As you are standing on the beach, where the water line is is always changing. Although millions of people have been to the beach and, as a by-product, have watched these swash processes - we still dont totally understand wave runup. And yet it is very important. Following the methodology of Sallenger (2000), wave runup and its constituents should, in theory, control when dune erosion occurs.
Depsite the number of "unrecorded" swash observations, we actually have a paucity of quantitative wave runup measurements. Using some data and some models, I've spent a bit of time exploring what some of the drivers of wave runup are. Although empirical equations (e.g., Stockdon et al., 2006) simplify everything down to a few simple factors controlling what water levels should be - perhaps not unsurprisingly there is a lot more complexity than first may be imagined on the environmental and morphological controls on swash processes.
There are much smarter people than myself working on this as well, but its clear that there is a subtidal morphologic control on wave runup which are not encapsulated by most numerical models. There is also undoubtedly a morpholgic control on infragravity swash, although infragravity energy is primarily generated outside of the swash zone and therefore the contributing morphology is located in deeper water.