SCOUR DOWNSTREAM OF BLOCK RAMPS * STEFANO PAGLIARA Department of Civil Engineering, University of Pisa, via Gabba, 22 56100 Pisa - Italy WILLI H. HAGER VAW, ETH-Zentrum CH-8092 Zurich, Switzerland The objective of the paper is to study scour downstream of a block ramp. Block ramps are often used for river restoration, because they dissipate a considerable amount of hydraulic energy. The experiments were carried out in two different channels. The first channel was 0.8 m wide whereas the second smaller channel had a width of 0.25 m. Different sediments for both the ramp and the downstream bed were used. Also, different ramp slopes were tested ranging from 1V:4H to 1V:12H; both the maximum and the medium scour depths were considered. The prominent non-dimensional parameters are highlighted first. Equations and graphs then demonstrate that the results can be interpreted by means of simple relationships for the condition in which a ridge is present downstream of the scour hole. 1 Introduction Block ramps are hydraulic structures often used for river rehabilitation to preserve an ecosystem and to produce an acceptable and sustainable river environment (Pagliara, Dazzini 2002). Block ramps may substitute the classical check dams and in parallel are often used as fish passages. The toe of these structures is characterized by supercritical approach flow with the potential for a scour. The scour hole due to a jet has been recently investigated with new techniques by Canepa and Hager (2003). The objective of this study is to describe the end scour hole downstream of a block ramp to allow for a hydraulic design. 2 Experimental setup The experiments were conducted at the Hydraulic Laboratory of Pisa University. Two channels were employed: Channel I was 0.8 m wide, 20 m long and 0.7 m high. Discharges up to 120 l/s were investigated. Channel II was 0.25 m wide and 3.5 m long, with discharges up to 10 l/s and continuously variable slope. In both channels filtration across the sediment matrix was inhibited to control the hydraulic conditions for the ramp * Work partially supported by Italian MIUR Cofin2001 “Non conventional stream rehabilitation structures with low environmental impact”. 1