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Virginia channel, from which they have been separated below the bridge by a broad grassy flat; a depth of 40 feet and over is then obtained in the main channel. The survey of last year did not extend below the Long bridge, but that of 1862 includes that part of the river between the bridge and Giesboro Point, about a mile below the arsenal. From the examination it is evident that the Washington channel, both below as well as above the bridge, is being seriously impaired; it has decreased in width as well as in depth. Some plan must be adopted for its improvement, and its execution should not be unnecessarily delayed. A very large part of the trade of the city is dependent upon keeping this channel open, and means should be adopted to prevent it from being further damaged by either artificial or natural causes. This part of the subject will be subsequently referred to. From the previous and already too extended description of the present condition of the river, it is also plainly apparent that in reality there is but one channel of the Potomac, the Virginia channel, which is practically useful at present for commercial purposes, so far at least as the entire city of Georgetown and a large section of Washington are concerned ; and how long this one will remain so, is a question which concerns every one interested in the welfare and prosperity of the two cities. The examination proves very conclusively that this one also is gradually becoming impaired by the continuous formation of a bar or shoal in the middle of the channel, which, extending now but a short distance, will finally assume greater proportions unless some artificial appliances be resorted to for its removal; during the period the surveys were being executed, sea-going vessels occasionally stuck fast on the bottom at low water. Any one who seriously studies the subject must beforcibly impressed with the great necessity of adopting some system of improvement for deepening either one or both of the channels, or by cutting a new one through which to force the large body of water which the Potomac will never cease to supply in sufficient quantities for all navigable purposes. Instead of allowing the river to spread out over a wide surface it should be confined in one channel, ample in width and depth ; this once accomplished by the engineer, it will be found that nature will come to his assistance and aid in the operation of perfecting his plans. In anticipation of such improvements legal measures should be taken for remedying all existing injuries to the channel, for the conservation of the shores and harbors, for preventing future encroachments in the construction of any works which may produce damaging effects. Every encroachment should be viewed with the greatest jealousy. It need not be stated that the general government has a great interest in encouraging the river improvements to be proposed, and by assisting with liberal appropriations their execution. Had this step been taken before the late war, it would be impossible to estimate the amount that would have been saved during that period ; by the expediting of transportation alone more than sufficient means would have acerued to compensate for the expense of the entire undertaking. Nor has the usefulness and necessity for such improvement on the part of the government ceased to exist; as long as the capital stands, so long will the need arise for a permanent and copious channel to lave its shores. As will be shown hereafter, the whole work will be remunerative in the greatest degree; the expense will be met by the advantages gained. It has been often stated that there are no more difficult and uncertain issues than those connected with the improvement of rivers, and, in submitting plans for those which will be proposed, it has been found desirable to endeavor to renew, as far as practicable, the old condition of the regimen of the river. As a great deal of time will necessarily be consumed in the preparation and obtaining of material, and the construction of such necessary works as may be needed to execute any contemplated plan of permanent improvement, even should the necessary appropriation or other meaus become available, it is recommended in the first instance to dredge out that portion of the Virginia or Georgetown channel where the existence of the shoal water demands it, and also to the same extent the Washington or city channel below the Long bridge, so as to obtain a suitable depth of water in each for sea-going vessels as are of a proper tonnage for the trade of the river. The present channels will then be preserved for immediate use in the event of no funds becoming tangible within a reasonable time for more extended and important improvements.
