The generation of longshore currents on a beach is investigated theoretically, using the concept of radiation stress to describe the flux of momentum associated with the incoming waves. Outside the surf zone, the longshore gradient of the radiation stress is always zero, and the flow is therefore driven only inside the surf zone....
Bowen, Anthony J.. 1969. "The generation of longshore currents on a plane beach." Journal of Marine Research 27, (2). https://elischolar.library.yale.edu/journal_of_marine_research/1153