the tunnel meeting within one-fourth of an inch, and the levels for grade as run from the two ends differing by only of a foot; also to Mr. Peter McAtee, tunnel foreman, for the very efficient performance of his duties. The oversight of the work, both by Mr. Johnston and Mr. McAtee, was so careful that there was no loss of life, and no man was injured from the beginning to the end of the work. This is a record of the use of high explosives in tunnel work that has rarely, if ever, been equaled. I am also indebted to Mr. John T. Ensor, U. S. attorney for the district of Maryland, for his prompt, efficient, and zealous assistance in examining and passing upon the titles to the lands that I was required to purchase for the work. The main drainage tunnel under Dalecarlia Hill and the shaft in the valley of Little Falls Branch, the most important and difficult parts of the project for the improvement of the Dalecarlia receiving reservoir, are now complete except the coping of the shaft and the retaining wall in the side of the hill in rear of the shaft, and the appropriation has been exhausted. There remains to be done for the completion of the project a short tunnel through the hill on the easterly side of Little Falls Branch; the permanent dams across the valleys of Little Falls Branch, Mill Creek, and East Creek, and the open channels between these streams that are to conduct all the polluted waters of the watershed of the reservoir into Little Falls Branch, from which they will pass into the shaft and thence by the main drainage tunnel around the reservoir and into the Potomac. When this has been done, the water of the reservoir will be drawn off, the reservoir will be filled with Potomac water from Great Falls, and the passage through the reservoir of this water, for which purpose it was originally constructed, will be renewed. The work can be completed in the next fiscal year if the necessary appropriation be made by Congress. Accompanying this report will be found plats showing the portal of the main drainage tunnel and the shaft in the valley of Little Falls Branch; also the short tunnel through the hill east of Little Falls Branch yet to be excavated. Respecting the latter, and also the open channels between the tunnel and Mill Creek and between Mill Creek and East Creek, of which a plan was shown in my last annual report, I have to remark as follows: The greatest quantity of water (see my last annual report) that is likely to pass through this tunnel in the heaviest rainfall, as found by the Burkli-Ziegler formula, Q=f.r()1, is 418 cubic feet per second. A If we make the interior diameter of the tunnel 7 feet, which is the diameter of the main drainage tunnel just completed, we can have the economical advantage of being able to use the "centers" constructed for the latter tunnel. The velocity with the tunnel running full would then be 10.86 feet per second. That with this velocity the invert would not be abraded by sand and pebbles carried along by the water, is shown by the fact that in Washington there has not been found any abrasion of inverts of sewers, when made of vitrified brick, from velocities as high even as 16 feet per second. By the Kutter formula V 41.6 + 1.811 0.00281 1 + (41.6+ I RI, and assuming 0.013 as the coefficient of the roughness of the brick lining of the tunnel, I find that the slope required to produce this velocity is 0.0041, or 0.41 of a foot in 100 feet. The open channel from Mill Creek to the tunnel will have to carry the same quantity of water, viz, 418 cubic feet per second. In determining the slope of this channel I have thought that it would be better to provide for a velocity too great than one too small. If it be too great and erosions of the bed should occur at points where the soil is less resisting, these places can be paved as successively may be found necessary, whereas if it be too small there would be required an annual expenditure from the appropriation for maintenance and repair of the aqueduct for removing deposits from the channel. I therefore propose to provide for a mean velocity of about 4 feet per second. This would be slightly excessive if the channel should run full, but as the calculation of 418 cubic feet per second was under the extreme supposition of a rainfall of 11⁄2 inches per hour over the entire watershed of the reservoir in a storm of several hours duration, and as the highest rate of rainfall recorded at the signal office in Washington between June, 1876, and November, 1892, was only 1.20 inches per hour for one hour, the probabilities are that the channel will never run full. When it does not run full or nearly full (during the major part of the year there will be but a few inches of water in the channel) the mean velocities will be less and there will be a danger, that can not be avoided in so changeable a stream, of deposits in the channel. For a mean velocity of 4 feet per second the waterway would require to have a cross section of not less than 104.5 square feet. If the channel be made 6 feet deep and 9 feet wide at bottom, with side slopes of one vertical to one and a half horizontal, its cross section would contain 108 square feet, and by the Kutter formula, assuming 0.03 as the value ofn, I find that