Erosion processes downstream of concrete spilways are a problem that affects a number of large dams around the world. Brazil in particular has a number of dams where they occurred and the Brazilian National Comittee of Large Dams recently established a technical commission to look at the problem in greater detail. Although directly related to the mechanical behaviour of rock masses, such erosion processes have however seldom been treated as a rock mechanics problem. The technical literature deals with them basically as a Hydraulic Engineering problem and a primary objective of the present paper is to emphasize the rock mechanics aspects of the problem.
Les processus d"erosion en aval des chutes en beton affectent plusieurs barrages autour du monde. Au Bresil en particulier il y a quelques barrages où le problème se presente et recemment le Conseil National Bresilien des Grandes Barrages a etabli un comite technique pour etudier le problème d"une façon plus detaille. Bien que ayant un rapport direct avec le comportement mecanique des massifs rocheux, ces processus d"erosion ont que rarement ete traites comme un problème des mecanique des roches. La literature technique, les decrit, en general comme problèmes du Genie Hydraulique et un objectif premier du present travail est de souligner des aspects de Mecanique des Roches du problème.
Erosionsprozesse stromabwarts von Schöpfkellen, bilden ein Problem, das weltweit mehrere Staudame bettreffen. In Brasilien gibt es insbesonderes einige Staudame, wo solches Problem auftri tt. Das Brasi"lianische National Verband fuer Große Dame grundetet neuerdings ein technisches Ausschuß, um das Problem im Detail zu studieren. Solche Erosionsprozesse sind allerdings selten als ein felsmechanisches Problem behandelt worden, trotz ihre direkte Beziehung zum mechanischen Verhal ten von Felsmassen. Die technische Literatur betrachtet es grundsatzlich als ein Wasserbauproblem und danach werden in der gegenwartigen Artikel die felsmechanische Aspekte des Problemes betont.
Energy dissipation in rock masses downstream of concrete spillways is probably one of the least known phenomena in Rock Mechanics. Nevertheless, spillways have been designed to dissipate floods with energy of many thousands megawatts during several hours or even days (e.g. Itaipu and TucuruÍ Projects in Brazil). In some projects, severe rock mass erosion ocurred during the first years of spillway operation, requiring expensive repair works.
Among the several cases reported in the literature, the reader is refered to Tarbela (Lowe et al, 1979), Picote (Lencastre & Cunha, 1966), Kariba (Hartung & Hausler, 1973) and twelve Russian projects mentioned by Taraimovich (1978). In Brazil, Jaguara (Pereira et al, 1991) a ski-jump spillway on fractured quartzite had a 23m deep hole developed in a few months due to imperfect gate operation. Erosion reached more than 30m deep after seven years not endangering the structures and is presently stabilized. On the other hand, there are some projects where spillways presented good performance, with minor erosion like for instance La Grande (Levay & Aziz, 1978) and several projects in Australia (Antunes So and Infanti Jr., 1986), who also reported cases of spillways on basaltic rocks in Brazil that presented almost no erosion even.
