Vízügyi Közlemények, 1997 (79. évfolyam)

4. füzet - Szlávik L.-Bálint G.: az 1997 tavaszi-nyári ár- és belvizek és a védekezési munkák

Az 1997 tavaszi-nyári ár és belvizek és a védekezési munkák 467 Floods, excess waters and fighting them in the summer of 1997 by Dr. Lajos SZLÁV I К PhD. and Gábor BÁLINT civil engineers Extreme weather conditions occurred in the summer of 1997 in Central- and Eastern-Europe resulting in 50-to 100 year floods at several locations. In some of the rivers of the region catastrophic floods caused high number of deaths and serious damages (Table I.). The spring and summer of 1997 was full in flooding and excess water events also in Hungary. In the first eight months of the year flood and excess water defence alertness lasted 107 and 168 days respectively. Cost of the defence activities was more than 400 million HUF. In the catchment area of the Upper-Danube June was, as usually, a rainy month around the average, but the heavy rains of middle June had serious consequences to the next months, namely it created very high soil moisture conditions in the mountains (Figure 1). In the Tisza river system and in the areas south, south-east of the Carpathian Basin the high rainstorms of middle June created local flooding catastrophes and played a role in maintaining, increasing or inducing excess water covered areas. The fierce flood hydrograph of the River Berettyó, was also due to this rainfall. In July two larger flood waves were launched from the Upper Danube and flood hydrographs of extreme intensity, height and duration were passing through the rivers Morava and Vág/Vah. In Middle-Europe the dangerous weather situation of the first days of July 1997 was essentially due to the standing cyclone, with its centre over the Carpathian Basin, that was developed between two motion-less anti cyclones which latter were dominating West- and EasternEurope, respectively (Figure 2.). Moist air masses arriving from the north and north-east caused heavy orographic rainfall (Figure 4.) in the spring area of the rivers Vistula and Odera, in the Morava catchment and in the Austrian sub-catchments of the Danube. By the 18th of July a situation very similar to that of the 7th of July occurred again in Middle Europe (Figure 3.). Investigating the precipitation time series of the sub-catchments of Danube (Figure 5.) it is seen that rainfall was very high in the Inn catchment, which is always deterministic to the flooding conditions of Danube, and the rest of the Austrian Danube tributaries as well as the catchments of the Morava and the Vág also had extreme rainfall. Upon the effect of the heavy rains of the first part of July (Figure 5.) very high flood hydrographs were passing through the Danube and its tributaries (Figures 6 and 7). Measured water discharges (Table II) and peaking water stages (Tables IV. and VI.) indicate this situation. The effect of the rains starting on the 18th of July resulted in a second flooding of Danube which was higher than the former and was peaking at Budapest with 7.55 m water stage. Flood peaks at the lower Hungarian stations were also higher than the former ones. The two flood hydrographs of June were considerable one and their return period was in the range of 12—25 years at the various gauging stations. This means that in this century this was the 4th-8th highest ice-free highwater and the flood levees of the lower Hungarian reach had to withstand high water stages of significant duration. Upon the recession of the flood water stages in the channel returned to the pre-flood conditions (as of early July) by the middle of August. In periods with flood the National Hydrological Forecasting Service, operating in the framework of the Institute of Hydrology of the Water Resources Research Centre V1TUKI, prepares regular forecasts for the gauging stations of the Hungarian part of the Danube. The forecasting system was developed int hte 1980s and is based on a rainfall-runoff model called GAPI, which was recently extended with a snow-melt module. Flood peaks are forecasted using a linear regression scheme (Tables III and IV). Experiences with the forecasting of the flood of the summer of 1997 indicated again the pressing need for the further and overall development of the hydrological forecasting system. The development programme is planned to last several years and will include a review of the applicability of forecasting methods, the data to be used and the most modern and up-to-date technical options. In September 1997 the National Water Authority initiated the launching of the „Forecasting 2000" programme and the preparatory work of the development has been started.