Oceanographers classify wave breaking into four main types; plunging, spilling, surging and collapsing. As surfers, we are interested only in the plunging and spilling breakers. The rule of thumb is that waves break when the depth is just less than the height of the wave (wave height = 0.78 * water depth). If the reef or sand bank gets shallow ver quickly, then the waves will tube (plunging breaker). On a gently sloping beach, the waves will spill (eg. Saunton Sands, Devon).