Étude sur les modèles aérodynamiques de la sédimentation des débits solides en suspension dans les retenues

Study of suspended load sedimentic in reservoirs on aerodynamic models

Institut de Recherches Hydrauliques, Bratislava (Tchécoslovaquie).

Résumé

By means of computation methods the amount of settled suspended load in reservoirs can be determined. The disadvantage is that with these computation methods the problem of suspended load sedimentation is too schematized and they do not take fully into account the shape of the reservoir and the velocity distribution in it. The authors established, by theoretical and experimental analysis that the suspended load sedimentation in reservoirs of hydraulic structures can be studied successfully on aerodynamic models. From the point of view of flow similarity conditions it is sufficient that the phenomena studied are in the automodel sphere of Reynolds numbers and to attain approximate similarity of resistance coefficients according to relationship (2). In view of the actual possibilities of attaining resistance coefficients on a model, this condition usually involves double to quadruple distortion of the model. It follows from analysis of the conditions of suspended load sedimentation similarity on aerodynamic models that the basic condition of suspended load sedimentation similarity is given by the relationship (12). This condition is a consequence of the similarity of the trajectories of suspended load grains, where turhulence is neglected. The aerodynamic model should he designed such that the distortion of suspended grain trajectories by inertial forces not in similarity can he neglected. The values of this distortion can be computed according to equations (21) and (27). A further part deals with the method of construction of the aerodynamic reservoir model. Experimental equipment is proposed for the direct proportioning of ungraded material representing the suspended load (Fig. 2). With this equipement, material with a larger diameter than permissihle for similarity is automatically eliminated. The same equipement can be also used for the grading of material into the requisite fractions. A computation procedure is proposed for the dimensioning of individual parts of this equipment. In 1962, hydraulic problems connected with the completion of the power station for the Samarra hydraulic scheme on the river Tigris in Iraq were investigated on a model. In view of the exceptionally large quantities of sediment carried by the Tigris, the problem of silt in the reservoir was a very serious one indeed. If the water level in the reservoir were to remain at maximum elevation, 75 per cent of the reservoir volume would silt up completely in 15 years. The authors have proposed reservoir regulation and dam operation methods to slow down and control silting in the reservoir. A 1 : 4 000/1 : 1 000 scale aerodynamic model was used for suspended load sedimentation research for urgently-required tests on a series of regulation methods. Fine sawdust was used to simulate suspended load. By means of the method described an approximately equal portion of the whole inflow of sedimented suspended load was attained in the reservoir as by computation without considering turbulence [4, 7]. As the reservoir sedimentation experiment itself takes very little time on the model, it was possible during both this and the investigation with streaming flow to study reservoir silting conditions for several alternative regulation dam layouts. The effects of various regulation measures showed up strikingly by the different types of sedimentation pattern they produced, which enabled the proposed measures to be reliably assessed (see Figures 5 and 6 for instance). The results of the study of reservoir sedimentation obtained by the above method were compared with the streaming flow investigation results obtained by smoke visualisation. It was estahlished that in the given case the most extensive suspended load sediment deposits formed in the regions of most extensive water flow, as is evident from the comparison of Figures 6 and 7. More comprehensive streaming flow tests with regulation in the reservoir found favorable on aerodynamic model were run on a hydraulic model to a scale of 1 : 200/1: 100. The overall results obtained on the aerodynamic model confirmed those of the hydraulic model study. Results of this study show that the described method of experimental research makes possible the solution of the prohlem of reservoir sedimentation with suspended load and the determination of the effect of regulation measures in reservoirs on the settlement of suspended loads. These data show that the method is rapid and not expansive. It provides a means of testing a series of alternative regulation measures in the reservoir from the point of view of their effect on suspended load sedimentation. The advantage of the proposed method over computation, is that it also allows for the velocity distribution in the reservoir, what is not entirely possible by computation alone.