The channel-ways should be in the minimum at least 100 feet in width, with a depth of 12 feet at mean low water. In view, however, of the improvement to be finally proposed, it will only be necessary at present, as far as the Washington or city channel is in question, to dredge such shoals or bars as immediately require alteration, without going into any extension of its width. This will be attended with very little expense, and not much loss of time, the machinery and labor to be employed becoming available during any temporary suspension in other adjoining works. In respect to the Virginia channel, the plan for its present improvement is represented on the accompanying sketch, which exhibits the plan and profile, marked B, for connecting the 12-feet water-curves. The distanee to be dredged is 1,900 feet, and, by calculation, the amount of soil to be removed will be about ten thousand one hundred and ninety-one (10,191) cubic yards. The distance is divided into sections of 100 feet, and the contents of each can be found by reference to the annexed table, marked channel B. Should the engineer in charge desire to accomplish the work without contracting for the same, it may be well to mention (as the question is frequently asked) that one of the best dredging machines is said to be Osgood's patent, which, according to the latest and most reliable information, can raise from 300 to 500 cubic yards of soft mud or sand per day. Each of the machines owned by the government, and used at present in the Patapsco river, cost about ten thousand five hundred (10,500) dollars. An estimate for this temporary improvement will be appended. Whatever opinions my be held, or have been expressed by others in regard to the obstructions or encroachments in the river, in consequence of the building of the causeway of the Long bridge, it has been clearly demonst rated to me that there is no doubt of that structure being very injurious to the Washington channel, and that the section referred to should, therefore, be removed, and replaced by either an arched bridge or one on piles of wood or iron. Those best acquainted with the river have pointed out a most marked increase, during the last few years, in the dimensions of the flats, and a consequent diminution in the width and depth of the channel. Whilst the causeway obstructs, to a considerable extent, the water coming down from the interior of the country, it also partia liy prevents the tide-water from flowing up; it therefore interrupts that continual scour which should result from the force of the one and the flowing and ebbing of the other. The tides should have a perpetual and unrestricted current. “The great object to be kept in view, in carrying into effect the improvements of the navigation of a tidal river, is the free admission of the greatest possible quantity of water from the sea, as reliance must be chiefly placed upon the scour produced by the tide, and not upon the current of the fresh water as the chief agent in keeping open the navigable channel of the river.” Many celebrated engineers have concurred in the above opinion, and, this being admitted, “it is manifest that all obstructions to the tidal flow upwards should be removed ;" such as shoals, and bars, or dams, and dikes, or any other cause of obstruction. The length of the causeway, as already stated, is one thousand nine hundred and sixty-seven (1,967) feet-a somewhat formidable obstacle to either river current or ocean tide; and the estimated cost of substituting for it the most simple plan of bridge—the pile, or trestle, or crib-workwill not require a very large expenditure, in proportion to the advantage to be gained. It is difficult to understand why the causeway was originally constructed. An estimate for its removal is herewith appended. In my last annual report, dated October 1, 1867, it was stated that orders had been given me to prepare a special report to the bureau in regard to the improvements of the channel of the river, and that the bridge question was so intimately associated with the matter as to necessarily form a part of the discussion. Since then two very interesting and comprehensive reports, in reference to bridging the Potomac at several points have been carefully examined, and the subject found to be very thoroughly exbausted. Should either of the plans proposed in refererence to the present site of the Long bridge be adopted, it would necessitate the removal of the causeway. These reports were made at different periods to the Department of the Interior. The one was prepared by Alfred C. Rives, esq., civil engineer, and submitted to Congress by the Hon. R. McClelland, then Secretary, on the 7th of February, 1857; and the other by Col. Silas Seymour, civil engineer, addressed to the Hon. O. H. Browning, the present Secretary, on the 18th of February, 1868. The first report was printed several years ago for distribution; and the perusal of the second, in the original manuscript, was kindly afforded me by the author.
We have now discussed the temporary expedients, at the same time the most economical and expeditious ones, for improving the navigation of the existing channels : firstly, by deeping the two channels—the Washington or city, and the Virginia or Georgetown-by dredging the shoals to the depth of twelve feet at mean low water; and secondly, by the removal of the causeway of the Long bridge, which contracts the current and obstructs the flow of the tide, in order to obtain a larger volume of water to assist in scouring them out. The velocity of the current, upon which mainly the scouring process depends, is not great; the greater the reason, therefore, for removing any obstacle to its free action. It may be here stated that the rise and fall of the tides is about three feet, and that the velocity of the current is one half mile per hour.
