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original surveys of Providence Harbor, made by the U. S. Engineers, and by the C. S. Coast Survey, to give our personal knowledge of the locality such precision as would enable us to make the practical suggestions which seem called for; but it is proper to state that questions of so general a character as those you propound—while admitting, designedly, no doubt, of great latitude in reply, make our task somewhat difficult. Nevertheless, we recognize in your action an indication of your very earnest sense of the importance of the work before you ; and we offer our suggestions as an introduction to this work, in which we propose to assist you hereafter to develop the details.
DESCRIPTION OF PROVIDENCE HARBOR.
From a physical point of view, the harbor of Providence is an interior basin of Narragansett Bay, and comprises the water space between Field's Point and the dam at Pawtucket, with the addition of Providence River, as far up as the “Cove”-a total water space of 1700 acres, in round numbers. For convenience of reference we shall divide this space into three compartments, viz: the Seekonk Basin, having an area of about 900 acres, between the before mentioned dam and the draw-bridges at India Point; the Providence River, from the head of tide water to the Point Street Draw Bridge, about 50 acres, and the harbor proper, from the bridges to Field's Point, about 700 acres, omitting lagoons.
The harbor proper is a thin sheet of water, covering a vast accumulation of silt or mud, through which winds a narrow channel, that the chart shows to be a continuation of that which issues from the Seckonk Basin.
This channel enlarges as we follow it down towards its outlet upon the bay, and the mind almost instinctively assumes that it has been hollowed out, or rather maintained by a stream, whose volume augments from point to point; and in the absence of tributary land waters, we naturally look to the tides for an explanation — recognying as axiomatic, the general rule that the section of a tidal channel through an alluvial bottom, should vary in area with the service it is called upon to perform as an aqueduct. This service, were the tides at different points synchronous and of uniform effective range, would vary almost directly as the superficial areas covered. For the case in question, the requisite conditions are so nearly fulfilled that comparisons of superficial areas and vertical cross-sections ought to be conclusive.
Selecting the two terminal sections, as near as proper regard to the definition of the channel will permit, we have the space above Bald Point, 950 acres, terminating in a vertical cross-section of 13,000 square feet, and the area of the space above Field's Point, 1,700 acres (including lagoons), terminating in a vertical cross-section of nearly 24,000 square feet; in the first case the ratio of our figures is 1.13, in the second case, 1.14. Intermediate sections are difficult of comparison, because the channel way is ill defined; but such comparisons as can be made offer the same general testimony, and we express without doubt the conclusion that: The main channel of Providence harbor is maintained by the tides; and as a corrollary, assert that: An increase or diminution of the tidal area of the basing would sooner or later be followed by a corresponding increase or decrease of the channel telono.
A tide of four and a half feet range visits Providence harbor, and the effective volumes--those for the hours of greatest filling and draining-are at the entrance section (Field's Point), eighty-nine millions of cubic feet on the flood, and ninety. six millions on the ebb; the former excluding or “backing up” three and a half millions of land water, and the latter including or bearing off the river supply.
Leaving the strictly physical point of view, we observe from the surveys that the harbor proper, although it presents a superficial area of 750 acres, affords for anchorage and maneuvering room for ordinary vessels only 138 acres at low tide.
By ordinary vessels, in this statement, we refer to those drawing only 12 feet when laden. The same area, 138 acres, is afforded for the manæuvering of good sized vessels (those drawing 16 to 17 feet,) at ordinary high water; but such vessels can find only 67 acres for anchorage, without taking the ground, at low tide.
Practically, the depth of a channel is that of its shallowest pass, and applying this rule, the depth of the main channel of Providence harbor is 14 feet at low water and 184 feet at high. While there is no generally recognized definition of a ship channel, no one would think of placing the requisite depth of a channel so designated at the figures just given. We are, therefore, constrained to state that:
The harbor of Proridence has no ship channel. There is no harbor, perhaps, in the country where dredging can be done at less cost; and since there is little silt naturally brought down by the inconsiderable fresh water streams, it is probable that the channel, once deepened effectually, could be maintained at a very small annual expenditure in the average.
