Annual Reports of the War Department

Statement of extent and net cost of dike work-Continued.

Statement of extent and net cost of revetment work.
CONSTRUCTING 520 LINEAR FEET OF SUB-BANK PROTECTION.

Statement of extent and net cost of revetment work-Continued. CONSTRUCTING 520 LINEAR FEET OF SUB-BANK PROTECTION-Continued. •

REPAIRING 790 LINEAR FEET OF UPPER-BANK PROTECTION.

ANNUAL REPORT OF SAMUEL H. YONGE, DIVISION ENGINEER, NEBRASKA CITY, NEBR.

MISSOURI RIVER COMMISSION,
OFFICE OF DIVISION ENGINEER,
Nebraska City, Nebr., June 30, 1897.

COLONEL: I have the honor to submit my report of operations conducted in improving the Missouri River in the vicinity of Nebraska City, Nebr., for the fiscal year ending June 30, 1897.

NOTES ON THE REGIMEN OF THE RIVER.

According to the map accompanying the annual report for 1891, the head of the Nebraska City Island revetment was situated where Dike No. 2 is shown on the map herewith submitted. The present head of the revetment is at "A" (Plate I), about 400 feet below Dike No. 2, that extent of work having been flanked or undermined. This damage, which has occurred since last fall, was brought about through the agency of an extensive reef reaching from the bluffs about a mile below the mouth of Walnut Creek to within a quarter of a mile of the Burlington and Missouri Railroad bridge at Nebraska City.

The main channel in this vicinity is usually between the reef, for almost its entire length, and the left bank; secondary channels through different parts of the reef, alternately forming and filling back for every slight change of stage.

In the fall of 1896 a pocket was formed by the reef's action, adjacent to and immediately above the head of the revetment, and about 100 feet of the work flanked. Between May 12 and 15, 1897, the 200 feet adjacent to and below the pocket were undermined by the part of the reef opposite, building up and extending about 100 feet toward the shore.

About May 19, a channel formed through the reef opposite the head of the pocket. The opening gradually enlarged until it attained a width of about 900 feet on May 26 (Plate I). This flow impinged against the city water front, causing a slight bank caving for about 2,000 feet above the waterworks pumping station.

In the early part of June, other channels were formed through the reef some distance farther upstream, while the one just described silted up considerably, the bank-caving along the water front coincidently ceasing.

From June 12 to 15, slight bank-caving just above the city waterworks pumping station resulted from a recurrence of somewhat similar conditions to those prevailing in the latter part of May.

A comparison of the maps of 1891 and 1897 shows that there has been some bank erosion during that period along the city front-probably the result of intermittent reef action as briefly outlined above.

DIKE CONSTRUCTION.

It was proposed to check the enlarging of the pocket above referred to, and the consequent flanking of the revetment below it, by three short dikes placed across it, approximately normal to the general direction of flow.

Soundings were made in March, 1897, on which the bills of piling and other material required for the dikes were based.

An examination made May 11, immediately after the arrival of the plant at Nebraska City, showed that since March the depth of water in the pocket had greatly increased by scour. In consequence, most of the piles that had been procured for the dikes were too short. As it was not practicable to get sufficient long piling in time to construct the dikes as originally designed before the June rise, the project was modified by your direction, by substituting two 3-row dikes for the three 2-row dikes. A short 2-row dike, across an eddy situated a short distance below the pocket, was added to the project.

The requirements for long piling were partly filled by procuring such cottonwood piles as were available within an economical hauling distance, and by splicing short pine piles.

Preparations for dike work were completed and pile driving at Dike No. 1 was begun May 19. The dike is 131 feet long, having the outer ten bents 3-row and the inner four 2-row. The pile penetrations vary from 15 to 28 feet, the lesser resulting from a scarcity of long piles.