The next questions to be considered have reference to some plan for the permanent improvement of the navigation of the river. Several have been recommended, but to the present time not one has been executed, or even adopted. There is a division of opinion as to whether one or both of the old channels shall be improved ; and again whether both of them ought not to be closed, and an entirely new one opened. One good, straight, broad channel will certainly be preferable to two interior ones ; and the one which promotes the greatest amount of benefit, and is most advantageous for the commercial interests of both cities—Washington and Georgetown-should be adopted, although it might not prove the most economical in the first instance to construct. If one can, therefore, be dredged out which will conform as closely as possible to the regimen of the river as it existed many years ago, before its waters were obstructed by the encroachments since made upon it, and should legal measures be taken to remove the latter, it would seem that such are the steps needful to be taken.
From the different surveys made it is evident that the current of the river has still a tendency to set in the direction of the old Washington or city channel, as laid down on the earlier maps, running parallel to the bold, curved shore which forms the city front. A pocket has been formed showing this tendeney; and within the last few years it has been gradually extending and deepening. The current is somewhat deflected from taking what otherwise would apparently seem to be its natural course by the encroachment of Easby's wharf, and by obstructions offered there by rocks above and below the surface of the water, which should be removed before any extended improvement is commenced. What would then seem to be the most natural plan is to connect deep water, a little above the east draw of the Long bridge, where the river washes against high bluff banks, with deep water below Easby's wharf, the course of the new channel following the general direction of the current, and taking advan
tage at the same time of the most favorable profile. The plan and profile of the proposed new channel is exhibited on one of the accompanying sketches. It should be at least 12 feet in depth at mean low water, and 100 feet in width. The length of the cut will be about 5,700 feet, and the quantity of soil to be removed is calculated to be one hundred and sixty-two thousand six hundred and twenty-one (162,621) cubic yards. An examination of the bottom of the river proves that it is perfectly feasible to dredge the channel to the necessary depth, frequent borings of the subsoil having furnished satisfactory results.
In computing the cubic contents to be removed, proper allowance has been made for the caving in of the sides of the channel, assuming that they will take a slope of 45°, the soil being composed of hard sand. The material removed can be conveniently deposited on the grassy marsh between the edge of the channel and the river shore, thereby embanking and reclaiming about 260 acres, the greater part of which would prove an extension to that part of the public grounds called the Mall
. Although it is not advisable to contract the banks so as to prevent the spread of the water and interrupt the tidal flow, still, as the river is so broad at this point, and the space reclaimed so limited, no very serious damage can be anticipated.
Among the several river improvements of the present century which have proved most successful, the greatest depth of water has been obtained by artificial means, principally by dredging, aided, when favorable, by the scouring process of the tides. Should the soil prove too soft, and not adopt a proper slope, quarter or half tide dikes can be used to guard the channel. A more substantial and permanently useful improvement for protecting it along the city front would be a sea wall. There it would also act as a retaining wall for the reclaimed land. By opening this new channel the force of the current will aid in scouring out that part of the old city channel below the Long bridge, which, as has already been stated, is being seriously impaired. The removal of the causeway of the bridge is a sine qua non in respect to the successful execution of any of the plans. Nor will this improvement produce any injurious effect upon the channel of the East Branch, as the two glide into each other at an acute angle, and together assume a gentle continuation of the curve of the shore as far down as Alexandria, where it is again deflected towards the opposite bank. Appended is a table (marked channel A) exhibiting the contents of each section of a hundred feet through the proposed cut, and an estimate for dredging the channel is annexed. Whether or not sufficient water will pass through the new channel when opened without the aid of artificial appliances to deflect it from the Virginia channel is a question requiring great consideration. The fact of the Potomac being influenced by the tides to a higher point than the proposed work renders it necessary to examine whether or not it will be expedient to construct dikes so as to give to the current a suitable direction. If properly directed the whole mass of water