Author(s): Arnd Hartlieb
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Keywords: No Keywords
Abstract: Large-scale hydraulic model tests were systematically performed to study the probability, process and shape of floating debris jams at spillways as well as their effect on discharge capacity. The test parameters were the characteristics of the floating debris (length, density, stiffness, number and length of branches, single logs or log groups) and the hydraulic conditions of the approach flow (mean velocity, Froude number). Concerning the jam probability it was shown in the tests that the characteristics of the floating debris are decisive. The length L of the floating debris related to the spillway width W is most important. Debris with L≤W has a low probability to get stuck of nearly 0% . But the jam probability increases rapidly for L>W. The test results indicate a linear correlation between L/W and the jam probability. The debris stiffness and branches are important, too: the stiffer the debris and the more and longer the branches, the higher the jam probability. An explicitly higher jam probability results also from log groups than from single logs floating to the spillway. The debris density and the hydraulic conditions of the approach flow have a smaller influence on the jam probability. In contrast they are the most important parameters for the process, the shape and the quantitative effect of debris jams. For a low velocity and Froude number of the approach flow and a low debris density an innocuous loose one-layer debris carpet swimming at the water surface is formed and the discharge capacity decrease caused by the jam is small. For a high velocity and Froude number of the approach flow and a high debris density a more dangerous compact multi-layer debris body is formed and the discharge capacity decrease caused by the jam is larger.
Year: 2012