Vízügyi Közlemények, 2002 (84. évfolyam)
1. füzet - Rátky István-Kovács Sándor-Váriné Szöllősi Irén: A Vezsenynél tervezett hullámtéri csatorna hidraulikai hatásának elemzése
60 Rá tky I. —Kovács S—Váriné Szöllősi I. 2000. The initial conditions were set as steady uniform flow: £>-730 mV 1 and at water level 0.30 m below the bank-line. At the given geometric data (Figure 1.) and channel bottom slope the variation of the smoothness coefficient with the water level was such in the model, which resulted in corresponding Q-H values (that of the vicinity of the Vezseny river bend). These were as follows: Overflow into the flood plain begins at ~ 780 m 3s _ 1; the overflow over the "summer dikes" of 2.2 m height begins in the range 1,500-1,600 m 3s _ 1; at peaking flow the water depth on the flood plain was 4 m. Table I summarises the results obtained with the assumption of various hydraulic roughnesses and of arrangements of the planned establishment. The exchange of flows between the main channel, the flood-berm and the flood-plain channel are shown on Figure 3. The more important conclusions are as follows: — A flood-plain channel of 500 m length could result the lowering of flood levels by about 0.10 m (at conditions when the bakcwater effect of the flow over the summer dikes causes, at the present, the rise of the water levels); — The spillway effect the better, the higher the summer dikes and the worse the flow-conveyance to the flood-plain; — Eventually the largest reduction of the water levels were achieved at channel width Bcsaf^ 700 m, but it is not worthwhile to increase the width over 500 m (as indicated by Table /.). The calculations were repeated with the actual data of the flood hydrographs of November 1998 and March 1999 of the Szolnok section of the river (Figure 4. ). Figure 5. shows the upper enveloping curve of the calculated water levels of the 1998 flood. The water level lowering effect of a floodplain channel of 500 m width is shown in Table II. in function of the height M of the summer dikes. Comparing the results of tables I. and II., one may state that the results are similar in their order of magnitude and trends: a flood-plain channel of 500 m width could have resulted during the floods of 1998 and 1999 in nearly 0.1 m drop of the flood water level. Figure 6. shows some important characteristics of the water exchange between the "spillway channel", the main channel and the flood-plain. The following conclusions can be drawn from these results: — In extreme conditions the presently "bad" water conveyance conditions of the flood-plain could cause, together with the effect of the high summer dikes, flood levels, which are by 0.11 m higher than the "permitted" levee-crest levels. These latter corresponded in the past to the "excellent" water conveyance conditions into the flood plain. — The value of 0.10 was obtained for only one site (over a ~10 km river length), with the assumption of the construction implemented. The favourable and unfavourable effects of subsequent control measures are superimposed on each other. Fortunately unfavourable impacts did not result with a maximum superimposement, due to the flattening out of the backwater curves. Nevertheless, they could result, cumulating over a longer river reach, in water level rise many times higher than the 0.10 m. — Water level reducing effects (drawdown surface curves) are also cumulating in the subsequent reaches, although not with their maximum drawdown. One of the preconditions of achieving these results is that the unfavourably impacted sections and the control structures (channels) are closely following each other. This can ensure that the water level lowering effect of an upper control measure will not diminish before that of the next one commences. * * *
