BREAKER ZONE 135 



other extreme, broken water may extend out 2 to 3 miles, or even 

 farther, if high waves are breaking in 3 to 4 fathoms or deeper on a 

 gentle slope with a strong onshore wind, i. e., so far out that an ob- 

 server standing on the beach near water level cannot see out past the 

 limits of the zone of breakers (fig. 35) . 



The effects of offshore bars are very important in this connection, 

 because these are developed by the actions of waves and tides, off many 

 river mouths, off the mouths of shoal harbors, and even off straight 

 beaches. It depends partly on the depth of the water over the bar 

 whether surf will break over it at any given time, which in turn de- 

 pends largely on the stage of the tide, but it also depends on the sizes 

 and shapes of the waves; likewise, on tidal currents, because breakers 

 develop in considerably deeper water if the tide is against the waves, 

 than if the reverse is true, as explained on p. 55. And tides often run 

 strong across bars and around them, especially off the mouths of large 

 rivers, where the velocity of the ebb out across a bar is often so great 

 as to cause a dangerous rip, when waves of any size are heaving against 

 it from seaward. 



Bars fall into two groups, as they bear on the surf problem: (a) 

 those that lie off beaches, and (b) those that front the mouths of rivers 

 or harbors. In the first instance the waves may break out along the 

 bar, with little or no surf on the beach behind it (fig. 36, above) . And 

 this often happens if the weather is calm, and the sea smooth. Or 

 they may break heavily on the beach, if they carry on across the bar 

 without breaking upon it, or if the development of breakers on the 

 bar does not destroy the wave forms there (fig. 36, below). 



The presence of a bar across a harbor mouth may not hinder access 

 to the latter so long as the waves are not breaking upon it. If they 

 are, however, such a bar may prove an impassable barrier, and many 

 "bar" harbors and river mouths are made inaccessible to shipping by 

 stormy weather for considerable periods of time. 



A heavy surf is also common well offshore, over submerged ledges 

 and reefs off rocky coasts. And the waves may be expected to break 

 in considerably deeper water there than on the beach, because of the 

 abrupt alteration in the angle of slope of the bottom that characterizes 

 such situations. Old swells, not over iy 2 to < 2 1 / 2 feet high offshore, that 

 we recently observed breaking in a depth of 5 to 6 feet, over a 

 ledge in the southern side of Massachusetts Bay, may serve as an 

 illustration. 



The behavior of waves, as they advance across bars or ledges, has 

 not received as much attention as it deserves, perhaps because close 

 observation on the spot is difficult, or even risky. In any event, great 

 caution is always advisable, when coming in across a bar, or especially 