Having indicated in the foregoing the premises upon which our answers to you should be based, we shall proceed to consider your questions in detail:
1. We recommend the enlargement of the main channel of the harbor, and an improvement of the passage ways through the bridges at the outlet of the Seckonk Basin.
2. In a port like Providence, employed for miscellaneous commerce, the greatest good to the greatest number is to be secured by a channel of eighteen feet depth at low tide.
In order, however, not to exclude vessels of greater draught, we recommend that the channel should have a central pathway of twenty-three feet depth, oue hundred and fifty feet wide.
The form of channel that would meet these requirements and have the most permanent slopes, would have the following widths naturally between its contours, in light alluvium:
23 feet depth.
.150 feet wide. ..600
18 12 Grade
725 .940 .1060
We recommend that this channel should be carried from the city to Field's Point, following generally the present channel below Fox Point, and using the above table as a guide, as far as possible, for computing the amount of material to be removed, (and not as a convenient plan of dredging,) we find from the original hydrographic sheet of the Coast Survey ("1326 A.”) executed in 1874, that the necessary dredging would then have been two and a quarter millions of cubic yards, bank measurement.
The average length of vessels likely to seek this barbor if improved, may be set down at 150 feet, and these, with average draught of eighteen feet, would require fifty-four feet of cable, giving for radius of swing room 202 feet. These vessels, anchored most carefully on both sides of the channel proposed, would leave 321 feet gangway for the passage of other vessels. If anchored with only the ordinary care (but still under supervision of a harbor master,) the width of the gangway would sometimes not exceed 150 feet. It is assumed that so few vessels of extraordinary draught would be in port at the same time, that the harbor master could so arrange them as to avoid having any two of them in the same section. It is proper to regard the vessels as lying transversely to the direction of the channel, because during the season when the largest fleet visits this port, winds from the W. S. W. and S. W. prevail.
In providing for anchorage room on either side of the gangway, rather than excavating a special basin for anchorage, it has been borne in mind that the whole channel, in the absence of vessels at anchor, will be useful for the passage of vessels under sail, and that this channel, traversed by tidal currents, will be less likely to freeze up than a sheltered basin, or more easily broken out when frozen.
3. We have no data from which to conclude that there has been any change in the range of the tide in the Seekonk river, and while the violence of the currents under the India Point bridges shows that the propagation of the tide has been, in a measure, interrupted, we are unable to state that this interruption lessens the flow through the channels, above and below, at time of maximum velocity (soon after half tide), when the effective scour occurs. This question is worthy of study, and we recommend comparative gaugings.
As things are now, an encroachment upon any of the tidal reservoirs will ultimately reduce the channels below by lessening the flow through them. Should such channels, however, be artificially increased in section, their rate of decline will be nearly independent of such encroachment until the present conditions are restored.
Since it would be eminently just and proper, under the present circumstances, to require of all who encroach upon tide water to provide by prepayment for the injury to the channels below, it is recommended that provision for maintaining the depth in any channel, hereafter dredged, should be made upon the same basis.
4. We recommend that such changes be made in the piers and bays of the bridges as the study indicated in No. 3 may show to be necessary to restore, so far as may be, the natural and orderly flow of the tides, not only because more useful scouring action may be induced, but because the present violent flux and reflux make the passage of vessels inconvenient, and sometimes injurious.
e are satisfied that the piers, by their angular resistance, and as abrupt obstacles, as well as inadequate water spaces, retard the tide unduly.
6. Not only in the neighborhood of the bridges, but throughout those portions of the channels where artificial structures, or unyielding banks, appear on one or both sides, the streams should be gauged to ascertain where these streams are out of register with the avenues they traverse. Slacks and eddies along the borders of such streams will indicate where such studies should be most carefully made, in view of occupation of territory for commercial purposes. Studies of this kind yield results which suggest their own application, and leave less to the uncertain judgments of men.
