The maximum discharge of outburst floods caused by the breaching of man-made and natural dams: Discussion

Abstract

The author is to be commended for developing a very useful relationship for quantifying the effect of a breach of an earth-fill dam. The high costs of safe dam construction exert pressure to reduce margins of safety. Dam safety programs are targets for budget cuts. Construction of higher dams and the need to use problematical sites can force designs beyond the margin of safe precedent. Developments on food plains below dams magnify the consequences of dam failures. Hence it is appropriate to examine the consequences of a dam failure, specifically to compare the maximum discharge associated with the breaching of a dam with the maximum discharge anticipated with a properly functioning spillway, which may also be taken to be a measure of the maximum river discharge to be expected in a state of nature in the absence of a dam. The design spillway capacity Q of several earth-fill dams in Manitoba (Mudry 1986), from the Gardiner Dam (Jasper and Peters 1979), and from the Teton Dam (U. S. Department of the Interior 1977) has been plotted against the reservoir storage volume V in Fig. 1 in a form similar to that used by the author. A line defined by the equation 20Q = 0.72V-5' has been drawn through the points. The equation fits the points reasonably well and has the same form as the author's [ l ] : Q ,,, = 0.72V , ,0.53. Comparison of the equations shows that if the outburst volume (V,,,) is equated to the reservoir volume (V) then the maximum discharge during breach (Q,,,,) exceeds the spillway design flood (Q) by a factor of 20. Comparison of the specific points for the Teton Dam on the figures indicates a factor of approximately 100. Hence it is clear that the effects of breaches of earth-fill dams are truly catastrophic and that design and inspection standards 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3