Hidrológiai Közlöny 1967 (47. évfolyam)

9. szám - Dr. Hankó Zoltán: A nagymarosi vízlépcső kismintakísérlete. II. Az építési körülzárás vizsgálata

394 Hidrológiai Közlöny 1967. 9. sz. Hankó Z.: A nagymarosi vízlépcső auf die Durchflussöffnung zu sichern (Abb. 4, Abb. 10.). Die Ausbildung laut „09" hat sich trotz der günstigeren Linienführung aus der Sieht der Verstopfung als gefáhrlieher erwiesen. Der Eistrieb konnte den kleinen Krümmungsradien und den plötzlichen Richtungsánderungen nicht gut folgen (Abb. 11.). Model Tests 011 the Nagymaros Barrage. II. Study of the construction pit enelosure By Dr. Z., Hankó On behalf of the Design Bureau for Hydraulic Planning (VIZITERV) model tests were performed at the Hydraulic Laboratory of the Research Institute for Water Resources Development (VITUKI) on the Nagymaros Barrage, to provide data for the design. On problems related to the preliininary studies and model design, construction and identification of the model a detailed account has been presented in Part I of this paper. Investigations associated with the construction pit enelosure will be desribed in the present Part II. Results of studies concerned with individual structures of the barrage will be dealt with in Part III. Construction of these structures is contemplated by enclosing successively different sections of the bed. It is required to exclude floods up to 10% probability — corresponding to a discharge of about 7000 cu m/sec — from the construction pit. The lieight of the enelosure is controlled by this requirement and has been set at erest elevation 106.8 m above mean Adriatic level (El. 106.8 m.A.f). Higher flood waves will be tolerated to inundate the construction pit and flow over the enelosure is expected to reduce the backwater effect ereated. The critical phase of different enelosure alternatives reflecting the organization and progress of construction work has been studied in a model to check whether the ensuing surface profilé, distribution of velocity, flow patterns and ice passage conditions comply with the eriteria specified by the designer. Two fundamentally different methods and scherlules of construction have been developed by VIZITERV. In the alternative illustrated in Fig. 1. construction is contemplated in two stages with one pit on each bank of the river enclosed in the first stage simultaneously. The flow cross section is located between the two enclosures in the vicinity of the originál main current line. In the ease of the alternatives demonstrated in Figs. 2. and 3. construction is contemplated in four stages. In the first stage there is only one construction pit adjacent to the left-hand bank, although the area enclosed is larger. In the cases illustrated in Figs. 1. and 2. it is the first stage, while in that according to Fig. 3. the second stage of construction whieh is to be regarded as critical. Different layouts corresponding to critical stages of construction were eomputed exclusively on the basis of results obtained from tests performed with the flood of about 10% probability. The considerations included in the comparison were the backwater ereated, the ensuing flow pattern, the distribution of surface velocity and seouring of sediment from the bottom (Figs. 4. to 6., Tables 1. and 2). Experiences gained during these comparative tests warranted a more detailed study of the designs "09" and "02". In the design "09" bank protection and parallel baffle dyke was suggested together with the most favourable trace for preventing unwanted bank erosion on the right-hand bank. Filling of the right-hand bank was recommended as an additional protective measure against bank erosion. By removing the alluvial material from the bottom down to the andesite bedrock with the help of dredging, headwater levels could be lowered appreciably, as a result of whieh design "09" satisfied similarly to design "02" the eriteria specified. Detailed studies were performed with ten different discharges ranging from the lowest discharge on record, 668 cu m/sec, to the flood discharge of 0.01% probability, corresponding to a rate of flow of 14 400 cu m/sec. The flow pattern, surface profilé and velocity distribution were deterrnined for each discharge. Conditions of passage of ice floes were alsó studied. The problem of seouring was neglected in the more advanced stages of study, since in the case of dredging down to the andesite bedrock before starting construction no subsequent seouring, or deposition was considered likely to oceur. Flow conditions are illustrated by the example presented in Fig. 7., showing the flow pattern ensuing during the passage of a flood of 10% probability. As will be perceived from the flow pattern, the flow pattern is considerably modified by the construction of enelosure, yet the arrangement thereof has no adverse consequences. Two methods were adopted for observing velocity conditions. One of these consisted of the evaluation of surface velocity patterns illustrated alsó in Fig. 7., while in the second velocities were observed using a Pitot-tube. Observed results were used in constructing Fig. 8. In Table 3. velocities pertaining to variáns typical cross sections have been compiled for the discharge of 6000 cu m/sec, beyond whieh navigation is no more possible. From the values tabulated it will be seen that the highest flow velocity for whieh navigation must be prepared is 4 m/sec. Results of observations concerning the surface profilé have been compiled in Fig. 9. Differences of stage — Zl — between the construction site and the Nagymaros gaging section have been entered on the abseissa, with ordinates representing the stage on the Nagymaros Gage, h. From these curves it was concluded that the maximum differential surface elevation at the enelosure should hardly exceed 1 m under conditions of peak flow. From tests performed in connection with the unobstructed passage of iee runs it was concluded that in the design "02" a row of cell eofferdams would be preferable to guide the ice towards the open cross section (Figs. 4. and 10.). In spite of the superior alignment of the cofferdam, design "09" presented a greater ice-jam hazard, since the ice cover in movement found difficulties in lollowing short radius curvatures and sudden changes of direetion (Fig. 11.).