242 



466. Dams. — The construction of a dam across an estuary, to 

 maintain the upstream reaches at low tide level in the interest of the 

 reclamation of tidelands, or at high tide level in the interest of naviga- 

 tion, recreation, and sightliness, obliterates the tidal storage up- 

 stream. The tidal currents in the downstream reaches are diminished, 

 and disappear at the dam. The tidal range at the dam is increased 

 by an amount dependent upon the length and depth of the remaining 

 part of the estuary. The accumulation of silt in the channel below 

 the dam ordinarily is to be expected. 



467. Character of computations of the efect of enlargements or con- 

 tractions. — In the preceding chapter a method was developed for 

 computing the tides and currents in an artificial channel of such 

 regular dimensions that the Chezy coefficient in the successive sub- 

 sections could be selected with sufficient assurance from precedent. 

 A somewhat different problem arises in estimating the changes in 

 the tidal ranges and currents that may be expected from projected 

 enlargements or contractions which merely will modify, without 

 essentially changing, the characteristics of the flow in a long tidal 

 channel. The latter problem is somewhat analogous to an estimation 

 of the changes in the slopes and currents of an upland river because 

 of similar enlargements or contractions. In both cases the imme- 

 diate effect upon the currents at the locality is easily determined, but 

 a reliable computation of the consequent effect in other parts of the 

 channel can be secured only from an elaborate and painstaking 

 analysis of the existing flow in the successive subsections, based on 

 adequate survey and records. 



468. Fortunately, a computation of the changes in the tidal ranges, 

 tidal datums, and currents because of projected enlargements or con- 

 tractions of a tidal channel is called for but rarely. If a closed chan- 

 nel is relatively short, the datum throughout it can be taken as the 

 established datum at the entrance, whatever the scope of the proposed 

 improvement; for while, as shown in paragraph 427, the tidal range 

 at the head of such a channel may be greater than at the entrance, the 

 increase in range and the lowering of the low water datum at the head 

 of the channel generally is too small to be of real consequence. 



Projected local enlargements or contractions of a long tidal estuary 

 rarely are so extensive that any material change in the low water 

 datums need be apprehended. Because of the daily variation in low 

 water, a change of even a foot in the low water datum, resulting from 

 major channel enlargements, is not immediately apparent. An early 

 redredging of the channel often is required in any event to remove 

 material which has slid in from underwater slopes or has been deposited 

 from other sources. If a projected improvement has been cut so close 

 that a shortage of a foot or so in the depth at low water is of any real 

 consequence to shipping, its further enlargement is to be foreseen. 



