Sediment Transport in the Swash-Zone of Natural Beaches

Abstract

Concurrent measurements of velocity, water depth, sub-surface pore-pressure and suspended sediment concentration (SSC) were obtained from the swash-zone o f a macrotidal, dissipaiive (tanP = 0.014), fine to medium grained (dso^ 0.24mm) beach at Perranporth, Cornwall, UK, in low (H^ = 0.8m) to high (Hj « 2.2m) energy conditions. Velocity, pressure and SSC were measured using electromagnetic current meters, pressure transducers and a four-level array of miniature optical backscaner sensors respectively. Results show an infragravity dominated wave field in the swashzone in both high and low energy conditions. In low energy conditions, the ratio between low ( f < 0.05Hz) and high ( f > 0.05Hz) frequency variance increased from 0.6 to 2.3 for both cross-shore velocity and surface elevation, and SSC at z = 1cm increased by a factor of about 5, between just shoreward of the breakpoint (h/hb = 0.6) and the swash-zone (h/hb = 0.2). Swash-zone suspended sediment fluxes at z = 5 to 6cm increased by a factor of about 2 from low to high energy conditions. Low frequency cross-shore velocity skewness values for low and high energy conditions were 0.04 and -0.81 respectively, suggesting an important contribution to offshore transport in high energy conditions, from large, low frequency backwashes manifest in the velocity time-series as low frequency negative skewness. Using 3D velocities from a similar experiment at Muriwai, New Zealand, it is found that TKE/velocity ratios in the uprush were about 1.7 limes larger than in the backwash, suggesting greater turbulence in the uprush. Turbulence and rapid flow reversal are important onshore transport mechanisms, manifest as high frequency negative asymmetry in the velocity time-series. Values for high frequency negative asymmetry at Perranporth were 1.8 times larger in low than in high energy conditions. The influence of inexflltration is investigated using velocity and sub-surface pore-pressure measurements, and a modified Shields parameter, which includes terms to account for the competing effects of stabilisation-destabilisation and boundary layer modification. It is found that there is dominance of stabilisation-destabilisation, resulting in a 4.5% increase in backwash transport and a 10.5% decrease in uprush transport, integrated over a swash cycle. Sensitivity tests show a reversal in this dominance if the median grain size is increased above about 0.55mm, suggesting the transport influence is onshore on coarser grained beaches and offshore on finer grained beaches. Calculations show that hydraulic jumps due to uprush-backwash interaction were about 2.1 times as likely to occur during high than during low energy conditions. Visual observations suggest that more sediment is advected offshore from these jumps in high energy conditions. An antidune ripple field, which enhances transport through increased bed roughness, may also form beneath hydraulic jumps, but this is shown to influence the total dimensionless transport over a 17-min. time-series by less than 5%. Overall results suggest that swash-zone erosion occurs more readily on relatively fine-grained beaches in storms, and accretion occurs more readily on relatively coarse-grained beaches in calm conditions, which is consistent with observations in nature. Large infragravity frequency backwashes manifest as negative infragravity skewness appear to be a key factor in linking enhanced offshore transport in the swash-zone with high energy conditions.