acteristic lowering of the water surface, and consequent decrease in depth during the last quarter of the flood, while the upstream current is still strong. Conversely during the last quarter of the ebb there is a rapid rise on the gauge, while a steady ebb current continues. This difference in time between high or low water and slackwater varies daily according to the winds and the phases of the moon, and is frequently overlooked, even by pilots and masters of vessels. It amounts to as much as an hour or an hour and a half. It follows that after highwater the tide will fall 18 inches or 2 feet before the ebb-current is entirely established. Furthermore, at or near high-water, although the flood-tide is running strongly up the Delaware in the channel where the depths are greatest, an eddy current in the opposite direction is commencing along the shores. The application of these facts to the Christiana jetty is evident. Since no effective ebb-current out of the Christiana is established until the tide has fallen about 2 feet, the upper 2 feet of the jetty, if built to high-water, will be of no effect in directing the efflux, this service being entirely performed by the jetty below reference 4. Again, the dike to high-water will at or near that stage of tide tend to interfere with the complete flooding of the Christiana basin by interrupting the eddy current back of the jetty. The upper 2 feet of the dike will therefore be under the first consideration useless, and under the second, if of any effect, disadvantageous. The economical aspect of the case is simpler. A dike built to reference 4, high-water being 6, will be entirely submerged twice a day for periods of at least four hours each, making a total of over eight hours out of the twenty-four. It will therefore have that protection against decay ensured by the saturation due to regular and constant overflowing. The durability of a structure so circumstanced, if of good material, may fairly be put at twenty or twenty-five years before decay will have materially impaired it, this period rapidly decreasing as the height is increased. With regard to the proper width to be given the jetty, the conclusions of the project are based less upon reasoning than experience. Under the ordinary conditions of a fresh-water construction on the banks of a tidal stream of moderate range and velocity, with bed of mud and sand, it is certain that a jetty having a width equal to the low-water depth would be secure. The presence of heavy ice in the stream introduces an element of danger to be considered. It seems probable that the apprehension expressed in the report of the Board that an unusual ice movement might sweep away the jetty, as originally proposed, is the basis of their recommendation of the much greater width. The contemplation of a large field of ice, moving at the rate of 1 to 24 miles an hour, might well suggest apprehension for the security of a structure opposing its progress, however massively constructed; but, as a matter of fact, there are numerous slightly-built structures, wharves, cribs, jetties, &c., in the Delaware that have stood for years, although within five years there have occurred two of the heaviest ice seasons ever known. The tramways at Forts Delaware and Mifflin, for landing material, built upon open piles 4 to 8 feet apart, have stood for three years without injury, although the ice has at times nearly covered them. An old tramway built nine years ago by a contractor near Fort Mifflin, when the battery was constructing, is still in existence, although the timber above high-water is thoroughly rotted. This apparent discrepancy between cause and effect is due to two facts. First. The main movement of ice, set in motion by the currents, is, necessarily, in the channel, while the movement along shore is controlled by the more sluggish current, and retarded by the friction of the bottom. Under these circumstances the ice lodges in the shoal water, and, from the constant pressure of the channel ice, is gradually pushed in and heaped up until, even at high-water, there is no movement. The shore structures are thus incased in ice. Second. The lodgment and consequent immobility of the ice are greatly aided by the difference in time previously referred to between high water and slackwater. During the later stages of the flood the ice is pressed shoreward as the tide rises, and is held in that position by the continu ing flood-current until the gauge shows a fall of a foot or more, and the ice is securely frozen to the underlying mass. In this way the ice will have reached, in some seasons, a depth of 8 to 12 feet without damage to the comparatively slight shore structure, against and upon which it rests. All the shoal water, both on the banks and in midstream, is in this manner occupied by stationary fields of ice. With regard to the Christiana jetty, the conditions, both general and special, favor stability. The river is wide, the shores low, the bottom flat and muddy. The flood-tide will continue in the channel after high-water, the slow eddy current aiding the shore movement of the ice, and its lodgment as the tide falls. Furthermore, during the winter northwest winds are the prevailing ones, and the west side of the river is frequently freed from floating ice by the pressure of the winds forcing it to the opposite shore. The only portion of the dike exposed to dangerous action by the ice will be the outer end, which, in the project, is supported by a heavy crib solidly filled, and its corners sheathed with iron plates to prevent cutting. It is assumed, since the ultimate object is the attainment of a 15-foot low-water channel, that the Board would have recommended both the extension of the jetty to the 15-foot curve in the Delaware and the construction of the crib at the end had they supposed the funds were available. This is inferred from the remark of the Board that, "to secure a depth of 15 feet will require the scouring effect" (of the Christiana outflow) "to be extended to the 15-foot curve of the Delaware." If the dimensions of the jetty, as recommended by the Board, can be reconsidered, a complete structure can be built to the 15-foot curve for about $35,000, which will do its full service and last for twenty years, while otherwise the $50,000 appropriation will be exhausted in reaching the 12-foot curve. It is not supposed that the whole jetty can be built this season, but it can be put under contract with economical advantages and completed next spring. In connection with the general subject of the improvement of the Christiana entrance, two other points have received consideration. First. The wharves belonging to the depot of the light-house establishment, which project into the