Variable swash motions associated with foreshore profile change

Variations in swash motions over a temporally changing foreshore surface are examined over the first tidal cycle following the onset of a storm using high-resolution video observations obtained at Duck, North Carolina. A dramatic example of profile adjustment was observed where nearly 1 m of vertical change occurred over a 4 hour time period. Swash edge excursion measurements during this study compare favorably with predictions from standard equations describing ballistic motions of an object on a plane slope under quadratic friction and showed no strong asymmetries between uprush and backwash that might explain the profile adjustment. Instead, these unique observations showed a tendency toward a predictable equilibrium between swash motions and profile response where the stability condition relates foreshore slope to incident wave period and initial uprush velocity using the ballistic equations. Under these conditions the foreshore adjusts to minimize the difference between swash duration and incident wave period. On the basis of this finding a simple morphodynamic model for changes in foreshore slope as a function of swash and surf conditions is presented and allows prediction of the transition from an erosive to stable condition.