Annual Transactions of IESL, 2005 © Institution of Engineers, Sri Lanka Effect on Wave Run-Up of Scaling Method of Core Material in Rubble-Mound Breakwater Model Testing D. M. R. Sampath and J. J. Wijetunge Abstract: The size of core material for rubble-mound breakwater models is usually found by employing geometric scaling. However, Burcharth et al. [3] suggested that the diameter of the core material in breakwater models should be determined based on Froude scale law for a characteristic pore velocity. In this backdrop, the present paper attempts to quantify the potential reduction in wave run-up associated with the new scaling method proposed by Burcharth et al. compared to the traditional method of linear geometric scaling. Accordingly, a series of wave run-up measurements were carried out in a 2D regular wave channel to examine the above for the type of rubble-mound breakwaters typically found in Sri Lanka. The measurements were first carried out with a breakwater model scaled using the traditional method, and thereafter, with the core of the model scaled using the new method under identical wave conditions, for a range of the surf similarity parameter representing both plunging and surging wave breakers. The results suggest that there is little reduction with the new method for values of surf similarity parameter between 1.3 and 2.2, though a marginal reduction of not more than 5% could be discernible for values of surf similarity beyond 2.2. Keywords: Wave Run-up, Rubble-Mound Breakwaters, Froude Scaling, Linear Geometric Scaling, Core Permeability, Notional Permeability 1. Introduction A complex and strong interaction takes place between wave field and wave damping structures such as rubble-mound breakwaters when they are exposed to wave attack. This wave structure-interaction leads to several hydraulic responses, namely, wave run-up, wave overtopping, wave reflection, wave transmission and wave breaking with associated energy dissipation. Of these, wave run-up, which determines the crest height, is a very important phenomenon in the design of wave-damping structures. Although design charts and formulae are used in the conceptual design of rubble-mound breakwaters, the final design is often based on extensive model testing to determine the crest elevation as well as the weight of armour stones. The permeability of the core material of rubble-mound breakwaters influences the wave run-up as well as overtopping and armour stability (van der Meer [9]). The size of core material for rubble-mound breakwater models is usually found by employing geometric scaling. However, Burcharth et al. [3] suggested that the diameter of the core material in breakwater models should be determined based on Froude scale law for a characteristic pore velocity. This is because, linear geometric scaling usually results in smaller core material, which reduces the flow in and out of the core, probably resulting in relatively larger run-up. Wave induced pore pressure in rubble-mound breakwaters has also been investigated by Jensen and Klinting [5], Oumeraci and Partenscky [6], and Burcharth and Andersen [2]. In this backdrop, the present paper attempts to quantify the potential reduction in wave run-up associated with the new scaling method proposed by Burcharth et al. [3] compared to the traditional method of linear geometric scaling. 2. Preliminary Considerations 2.1. Typical Structural Details of Rubble- Mound Breakwaters in Sri Lanka Breakwaters in most of the small-craft fishery harbours in Sri Lanka are of the rubble-mound Eng. D. M. R. Sampath, AMIE(SL), B.Sc. Eng. (Hons) (Peradeniya), Temp. Lecturer, Department of Civil Engineering, University of Peradeniya. He has fulfilled all requirements for the award of the M. Sc. Engineering degree from University of Peradeniya. Eng. (Dr) J. J. Wijetunge, AMIE(SL), B.Sc. Eng. (Hons) (Moratuwa), Ph.D.(Cambridge), Senior Lecturer, Department of Civil Engineering, University of Peradeniya. ENGINEER 1