stream from the north bank, interfere with the proper flow of both tides, and should be removed. Their injurious effect will be still more evident after the construction of the jetty than they are now. It is not anticipated that any objection will be made to the removal, since the light-house establishment has built a new depot near the Cherry Island range lights, some miles above, on the Delaware shore, with the intention of abandoning the Christiana depot so soon as the other shall have been completed. Second. To perfect the entrance some works are necessary on the south bank. On the tracing prepared to accompany this report are drawn lines intended as dike or bulkhead lines. The purpose of these is to regulate the flow of the Christiana, and in connection with the proposed jetty, to give its mouth that ear-trumpet shape, opening towards the flood-tide in the Delaware, which is desirable for the embouchure of a secondary stream almost entirely dependent upon the tides of the primary for its water supply. No project for the construction of a jetty along these lines has been submitted, for the reason that the bordering lands are owned by wealthy manufacturers, who propose the erection of works in the vicinity, and who will, in time, doubtless see the desirability of taking in the area inclosed between the present dike line and those proposed as bulkhead lines, since they will gain a large additional acreage, at the expense only of the dike. As this will join the main land at both ends, the common mud dike, pitched with stone on the river side, as usually constructed on the Delaware, will answer all requirements, and the gov ernment will not be called upon to build it. The accompanying schedule gives the several methods of construction and total cost, in parallel columns. No. 1 is that recommended by the Board to the 12-foot curve, with the additional amount necessary to carry it to 15 feet. The height is carried in the estimates to 7 feet, since the high-waters frequently reach that elevation. It has the crib superstructure proposed by the Board. No. 2 is a modification of No. 1, using sheet piling and part mud filling. The expense in both cases exceeds the appropriation. The remaining methods have dimensions as in the original project. No. 4 is cheaper, and would be entirely effective even if built with hemlock piles, the frequent submergence protecting it. No. 5 is a combination, having the piles cut off at low-water and a crib superstructure as proposed by the Board, but with the dimensions of the project. It is rather more expensive than No. 4 (b), and the crib superstructure cannot have the strength against pressure of the solid piles, the material in both cases being the same. It is believed that on the whole No. 4 (b) will answer all requirements and possess sufficient durability to warrant its construction, which is, therefore, respectfully recommended. Very respectfully, your obedient servant, J. N. MACOMB, The CHIEF OF ENGINEERS, U. S. A. REPORT OF THE BOARD OF ENGINEERS. OFFICE OF BOARD OF ENGINEERS FOR FORTIFICATIONS In our report of June 7, 1881,* we concurred generally with Colonel * See Annual Report Chief of Engineers for 1881, page 775. 1 Macomb as to his views upon the improvements that could be effected in the way of deepening the above river, except that our confidence in the maintenance of a 15-foot channel by the erosive power of the current was not as great as his. We also concurred as to the use of the north jetty to remove the bar in the Delaware at the mouth of the Christiana, with the following changes: First. That the jetty be built at the present time only as far out as the 12-foot curve. It was thought quite probable that the erosion would extend a little beyond the jetty's end, but whether it would or would not it seemed very suitable to let its extension be dependent upon the successful maintenance of 15 feet depth of channel to Wilmington. Second. To make the jetty more effective it was proposed to raise it to high-water (mean high-water). Third. As the portion above low-water would be alternately immersed and exposed to the sun's rays, this Board suggested that the pile structure of Colonel Macomb's project should be terminated as near the ordinary low-water as "economy of construction will admit, and to put on a superstructure of crib-work rising to high-water. This last when decayed can be removed." Now the ordinary rise and fall of the tide at this place is 5 feet 6 inches. As the finished pile structure will rise about 1 foot 6 inches above mean low-water, the superstruct ure will be but 4 feet high. A method of construction in conformity with our design is exhibited on accompanying sketch. By the change thus indicated the Board thought that oak might be dispensed with and much saving effected. Fourth. It was also suggested that while the width of the jetty as originally designed by Colonel Macomb would endure all ordinary pressures it might be insufficient as against ice floes. The widening by the Board bore reference to the possibility of its being subjected more or less to ice piled upon it or borne down against it by reason of rising river, excessive currents, and high storm winds. The experience on the . Hudson was the Board's guide in suggesting additional width of section to jetty at the mouth of the Christiana. The question was fully discussed whether to make a return as a protection against ice or to widen the jetty. It may not be necessary to widen the inshore portion where the ice grounds. If the constructing officer, however, knows, from his own experience, that there is no danger to his jetty from ice floes, there seems to be no reason why he should not build it with the width shown in his project. Stone is in our opinion altogether preferable to mud for filling between the piles. We submit the following as an approximate estimate: NORTH JETTY. One thousand four hundred and seventy feet long and extending to 12-foot curve, and rising to level of mean high-water. 1,286 feet 1-inch bolts, 1,960 feet 1-inch tie rods, 10,800 pounds, at 6 cents.. 648 00 Stone, 12,675 cubic yards, at $1.75.. 22,988 27 22, 181 25 45, 169 52 The timber, piles, &c., are at the contract prices for the Savannah River. The price of stone is assumed at 75 per cent. greater than contract prices on Long Island Sound, and more than twice as great as those paid on the Hudson River. The Board presents the annexed sketch to accompany its estimate and to show how its results were obtained. Respectfully submitted. Z. B. TOWER, Colonel of Engineers, Bvt. Maj. Gen. Colonel of Engineers, Bvt. Maj. Gen., U. S. A. Lieutenant-Colonel of Engineers, Bvt. Brig. Gen. Brig. Gen. H. G. WRIGHT. Note: 20% more stone than shown in this 56 N.W. L.W. |