MODEL TEST OF CONCRETE OVERFLOW COFFERDAM WITH DIFFERENT CREST SHAPES

摘要

Cofferdam is a temporary protective structure, which always has been built before the major hydropower project for preventing water into the foundation pit during construction. For a hydropower project with several years construction period, the cofferdam is always required to discharge flood from the crest, so as to reduce the size of diversion structures, reduce the construction duration and reduce the project investment. A concrete cofferdam with poorly designed crest shape may result in downstream protection slope scouring and cofferdam downstream slope erosion. Combined with a hydropower project, hydraulic model tests of a cofferdam with 6 different crest shapes were conducted on a physical model with scale of 1:60. For each crest shape, velocity fields on the crest and the downstream protection slope were measured, flow patterns near the cofferdam were detailed described, and the mean and fluctuating pressure on the cofferdam downstream slope were measured and analyzed. The test results show that: (1) crest shape with a smaller geometry size compared with cofferdam has a relatively little effect on the discharge capacity of cofferdam; (2) the height of crest and its transition to cofferdam may influence flow patterns near the cofferdam and hydraulic parameters (including velocity fields on the crest and the downstream protection slope, and the mean and fluctuating pressure on the cofferdam downstream slope) more; (3) an appropriate depth in downstream foundation pit is very beneficial for reducing the fluctuating pressure and downstream protection slope scouring. From comparative analysis, shape II is regarded as a relative optimum one not only for the satisfactory discharge capacity of cofferdam, for a lesser degree scouring on downstream protection slope, but also for its significantly decreasing velocity on the crest and the downstream protection slope (in the case of flood overflowing crest) and for a lower fluctuating pressure resulted by flood into downstream channel.