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The Neva flooding

The ontribution Evaluation of the different temporary scales oscillations in the forming of the Neva floods

E.A. Zakharchuk, N.A. Tikhonova

According to existing submissions, oscillations with periods from several hours to several day connected to a variability of a wind and atmospheric pressure into the intensive cyclones moved above Baltic Sea contribute dominant to forming of the Neva floods. However, special quantitative valuations about a ratio of the contributions of oscillations different temporary scales in shaping of the Neva floods was not conducted. To fill this gap we estimate by of statistical analysis the sea level oscillations magnitudes in ranges of different temporary scales on an example of a 25-year"s run of hourly sea level measurements in the Kronstadt Stantion.

Taking into account length and discreteness of sea level realization we well subdivide sea level oscillations according to a Monin" classification into the following temporary ranges:

1. Multi-year oscillations (temporary scales from years to decades of years).

2. Seasonal oscillations (1 year periods and multiple by it harmonics)

2. Synoptic perturbations (tidal and not-tide oscillations; periods - from several days to months)

3. Mesoscale of oscillation (tidal and not-tide oscillations; periods - from hours to several days)

In the beginning, the predetermined runs of the short-period and long-period tides and its amplitudes and phases were obtained with the help of the least-squares method of the harmonic analysis of the tides.

Then with the help of Butterworth"s filter, sea level oscillations with periods more than 2 day were eliminated from an obtained residual hourly run, that has allowed to receive a series of not-tide mesoscale sea level oscillations.

Also with the help of Butterworth"s filter oscillations with periods less than 2 day, and more than 120 day was excluded from the obtained residual series. Thus, the series of synoptic oscillations with eliminated long-period tide was obtained.

To derive of seasonal oscillations initial series of sea level measurements was averaged to day Then, with the help of non-stationary harmonic analysis a components with periods 1 year, 1/2 and 1/3 years were selected with an evaluations of their amplitudes and phases.

The annual averaging of a residual daily mean sea levels was conducted to obtain series of multi-year sea-level oscillations and to estimate its maximum magnitudes.

The results of the analysis have shown that longtide and short-period tides make an unimportant contribution to formation of the Neva floods. The elevation of level over zero significance for the account tides during floods make no more than 1-7 cm (1 - 4 %). The contribution of multi-year oscillations can be both positive, and negative and to vary from -12 up to + 16 cm (-8 ¸10 %). The contribution of seasonal oscillations makes from -13 up to + 58 cm (-8 ¸ 35 %), synoptic - 5 - 68 cm (2-42 %). The greatest contribution to the Neva floods is necessary on mesoscale oscillation: from 36 up to 115 cm (22-58 %).

The ratio of deviations of a level from zero significance in oscillations of different temporary scales during the Neva floods shown in the figure 1.


Fig. 1. A ratio of deviations of a level from zero significance in oscillations of different temporary scales during the Neva floods

It is well visible, that the ratio of the contributions of oscillations of different temporary scales in formation of the Neva oscillations can very considerably differ depending on one year. So, for example, in autumn of 1990 and in winter of 2000 mesoscale oscillation make an important contribution to formation of the Neva floods, while the contribution of oscillations of other temporary scales was insignificant. And, on the contrary, in autumn of 1983, in winter of 1993 both in autumn of 2001 and in winter of 2002 the contributions of mesoscale, synoptic and seasonal oscillations were close on significances.