Coincidence of Flood Flow of the Danube River and Its Tributaries (original) (raw)

Within the regional cooperation of the Danubian countries related to hydrologic issues, and in accordance with the UNESCO IHP program-phase IV and V, Yugoslavia was put in charge to realise Project No. 4 entitled "Coincidence of Flood Flow of the Danube River and its Tributaries". This project was completely realised by the Yugoslav experts. The experts from other Danubian countries provided assistance regarding data collection and the project review. The results of the study is presented in this publication, which also represents fourth-supplementary part of the monograph "The River Danube and Its Basin-A Hydrological Monograph", which was published in 1986 within the regional cooperation of the Danubian countries. The starting point for the analyses is that the classical approach to the risk assessment aims at determining the probability that a pre-selected value of the flood characteristic will be exceeded by the flood event. This is actually equivalent to determining of the return period of that event. The applied procedure consists of statistical analyses of hydrological data on nearby gauging stations. This approach, from the point of view of engineering, gives satisfactory results in accomplishing a large number of tasks. In particular, it appears to be the most frequently applied approach when dealing with reasonably simple river systems, i.e. when the river does not receive large tributaries within the examined reach. However, when the system encompasses confluences of large rivers, the described approach cannot give a reliable flood characteristics estimate. Namely, for various reasons flood wave geneses in two catchments are different, so that flood peaks do not occur simultaneously at both the main river and its tributary. Yet, the flood wave of one river may greatly affect water level of the other stream. In addition, hydrological data are usually collected at gauging stations located outside of the reach where influence can be realised, so that the interaction of one river upon another is not accounted for by the gathered data. If this is the case, reliable estimation of flood events coincidence on both the main river and its tributary becomes a vital issue. To that end, a methodology to be presented in the publication pertains to the estimation of the coincidence of flood flows at the main river and its tributary. The term "coincidence" is used to denote the probability of simultaneous occurrence of the two random variables, X and Y, which denote random events at the main stream and its tributary. According to the theory of statistics, a two-dimensional probability distribution function of the normally distributed bi-variate random process, X and Y is defined as follows: