Rensselaer Republican, Volume 21, Number 44, Rensselaer, Jasper County, 4 July 1889 — THE CONEMAUGH DAM. [ARTICLE]

THE CONEMAUGH DAM.

Report of in Engl-jearlng Expert as to the Cause# of the Jdhnstown Catastrophe. H. W. Brinckerhoff, C. E., of the editorial staff Of the Engineering and Building Record has imfle a thr ‘ t - rm g K "Investigation of the broken Conemaugh dam and thus reports to his paper the results of his observations: “Arriving at the scene of the break we found an earthen embankment stretching a distance of about 901) feet on its crfe3t across the valley of the Conemaugh. Its central portion had been completely washed out, and even below, the original bed of the stream, the rush of the released water having scooped out quite a basin just below the dam.

“The width of the break at the crest was about odO feet, narrowing to perhaps less than one-fourth of that amount at the bottom. The structure of the remaining portion thus exposed in section showed a .tolerably homogeneous mass of stiff, gravelly clay, well-covered with rip-rap oiq both slopes, the lower slope having quite a growth of trees and bushes on it. My first Look was for the heart wall of puddle or masonry that is usual in large earthen dams, but a closer examination of the material left no doubt in my mind that the omission was due to the fact that the dam was practically all puddle of very good quality, “This had apparently been deposited in pretty uniform horizontal layers and the exposed portions showed a high resistance to the erosion of the flood, standing with very steep, and in somo cases perpendicular, slopes. •Around the north end of the dam a spillway or waste weir had been cut through the solid rock, of which both sides of the valley at this point appear to be composed, more or less covered, however, with soil. This waste weir was sixty-five feet wide at its narrow*" est point before it commenced to descend materially. It was somewhat wider at its upper end, enough probably to amply offset_the obstruction pc-_ easloned by a fish-screen three feet high or so, attached to the postsroflT'

carriage bridge which crossed at that point. The bottom of the waste weir at its inlet is about ten feet below the crest of the dam at its nearest pal nl. The crest of the dam, some ffOU feet long between the weir end and the break, sloped uniformly from the former to the latter point, as could be seen by sighting along it a line from the weir end over the edge at the break falling several feet below the edge of the crest on the opposite side

of the break. As near as could be estimated by measuring up from the water-mark on_.the rip-rap. and by sightiiig across from the bottom of the waste weir to the water-mark on the other side of the reservoir the crest of the dam at the side of the break next to the weir was about seven feet above the bottom of the weir. “At the bottom of the break, the line of the inner toe, were the remains of a foundation of a gate-house, from which some large iron pipes had originally extended through the dam for the purpose of drawing off the water

when the reservoir was used for its original purpose of supplying the state canal. According to the statement of an old resident of the vicinity some time after its use for this purpose had • been discontinued a leak along these pipes or the tunnel in which they were laid oaujod.the tral portion of the dam; the break, iFowover, does not apjiear to be w extensive as the present one, and caused no serious damage. Wheiilts breaeh was closed to form a lake for its recent use of boating and fishing, the pipes for drawing offetvere no longer needed, and were therefore omitted. “What sort of work was done and what material was used in repairing the break, there is nothing now loft to show, as far as can be judged from the general appearance of things, and the fact that both leaks occurred at the same point. It will therefore be understood that all that has been said so far probably applies exclusively to tfye original work. “The depression of the crest of the remaining portions of the dam toward the center is most likely due to the settlement of the embankment, which would naturally be greater where the dam was highest. If, as is possible, the crest of the new work was made fair with tho old at the start, it would in course of time settle to even a lower point, perhaps four or five feet below the elevation of the dam at the ends. This would, in a measure at least, account for the rapid destruction of tho dam. The: old resident before mentioned, whd isaVv the dam go, observed no leakage through the dam, but said that the water commenced to flow over the crest of the dam at its center. This continued a couple of hours or so before the flow becamo serious; then tho lower slope began to cut away rapidly under the increased flow, the sides of the breach falling la mm time iio time, until In about half an hour the breach was complete. • - “The overflow was due to a combination of two causes; first, the inadequacyjof the waste weir, which, however ample It may seem to tfiave been, was, plainly unable to carry off the' vast volume of water that poured into the reservoir, and second, to the .depression reduced not only the effective discharge area of the weir but also the velocity of the discharged water, thus

very greatly reducing the weir’s capacity for relief. ‘“The position as well as the amount of this depression wa,s a serious matter. Being in the center, the overflowing water could wash the widest portion of .Jtllfi outer slope imeHnit the dam to the bottom in the shortest possible time, both sides of the breach caving in once; while, had the overflow occured at either end, nearly two thirds of the dam weald h ive had to have been washed before the resevoir could have been emptied; and, as in addition to this the water could only act on one part of the dam at once, the time requiredto empty the reservoir would have been increased in a proportion even greater than that of the material to be washed away.

“An effort was made in this direction when the water began flowing over the dam by cutting through at or near the end o f the darn farthest from the waste . weir, but rock was soon met with and a sufficient outlet could not be made in season to prevent the overflow. “Though the section of the dam appeal’s to have been ample to resist overturning or sliding even with the reservoir full to overflowing it is easy now to see that an increase in the length of the lower or outer slope, while it would not have prevented th.e destruction of the dam, would have materially retarded its progress, and thus not only have reduced the volume of the ensuing flood by letting the water out more slowly, but have given more time for those below to escape its coming- , “To sum up, it may be said that the dam as originally designed seems to have had sufficient stability and to have been well built. I-n grading the crest, however, not enough allowance appears to have been made for subsequent settlement, and this defect was probably increased when the dam was repaired, even supposing that no other elements of weakness may not have been introduced. This concentrated the destructive effect of the overflow at the worst possible point and the disaster followed.

—“As-cumpareJ to the TbTumeTlFtEe" stream when observed, the waste weir appeared to be of ample dimensions, especially if the crest of the dam had been everywhere' ten feet above the bottom of the weir. “Whether the size of the weir bore any reasonable proportion to the area of the water-shed of the South fork, especially if denuded of trees, or to any observed flood flow of the stream itself, it is of course impossible to determine it present.”