THE SAVA RIVER FLOOD FORECAST AND SREDNJE POSAVLJE FLOOD CONTROL EFFICIENCY

Dušan Trninic

Meteorological and Hydrological Service HR 10000 Zagreb, Grić 3, CROATIA

Phone:    +385 (1) 4851 523

Fax: +385 (1) 4851 520

E-mail:      trninic@cirus.dhz.hr

Abstract: The disastrous Sava River flood near Zagreb, which reached its peak on October 26, 1964, was one of the heaviest floods recorded in the Republic of Croatia. On that day, the absolutely highest water level of 514 cm was recorded, while the flow rate was 3.128 m³ s^-1. Total water wave volume was 1.228 10⁶ m³. It should be mentioned that the volume of the previous wave of 426 10⁶ m³, was also considerable. The direct damage caused by the flood alone were immense: 17 casualties, 50.000 homeless, 10.000 completely destroyrd residential units, 3.297 farm buildings, 120 damaged company properties and the like. The intensity of the flood is best illustrated by the fact that the direct  damage from 1964 flood reached 9.18 % of the national product of the Republic of Croatia. Following the flood, the development of the Multipurpose Hydrodevelopment Solution of the Srednje Posavlje was initiated, including construction of dykes, flood release canals, retentions, spillway embankments, weirs, hydropower structures, and other water structures. Although the system has not been finished, the experts forecast with high probability that recurrence of the 1964 flood in not probable. However, account should always be taken of possible major floods in the future. The paper also describes the lateral Jankomir Spillway, a key structure in the Odra Flood Diversion Canal and the Srednje Posavlje Flood Control System intakes. Particular attention has been paid to development and application of hydrologic forecasting of floods and undertaking of adequate flood control measures.

Keywords: sava river, flood, forecast, flood control, odra canal, lateral Jankomir spillway

1    INTRODUCTION

The article analyses flood waves of the Sava River near Zagreb for the period 1926-1997, highlighting partyculary the disastrous flood of October 1964. It should be mentioned that high and flood waters of the Sava River near Zagreb had also been recorder before 1926, i.e. in 1876, 1880 (flood and earthquake), 1895 (two floods) and 1923. The Zagreb floods caused by the Medvednica Mt brooks of 1898, 1936 and 1989 also should be mentioned. The autors who analysed the problems related to the Sava floods and the referenced works which dealt with the problem are The Study of the Sava River Training and Regulation in Yugoslavia (1972), the first overview of disastrous Zagreb flood of 1964 by Vadlja (1965), and works of Srebrenović (1986), Tusić (2000) and Brkić (2000).

2    DATA

Analysis of flood water hydrographs

Fig. 1 shows the five highest flood waves placed on the common ordinate which corresponds with maximum discharge. Some of the flood water from January and November 1979 and from 1980 was discharged into unfinished Odra relief canal. It should be mentioned that the Odra Canal was first put into operation in January 1979. It should be mentioned that only 31.0 km out of designed 51.4 km of the Odra Flood Diversion Canal has been built.

Analysis of water wave volume above discharge for a determined duration

This analysis is very important for the operation of the flood protection service, so it is covered in more detail. For the one reference discharges, Qr = 856 m³s^-1 corresponding for 5 % of the average discharge duration curve (1926-1992), respectively, the flood wave volumes above these reference discharges have been calculated. The volumes of preceding waves (30 days prior to the maximum discharge of the main wave) have also been calculated for the same reference discharges. The results of this analysis for all 10 waves are presented in Table 1.

Table 1    Ten maximum volumes of analysed waves, the volumes of preceding waves on the Sava River near Zagreb above the reference discharge Qr = 856 m³ s^-1 (5% duration) for the period 1926-1997

The data presented in Table 1 indicate that the maximum volumes of the Sava River near Zagreb were encountered in 1964 and 1933, while the volumes of preceding waves were recorded only for 1964, 1926, and 1974. The maximum preceding wave volume was the one in 1926, and then in 1964. Fig.3. shows the final concept, so far, 40-50 % of designed structures have been constructed. The basic idea behind the Multipurpose Hydrodevelopment Solution was achieving of flood control with safety level of 99.9 % for the Central Posavlje region, including the cities of Zagreb, Karlovac and Sisak.

