Virág Árpád: A Sió és a Balaton közös története. 1055–2005 (KÖZDOK Kft., Budapest, 2005)

The common history of Lake Balaton and the Sió Canal (1055–2005)

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THE COMMON HISTORY OF LAKE BALATON AND THE SIÓ CANAL (1055-2005)

operated according to detailed rules by a state agency, though distrust persisted to these days and was transferred to the operators of the new headworks commissioned in 1947 as well.

The Siófok headworks plays an essential role in controlling the water level in Lake Balaton and in draining the marshes in the Sió Valley, though conveying the discharge released into the Sió Canal without overtopping the banks is of comparable importance. The significance of these two issues had been recognised in the early 19th century already, but owing to the lack of adequate hydrological information no quantified project formulation was then possible. Scientific advances up to second half of the 20th century were needed to provide a satisfactory answer. The first sluice with both gates wide open could release a discharge of 17-18 m3/s, while 8-10 km farther downstream flows higher than 6-7 m3/s have already overtopped the Sió banks. The highest capacity of the second sluice was 50 m3/s already, but the conveying capacity of the Sió Canal was still limited to 20-25 m3/s in the early 1920’s, though the professionals were aware of the fact that for controlling satisfactorily the rising lake level the desired conveying capacity of the canal would be at least 50 m3/s along the upstream reach and 90 m3/s farther downstream.

The sluice in the present headworks was commissioned in 1947 with 50 m3/s capacity at the lowest lake level. Parallel thereto work was started on the 40 km long middle-and upstream reaches of the Sió Canal to enlarge it to 80 m3/s capacity. Frequent bankslides owing to poor soil conditions have called for repeated re-dredging operations delaying completion over four decades. The capacity of the sluice in the headworks was increased meanwhile to 80 m3/s in 1977, though no use was made thereof since. Over the more than 100 years long period from 1892 to 2005 the gate operation rules have been changed repeatedly in response to changes in the requirements and public expectations as regards lake levels. The demands of recreation and bathing have grown steadily since the early years of the 20th century. The rules have nevertheless failed to prevent the lake level from rising to El. 105.65 m and 105.60 m in 1916 and 1917, respectively (when the second sluice was still operating), or from dropping to El. 103.70 m Adriatic datum in 1922. The 1.95 m wide water level change over five years has prompted the hydrologists to develop gate operating rules better adjusted to the natural lake regime. The rules in force since 1940 specify for the first day of each month the water level on the Siófok gauge (the „0” mark of which is at El. 104.09 m above the Adriatic datum). Accordingly, the lake level on the 1st November shall be between +45 and +55 cm (El. 104.53-104.63 m) to minimise destruction by any expanding ice cover, and can be raised gradually after ice-melt to +70 - +80, by the 1st of May to +90 - +115 cm, on the 1st of July to +90 - +95 cm limiting thus the range of level change to 50-70 cm within a year. The gate operating rules were amended four times between 1941 and 2005, in that the water levels prescribed for the 1st of each month were modified and the range of change over the years was reduced.

Under the mles adopted in 2005 the highest allowable water level in each month is +110, the optimal one from September to February is +95 and from March to August +100 cm. The lowest allowable levels should be +70 to +85 cm, resulting in an annual fluctuation of 25^40 cm. Up to May, 2005, no experiences have been gained with these rules, in that owing to the sequence of dry, warm years, after May, 2000 no water was released through the Siófok gates and during these five years the water level has remained consistently below the lowest level prescribed for a particular month. Thanks to the more abundant rains in 2004 and the first months of 2005, the water level rose to +100 cm by the 1st of May and is expected to continue rising, making it possible to release again water to the Sió Canal.

Closure of the canal headworks for periods of comparable length is unparalleled on the over 200 years long record of Balaton water levels. The recurring low lake levels, besides causing inconvenience to bathers, have enabled hydrologists to study the changes in the water-balance components, further in the chemical and biological parameters under such conditions. The annual rainfall has fluctuated about the long-term mean, while the inflow has dropped and evaporation intensified considerably. The dramatic