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Rock Channel would deprive the Main Ship Channel of no advantage in point of currents, and would perhaps check the present injurious action of waves; it might cause such a change in the course of the currents as to introduce a scouring power into the Main Ship Channel; it would at least, render the navigation less troublesome, by removing cross drifts which now frequently sweep vessels from their course and cause them to go on shore.
That part of the flood.stream, which runs along the north side of Brewster Bar, sets directly over Tower Rock, and thence it sweeps over among the rocks to eastward of George's Island, where it exhausts itself. A float, two feet in diameter, drawing twenty-two feet of water, which was suffered to drift through Black Rock Channel on the strength of the flood current, grounded and held fast upon Tower Rock -an object which is so small in circumference, that even with good ranges, one may find it only with great difficulty. That part of the Ship Channel in the immediate neighborhood of the Narrows Light is the scene of continual whirls and eddies, which, although not violent, are bewildering to the navigator. To vessels of great draught the navigation is rendered doubly critical from the close proximity of these whirls to Corwin and Tower Rocks. These whirls, to which we have given the closest study, are, in the main, due to the influence of the streams through the Black Rock Channel; their characteristic forms and their epochs will be made the subject of a special note in a future report.
The chief characteristics of the estuary of the Charles River are sucessions of blind channels with shallow bulkheads. The causes of these sudden alternations of channels and banks are exceedingly simple. When the channel of a river is confined, and is of uniform width, the depth will be more or less the same from point to point, and this depth will be but little greater than that required for the conveyance of the inland supply - the equal tidal forces effecting little or no scour — but where a river terminates in an estuary, it frequently happens that the ebb and flood currents, entering at different angles, choose different paths, digging distinct trenches for considerable distances until a point is reached where the two paths concur, at which point they counteract each other's efforts, and a bulkhead is
left. In the case of Charles River the flood and
ebb have, at some points different paths, but the former conflicting with the river stream, leaves little or no trace of its power. The ebb on the other hand, when traversing a region where its path is distinct, effects in conjunction with the river waters,
a considerable scour.
Our observations reveal these
simple operations in a manner remarkably distinct and conclusive. As an illustration, we give upon Diagram H the curves of velocities across the channel of the Charles River, off the foot of Mt. Vernon Street. It may be seen from this, that the volumes of ebb and flood do not differ widely, but the position of the axis of the flood is more central than
that of the ebb.
These curves illustrate the influ
ence of the momenta in directing the course of tidal streams. There is no reason why the axes of the flood and ebb at this station should differ in position, except that they each preserve for a time the velocity and direction acquired in an earlier stage of the journey. Why, it might be asked, after an inspection of our diagram, are there not in all cases two channels, one lying along the axis of the ebb, the other following that of the flood? The answer is simple ; although the flood at a certain place may at maximum exceed in velocity the highest rate of the ebb, the duration of its flow is less, and consequently the resultant may be zero, and no permanant scour be effected.
effected. The shallowness of the muddy bulkheads indicates a lack of inland scour, that is, a scanty supply of river water.
In the Charles River there occur indigenous oyster beds here and there - usually near mid channel, , and usually in ridges at right angles to the direction of the stream.
The bridges over the Charles River are not arranged with suitable reference to the natural flow of the currents, and it frequently happens that the momenta of the streams are destroyed by encounters with the piers and piles of the bridges and adjacent wharves. There is reason to believe that the river current, in the absence of the bridges, would distribute its deposit gradually, as its velocity slackened in its progress towards the sea, instead of throwing it down in abrupt piles, as now. The ebb and flood tidal currents, approaching from nearly opposite directions, are differently deflected by the artificial structures, so as to complicate their relative paths and make it impossible to lay down a uniform rule for future structures in different portions of the estuary, as each case of a future structure must be a special one, and have relation to those already established in the neighborhood.
There are some localities where the building out of the river banks would improve the course of the stream, and there are other points where permanant stone walls now interfere with its course.
The current at any point of the channel is moving not only with an action due to the local iufluence of gravity, but also with the power accumulated at points higher up the stream. Of these two elements the momenta is the conservative power, that which equalizes the depths of the water-ways and gives uniformity to the channel. It is this particular element of power which may be destroyed by bridges and open piling across the channel, which, although they suffer all the water to find its way through, do nevertheless produce much mischief. Among the railway bridges over the Charles River the utmost confusion is observable in the course of the tidal drift — its unity of movement is destroyed, it is local in character. To vessels towing through these bridges, the abrupt changes in the directions and velocities of the currents between the draws is a source of delay and vexation.
The tables and the diagrams appended to this report will be found to contain much that is suggestive and useful in unraveling the difficult questions concerning the tendencies of the harbor, but we have refrained from presenting some of our views because additional observations are requisite to furnish some of the intermediate steps in the arguments. We are convinced, from much experience in these studies, that all arguments and conclusions should rest upon numerical data obtained from careful observations and interpreted according to well founded doctrines of mechanics.
Besides the special localities, already referred to, we believe we have determined the part taken by