Most of the -work with tides in this country is done by the Uo S. 

 Coast and Geodetic Survey o They publish yearly a set of predicted tide 

 tables that cover the coast of the TOjrld that are of conmiercial signifi- 

 canceo The Survey also has the job of defining the various datum planes 

 which serve as the working datums throughout the country as well as along 

 our seacoastSo Mean sea level at Sandy Hook is the standard datum for 

 this country whether in Boston, on the top of Pikes Peak, or in Death 

 Valleyo A full hour's lecture and more could be devoted to the subject 

 of the importance of datum planes and the proper respect of them when 

 making beach studies » Over one square mile of beach, an unrectified 

 error of Od of a foot in datura will produce a cubage error of 100,000 

 cubic jardso Yardages of this magnitude are of primary importance in 

 beach studies and errors of this magnitude can produce seriously mislead- 

 ing interpretations of basic data. 



The other feature of tidal action, which we will touch on briefly, 

 is tidal currents These currents are generally noticeable along the 

 coasts only in the vicinity of bays, estuaries, or inlets <, The rise 

 and fall of the tide produces a flood and ebb of the current filling and 

 emptying the estuary. This current can reach sufficient magnitude in 

 the vicinity of the estuary to cause erosion ty its own action o However, 

 the effect of the current is felt over a much larger beach area where 

 the currents are too small to affect the movement of the beach sand by 

 themselves but can contribute to a significant action hy working in con- 

 junction with the waves breaking on the beach » 



The mouth of an inl^t or estuary is usually a meeting point of 

 beach interest and navigation interest o Attempts to improve one feature 

 frequently work to the detriment of the other o Since the Corps of 

 Engineers has a major responsibility in both cases, it is easy to see 

 that we are frequently found in the somewhat uneviable position of 

 serving two masters o 



Of course some streams and rivers have sufficient fresh-water 

 discharge so that this fresh-water discharge dominates the picture 

 rather than the tidal flow„ However, the hydraulics of the two can be 

 studied on somewhat the same basis o 



Beach material in the shore environment is subjected to the action 

 of various forces^ a dominant one being the wind-generated ocean waveso 

 The heights of waves generated over the open ocean by the wind is con- 

 trolled by three factors?- the wind velocity j, the duration of the wind, 

 and the fetch o The "fetch" is the length of the stretch of open water 

 actually in contact with a specific wind developmento 



