Author(s): James Yang
Linked Author(s): James Yang
Keywords: Floods; Dam safety; Spillway; Floating debris; Countermeasure
Abstract: During extreme floods, a large quantity of floating debris can be produced due to erosion, drifting with the flood water and eventually accumulating upstream of the dam. In Sweden, the debris is typically composed of spruce and pine trees of lengths up to 25 m. The debris may clog up the spillway openings and reduce the discharge capacity. As this is a dam-safety risk, proper countermeasures need to be taken to handle the debris. An innovatively designed visor is devised to prevent the spillway from clogging with debris. It is formed to have a horizontally semicircular shape, with sloping beams supported on a platform. In one project, a concept model is tested, while in the other, a more realistic model according to scale is examined. Taking account of prototype tree lengths, 100-130 small trees with a density of between 650-1000 m3/s are used. Without the visor, the trees block all the spillway openings from the threshold to the water surface and the discharge capacity is reduced by some 30% . The tests have shown that the visor effectively stops the floating debris and the spillway openings are kept free from the debris. Trees with low density approach the visor afloat and are intercepted in the surface water. Debris with higher density is dragged down when approaching the visor. As a result, it covers the visor height, leading to a reduction in the flow passage. The study demonstrates the use of the visor to maintain relatively free spillway flow and a marginal reduction in the discharge capacity.