Hydrokinematic regions within the swash zone

A method for delimiting the swash zone and regions within is presented. Two regions are recognized and distinguished by their differing flow kinematics. The outer swash region involves wave-swash interactions and related processes, whereas the inner swash region consists of pure swash motion (i.e., free from interaction with subsequent waves). The boundary between these two hydrokinematic regions can be determined from shoreline elevation time series. The vertical extent of the outer swash was found to scale directly with inner surf zone wave variance and beach slope. Since the vertical extent of the entire swash zone also varies directly with the former, the relative extents of the outer and inner swash are approximately constant for the range of beach slopes investigated here. The efficacy of a previously utilized method for determining the location of instruments in the swash zone, based on the percentage of time the bed is inundated, is established here for the first time. A new method for determining the location of an instrument station within either of the hydrokinematic regions is also presented, and requires only a single pressure sensor time series. The data discussed here include over 140 runup time series collected from five different sandy beaches with beach face gradients ranging from 0.03 to 0.12. The results are expected to be generally applicable to swell-dominated sandy beaches, where swash is driven by a combination of short and long waves in the inner surf zone. The applicability of the results at either extreme of the reflective–dissipative continuum remains to be established.