Vízügyi Közlemények, 1999 (81. évfolyam)

3. füzet - Papp Ferenc: Árvízvédelmi biztonság és kockázat

Árvízvédelmi biztonság és kockázat 349 was first rapidly increasing. This was followed by the increasing safety of these facilities, which resulted in the decrease of the area and the number of events of flood inundation. The frequency of flood-inundation, as indicated by the figure, was decreased with an order of magnitude and it was only 0.3% in the 20th Century. Safety means a condition without hazards. The height-safety of flood defence facilities (mb) specifies the height (#

k) with which the crest level of the levee is (or should be) higher than the design flood level (#

m). Stability is calculated in the practice, still today, by relationship 2. (Table /.). In the field of flood defence the determination of the flood-load (water level and duration) and the resistance (of earth works and subsoil) is subject to uncertainties. Consequently it would be highly justifable to specify safety in probabilistic tenns. As opposed to thsi, the present Hungarian practice (in compliance with the respective planning and control regulations) consider safety with fxed values and factors. Comparing safety requirements with the results of the survey of the state of the levees one can specify the level of flood-resistance for each river section and river reach. The costs of eliminating the deficiencies can also be calculated on this basis. The results of a detailed survey made in 1995 indicated that the maintenancq; cost of the first-order flood defence facilities is 2 billion HUF in a year and the development cost (to the level requested by the relevant technical regulation) would be 58 billion HUF. As opposed to this the Government supported development activities were continuously decreasing until 1995 and started to increase slowly afterwards (Figure 2.). Under such conditions the technically needed âevelopment can only be made in 45—50 years. Considering the extraordinary natural damages of the Earth, in 1997, with a 2 year return period (with probability p=0.5) one obtains the experimental values given in Table II. Although these data also include storm-surges (tsunami), the fact is that damage and risk data are the highest here, in every column. Table III. presents some of the experimental flood risk values, as derived on the basis of foreign damage data. In respect to Hungary old data on the economic damage done by floods are rather unreliable. Consequently it was desirable to calculate the flood damages for the period before 1950 on the basis of the inundation data, which were collected for Figure /, making use of area-specific unit damage rates. For the period after 1950 the flood damages could be assessed on the basis of actual losses, as included in the inventories (flood-inundation damage + the cost of flood defence), and compared to the relevant GDP. The data obtained in this marner are shown in Table IV. On the basis of Table IV it can be stated that the flood defence conditions of Hungary are favourable also on the basis of empirical risk characteristics. The large-scale river regulation and flood control works of the last century contributed to this to a large extent. By the end of the last century the flood control system of Hungary has practically been completed, as of the present stage. This resulted in a decisive change in the flood hazards in the areas prone to inundation. Successful flood defence activities were largely due to the high professional and technical level of the defence actions and to the high (evel of organisation. The economic risk has been considerably decreased after the establishment of the flood control system and the technical-professional development of the defence actions. The risk of human casualties was practically reduced to zero. Nevertheless, population and the value of national assets have been increasing, accumulating in the protected areas. Straightening of defence works could not cope with this development rate and thus the economic risk of flooding was continuously increasing. Today the economic risk is much higher than that before the construction of the flood control system. The evaluation of the state of flood control on the basis of risks involved means that economic and other consequences must also be taken into consideration in addition to the technical safety. The most important conclusions, which can be drawn on the basis of the analysis made, are as follows: — Among the damages caused by naUiral phenomena the highest economic and human-life risk is due, both nationally and globally, to floods; — In Hungary there was no human life lost in floods after the completion of the flood control system;