A Theoretical and Experimental Study of Undertow

It is well known that on a three-dimensional beach large volumes of water carried shorewards by the breakers feed longshore currents, which eventually escape back through the breaker line, often as rip currents. In a steady two-dimensional situation, however, the mass flux represented by (among other things) the surface roller in the breakers returns as a seaward current close to the bottom. This current is called the undertow. In this paper theoretical results for the undertow are compared with the results of recent laboratory experiments.