Sediment imprint of the severe 2002 summer flood in the Lehnmühle reservoir, eastern Erzgebirge (Germany)

Abstract

A series of 18 short cores has been obtained from the Lehnmuhle reservoir (operation since 1932) in eastern Erzgebirge (Germany) in order to investigate the effects of the severe flood event in August 2002 on sedimentation by combining microfacies and high resolution μ-XRF scanning techniques. A distinct graded detrital layer, unique for the whole record, appears in almost the entire reservoir basin, ranging in thickness from 33 mm at proximal sites close to the river inflow to 5 mm at distal sites. The total sediment influx during this event was estimated to approximately 2,400 tons. Around two-thirds of the sediment was deposited in the southern-central part of the basin (approx. 32 % of the basin area) due to basin morphometry and proximity to the inflow. An enhanced flux of fine silt and clays to areas near the dam was observed and is likely driven by a steady current towards the dam. Occurrence of detrital material in a lateral bay reveals that sediment derived not only from the main inflow but also from surface runoff through non-permanently water bearing stream channels around the reservoir. In addition to the exceptional 2002 flood layer, 22 microscopically thin detrital layers were detected in the sediment cores, most of them at the deepest core locations close to the main dam. A chronology of detrital layers was established by 137Cs dating of three core sequences and was transferred to other cores by detailed correlation based on four lithological markers. The comparison with instrumental data reveals that 64 % of the total 22 flood events over the last three decades with a daily discharge > 8 m3s-1 of the main inflowing stream resulted in deposition of a layer of detrital material in the reservoir basin. Copyright: