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Additional parameters for the design of straight ogee spillways
Expanded Title:The Ogee spillway relationship (USBR, 1987; Vischer & Hager, 1999) is used to define the required profile of the spillway section of a dam or hydraulic structure. The Ogee relationship describes the bottom nappe associated with a sharp-crested weir. The current relationship accommodates the influence of the unit discharge, the angle of inclination of the upstream wall face, as well as the relationship of upstream pool depth to the total upstream energy at the apex of the structure. In cases where the discharge flow rate exceeds the design flow rate the nappe coheres to surface of the spillway and a sub-atmospheric pressure region is generated that could lead to cavitation (Savage & Johnson, 2001; Momber, 2000). Cavitation usually occurs during a unit discharge, in excess of the design head, when the surface pressure could reduce at positions along the spillway to sub-atmospheric pressure. This may cause the formation of vapour cavities. The vapour cavities (also referred to as miniscule air bubbles) will progress along the flow path due to the high flow velocity on the spillway to a region downstream where sufficient pressure is available leading to the collapse of the air vacuum. This generates localized high pressures. Should these vapour cavities collapse near the spillway structure, there will be some superficial damage to the spillway’s surface where the vapour bubble has collapsed. This cavitation damage can ultimately result in substantial erosion and, if ignored, will subsequently cause failures of the spillway chute. Minute cracks, offsets and increased surface roughness intensify this cavitation process. The extent of cavitation damage is a function of the cavitation indices at key locations on the spillway chute and the duration of flow over the spillway. This emphasizes the need for a geometric, accurate and precise spillway profile to reduce the possibility of sub-atmospheric pressure formation (U.S. Army Corps of Engineers, 2009). The current Ogee spillway relationship lacks to incorporate the asymmetrical cross sectional upstream geometry of the spillway, the relative orientation of the spillway with regard to the approaching flow and the curvature of the spillway in relation to the depth of the structure.
Date Published:01/12/2015
Document Type:Research Report
Document Subjects:Water Resource Management/IWRM - Planning and development, R & D - Methodology
Document Keywords:Guidelines
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:2253/1/15
ISBN No:978-1-4312-0724-4
Authors:Van Vuuren SJ; Coetzee GL
Project No:K5/2253
Originator:WRC
Organizations:University of Pretoria
Document Size:4 866 KB
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