Various processes are described by which a drop of water, impacting on a water surface, may entrain a bubble. There are: (i) irregular entrainment, in which the complex details of a splash entrain bubbles; (ii) regular entrainment, in which the crater formed by the drop impact develops in a predictable way to form a single bubble; (iii) entrainment of large bubbles, in which most of the volume of the crater is trapped as a bubble; and (iv) Mesler entrainment, in which many very small bubbles are trapped in the very early stages of the impact process; possibly between wave crests of capillary waves which develop on the drop and on the water surface. This last process is different from the preceding three in that it produces very little sound. All of the processes except the third one have been described individually in the literature; here we present them together for comparison. The regular entrainment process is discussed in the greatest detail and some new experimental results are presented which concern the size and nature of the entrained bubbles. The significance of the regular and irregular entrainment processes to the generation of underwater sound by rain is discussed.

A simple parameterized model for wave-induced burial of mine-like cylinders as a function of grain-size, time-varying, wave orbital velocity and mine diameter was implemented and assessed against results from inert instrumented mines placed off the Indian Rocks Beach (IRB, FL), and off the Martha's vineyard coastal observatory (MVCO, Edgartown, MA). The steady flow scour parameters provided by Whitehouse (1998) for self-settling cylinders worked well for predicting burial by depth below the ambient seabed for (0.5 m) diameter mines in fine sand at both sites. By including or excluding scour pit infilling, a range of percent burial by surface area was predicted that was also consistent with observations. Rapid scour pit infilling was often seen at MVCO but never at IRB, suggesting that the environmental presence of fine sediment plays a key role in promoting infilling. Overprediction of mine scour in coarse sand was corrected by assuming a mine within a field of large ripples buries only until it generates no more turbulence than that produced by surrounding bedforms. The feasibility of using a regional wave model to predict mine burial in both hindcast and real-time forecast mode was tested using the National Oceanic and Atmospheric Administration (NOAA, Washington, DC) WaveWatch 3 (WW3) model. Hindcast waves were adequate for useful operational forcing of mine burial predictions, but five-day wave forecasts introduced large errors. This investigation was part of a larger effort to develop simple yet reliable predictions of mine burial suitable for addressing the operational needs of the U.S. Navy.