4. Discussion and conclusions The development of any empirical equation is strongly dependent on the quality of the data upon which it is based. The field dataset from a range of sites presented here includes information on beach morphology and sedimentology, wave conditions and extreme runup, and all parameters extracted from these measurements are subject to a natural variability, as well as measurement error. Not surprisingly, therefore, considerable scatter remains in the data, even when fitted to the ‘best’ equation or the XBeach-G model. The largest cause of scatter in the data is considered to be the estimation of the runup elevation from the video data. Apart from pixel resolution and issues with appropriate identification of the leading edge, there is also the assumption that the beach is planar, two-dimensional and stable during the experiment. Additionally, considering the relatively short time series (17 min) and the long wave periods generally encountered under the more energetic wave conditions (Tp N 10 s), the 2% runup statistic is often based on only a handful of runup events. Nevertheless, the methodology used is identical to that deployed in the more recent previous runup studies (Aagard and Holm, 1989; Holland and Holman, 1993; Ruessink et al., 1998; Ruggiero et al., 2001; Stockdon et al., 2006).
