Exchange of momentum in the surf zone due to organized water motion

The prediction of longshore current velocity distributions often relies on a momentum exchange coefficient E to describe the shore normal momentum exchange due to the shear in the velocity. This note treats an additional contribution to the momentum exchange due to the organized circulation in the surf zone caused by the water carried shore-ward with the surface rollers of spilling breakers and broken waves. The contribution from the surface rollers to the momentum exchange in the surf zone is found to be important. It is estimated to be larger than the exchange due to the turbulence for a considerable range of parameters.