Evaluation of a Predictive Equation for Runup

Extreme runup occurring during storms and hurricanes is likely to be responsible for the most dramatic erosional events, impacting both the beach and dunes and forming an important design criterion for coastal structures and set back. Yet one of the most commonly used predictive equations for runup (Holman, 1986) is based on data from a single site and has not been broadly tested. We will examine the consequences of the extension of Holman's equation to other beach and wave conditions by comprehensive testing using data from seven field experiments: Duck, NC (1982, 1990, 1997); Scripps Beach, CA (1989); San Onofre, CA (1993); Gleneden, OR (1994); and Agate Beach, OR (1996). Special attention will be given to data collected during high tides and large wave events as they represent times of highest runup and most significant erosion. Holman's equation shows a relationship between extreme runup and the Iribarren number, which includes a linear dependence on beach slope. However, on more complex topographies, it is unclear whether runup is more dependent upon the foreshore or surf zone slope. Our analysis will investigate the most appropriate definition of beach slope by testing both in the single horizontal dimension equation. We will then expand the one-dimensional equation to examine beaches with longshore variable topographies. Here, the equation predicts significant variations in runup with possible consequences to short scale variability in beach erosion. Using the improved equation and data from sites with multiple longshore locations, we will examine the longshore variability of beach slope and how it relates to both predicted and observed runup statistics.