This study provides a conceptual model of the functioning of gravel-bed rivers during the post-regulation period in Poland and forecasts their subsequent evolution. The main difference between fluvial processes during the pre-regulation and post-regulation period is that they are limited to a zone that is currently several times narrower and trapped in a deep-cut channel. During the river post-regulation period, the construction of additional river training works was significantly limited in river channels. Moreover, all forms of economic activity were significantly reduced in the channel free migration zone, particularly bed gravel extraction operations. As a result of these changes, a limited recovery of the functioning and hydromorphology of the river channel occurred via a return to conditions in effect prior to river regulation. In recovering sections of river, the channel gradually broadens, and its sinuosity and number of threads increase. The overall process can be called spontaneous renaturalization, which yields a characteristic post-regulation river channel. The conceptual model was developed on the basis of the evolution of the gravel-bed river, the Raba River, during the post-regulation period in the Polish Carpathian Mountains.
thermal extremes in the Świętokrzyskie mts ( central poland ) at the turn of the 21 st centuryAbstract : The aim of this study was to determine the number of hot days, very hot days and very frosty days as well as heat waves and cold spells in the Świętokrzyskie Mts ( central Poland ) during the 1981 -2013 period. Datasets of minimum and maximum daily air temperatures from the IMGW-PIB stations : Kielce-Suków, Bodzentyn and Święty Krzyż were used in the study. During the last 35 years 17 heat waves and 12 cold spells occurred in the study area. The number of hot and very hot days and heat waves on the summits of the Świętokrzyskie Mts ( Święty Krzyż weather station ) was 2 -5 times lower than at the valley stations. There was an evident increase in the occurrence frequency of hot weather extremes in the 2001 -2010 period as compared to the end of the past century. There is no clear tendency to change in the number of very frosty days and cold spells. Two indices :WSNC and WSNZ were proposed as tools for the assessment of the intensity of heat waves and cold spells.
Seasonal and multiannual variability of the number, duration and severity of cold spells in Europe were analysed in this paper in cold seasons starting with 1981/1982 through 2017/2018. Data were used from seven weather stations characterised by different degrees of climate continentalism. Cold spells did not occur in the oceanic climate of southeastern England. In the Netherlands only a few cold spells were identified in the entire study period lasting 31 or 67 days in total, depending on the method used. The number, duration and severity of cold spells increased toward the east. In the continental climate of the European part of Russia, 163 or 74 cold spells were identified. Their total duration exceeded 1300 days. In Western Europe the potential period when cold spells occurred lasted about 50 days, from the middle of December to the beginning of February. In Eastern Europe the period when cold spells occurred lasted about 140 days, from the beginning of November to the second half of March. The longest and the most severe cold spells were concentrated in four periods: 1984/1985 to 1986/1987, 1993/1994 to 1996/1997, 2001/2002 to 2005/2006 and 2009/2010 to 2011/2012. Statistically significant trends in the number, duration and severity of cold spells were not evident for the locations analysed over the period from 1981 to present.
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