Geoscience Reference
In-Depth Information
Fig. 7.3
The broken South Fork dam, as seen from inside the empty reservoir. The opening was about 130 m
wide near the top; the spillway can be seen on the right just below the bridge. It took the flood surge
roughly 53 min to reach Johnstown. Drawing by Schell and Hogan. (From
Harper's Weekly
, 1889.)
took place in the afternoon of May 31, 1889, after a night of heavy rain, and was caused
by the failure of a badly maintained dam on South Fork Creek, some 23 km upstream
along the Little Conemaugh River and some 135 m higher than the town itself. After the
dam suddenly gave way completely (at 1510) (Figure 7.3), a wall of water, exceeding
15 m in some places, raged down the valley; as it reached Johnstown (at 1607), in a little
less than an hour, it spread out somewhat over a wider area, but its center was still at
least 10 m high. While the main event in Johnstown was over in 10 min, it left more than
2200 dead in its aftermath and near total destruction of the city.
Example 7.1. Some features of the Johnstown flood
Interestingly, the reported features of the Johnstown flood are not unreasonable in light
of Equation (7.7), and some of them can be reconstructed with a few rough estimates of
the effective parameters. Dam breach problems like this are highly unsteady in nature,
and they have been the subject of intensive study (see, for example, Yevjevich, 1975).
However, assume for the present example that the reservoir was large enough, resulting
in a steady inflow into the river channel after the dam had failed. From the eyewitness
accounts, the height of the surge appears to have been of the order of
h
1
=
10 m. With a
mean slope of
S
0
=
0.07 (see Table 5.1),
Equation (5.41) yields a velocity of the water behind the surge of
V
1
(135
/
23 000) and an assumed roughness of
n
=
5.08 m s
−
1
.
=