might be usefully employed instead of wasting its power by being spread over the present broad surface of its bed. In the very able and suggestive report by Mr. Alfred Rives, to which reference has already been made, upon the subject of opening a new channel, he proposes the building of a breakwater from the southern extremity of Mason’s island to deflect the current. In the same connection he remarks that the advantages resulting from a combination of the arrangements suggested would be permanent, “because the river will be confined in a sufficiently narrow channel for more than a mile below Easby's wharf by the proposed breakwater, which it leaves in a direction normal to the Long bridge, finally passing through the opening reserved for it, and becoming tangent to a bold curved shore of wonderfully regular outline. It is well known that currents in curves form stable and deep channels.” The building of such a breakwater would have the tendency, sooner or later, to close entirely the Virginia channel. Instead of being preserved for navigable and commercial purposes, its place would be occupied by marsh or meadow land which would eventually form. This would be but a continuation of the process already going on in consequence of the building of the causeway at the head of Mason's island. Again, in constructing such a breakwater or dike and thus contracting the width of the stream, what would be the general effect in the course of time upon the new channel should one be opened? And would not such a work perhaps prove an impediment instead of an improvement to navigation by preventing and thereby decreasing the flow of the tide ?
Considering, however, the very great width of the Potomac opposite the two cities, where the upward tidal flow and the downward current of the river encounter and apparently hold each other in check, and also the short distance above the proposed improvements to which the tide ascends, to say nothing of the small rise, it does not seem probable or possible that any very great damage will result from building a breakwater to deflect the entire mass of water into the new channel.
It is to be conceived that a river can be too wide at certain places, which is especially the case at the head of tide navigation on the Potomac.
The first and great injury to this portion of the river was the construction of the causeway which closed the old main channel between the island and the Virginia shore. This should be lowered so that its upper face would not exceed the level of mean high tide; it would then offer no obstruction to the passage of the waters of the overcharged river during freshets, thus preventing inundations and great damage to property. It would then have to be strengthened, and in fact rebuilt on a different plan. For the same reason, the elevation of the proposed breakwater should be but very little higher than the same level of mean high tide. In constructing the work it might be built in sections, and as it progresses the practical effect upon the action of the current could be observed and any needed change in the plan would suggest itself. The section across the Virginia channel should be the last one completed, in order not to interfere until the last moment with the navigation through it.
It is a question, especially in an economical point of view, whether a breakwater from the southern extremity of Mason's island and parallel to the current of the river, will be more advantageous than a dam or pier built perpendicular to its general direction and extending from the Virginia shore across the old channel to near the edge of the new one. On the sketch, the position of such a dam is located, it being laid down along the line of least water, as the profile will show. The selection of a dam in preference to the breakwater proposed by Mr. Rives, is that the first will be much shorter, more conveniently built, and consequently a more economical structure. The difference in the practical working of each will not be very great.
The report which gained some currency during the early part of 1867, and assumed an official recognition by the action of the Board of Trade of the city of Washington in offering a preamble and resolution on the 11th of February of that year, that the previous heavy “freshets in the Potomac had caused the channel on the Virginia side of the river to be blocked up so as to cause the flood to seek an outlet across what is known as the “flats,' thereby opening the original channel of the Washington side of the river,” was not entirely based upon the facts as they exist. The recent survey proved that no such change had taken place to the great extent above quoted. It was also stated that the heavy ice formed a natural breakwater, similar in its effect to the one proposed by Mr. Rives. The combined action of the freshets and ice will no doubt account for the tendency, already mentioned, in the river to resume its old channel, and offers a strong argument in support of the plan proposed.
If the plan for a dam should be adopted, the following would be its dimensions: the required length is 2,560 feet, with a width of 15 feet on top and 15 feet in height, the average depth along it being nine feet, and the slope being