296 caldwell. SEDIMENTATION IN HARBORS [Ch. 16 



(3) Improvements in tidal theory to enable more reliable computa- 

 tions of effect of changes in tidal regime on resulting tidal prism. 



(4) A better definition of the laws of intermixing of sea water and 

 fresh water and of the action of density currents. 



OFF-CHANNEL HARBORS IN TIDAL ESTUARIES 



In an attempt to avoid the high rates of shoaling due to excessive 

 movement of silt-laden tidal waters through the harbor area, many 

 off-channel harbors have been created in tidal estuaries. Examples of 

 such off-channel harbors are: Washington Channel Harbor, Washing- 

 ton, D. C; Texas City, Texas; Houston, Texas; London, England (in 

 part) . 



Generally the off-channel harbor is subject to shoaling from sus- 

 pended silt and clay brought into the harbor by the interchange of 

 tidal waters between the harbor and the estuary. Moreover, the prob- 

 lem of the shoaling of the channel connecting the off-shore channel 

 with the estuary remains a problem. 



Improvements to such off-channel harbors have been attempted along 

 the same lines as for off-channel river harbors, principally: 



(1) Dredging to remove excess shoaling. 



(2) Use of training works to protect entrance channel (Texas City, 

 Texas) . 



(3) Locks to reduce interchange of water between harbor and estu- 

 ary (London, England, in part). It is to be noted that such locks are 

 at times designed primarily to maintain a constant water level in the 

 harbor where the tidal range is excessive. 



It would appear that investigations of the general types described 

 above for channel harbors in estuaries would possibly be the most use- 

 ful in this type of problem also. An evaluation of the factors govern- 

 ing the interchange of water between the harbor area and the estuary 

 would also be very helpful. 



SHORE-LINE HARBORS 



Shore-line harbors are intended to include those harbors fronting 

 directly on the open shore of the oceans, bays, or large lakes and having 

 relatively small tidal flow or fresh-water flow from the tributary water- 

 shed. These harbors are generally protected from wave action by some 

 natural feature or man-made structures. Notable examples of such 



