Sedimentary records of 2010 and 2011 Warta River seasonal floods in the region of Poznań, Poland

Abstract

The Warta River near Poznañ (West Po land) rep re sents a me an der ing low land river mod i fied by hy dro-en gi neer ing pro jects. Re cently, two large floods oc curred dur ing the sum mer of 2010 and the win ter of 2011. Rapid re sponse sur veys were conducted to doc u ment ero sion and sed i men tary re cords of the floods (spa tial ex tent, grain size, com po si tion, grain microtextures). Fol low-up stud ies, which were con ducted dur ing the two years af ter the floods, as sessed postdepositional changes in the flood de pos its. A thick sand layer formed that ranged from an av er age of 10-15 cm (sum mer) to 30-35 cm (win ter), build ing nat u ral lev ees, side bars and cre vasse splays. The sand lay ers con sisted of fine-and me dium-grained sands that were well-sorted, fine skewed and sourced from the river chan nel. The lon ger but smaller win ter flood pro duced thicker and coarser de pos its at dif fer ent sites com pared with the sum mer flood. An or ganic-rich mud layer and al gal mats, which were short-ex ist ing, were also de tected on the floodplain. The study proved that the flood re cord on en gi neered rivers may be only in the sand bod ies near the river chan nel and their pres er va tion is mainly con trolled by their thick ness. A sin gle site may not re cord all floods, as par tic u lar events tend to leave de pos its in var i ous places.

Sedimentary evidence of extreme storm surge or tsunami events in the southern Baltic Sea (Rogowo area, NW Poland)

“…The geomorphological po si tion, ex tent, grain size char acter is tics and pres ence of ma rine in di ca tors in layer A seem to fa vour a storm or tsu nami rather than a flood (e.g., Skolasiñska et al, 2015;Matsumoto et al, 2016) or wind-blown or i gin of the event layer. How ever, one can not ex clude some post -depositional mod i fi ca tion of the event layer due to wind trans port after de po si tion, as was ob served for in stance af ter the 2011 Tohoku-oki tsu nami on Sendai Plain, where sandy tsu nami depos its were partly re worked by winds (Rich mond et al, 2012).…”

Environmental impact of flooding: studies of ‘self‐cleaning’ of river ecosystem during consecutive flood events

“…The grain‐size analysis has been provided on a population of 165 samples (not only those whose geochemical analysis results are presented in this paper but also others collected during investigation – Skolasińska et al ., ) for the four groups mentioned above (sand, mud, organic, anthropo). The results of grain‐size analysis demonstrate that the groups differ significantly in terms of grain size distribution (Figure e).…”

“…It may, therefore, be stated that the two floods (in summer and winter), which differed in magnitude and duration period, also differed significantly in terms of sedimentological record (Skolasińska et al ., ). The sediments themselves did not exhibit differences in textural features (Figure f), but there was a clear difference in the amount of deposited sediments.…”

“…Each averaged sample from the surface layer of the sediment was placed in a polyethylene container using plastic tools. Samples for geochemical analysis were taken parallel with the samples for sedimentological studies -details on the location of sampling points can be found in this article's supplementary material (Skolasiń ska et al, 2015). The sites used for sediment sampling in the above areas were slightly different for the summer and winter flood events (Tables 1 and 2), as the locations of major deposition during the winter flood did not precisely correspond to those of the summer flood [an exception was study site (E) where soil samples were collected outside of the city agglomeration].…”

“…The flooding of two infiltration water intakes for Poznań -Krajkowo and Dębina was problematic for the city agglomeration (Kozak and Niedzielski, 2008;Kozak et al, 2012). In addition to immediate effects (direct damage), the flood wave transported large quantities of sediments that were deposited in proximal flood plains forming levees and crevasse splays (Skolasiń ska, 2014;Skolasiń ska et al, 2015). Flood deposits may be a significant source of environmental pollution, and the toxic elements they contain may pose a potential threat to ecosystems (Zerling et al, 2006;Navratil et al, 2008).…”