The Passaic Flood of 1903 | Page 7

and are placed upon the dam by one crew of workmen, it may be safely assumed that they are of approximately equal stability and might be expected to fail almost simultaneously along the length of the dam crest. So sudden a decrease in the effectual height of the dam must lower the water on the dam crest markedly, and as every other probable cause has been eliminated in the case of the recent flood, the explanation of the check in the progress of floods over this dam may be safety accepted as due to carrying away of flashboards. This effect should be apparent in the gage-height records only.
In the flow diagrams (figs. 1 and 2) the effect would not be the same, but the curve would rise more sharply. Similarly, the measurements at the beginning are not correct, as they are calculated according to gage heights measured from the stone crest of the dam. Therefore, a true flood curve at this point would be much flatter at the beginning and rise sharply at a period coincident with the carrying away of the flashboards.
An important difference between the two floods is that the earlier continued longer, but the later one was much higher. The flood of 1902 was caused by the turning of an equivalent of approximately 6 inches of precipitation into the main channel during a period of six days. In the deluge of 1903 there fell 11.74 inches of rain, the greater part of which was precipitated in 36 hours. Thus it is seen that there was in the flood of 1903 a larger rainfall during a much shorter period than in the flood of 1902. Computation shows that the total run-off from the drainage area above Dundee dam during the earlier flood was 13,379,000,000 cubic feet, and that on account of the frozen condition of the ground at that time this amount of water represented practically all of the precipitation. During the flood of 1903 there was a total run-off for the same area of 14,772,000,000 cubic feet, which represents about 66 per cent of the observed precipitation. According to these figures the total amount of run-off in the 1903 flood was only 10 per cent greater than that in 1902, while the actual flood height during the 1903 flood was 27 per cent higher than during the flood of 1902. The above comparison shows, in a striking manner, the effect of the condition of the surface. In the case of the later flood we had, as has been stated in previous pages, an area which had been well watered during the previous summer, and the observed ground-water levels were fairly high. There was, however, sufficient storage capacity in the basin to retain about 34 per cent of the precipitation occurring between October 7 and 11. This water must have been largely absorbed by the earth. The general relations of the floods of 1903 and 1902 can therefore be briefly stated as follows:
General relations of floods of 1903 and 1902.
-----+--------------+--------------+-------------+-------------> |Average | Duration of | Maximum | Total |precipitation.|precipitation.| flood flow. | run-off. | | | | -----+--------------+--------------+-------------+-------------> | Inches. | Days. |Sec.-feet. | Cubic feet. 1902 | 6 | 6 | 24,800 |13,379,000,000 1903 | 11.74 | 3 | 35,700 |14,772,000,000 -----+--------------+--------------+-------------+------------->
<-----------+------------- Run-off. | Duration of | flood at | Dundee dam. <-----------+------------- Per cent.| Hours. [B]100 | 270 66 | 225 <-----------+------------- [Footnote B: Approximately]
In the following table and fig. 2 are recorded gage heights taken at hourly intervals during the crucial part of the flood and the amount of water expressed in cubic feet per second flowing over the crest of the dam at each gage height.
[Illustration: FIG. 2.--Diagram of flood flow at Dundee dam, flood of 1903.]
Flow of Passaic River at Dundee dam, 1903.
------------------------------------------- Date and hour. | Gage. | Flow. ---------------------+-------+------------- |Feet.| Sec.-feet. Oct. 8. 6.30 a. m. | 0.66 | 780 1 p. m. | 1.50 | 3,175 6.30 p. m. | 2.17 | 5,500 8 p. m. | 2.59 | 7,300 10 p. m. | 3.00 | 9,125 11 p. m. | 3.33 | 10,700 12 p. m. | 3.50 | 11,525 9. 1 a. m. | 3.50 | 11,550 2.30 a. m. | 3.59 | 11,950 4 a. m. | 3.50 | 11,525 6 a. m. | 3.66 | 12,300 8.30 a. m. | 3.75 | 12,775 9.40 a. m. | 4.00 | 14,075 10.55 a. m. | 4.66 | 17,650 12 m. | 4.75 | 18,200 1 p. m. | 5.25 | 21,050 2 p. m. | 5.37 | 21,750 3 p. m. | 5.45 | 22,250 3.45 p. m. | 5.37 | 21,750 4.25 p. m. | 5.29 | 21,300 5 p. m. | 5.23 | 20,950 5.45 p. m. | 5.19 | 20,700 6.30 p. m. |