Beach slope effects on sand suspension and transport rate by infragravity waves

The suspension and transport of sand by infragravity (IG) waves has been investigated in various studies. The diversity in boundary conditions has resulted in observations that contrast in direction and relative importance of IG sand transport as well as in the underlying suspension mechanism. We hypothesize that the relative importance of IG wave energy to sea-swell wave energy, which is affected by beach characteristics and offshore wave conditions, explains these contrasting observations. Here, we investigate this with data from the moderately sloping (1:35) Sand Motor beach, and the gently sloping (1:80) Ameland beach, both in the Netherlands. On each site, wave characteristics, velocities and sand concentrations were collected at up to 4 locations in the intertidal zone. On the moderately sloping Sand Motor beach, the IG waves were typically small compared to sea-swell waves (Hm0_IG/Hm0_sw< 0.4), despite offshore wave heights close to 5 m. Accordingly, the sand was suspended by the sea-swell waves and the IG transport direction is linked to the correlation of the IG orbital motion with the sea-swell wave envelope. In the shoaling and outer surf zone, the correlation is negative and the IG transport is offshore directed. Further shoreward, the correlation becomes positive as the largest sea-swell waves are present on the IG crest, resulting in onshore IG transport. However, in both cases the IG transport hardly contributed to the total transport. This contrasts with the inner surf zone of the gently sloping Ameland beach, where the IG waves were more energetic (Hm0_IG/Hm0_sw > 0.4), and their transport contribution was substantial. The IG transport was predominantly offshore directed as the sand was mostly suspended by the negative IG-wave velocity phase. On the whole, our data confirm that the local ratio of IG to sea/swell wave height is a good proxy for the sand suspension mechanism, the IG transport direction, and the relative importance of IG waves to the total sand transport.