Vízügyi Közlemények, 1973 (55. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

(10) FORECASTING FLOOD HYDROGRAPHS ON THE DANUBE BY MEANS OF THE PASSAGE THEORY By János Csorna, Ph. D., Gézáné Juracsik (Mrs.), Zoltán Szígyárló, Ph. D. (For the Hungarian text see pp. 3) A brief review is presented in the introduction of the forecasting methods published in the relevant literature. In order to compare the different methods these have been applied to an actual example and the regular errors and standard deviations have been compiled in Table 1. The theoretical foundations and the procedure of the passage theory are hereafter described. The functional relation defining the transformed flood hydrograph is given by Eq. (1), the only parameter A involved being computed from Eq. (3) with the hydraulic characteristics of the channel section. The shortest passage time values are found from Eq. (2), while the average passage time is given by Eq. (4). For practical computations the simplified Eq. (5) is suggested. Forecasts have been prepared for the 188 km long section of the Hungarian Danube between Nagymaros and Dombori Puszta, in connection with which the hydraulic characteristics of the experimental river section have been determined. The method used in establishing the basic data needed for performing the computations is described. The parameters AT and A needed for applying the theory are determined first. The results obtained are compiled in the manner shown in Figs. 1 and 2. The ranges of discharge and stage determined from these characteristic curves and the parameters AT and A pertaining thereto are shown in Table II. After the determination of the hydraulic basic data, the composition of unsteady discharge time series is described, for which purpose the use of the approximate formula of Eq. (7) is considered advisable. The practical execution of the computations is dealt with next. The stage records on the Nagymaros, Budapest, and Dunaújváros gages of the three flood waves used for the computations are shown first in Figs. 3 to 5. Concerning the practical performance of flood routing on the basis of Eq. (11) detailed information is provided with reference to Fig. G and Table 111. Out of the routed flood hvdrcgraphs six are illustrated as examples in Figs. 7 to ,9. The results obtained are discussed next, characterizing the reliability of forecasting by considering the regular error and standard deviation obtained from a comparison of the observed and transformed flood hydrographs. These values are shown in Tables IV and V and the following conclusions are arrived at: a

) The regular error of forecasting is practically unaffected by the distance, whereas the standard deviation increases as the square root of distance. b

) The approximate formula in Eq. (7) used for composing unsteady discharge records computed from stage data is perfectly suited for practical computations. c

) The reliability of the forecast is unaffectes by the magnitude of the parameter A, as long as this remains within the range 0,4sAs

0.03. In the interest of comparing the results obtained with data published in the relevant literature, the percentage regular errors and standard deviations of the forecasts considered here have been deteimined and ccmpiled in Tables VI and VII, in a manner similar to that adopted in Table 1. From these tables it has been concluded that the forecast prepared with the help of the passage theory are comparable in accuracy with the results of the methods currently recognized as the most reliable. On the grounds of the foregoing considerations it is claimed as a considerable advantage of this method over the semi-empirical methods that in the knowledge of the hydraulics of the channel the computations can be performed directly without the necessity of knowing the passage conditions of previous flood waves. The advantage of the method described over those based on the solution of the differential equation of unsteady flow is, on the one hand, that the solution does not require the knowledge of the lower boundary condition and, on the other, that