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Title: ESTIMATING FREE OVERFALL JET SCOUR OF A COHESIVE SOIL

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

Submitted to: American Society of Agri Engineers Special Meetings and Conferences Papers
Publication Type: Proceedings
Publication Acceptance Date: 7/13/2000
Publication Date: 7/13/2000
Citation: Hanson, G.J., Robinson, K.M., Cook, K.R. 2000. Estimating free overfall jet scour of a cohesive soil. American Society of Agricultural Engineers Meetings. Paper #002065. 22 p.

Interpretive Summary: As flowing water falls over an overfall, a scour hole can form downstream. The scour occurs because of the forces created by the falling water. The primary objective of this study was to relate the forces of the water at the overfall and beneath a submerged jet apparatus to the scour that occurs. The secondary objective was to determine the potential of using the ejet apparatus to predict scour at an overfall. Large scale overfall tests were conducted in an outdoor flume, and small-scale submerged circular jet tests were conducted with an orifice. The scour tests were conducted on a cohesive soil, prepared at various compaction moisture contents and densities. Soil placement and compaction methods were observed to influence rate of scour in similar fashion for both environments. The results indicated that the jet test can be used to predict scour in the overfall environment. These results should be of value to those engineers and researchers interested in soil erosion, headcut advance, and scour.

Technical Abstract: Scour from impinging jets is the driving mechanism causing scour below grade control structures, dam spillways, gullies, pipe outlets, etc. The primary objectives of this study were to 1) compare cohesive soil scour of a free overfall jet with the scour of a submerged circular jet and 2) predict free overfall jet scour based on smaller scale submerged circular jet test results. Large-scale overfall tests were conducted in an outdoor flume 1.8-m wide, 29-m long with 2.4-m walls. Small-scale submerged circular jet tests were conducted with a 13 mm diameter orifice. The scour tests were conducted on a cohesive soil, prepared at various compaction moisture contents and densities. Soil placement and compaction methods were observed to influence rate of scour in similar fashion for both environments. Analysis is based on the investigation of theoretical jet diffusion characteristics of a free overfall jet and of a submerged circular jet. Comparison of the excess stress erosion parameters, critical stress and erodibility, from the analysis of both environments indicates that they are both the same order of magnitude. Therefore small scale jet test can be used to predict scour in the overfall environment. These results should be of value to those engineers and researchers interested in soil erosion, headcut advance, scour, and water management.