Flow velocity and sediment transport in the swash zone of a steep beach

Abstract

Detailed measurements of flow velocity and total sediment load were obtained in the swash zone on a steep beach. Swash motion was measured using ducted impeller flow meters and capacitance water level probes. During wave uprush, the onshore flow increased almost instantaneously from zero to its maximum velocity after the arrival of the leading edge of the swash lens and subsequently decreased gradually to zero for the remainder of the uprush. During backwash, the offshore flow increased steadily from zero to its maximum towards the end of the backwash and dropped rapidly to zero as the beach fell “dry”. The duration of backwash was typically longer than that of uprush and maximum water depth on the beach was attained just prior to the end of the uprush. The total sediment load was measured for 35 individual wave uprush events using a sediment trap. The amount of sediment transported by a single uprush was typically two to three orders of magnitude greater than the net transport per swash cycle (difference between uprush and backwash) inferred from surveys of beach profile change. The measured immersed weight total load transport rate displayed a strong relationship with the time-averaged velocity cubed, which is consistent with equations for both bedload transport and total load transport under sheet flow conditions. The Bagnold (1963, 1966) bedload transport model was tested against our field data and yielded Ib = kū3Tu(tanφ + tanβ), where Ib is the immersed weight of bedload transported during the entire uprush (kg m−1), k is a coefficient (kg m−4 s2), ū is the time-averaged flow velocity for the uprush (m s−1), Tu is the uprush duration (s), φ is the friction angle of the sediment and β is the beach slope. The empirically determined value for the coefficient k was 1.37 ± 0.17.