Observations of wave run-up and groundwater seepage line motions on a reflective-to-intermediate, meso-tidal beach

• Swash (SW)/ground water seepage line (GWSL) motions monitored using coastal images.
• The Kumaraswamy probability distribution proposed for SW and GWSL height PDFs.
• SW zone saturation related to tidal gradient, wave weight and infragravity motions.
• The beach-face slope controls the uprush, backwash and GWSL velocities.
• Tidal gradient affects swash and GWSL vertical velocities/elevations.

Swash (SW) and ground water seepage line (GWSL) motions have been recorded at an exposed, meso-tidal, reflective-to-intermediate beach, using a coastal video monitoring system, during a 15-month period. The monitoring period allowed the collection of imagery under a wide variety of wave and beach-morphological conditions and SW and GWSL velocities were extracted on a wave-by-wave basis. The continuous, double-bounded Kumaraswamy (Kw) probability distribution is proposed to parameterize the SW and GWSL height distributions and since the distribution shape of the latter was shown to be influenced by the tidal elevation, generic, tidal elevation-dependent Kw PDFs are proposed. The beach-face slope, more than any other factor tested, was found to apply certain control to the mean uprush, backwash and GWSL velocities. The results also indicate that in reflective-to-intermediate beaches, the GWSL lies between the mean and 2% exceedance swash elevation; with the occurrence of swash running above a saturated beach-face being controlled by the tidal gradient and the significant wave height.