L emporarily improving navigation during the low-water season. 'he principal obstructions met with are the sand bars built up durng high stages, through which the river slowly cuts a channel as he flood recedes. If this process could be assisted and hastened, navigation would be improved thereby, and to this end experiments were made with dredges. These experiments, started in 1894, were successful and as a result the Commission was led to reconsider the whole subject of the river improvement, and as a result of this study the Commission decided to adopt a new plan for future operations, namely, to depend for the relief of the immediate needs of navigation upon dredging, and to restrict the improvement by bank revetment to special localities, where local interests or necessities demanded it. The reason for this action of the Commission, with the reports of the committees who had specially investigated the subject and the dissenting views of some members, are stated in full in the Report of the Chief of Engineers for 1896, pages 3419-3423, 3430-3432, and 3439-3459. Of the above the following will be quoted: * One important result of experience in respect to these two kinds of channel work has been to emphasize more and more the relative importance of bank protection as compared with work for contracting the channel way by the upbuilding of new banks. It is now considered settled that in a plan for the permanent improvement of the channel by the contraction of its low-water width the work of bank protection should precede the permeable dikes and other structures of that class, and that the latter may be regarded as supplementary in function and as necessary only in a minority of cases, If the works for bank protection originally projected had proved sufficient, as it was hoped they might, to hold the banks of the river with permanence, a substantial improvement of the channel throughout the worst reaches of the lower river would have been possible by this time. When they failed to realize those expectations the problem became a new one. No precedents existed to throw any certain light upon.it. It had to be taken up and worked out by original methods, experiments, and trials. As these proceeded, the necessary cost of the work increased, step by step. It was taken for granted that the disposition of Congress to undertake finally the permanent improvement of the entire river in this manner would depend in large degree upon the anticipated cost of the work, and that it was incumbent upon the Commission to find out, if it could, not only whether or no the work is possible, but the lowest cost at which it is possible. With this view the progressive changes in structure and methods which have been made have been conservative, proceeding by short steps as necessity seemed to require. A point has been reached now at which some important facts can be definitely stated. While it is not possible to say that the form of bank protection *st form of mattress work and secured by the sUGNE my way upper bank riprap. it stelened to be puma A back the riser, barr.ng extraord tart wriPETA STRAN Sup und Lumbers, $30 per rasting foot of task postaus r. **s had a half times the evet originally estimated by the FULLI BUL Is atatums were large interests depend upon midng a mean man, sayĺ back, as at city harbors, points where estofa am ramasne ma u kurgjonal importance are liable to be breased by far og tek My Ava protection at a cost not incommensurate with the ne Bonstrated fact. kuper fun of importance now definitely settled is that in the engr of maltreams of the type now employed on a large scale the sappy of me NAKASAɛ in katent of the problem presenting serious difficulty. The me | an studung bar necessary to supply material for a mile of fases: *aker til feet wide is probably five times as great as that required for an e ALNÁ NA Mattresses built in the early years of the work. The quant growth used in this form of mattress is so great that the est yout y *. -Ply available along the whole length of the river from Cairo to ï Volg * WA more than sufficient for the making of 15 miles per a krym w theme facts together it became apparent several years ago-not W MAR, ha gradually, as facts accumulated and the deduction of experie that it was not possible by any or all of the methods which 523 maxi, kingonped to accomplish such permanent improvement of the river fr " •16 down as was necessary to meet the urgent demands of commerce with: wy #ble time to come, It was this disagreeable but unavoidable conci. suk from the work of preceding years, together with the recent and wonderful de vejopment of the hydraulic dredge, that led the Commission to undertake the experiments of dredging the bars of the river with the view to the temporary movement of its low water channel which has been detailed in recent re port of the Commission, and to which more extended reference is made else where in this report. Those experiments have proceeded favorably so far as te hold out a reasonable probability of entire success. If it is possible in that way to meet the immediate necessities of commerce by securing a channel of ample depth during the low water season at reasonable cost, it is considered by the Commission to be its first duty to advise such use of the money appropri ated by Congress as will accomplish that end with the least possible delay. With this view the Commission has made no recommendation of any allotment from the appropriation for the year ending June 30, 1897, for bank protection or contraction works for the general improvement of the river except such sums to Plum Point and Lake Providence reaches as are supposed to be necessary to make such repairs and extensions of existing works in those reaches as will rly test their efficiency and permanence. And it is the general recommenda1 of the Commission now, to be followed hereafter by such special recomndations as circumstances may require, that out of the funds appropriated Congress for the improvement of the river below Cairo such sums shall be voted to the work of dredging the low-water bars as shall be sufficient, first, thoroughly test the practicability of obtaining a good low-water channel by at means, and then, if the work shall be found to be practicable, to build, uip, and operate a plant sufficient for its secure accomplishment before any >w and further revetment and contraction works for the general and permaent improvement of the channel shall be undertaken. In considering the question of going forward now, or at a later date, with ie general and permanent improvement of the channel by protection of its anks and regulation of its low-water width, it is manifest from what has been aid that it must be done, if at all, upon largely revised estimates of cost and ime. It was recognized from the beginning of the Commission's work that the onstruction of works of bank protection and channel contraction in detached eaches would be subject to