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Title: EROSION OF STRUCTURED MATERIAL DUE TO IMPINGING JET

Author
item Hanson, Gregory
item Robinson, Kerry
item Cook, Kevin

Submitted to: International Conference on Water Resources Engineering Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/3/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: There is presently considerable interest in evaluating the erosion of materials in several engineering applications such as earthen spillways, dam embankments, and bridge scour. This interest is due to safety concerns related to these structures. Erosion of earthen spillways and dam embankments has the potential of breaching a reservoir and endangering lives downstream. Scour of bridge piers and abutments endangers the structural integrity of the bridge and the lives of people who rely on it for safe transit. Structured materials such as fractured rock are often encountered in these environments. A simple study was conducted to evaluate the point at which blocks are removed from their resting position due to hydraulic forces created by an impinging water jet at an overfall. The blocks were tightly arranged in a grid pattern. As the jet impinged on the blocks, pressure created by the jet was transmitted through the cracks between blocks, resulting in uplift pressures on individual blocks. Based on evaluation of the data, block removal was determined to be a function of the thickness of the block dimension perpendicular to the horizontal bedding plane and the uplift pressures created by the jet flow. This study provides engineers with some baseline information on what hydraulic actions may be expected in these situations.

Technical Abstract: Characterizing the effects of material structural properties on hydraulic threshold is important in quantifying the performance of earthen spillways, embankment foundations, headcut migration, road ditches, drainage channels, and river beds. A simple threshold study in an overfall setting was conducted in the laboratory using five different sizes of blocks, varying from 8-cm cubes to 9- x 20- x 40-cm rectangular blocks. The blocks were systematically arranged without cohesion. Tests were conducted on rectangular blocks in a maximum of four orientations downstream of overfalls varying in height from 30 cm to 100 cm. Block stability increased as the block dimension orthogonal to the bed surface plane increased and as the estimated stagnation pressure in the impingement zone decreased. A dimensionless stability factor was developed based on a simple analysis of the forces on the block.