Hydraulic controls on the entrapment of heavy metal-polluted sediments on a floodplain of variable width, the upper Vistula River, southern Poland

Distribution of heavy metal-polluted overbank sediments deposited by a major flood in July 2001 in three gauging sections of different width on the Vistula River, southern Poland, was investigated and related to reconstructed pattern of flow velocity during the event. The mean velocity of the flow over the floodplain was low and showed a marked contrast with mean velocity in the channel zone in the Smolice and the Sierosławice cross-sections, 4.5 and 9 times wider than the respective channel widths. High rates of lateral thinning and fining of overbank deposits were observed. At Sierosławice, the heavy metal concentrations first rapidly increased with distance from the channel, and then remained high over most of the floodplain, At Smolice, concentrations were relatively low within the levee deposits and high over the rest of the floodplain. In both cross-sections the loads of heavy metals were highest close to the channel margins, and decreased exponentially with increasing distance from the channel due to the rapid lateral thinning of the flood deposits. The mean velocity of the flow over the floodplain was high in the Bielany cross-section, only 2.6 times wider than the channel width. Sediments deposited on the narrow floodplain were mostly sand, slowly thinning laterally. While the concentrations of various heavy metals increased gradually with increasing distance from the channel, their loads varied across the floodplain. The highest load of some metals occurred near the channel margin and that of others away from the channel. This study indicates that floodplain width and the resultant flow hydraulics exert a significant influence on the lateral distribution of floodplain sediments and entrapped heavy metals. In wider floodplain sections, the largest amounts of heavy metals accumulate near the channel margin and may be remobilized in the future due to bank erosion.