Prognoza wpływu stopnia piętrzącego "Wyszogród" na wody gruntowe

Abstract

Wpływ stopnia "Wyszogród" obejmie przede wszystkim taras zalewowy, a tylko lokalnie nadzalewowy. Obszar wpływu ma szerokość od 0,3 do 4,5 km, zależną od projektowanych rzędnych zwierciadła wód w sieci drenażowej. Sieć ta będzie likwidowała wpływ zbiornika na wody gruntowe, nie powodując większych wahań zwierciadła wód gruntowych, które najczęściej wyniosą ok. ±0,5 m w stosunku do rocznego stanu minimalnego z 1966 r. W całym obszarze wpływu stopnia zwierciadło wód gruntowych będzie stabilne, zależnie od sieci drenażowej, a jego wahania wyniosą ok. 0,6 m w stosunku rocznym. Na przeważającym obszarze wpływu omawianego stopnia zwierciadło wód gruntowych wystąpi na głęb. 1 - 3 m.

A PROGNOSIS OF INFLUENCE OF THE WYSZOGRÓD CASCADE ON GROUNDWATERS

The second version of the project of construction of cascades on the Vistula River involves location of front dam of the Wyszogród cascade at Kępa Polska (about 20 km NW of the mouth of the Bzura River) in lower section of that river. This dam should lead to about 12 m rise of water level in the river, the effects of which arc to spread back as far as the next planned cascade (Cząstków cascade) on that river and the existing Dębe cascade on the Narew River. Water reservoir will be delineated by embankments (so called lateral dams) and, in the case of a marked part of this section of right bank of the Vistula River, Pleistocene morain plateau. Areas adjoining the embankments will turn into depressions in relation to rised water level in the reservoir so it is planned to drain groundwaters by means of newly made or adjusted trenches. Water from drainage network will be pumped back into the reservoir by pumping stations or it will freely flow into the river below the front dam.

Quaternary horizon in the first and most important aquifer hers. The horizon, mainly built of sandy-gravel sediments, is markedly varying in thickness (from 8 to almost 100 m). Groundwater table is free here, except for areas of occurrence of organic muds, and usually situated close to terrain surface. Groundwaters are in hydraulic contact with river ones so oscillations of ground water table in areas directly adjoining the rivers depend on changes in water level in the latter (Tables  1, 2; Fig. 1). The maximum extent of influence of the Vistula River on ground water table oscillations is generally estimated at up to 1.5-2.0 km from embankments.

Percolation properties of Quaternary sediments were studied by field methods in boreholes (by overflooding and pumping) under natural conditions and in laboratory, on samples with disturbed structure but appropriate density (Tables 3, 4, 5).

The minimum annual water levels in river, especially many-years' minimum levels, appear to be the best for prognostic comparative analysis of influence of a dam on groundwaters in direct neighborhood of river. For the analysis, there were available data on annual water levels (minimum and maximum) as well as results of weakly measurements of groundwater table, taken in 1966. Moreover, it became possible to take into account for the first time in such analysis the design of drainage network. Ordinates of curves of depression of prognostic groundwater table were calculated using mathematic equations proposed by A.W. Romanow (fide S.K. Abramow et al., 1964), which take into consideration one or two rows of drainage trenches (Fig. 2).

Three hydrogeological maps in the scale 1 : 50,000 were compiled in order to obtain spatial reconstruction of changes in prognostic ground water table after filling up the reservoir. The maps, compiled assuming appropriate work of drainage network, show:

-prognostic hydroisohypses,

-changes in position of prognostic ground water table in relation to the minimum annual table in 1966, and

-prognostic variability in depth of groundwater table after filling up the reservoir.

The characteristic minimum and maximum annual water levels and prognostic ground water table are shown in Fig. 4. It follows that the cascade will influence ground waters in the floodplain and, locally, first terrace, i.e. in area varying from 0.3 to 4.5 km in width. The width of that area and the scale of changes of ground water table will depend on efficiency of drainage network. The designed network should surpress the influence of water rise in the reservoir so it may be expected that the groundwater table will be situated ±0.5 m in relation to the minimum annual level in 1966. In the floodplain, the table should be situated at depth of 1.0 - 3.0 m, i.e. close to the present one.