63. Waves and currents provide the forces which move the material. 

 The mechanics of tran.sport are not precisely known^ but it may be generally 

 stated that there are three basic modes of transport^ that which occ^ars on 

 the foreshore in a more or less scalloped path due to uprush and backv/ash 

 of obliquely approaching waves known as "beach drift"; that which is moved 

 principally in suspension in the surf zone by littoral currents and the 

 turbulence of breaking waves| and that which is moved close to the bed in 

 depths beyond the surf zone by the oscillating ciirrents of passing X;raves. 

 Significant bottom movement has been observed in depths exceeding 100 

 feet on exposed sea coasts. Figure 35 illustrates the three basic modes 

 of transport^, and Figure 36 depicts trends of proportionate bed load 

 transport related to wave energy and steepness from laboratory experiments. 



6i4.. It has been observed that changes in the shore profile occur 

 with changes in water depth and ^^rith changes in wave characteristics. 

 Profile adjustment due to change in water depth is relatively slow, and 

 therefore minor with respect to single tidal cycles. Measurable change 

 has been detected when comparisons are made with water level expressed as 

 daily mean sea level. The profile adjustment due to change in wave 

 characteristics is rapid^ and a single storm of a few hours duration may 

 cause a major change in profile. Profile changes may be ascribed primarily 

 to the onshore or offshore shifting of beach and bottom material. 

 Generally^ the shift is offshore as the water level rises or the waves 

 steepen, and onshore as the water level lowers or the waves are flatter. 

 Laboratory studies indicate that the critical X'vrave steepness defining 

 the boundary between onshore and offshore movement is in tlie order of 

 Hq/Lq « 0.025. This has not yet been confirmed in nature. 



65. Onshore and offshore shifting of material results in longshore 

 movement at a sluggish rate and may be classed as a mode of transport, 

 the relative importance of which is unknown at this time. Suspended 

 material moving with the littoral current in the surf zone, beach drift 

 and wave current bed load transport seaward of the surf zone all represent 

 more rapid transport, and are thus presently considered as primary factors 

 in resultant littoral drift. 



66. Regardless of the mode of transport^ the direction and rate of 

 littoral drift depend primarily upon "the direction and energy of waves 

 approaching the shore. Exceptions exist on short reaches of shore 

 adjoining tidal inlets x^rhere the tidal current pattern may be dominant. 



67. Slope Sorting of Littoral Ma terial. ~ Wherever sandy beaches 

 exist or where the surf zone and nearshore bottom is composed of sand, 

 the grain size of material along a shore profile decreases generally as 

 the water depth increases until depths are reached where wave currents 

 are incapable of moving bed material. The coarsest material is usually 

 found in the surf zone in the vicinity of the plunge point of waves, 

 though in protracted periods of mild wave action the foreshore and surf zone 

 m.aterial may be nearly equal in size. Foreshore and nearshore slopes are 

 related to the grain size of material of which they are formed, but the 



