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Title: INVESTIGATION OF SCOUR BELOW AN OVERFALL

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

Submitted to: American Society of Agri Engineers Special Meetings and Conferences Papers
Publication Type: Popular Publication
Publication Acceptance Date: 7/22/1999
Publication Date: N/A
Citation: Robinson, K.M., Hanson, G.J., Cook, K.R. 1999. Investigation of scour below an overfall. American Society of Agricultural Engineers Meetings. Paper#992037. 12 p.

Interpretive Summary: As flowing water encounters an abrupt overfall, the water accelerates over the fall and drops into a plunge pool. The hydraulic forces transmitted to the plunge pool surface can cause excessive erosion and scour. Research was conducted to examine the rate of vertical scour when the plunge pool water level remains constant. As additional scour occurs, the pool depth increases. The amount of scour observed over a period of time increased linearly as the tailwater depth increased. Equilibrium scour depths were not obtained. As the moisture content of the compacted soil in the plunge pool was decreased, the observed scour rate increased. As the soil density was increased, the scour rate decreased. This study provides information that will improve our understanding of plunge pool scour and headcut erosion. Researchers, engineers, and managers of hydraulic structures should find this information of interest.

Technical Abstract: Scour below an overfall contributes to headcut instability and gully advance. Five large-scale tests were conducted by preforming an overfall in a compacted cohesive soil bed. Different soil conditions were examined while attempting to hold the tailwater at a constant elevation. Water traversed a straight drop overfall and impinged on a horizontal surface downstream. The rate of vertical scour was measured in the plunge pool downstream of the gully overfall, and typical erosion behavior is discussed. Scour tests ranged from 1 hour to over 53 hours in length. The vertical scour rate was observed to increase linearly as the tailwater depth increased. The moisture content at the time of compaction was indirectly related to vertical scour rate. Increasing the soil density caused the scour rate to decrease. This study provides additional information for topics such as plunge pool scour and headcut erosion.