The Passaic Flood of 1903 | Page 7

curves in fig. 1 were drawn it has been found by careful
observation that the depressions which occur in the rise of every flood
over Dundee dam are probably due to the carrying away of the
flashboards which are placed upon the dam crest in times of low water.
A review of the gage heights recorded by floods for several years past
shows that the break occurs when the height of water over the dam
crest reaches from 40 to 60 inches. The flashboards used upon this dam
are usually 18 inches wide, and as they are supported by iron rods,
which are of approximately the same strength 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