FLOW VELOCITY AND SEDIMENT TRANSPORT IN THE SWASH ZONE OF A STEEP BEACH

Detailed measurements of flow velocity and total sediment load were ob tained in the swash zone on a steep beach. Swash motion was measured u sing ducted impeller how meters and capacitance water level probes. Du ring wave uprush, the onshore flow increased almost instantaneously fr om 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 how 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 bac kwash 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 to tal sediment load was measured for 35 individual wave uprush events us ing a sediment trap. The amount of sediment transported by a single up rush was typically two to three orders of magnitude greater than the n et transport per swash cycle (difference between uprush and backwash) inferred from surveys of beach profile change. The measured immersed w eight total load transport rate displayed a strong relationship with t he time-averaged velocity cubed, which is consistent with equations fo r both bedload transport and total load transport under sheet flow con ditions. The Bagnold (1963, 1966) bedload transport model was tested a gainst our field data and yielded I-b=ku(-3)T(u)/(tan phi+tan beta), w here I-b is the immersed weight of bedload transported during the enti re uprush (kg m(-1)), k is a coefficient (kg m(-4) s(2)), (u) over bar is the time-averaged flow velocity for the uprush (m s(-1)), T-u is t he uprush duration (s), phi is the friction angle of the sediment and beta is the beach slope. The empirically determined value for the coef ficient k was 1.37+/-0.17. (C) 1997 Elsevier Science B.V.