The surf zone distortion of small scale coastal models

In recent costal modeling practice physical models are constructed in undistorted scales preserving the Froude similarity. Correction for scale effect is obtained a posteriori by means of a distortion coefficient, which multiplies the final quasi-equilibrium profile abscissa and provides a correction of the predicted prototype beach slope. The choice of this distortion coefficient is therefore crucial as it highly affects the reliability of the prototype reproduction. Furthermore, it has been observed that, because of the complexity of coastal sediment transport phenomena and beach profiling in the surf zone, a single coefficient does not succeed in correcting the whole beach profile. The scale effects affecting the surf zone of Froude models has been examined in this paper. A method for correcting scale effects in the surf zone is proposed. It is based on a distortion parameter, alpha(surf), to be applied only to the beach surf zone, leaving the offshore and foreshore parts undistorted, above and below the surf zone. An empirically based formula is found for calculating alpha(surf) as a function of the ratio between a parameter representing bed load transport and the characteristics of the model sand, expressed by the mean diameter D-50 and the Kurtosis parameter of the particle-size curve. It is commonly accepted that scale effects can be minimized if, besides Froude similarity, the Dean parameter is the same both in the prototype and in the model. However, this often implies very large model scales, not compatible with laboratory facilities. It is shown that the introduction of the alpha(surf) parameter allows to employ a more practical small model scale by properly correcting scale effects, even though Dean similarity is not respected.