Mach Stem in the Near Shore Zone
While the region past the breakers has much smaller waves, they still retain considerable energy. The waves propagate in various directions and at times interact in a complex, nonlinear manner. In this picture two counter-propagating wave trains have a small angle between their wavenumber vector and a very clear Mach stem pattern results.
Marek Stastna (distributed via imaggeo.egu.eu)