ABSTRACT:

The behavior of unlined rocky foundations in plunge pools and stilling basins of high-head dams is governed by an interaction between aerated high-velocity turbulent flow and a fractured rock mass. During dam crest overflows or spillway functioning, the rock mass located immediately downstream is generally subjected to scour. In some cases, the so formed scour hole may even endanger the stability of the dam and its appurtenant structures. Prediction of the behavior and time evolution of fractured rock in plunge pools is complex and, today, remains very difficult to assess by means of straightforward mathematical techniques. Major problems in developing rock scour assessment methods are the geo-mechanical behavior of the in-situ rock mass and the fact that the physics involved cannot be described and tested on a laboratory scale. The present paper first outlines the major physics and the behavior of scour formation of fractured rock in plunge pools and stilling basins. Second, a physical based and prototype scaled rock scour prediction method is presented. The model has been developed based on prototype-scaled measurements of turbulent pressure fluctuations at water-rock interfaces and inside single rock joints, both of which have been reproduced in an experimental facility. The facility allows generating turbulent air-water flows of up to 35 m/s and is capable to record the corresponding water pressures and hydraulic jacking effects generated by the turbulent flow inside the underlying rock joint.

1 PHYSICS OF ROCK SCOUR
1.1 Introduction

Rock scour occurs when the erosive capacity of water exceeds the ability of the rock to resist it. Typical environments where rock scour is a concern are downstream of overtopping dams, downstream of spillways, in plunge pools, around bridge piers, in unlined rock tunnels, and in channels and at other structures constructed in rivers and marine environments.

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