Hydrodynamics and sediment transport on a macro-tidal, mixed (sand and shingle) beach

A field investigation of a macro-tidal, ridge and runnel beach at Morfa Dyffryn, North Wales, is described, which examines the distinctive characteristics of a mixed (sand and shingle) beach.The moments of the velocity field and wave reflection at Morfa Dyffryn are compared with those from a similarly macro-tidal, sandy beach at Neiuwpoort-aan-Zee, Belgium. Strong, wind-enhanced, mean longshore currents persist into shallow water. Results of the velocity moment analysis indicate that the mean flows dominate the sediment transport patterns, particularly in shallow water, early in the flood and late in the ebb phases of the tide. The mean longshore current can be responsible for mobilising (in addition to transporting) sediment, when in excess of 0.3 m s-1. Mean fluxes dominate the measured suspended sediment transport patterns, in common with other macro-tidal beaches. Suspended sediment fluxes are generally at their highest in shallow water, although extremely high fluxes were observed just after High Water. These patterns are attributed to increased re-suspension due to wind/resolved mean current interaction. Therefore, high transport rates can occur, even in the absence of storm conditions. The relict sand/shingle ridge has little influence on reflection of wind wave energy. However, reflection of swell waves is enhanced, once the swash zone inundates the sand/shingle berm, due to its steeper gradient.Fluctuations in the beach water table due to tidal inundation are compared to groundwater behaviour on a sandy beach at Canford Cliffs, Dorset. The GRIST I model (which was derived for sandy beaches) predicts well both the location and translation of a seepage face across the sand/shingle profile. The sand fraction of the sediment determines the response of the water table. Laboratory hydraulic conductivity tests undertaken on sand/shingle mixtures indicate that the shingle content of a beach should be in excess of 80% to 90%, before increased dissipation of energy through percolation can occur.The main significance of the sand/shingle mixture, in relation to the morphodynamic response of the ridge is through its ability to maintain a steeper slope than would be supported by a sandy beach. The change in gradient across the mixed beach profile has more influence on the hydrodynamics, than does the sediment composition.