Since it is not possible to elaborate on the Srednje Posavlje Flood Control System into more detail, a brief description will be given of the lateral Jankomir Spillway, a key structure in the Odra Flood Diversion Canal and the Srednje Posavlje Flood Control System intakes. Its primary purpose is to reduce maximum discharges through the Croatian capital, the City of Zagreb. The existing lateral Jankomir Spillway will undergo certain modifications in the future due to the changes in the Sava River catchment and the riverbed, and realization of the design characteristics of the Srednje Posavlje System. The first modification alternative assumes setting up a new, lower level line of the lateral Jankomir Spillway crest, the other alternative envisages construction of a weir, while the third proposes construction of a weir and reconstruction of a section of the spillway embankment.

3    FLOOD FORECAST

The hydrological forecasrs are clearly an important factor in a struggle against detrimental impact of high and flood water. In addition to a well organized and upgraded network of weather and hydrological stations and availability of adequate forecast models, appropriate flood control (preventive and protective) measures may be undertaken in order to reduce total flood damage. The research has shown that use of the control measures based on hydrologic forecasts could make a 30 % difference in total demage. The forecast models are being improved concurrently with the Srednje Posavlje Flood Control System construction. Presently, two models are used for implementation of operational measures for mitigation of the Sava River flood damage, ie. (1) linear-type multiple regression model used to forecast the water wave peak in a particular system point (its disadvantage is that it focuses exclusively on the maximum water wave forecast), and (2) forecast model for simulation of flood control management within the area bordered with sections Zagreb, Jamnička Kiselica and Crnac.

4    CONCLUSION

Economic and social development and increased urban growth result in flood control turning into ever more complicated and expensive issue. It is further affected by an increasing number of structures located in the flood areas. Therefore, construction of the remaining hydraulic engineering structures within the Srednje Posavlje Flood Control System need to be continued, while higher efficiency and timely flood forecasts triggering adequate flood control measures should become an objective.

The futher scientific research should be focused on antropologic factors that considerably change the runoff conditions and possible consequences that could result from the global climate change.

References

Srebrenović, D. (1986) Primjenjena hidrologija (Applied hydrology) Tehnička knjiga, Zagreb

The Study of the Sava River Training and Regulation in Yugoslavia (1972) Studija regulacijll uređenja rijeke Save u Jugoslaviji, Polytechna-Hydroprojekt-Carlo Lotti & Consulting Company, Praha-Roma-Zagreb.

Trninić, D. (1991) Hidrološka analiza poplavnih i velikih voda na Savi od Radeča do Zagreba u jesen 1990  (Hydrological analysis of high and flood waters on the Sava River from Radeče to Zagreb in autumn 1990), Zagrebačka vodoprivreda 39, 29-36, Zagreb.

Vadlja, J. (1965) Poplava u Zagrebu 1964 (1964 Zagreb flood), Građevinar 1, 26-31.

Tusić, V. (2000) Prognoziranje velikih voda (Flood forecasts), Okrugli stol: Hidrologija i vodni resursi Save u novim uvjetima-Zbornik radova, 143-154, Slavonski Brod.

Brkić, B. (2000) Tehničko rješenje uređenja hidrotehničkog čvora Jankomir (Technical Concept of Developing the Jankomir Hydraulic Engineering structures), Okrugli stol: Hidrologija i vodni resursi Save u novim uvjetima-Zbornik radova, 393-402,  Slavonski Brod.

Fig. 1  Five highest flood waves of the Sava River near Zagreb in the period 1964-1980

Fig. 2   The Sava flood near Zagreb (26 October 1964) – a detail