Sediment Transport and Bedform Dynamics in Rip Currents

Abstract

Simultaneous in-situ measurements of waves, currents, water depth, suspended sediment concentrations and bed profiles were made in a rip channel on Perranporth Beach, Cornwall, UK. Perranporth is a high energy beach (annual offshore Hs = 1.6 m) which is macro-tidal (mean spring range = 6.3 m) and the grain size is medium sand (D50 = 0.28 – 0.34 mm). It can be classified as a low tide bar – rip beach and exhibits a relatively flat inter-tidal zone with pronounced rhythmic low tide bar - rip morphology. Data were collected over two field campaigns, totalling 14 tidal cycles and including 27 occurrences of rip currents, in a range of offshore wave heights (Hs = 0.5 – 3 m). The in-situ measurements were supplemented with morphological beach surveys. Sediment samples were taken for grain size analysis. The rip current was found to be tidally modulated. The strongest rip flow (0.7 m/s) occurred at mid to low tide, when waves were breaking on the adjacent bar. Rip flow persisted when the bar had dried out at the lowest tidal elevations. The rip was observed to pulse at a very low frequency (VLF) with a period of 15 - 20 minutes, which was shown to be influenced by wave breaking on the adjacent bar. The rip was completely in-active at high tide. Bedforms were ubiquitous in the rip channel and occurred at all stages of the tide. Visual observations found bedforms to be orientated shore parallel. When the rip was active, mean bedform length and height was 1.45 m and 0.06 m respectively. The size and position of the bedforms in the nearshore suggested that they were best classified as megaripples. When the rip was not active, the mean bedform length and height was 1.09 m and 0.06 m respectively. In rip conditions, with typical mean offshore flow rates of > 0.3 m/s, the bedforms migrated in an offshore direction at a mean rate of 0.16 cm/min and a maximum rate of 4.6 cm/min. The associated mean bedform sediment transport rate was 0.0020 kg/m/s, with a maximum rate of 0.054 kg/m/s. In the rip, migration rates were correlated with offshore directed mean flow strength. In non-rip conditions, bedform migration was onshore directed with a mean rate of 0.09 cm/min and a maximum rate of = 2.2 cm/min. The associated mean bedform transport rate was 0.0015 kg/m/s, with a maximum rate of = 0.041 kg/m/s. The onshore bedform transport was correlated with incident wave skewness, and was weakly correlated with orbital velocity. Over a tidal cycle, the offshore directed bedform transport was only marginally larger in rip currents than when it was when onshore directed in non-rip conditions. Sediment suspension in the rip current was shown to be dependent on the presence of waves. Suspended sediment transport was dominated by the mean flux. The mean flux contributed > 70% of total suspended transport on 19 out of the 27 observed rip current occurrences. The net contribution of the oscillatory flux was small compared to the mean flux. Within the oscillatory component, a frequency domain partitioning routine showed that the VLF motion was an important mechanism for driving offshore directed sediment transport. This was balanced by onshore directed sediment transport at incident wave frequency of a similar magnitude. Depth integration showed that the mean total suspended sediment transport was in the range of 0.03 kg/m/s to 0.08 kg/m/s. At high tide, when the rip was inactive suspended sediment transport rates were minimal compared to when the rip was active. Bedform transport was (on average) 6% of the total suspended sediment transport in a rip current. The new results presented here show that rip currents make an important contribution to offshore directed sediment transport. The magnitudes of transport indicate that future improvements to morphology change models should include rip driven offshore sediment transport.