Ouvrages de dérivation et transports solides

Diversion works and sediment transport

Professeur à l'Ecole Nationale des Ponts et Chaussées

Abstract

The author has been commissioned by the International Hydraulics Research Association to draw up Directives for the Development of diversion tunnels for rivers carrying drifting solids. The study examines the present state of the techniques involved in these problems. - The paper also examines the hydraulics and civil engineering techniques concerned, in particular with the traditional delicate balance between the sometimes quite difficult theoretical concepts and purely empirical approaches. The paper studies the different approaches adopted in this regard to the design and execution of a diversion tunnel, recently built on the Rhine river (Strasbourg dam). Although progress is made continuously, the traditional approaches are by no means discredited, and one is faced with the usual problems of (cutoff) below-average pressures and energy waste. The major improvements basically lie in the use of modern construction techniques, e.g., for installing waterproof grout curtains in the ground (cutoff) for systems to minimise wear on concrete aprons. In addition, there has been a trend in the gates used towards a well defined type : sector gates with flaps. The paper discusses the "tertiary" techniques, an expression which covers the many activities involved in the daily operation of facilities including among others : - maintenance of equipment (leakproof quality of gates), access, - elimination of floating debris. i.e., any activity into which experience enters. Substantial progress has been achieved in the analysis of this type of problem. Automatic systems can now be used for many current operation. Such as operation of gates, cleaning of grates. - Questions relating to the elimination of solids are considered, of course, on the basis of the laws governing their movement. The example of one of the largest facilities for eliminating suspended solids for diverting a maximum flow of 460 m3/sec from the Colorado river for hydro-agricultural purposes provides evidence of the cost involved in this type of project, and therefore of the magnitude of the problems which designers try to avoid when building them. The paper thus focusses on the problems of travelling solids because of the many effects they have on the type of facility examined. The following developments can be observed as regards drifting : confidence in conventional formulae is diminishing, and the too rare comparisons of these formulae with experimental results show that they overkill Whereas the present theory has it that serious difficulties should be encountered downstream from diversions of substantial flow (raising of the beds), they virtually never occur in practice, despite the number of books examined by the author. In his view, the explanation is that the present theories are all based on results provided by models and the hypotheses on which these models have been formulated could be inaccurate. In particular, paving phenomena - or the change in the size of the material forming the bed-could have a considerable effect on the river. The current theoretical approach to suspended substances travelling in the water is somewhat rigid, and is perhaps based on over-simplistic hypotheses. In any event, it does not allow the treatment of some fundamental aspects encountered when setting the dimension of facilities such as sand traps. These conclusions are fully in line with those reached at the seminar organised by the River Hydraulics Committee of the AIRH in Lisbon in June 1982, on the "Needs for Research in River Hydraulics". On-site measurement of the two types of drifting mentioned is the only way of enhancing the present standard of knowledge about the transport of solids. The paper ends by underlining that, although there has been some progress in the study and construction of diversion tunnels, much more needs to be done to understand the basic mechanisms. Until then, a systematic study of existing diversion tunnels would allow either research on the motion of solids to be carried out or would provide examples on which the study of new facilities could be based. The French Version of the "Directives" should be ready before end 1983, followed by the English at an as yet undetermined date.