294 caldwell. SEDIMENTATION IN HARBORS [Ch. 16 



area, where the increased stream cross section results in much less 

 velocity with an accompanying deposit of shoal material in the harbor. 

 In some areas the conditions are such that the tidal flow in the har- 

 bor area, supplemented by normal river flow, would in time flush out 

 the excess material moved in by the river floods; however, this is of 

 little real benefit as the excess shoals in an active harbor cannot be 

 tolerated even for short periods. As a result, frequent and often ex- 

 pensive dredging has to be resorted to in order to maintain usable 

 depths. 



Solutions to problems at fall-line harbors have been: 



(1) Dredging as necessary to remove accumulated shoal. 



(2) Use of training walls and dikes in an attempt to divert mass of 

 shoal material away from the harbor proper. This, of course, can 

 result in the shoal forming in the ship channel connecting the harbor 

 with the sea. 



(3) Creation of off -channel harbors. Such construction is expen- 

 sive and, of course, does not eliminate the deposit of the shoal material 

 in the ship-channel proper; it generally results in the same problem 

 described above for off-channel river harbors. 



It would appear that the most useful information leading to solutions 

 of this type of problem would be that dealing with a study of com- 

 petence of flow to move sand and gravel. This information might con- 

 ceivably lead to the design of training works or channel alterations 

 that would spread the shoal over a greater distance, with accompany- 

 ing greater navigation depths, thereby giving the tidal flow plus normal 

 fresh-water flow a chance to flush the shoal material down the river 

 without excessive shoaling. 



CHANNEL HARBORS IN TIDAL ESTUARIES 



Most of our larger harbors are located on tidal estuaries, the term 

 estuaries being taken in its broadest sense to include tidal rivers, bays, 

 and lagoons. In most harbors in tidal estuaries, a sizable tidal prism 

 moves through the harbor as a result of the flood and ebb of the tide 

 filling the tidewater area farther up the estuary. Examples of such 

 harbors are numerous, including New York, New York; Philadelphia, 

 Pennsylvania; Savannah, Georgia; Jacksonville, Florida; Galveston, 

 Texas; Mare Island Straits, California; Canton (on the Yangtze), 

 China; St. Nazaire (on the Loire), France; Antwerp (on the Scheld) , 

 Belgium; and London (on the Thames), England. 