the slope required to enable this channel to carry 418 cubic feet per second is 0.0012, or say 1.2 feet in 1,000 feet. I therefore propose for the tunnel an interior diameter of 7 feet and a slope of 0.0041; to make the open channel between Mill Creek and the tunnel 6 feet deep and 9 feet wide at bottom with the side slopes just mentioned, and that the channel shall have a slope of 0.0012. Under the same supposition of 13 inches per hour rainfall, the channel from East Creek to Mill Creek will have to carry (see also my last annual report) 110 cubic feet of water per second. For a mean velocity of 4 feet per second the cross section of the channel must therefore have an area of not less than 27 square feet. A channel 3 feet deep and 5 feet wide at bottom with the same side slopes as before would have an area of 28 square feet. The proper slope or inclination in this case would be 0.003, or 3 feet in 1,000 feet, and I propose to make the channel accordingly. Estimates for completion of the works of improvement of the Dalecarlia receiving reservoir and purchase of land authorized by the act of March 3, 1893, for lowering the height of the cross dam at the distributing reservoir, and for cleaning out the distributing reservoir, will be found in the list of estimates appended hereto, and explanations of the same will be found further on in this report under the title "Expla. nations of estimates." THE CONDUIT AND THE CONDUIT ROAD. For want of funds nothing has been done during the last fiscal year in the work of removal of deposits in the conduit, which my inspection of its interior in September, 1891, found to amount to about 15,000 cubic yards. The deposits interfere with the full flow of the conduit, and, with the want of height of the dam at Great Falls, although not to so great a degree as the latter, they are a cause of a deficiency of water in the distributing reservoir, which in turn give rise in summer to complaints to the District Commissioners from householders in the city. Their removal requires the emptying of the conduit throughout its length and the digging up and loosening of the deposits and sluicing them out through the waste gates and valves in the conduit, and is very expensive by reason of the necessity of employment of night labor, which costs more than day labor, and by reason, also, of its frequent interruptions during the refillings of the distributing reservoir required to keep up the supply of the city. The work can not be done by means of the small annual appropriations for repairs of the aqueduct, and in my estimates of 1892, and again in 1893, I asked for an appropriation of $14,000 for this purpose, but it has not yet been granted by Congress. It is a most important work, and I again include the item in my annual estimates. The deposits in the 7-foot by-conduit at the distributing reservoir were removed in July. There was a depth of about 2 feet at the influent gatehouse, and it decreased to about 6 inches at the auxiliary gatehouse. The trouble heretofore had in opening the waste gate in the dam of wasteweir No. 3 was ended by the making of an iron ratchet for maneuvering the gate. Seven hundred and eighty-four cubic yards of flint rock, purchased in February, were crushed in April and piled on the side of the Conduit road above the distributing reservoir, for use in the repair of the road from this reservoir to culvert No. 24 during the next winter. The stone cost 93 and 95 cents and $1 per cubic yard, and the cost of setting up the steam crusher, crushing the stone, and piling it was 41 cents per cubic yard. Sixteen boundary stones were planted between Cabin John bridge. and Griffiths Park bridge (bridge No. 3), and six were planted between the distributing reservoir and the Dalecarlia receiving reservoir. The Conduit road, from the intersection of the Foxhall road to the upper end of the distributing reservoir, was repaired in February with 1,088 cubic yards of crushed bluestone, from the quarries on the Virginia side of the Potomac, instead of the white flint rock heretofore used on this road. The distance is about 5,000 feet, or about 1 mile. The stone was put on about 4 inches deep, and it was thoroughly rolled with the 15-ton steam roller kindly loaned me by the District government. I was induced to use bluestone for this repair of the road by the exorbitant demands of the owners of flint rock in the vicinity as to prices and by an experiment I made respecting the comparative resistance to abrasion of bluestone and flint rock. This was made at a foundry in a large cylinder termed a "rumbler," used for cleaning castings. The cylinder, partially filled with 300 pounds of broken stone and 100 pounds of broken iron castings, was revolved at the rate of 30 revolutions a minute, the fine material as fast as it was worn from the stone falling out through interstices in the cylinder. The loss of weight of the bluestone by this process was found to be considerably less than from the flint rock, but experience has since proved that the latter makes by far the better pavement. It is not so dusty in summer or muddy in winter, and this is doubtless due to the fact that the particles worn from the flint rock are in the form of sand, while those ENG 94--201 |