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Feet.

Vith

1 and 3 of more than 8 inches. To this incline on the surface of the water, occasioned by retardation, are due the currents during both flood and ebb, which at times attain the velocity of five miles an hour. When it is remembered that in a clear way a current of six knots an hour is navigable only with great difficulty, one appreciates that to pursue a sinuous course which runs athwart the line of a five-mile current, at varying angles, and which is thickly studded with obstacles, is attended with great peril.

In considering the loss of scour which the alteration of the original tidal condition of the Seekonk river occasions it should be borne in mind that the regime of a channel is a function, not of the mean tidal flow, but of the flow of the highest spring tides at the time of maximum scouring effect. The figures given in this matter of scour, therefore, which are obtained from the tide, the observations of which are platted on the appended diagrams, and which was an ordinary spring tide, rather under-state than exaggerate the extent of this loss.

By reference to the accompanying diagram representing the spring tide of September 27, it will be noticed that the tide at Station No. 1 had sensibly slackened in its rate of fall at 1:20 P. M. After that time it may be assumed that the volume of water remaining in the Seekonk basin would have very little value in scouring effect upon the channels below. The high tide level at Stations 1 and 3 was about the same and the whole fall of the tide at Station 1 was 6.84 feet, but at 1:20, when the scouring value had ceased, the height of the water at Station 3 was 0.70 foot above that of Station 1, and it may be said, therefore, that one-tenth of the volume which might have escaped from the Seekonk had failed to do so at a time to be of value in scouring the channels below.

During the hour of greatest scouring effect-soon after half-tidethe flood at No. 3 flows 63,720,000 cubic feet. Taking the tide at No. 1 to indicate the normal condition, it should flow 70,920,000 cubic feet. This shows a loss in the scouring effect of the flood tide of ten per cent. In this tide the rate of flood flow is more rapid than As a rule in our harbor the ebb flow is most rapid.

that of the ebb.

During the hour of greatest scouring effect of the ebb tide the discharge at No. 3 was 58,520,000 cubic feet. The tide at No. 1 indicates that it should have been 67,320,000 cubic feet, showing a loss in scouring effect of 16 per cent. The city of Providence has expended upward of $99,600 in dredging the harbor since 1870. What percentage of this amount might have been saved had not the scour of the current been thus greatly impaired, it would be interesting but fruitless to inquire; but in considering-now that a new condition of channel is in process of establishment by dredging in the main harbor-how far the restoration of the lost sixteen per cent. might avail to maintain it, there may be much profit. We do not pursue this question at the present time for the reason that much of the information we have obtained bearing upon this and other questions in regard to the effect of the obstructions has not been so fully studied as is desirable before stating our conclusions. Many useful observations in regard to the action of the tide in the river between Washington bridge and Pawtucket have been made by Mr. H. A. Bentley, assistant to Gen. G. K. Warren, U. S. Engineers, which we expect to avail of.

BRIDGES IN SEEKONK RIVER.

An examination of the sketch appended to this report and entitled, "Obstructions at the Mouth of Seekonk River," reveals the substantial cause of the change in the "natural and orderly flow of the tides" which the observations discussed above indicate. This sketch shows in plan the two bridges at India Point, and in profile exhibits the forms of the foundations on which they are reared. Complaint as to the location of the draws of these bridges is frequent; so they appear objectionable in respect to both foundation and superstructure, and in these two respects are to be considered.

First, as to foundations: The considerable formation on which the Washington bridge is imposed is the product of the intermittent labor of eighty years past. The first Washington bridge was built in 1793. It had six piers, entirely of piles, and a draw on the east bank where the present draw is. In 1807 a storm carried it away. The piers were repaired, and a new superstructure supplied. This was carried away in

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