284 mason. SHORE-CONTROL PROBLEMS [Ch. 15 



land, including specifically dune areas and other marginal-land fea- 

 tures; (4) the rate of supply of material contributed by drainage 

 features. Unfortunately it is not possible, in the present state of the 

 science, to evaluate these factors in other than a qualitative fashion. 



Comparative study can indicate the probable relative importance of 

 each of the factors, but only rarely are quantitative data available. 

 In many areas only cursory examination is required to establish the 

 littoral drift as the most important item, for example, along the New 

 Jersey coast; whereas other areas are very complicated, for example, 

 the Monterey Bay, California, coast where a submarine canyon and 

 large supplies of material brought to the coast by streams greatly in- 

 fluence the coastal regimen and probably completely dominate the 

 littoral phenomena. 



Littoral drift is now evaluated by the amount of material trapped 

 by shore structures, either natural or man-made, and from knowledge 

 of the rate of depletion of sources of supply, for example, eroding 

 cliffs or headlands. 



Estimation of the material volume derived from or lost to the ad- 

 jacent sea bottom is unsatisfactory at present. Experiments are in 

 progress at reduced scale in the laboratory which give promise of con- 

 firming some conclusions reached from theoretical considerations re- 

 garding material transportation on the sea bottom. However, there is 

 little, if any, evidence of a conclusive nature with respect to this 

 phenomenon in nature. 



The quantity of material derived from or lost to the land is now 

 evaluated with fair accuracy by comparative study of the topography 

 of the area at various times, coupled with knowledge of the character of 

 the land and shore material. 



The determination of the volume of material supplied to a shore area 

 by streams, gully washes, or other drainage features is facilitated by 

 knowledge of the suspended and bed load carried by the flow. Some- 

 times this information can be gained from study of the stream mouth 

 delta, more easily from records of the rate of silting, or filling, of the 

 stream upstream from the mouth. The wide variation of material load 

 carried, between flood and normal stages, must be kept in mind. 



Wave action is the primary source of energy at the shore. Its im- 

 portance is overriding everywhere on the open coast and is not seriously 

 diminished even in the immediate vicinity of large tidal entrances. 

 The waves of concern are the characteristic progressive oscillatory 

 surface waves so familiar to all, the long-period swell not usually so 



