241 



coasts of England and France, bores occur in a number of the shallower 

 estuaries of these countries. The most noteworthy tidal bore re- 

 ported is that in the Tsien-Tang-Kiang River, which enters Hangchau 

 Bay, some distance south of Shanghai, in China. The tidal range in 

 the mouth of the river, at ordinary spring tides, is given as 25 feet, 

 sometimes reaching as much as 34 feet. The bed of the river runs 

 nearly bare at low tide. The incoming tide advances in a breaking 

 wave which is described as from 8 to 12 feet in height, rushing with a 

 loud noise up the estuary at a rate of 14 miles per hour (Wheeler, 

 Practical Manual of Tides and Waves, pp. 142-144). 



EFFECT OF ARTIFICIAL CHANGES IN AN ESTUARY 



464. Comprehensive enlargement. — A comprehensive enlargement of 

 the channel in a long tidal estuary, to afford greater depth and width 

 for navigation, generally increases the tidal range in the upstream 

 reaches and increases the rate at which high and low water travel up 

 the channel. The increase in range depresses the plane of mean low 

 water, and other low water datums. Additional excavation is there- 

 fore required to afford the projected increase in depth at a designated 

 low water datum. Thus, after the navigation channel in the upper 

 part of the estuary of Hudson River was deepened from 14 feet to 

 27 feet at mean low water, the mean low water datum at the head 

 of the improvement was lowered by a foot. The increase in the depth 

 of the navigation channel between Philadelphia and Trenton, from 12 

 feet to 25 feet, also depressed the low water datum at Trenton by a 

 foot. 



465. Contractions. — A radical local contraction of an estuary by 

 training works, piers, or land reclamation, decreases the tidal range 

 upstream. The consequent reduction in the volume of the tidal prism 

 decreases the currents below the contraction and tends to increase 

 the tidal range at and below it. The removal of a marked local 

 contraction at midlength of a long estuary similarly increases the 

 storage and release of water upstream, increases the currents below 

 the contraction and may decrease the tidal range at and below the 

 site. A decrease of about half a foot in the mean tidal range on the 

 Delaware at and below Philadelphia, shown by a comparison of the 

 tide gage records prior to 1890 with those after 1900, usually is 

 ascribed to the contractions at the extensive training works which 

 were constructed in the lower part of the estuary during the interval. 

 The decrease in range may have been due partly, as well, to the major 

 enlargement of the river at Philadelphia during this period. This 

 enlargement included the removal of several islands which had so 

 contracted the cross section as to create excessive currents. The 

 enlargement was followed by a considerable increase in the tidal range 

 at the head of the estuary. 



