Vertical structure of the undertow outside the surf zone

The vertical structure of the undertow in the shoaling region outside the surf zone is quite different from that inside the surf zone. Inside the surf zone the undertow typically has a large seaward directed velocity near the bottom and either a shoreward directed or a small seaward directed velocity at trough level. Measurements show that outside the surf zone the undertow has a small seaward directed velocity near the bed and a large seaward directed velocity at trough level. In this paper we develop theoretical expressions for the undertow outside the surf zone and show that the steady streaming from the bottom boundary layer which was earlier found to have a negligible effect in the surf zone (Svendsen et al., 1987) does have a significant influence on the vertical structure of the undertow in that region.