, Vol. 2, 1449-1471.

Van der Meer, J.W., and J.P.F.M. Janssen. 1994. Wave Run-Up and Wave Overtopping at Dikes, Delft Hydraulics No. 485.

Van der Meer, J.W., and T. Bruce. 2014. New Physical Insights and Design Formulas on Wave Overtopping at Sloping and Vertical Structures. Journal of Waterway, Port, Coastal and Ocean Engineering, 140 (6), 04014025, 18 pp.

Van Gent, M.R.A., Van den Boogaard, H.F.P., Pozueta, B., and J.R. Medina. 2007. Neural network modelling of wave overtopping at coastal structures. Coastal Engineering 54 (8), 586-593.

Authors retain copyright and grant the Proceedings right of first publication with the work simultaneously licensed under a Creative Commons Attribution License that allows others to share the work with an acknowledgement of the work's authorship and initial publication in this Proceedings.