Mat weaving was begun May 21, and, with the ballasting, completed May 23. Dike No. 2 is 130 feet long. Its outer eight bents are 3-row, its inner six 2-row work. The piles have an average penetration of 29.8 feet, the minimum being 234 feet. After the first six bents of piles were placed, the driving became excessively hard, and a water jet supplied by a steam pump was used in connection with the Vulcan Nasymth hammer to increase the speed.

The bracing of the Nebraska City dikes consists of top direct braces and wales, diagonal cable stays, and direct ground braces, the latter detail being used for the first time. They are designed to diminish distortion of the pile bents frames that would result from the ground behind the piles of the front row yielding when the dike is subjected to stress tending to overturn it. Each ground brace consists of two 4 by 8-inch scantlings bolted and spiked to blocks near their ends, which separate them slightly more than the diameters of the piles which they are to brace. Short plank spiked at intervals to the under edges of the scantlings form a crate between the blocks to hold stone ballast for sinking the brace to place. (Fig. 3, Plate II.) Preliminary to framing a ground brace, the space which it was to occupy on the ground between the two piles was measured. Several apparatus were made for this purpose, but none were entirely satisfactory, as they frequently failed in swift, deep water to furnish a close enough measurement to insure the requisite snug fit between brace and piles to make the former fully effective. That finally adopted is illustrated in figs. 5 and 6, Plate II. It consists of an iron ring 2 feet in diameter, made of 4-inch iron, bolted to a long straightedge. The ring being slipped over the pile and the straightedge plumbed with a spirit level, the measurement a is made, the constants b and b being added to it to give the total length of brace outside the separating blocks. To place the braee, it was first swung by -inch wire strands passing over two rollers, one near each end of the brace, supported by two punts. (Fig. 4, Plate II.) It was then loaded with enough stone to give a sinking weight of 300 or 400 pounds in excess of its buoyancy and lowered, being kept approximately level to prevent upsetting by the current and dumping the ballast. This happened whenever the downstream end of a brace was allowed to get much below its upstream end.

As the river was near a flood stage during the construction of the dikes, the conditions for placing the ground braces were unfavorable. They were not infrequently, while being lowered, prevented from reaching the bottom by being bound

against the piles by the current, or jammed between the piles either on account of their being crooked or having been driven out of plumb. Whenever this occurred, the appliance shown in figs. 10, 11, and 12, Plate II, was used with more or less advantage to clear the brace by springing the piles apart. It consists of two arms made of 4 by 4-inch pine, each arm having an iron sleeve attached at one end, and a screw working through a nut at the other.

The spreader is adjusted to fit approximately different widths of bent by sliding each arm through the sleeve of the other. The piles are forced apart by turning the screws, the arms being prevented from slipping by an iron link lying diagonally across them, fitting into notches on their outside faces.

After getting the ground braces to the mat, or as near it as possible, the strands used for lowering were spiked to the heads of the adjacent piles.

By your direction the screen was omitted from the outer fifty feet of Dike No. 1 and the outer 30 feet of Dike No. 2. Dike No. 1 was completed May 29.

Work on Dike No. 2 had to be several times suspended on account of high water, and was not completed until June 21.

An improved form of clamp, consisting of a pair of cams with serrated faces, for attaching to the head of the jackscrew used in tightening the stay cables, to prevent them from slipping while being tightened and fastened to the back piles, was used with satisfactory results. Its details are shown in figs. 7, 8, and 9, Pl. ÎI.

To meet the emergency caused by bank-caving along the city water front, previously referred to, by which the pumping station supplying the city's water and the plant furnishing electric light were threatened, I was directed by you to construct three short dikes immediately above the pumping station.

Subsequently, bed rock being found at the sites of the dikes so close to the surface as to make it impossible to give the dike piles proper penetrations, the construction of the dikes was, by your direction, abandoned, and instead a section of bank at the site of each dike, 50 feet long, was ordered protected with 2 cubic yards of stone riprap per linear foot.

This work was begun June 24, and about one-third completed at the close of the fiscal year.

The moving of plant to Leavenworth, Kans., by the steamer Hugo began June 24. The following statement shows the extent and net cost of the improvements: