The on-offshore and the vertical components u, w of the velocity in a surf zone on a uniformly sloping beach in a wave tank were measured simultaneously with a laser-doppler velocimeter under two conditions.The time variation of the Reynolds stress -u'w' during one wave period is discussed.The Reynolds stress behaves as that in the oscillatory pipe flow does.The magnitude of the terms including the Reynolds stress terms in the on-offshore momentum equation is estimated.The Reynolds stress terms does not play any important role in the on-offshore momentum transfer during one wave period in the surf zone.

The on-offshore and the vertical components u, w of the velocity in a surf zone on a uniformly sloping beach in a wave tank were measured simultaneously with a laser-doppler velocimeter under two conditions. The time variation of the Reynolds stress -u'w' during one wave period is discussed. The Reynolds stress behaves as that in the oscillatory pipe flow does. The magnitude of the terms including the Reynolds stress terms in the on-offshore momentum equation is estimated. The Reynolds stress terms does not play any important role in the on-offshore momentum transfer during one wave period in the surf zone.