6. The channels of Providence harbor must ultimately be preserved by dredg. ing, if the commerce of the port continues to increase; and if provision is made, as suggested in our reply to No. 3, for creating a harbor fund, liberal provision may be made for the occupation of the frontage, reserving in all cases an ample channel way, with allowances for anchorage and winding room.
For the location of every portion of the harbor line a reason should be stated; or should no adequate reason exist in any particular case, this should be frankly acknowledged, in order that a change may be made wherever the wants of commerce demand, without disturbing public confidence in the permanence of the system as a whole.
The element of permanence is so important, as affecting the value of riparian privileges, that every endeavor should be made to give a final solution to the problem at each point, and to this end a physical reason is the best because most permanent
The studies indicated in Nos. 4 and 5, are among the requisites for the proper location of harbor lines, limiting the undue extension of wharves, &c. It is very often the case that parties on opposite sides of a basin or channel, can present equal claims, based on commercial wants, for the privileges of extension, but that both cannot be granted without impairing the navigable facilities of a public thoroughfare. In a case of this kind your Board will find that careful gaugings of the water at rest, or in motion, will enable you to determine lines which shall represent equal displacements or diversions of the streams, or balance other physical elements, and thus you will be able to secure, as a compromise of all lines, an equitable adjustment.
The above are designed to be the general hints and suggestions which your question seems to call for, but we shall be rearly whenever you have arranged a scheme of lines to go over the ground, examine the data upon which your reasons are based, and weigh all the arguments with you.
7. There should be prepared a full statement of the present obstructions or deficiencies as developed by past experience, and the public importance of relief and improvement of this as a national harbor should be set forth.
You will best commend any project to the attention of Congress by indicating your willingness to assume a share of the burden, either by contributing towards the work directly, or by providing for the maintenance of the improvement, as we have heretofore suggested.
8. The Board, if advised of the progress of the studies indicated and the improvements as they progress, will be most happy to yield any assistance in its power.
Very respectfully, your obedient servants,
DANIEL AMMEN, Chief of Bureau of Navigation.
NEWPORT, R. I., December 22, 1877. I will make a special communication to the Honorable Board of Harbor Commissioners, noting what part of the above I agree with, and what I do not.
G, K. WARREN, U. S. Engineer Corps.
BUREAU OF , .
WASHINGTON, December 28, 1877. To the Honorable Board of Harbor Commissioners of the State of Rhode Island:
GENTLEMEN: I have the honor to forward the reply of Gen. G. K. Warren, U. S. Engineers, to the questions given the Board by you. In my belief, your honorable body will derive advantage from whatever differences of opinion may exist when clearly and fully expressed, as is the case in the enclosed paper.
It indicates, on the part of our co-laborer, a careful study and full knowledge of the localities, and doubtless will receive, as it should, that careful consideration to which it is justly entitled. I have the honor to be, Very respectfully, your obedient servant,
DANIEL AMMEN, Chief of Bureau of Navigation.
ENGINEER OFFICE, U. S. ARMY,
377. } To the flonorable Board of Harbor Commissioners of the State of Rhode Island :
GENTLEMEN: I was unable to attend the meeting of the U. S. Advisory Council this month, to consider the questions and prepare the answers which you desired of us. I, therefore, sent my proposed answers without the benefit of consultation with my co-members, expressed briefly. To some of these answers as thus made by me, my colleagues have made important additions, which I adopt. But in the matter of natural physical influences, I differ so much with them that I cannot subscribe to the part they have presented preliminary to the answers they have given, and this difference also prevents my agreeing with them in certain of the answers. I therefore make this special communication.
As a preliminary to considering the question of Providence harbor, I will say briefly, that in my judgment its improvement is involved in no problem of physical laws, and that it is simply a question of capacity for commerce and convenience of navigation. Points to be cut off; crooked places to be made straight; nar