influences more or less unfavorable from changes n unprotected bends above. In commencing work in the detached reaches of Plum Point and Lake Providence the Commission was not oblivious of this fact, but was influenced by other considerations which seemed to overbalance this disadvantage, and it has not since seen good reason to doubt the wisdom of that decision. But the experience gained in those reaches emphasizes the fact that it is highly important in any project for general and permanent bank protection to leave no unprotected reaches above the work done from changes in which new conditions may be produced in improved parts of the river. And it is considered by the Commission that in undertaking a complete and permanent system of bank protection the work ought to begin at the upper end of the river, at or near Cairo, and be carried thence downstream with substantial continuity. The distance from Cairo to Vicksburg is 600 miles. In a project for the complete and permanent improvement of that portion of the river by the means stated some portions of it would require work on both sides of the channel, though this would not usually be the case. And it may be that in some portions of it no work would be needed at all, at least for a number of years to come. As to these matters an exact judgment can not be formed in advance. But for the purpose of a preliminary estimate it is considered conservative to count on the ultimate revetment of a bank line equal to two-thirds of the whole distance between the two points. Based on this assumption, and a cost of $30 per running foot, the cost of a complete revetment from Cairo to Vicksburg would be $63,000,000, and at a rate of progress of 15 miles per annum, which limit, or less, it is believed would be imposed by the available supply of material for mattress work of the construction now employed and believed to be the best, the time necessary for the completion of the work would be forty years. It is considered that the annual cost of maintenance of completed work could not be estimated at less than ten per cent of the original expenditure. Upon the general principles described the work of the Commission has since been conducted with but one slight alteration. In 1899 experiments with contraction work were again begun, this time the work being designed only for one special purpose The dredging operations will, for reasons stated at the beg of this paper, not be discussed, but the contraction works and revetment constructed in the First and Second districts will b described. The attempt will be made to relate the history of t works, their beginning, their development, and their successes failures and the causes of such failures. CONTRACTION WORKS Of the contraction works which have been built in the First Second districts nearly all have been located in Plum Point R and in preparing the history of these works, and also of the c revetment constructed in this reach, much of the compiling writing has been done by Assistant Engineer Aug. J. Nolty, has been connected with this work continuously from its incept to the present time. In selecting this reach of the river for the first systematic periments in regulation the Commission followed the recommend tion of the Board of 1879, who in their report stated: That such a trial may thoroughly test the practicability and the cost of reg lating the river and increasing its low-water depths, one of the worst pla on the river should be selected. Such a place is the Plum Point Reach (15 miles below Cairo). This reach is about 20 miles in length, and presents many places excessive width, reaching two miles at high water and one and s half miles at low water. The case is made still worse by the fact that the river at one place seems to be filling up a concave bend, thus giving rise to miles of nearly straight low-water channel, some of which is nearest the conver high-water bank. On Plate V will be seen a map of this reach, made from the survey of 1880. For the purposes of this first scheme for river regulation this reach of the river was supposed to begin at the foot of Forked Deer Island and Chute. Here the channel of the river was near the left bank, and for the next 4 miles it hugged this, the concave, bank in what is known as Ashport Bend, the bank being a steadily caving one. At the foot of Ashport Bend or Gold Dust Landing the river ened out quite suddenly, and in the middle of this wide river 3 located the large and high Elmot Bar. This bar was of comatively recent growth, the 1874 map (Plate IV) showing it to rather low and scattered, but it has since grown in size and ght and is now as high as the main banks of the river and thickly abered. To the left of the bar was what is known as Gold Dust ute, carrying at about the bank-full stage 40 per cent of the disharge of the river, but being at low water narrow, crooked, and not vigable on account of snags and insufficient depth. In the mide of the lower end of Gold Dust Chute was located Island 30, one the older islands of the river. This divided Gold Dust Chute into vo outlets known, respectively, as Elmot Chute and the Chute of sland 30. From Gold Dust Landing the main channel of the river assed by the head of Elmot Bar over to the right bank of the river t Mill Bayou, this crossing at low water being usually shallow and lifficult of navigation. Starting at Mill Bayou the channel hugged The right, a concave, bank along what is known as Fletchers Bend, his bank being also a steadily caving one. After following this bank for some 4 miles or so, the channel forsook it, and for the next 4 miles pursued a generally straight course, passing across the foot of Island 30, skirting the convex left bank at Plum Point, and finally coming back again to the still concave right bank along the upper side of Craighead Point. This part of the river, from the point where the channel left the concave right bank down to the point where it returns to this bank, was of above the average width, and in it were located numerous sand bars that shifted in place from year to year, causing a similar shifting of the low-water channels that were formed through these bars. The manner in which these bars and channels changed from place to place can be seen by a comparison of the maps of the reach made from time to time. (Plates LVII to LXIX.) In addition to these small and shifting sand bars there were two large and permanent ones, Osceola and Bullerton, located close to the concave right bank. The channel behind them, though narrow, was fairly deep, and in some years they formed the best, if not the only, available low-water channel in this part of the river. The reach had for years been one of the worst parts of the river, and at low-water channel depths in the various crossings of 5 feet, and even less, had been frequently noted. Certainly a part of the river in greater need of improvement could not well be found, and it was accordingly selected for regulation. |