Character, Distribution and Significance of Coarse Sedimentary Material in Mid-Mountain Valleys: Case Studies from Bohemian Forest Mts.

Abstract:

The aim of this study is to collect information on the distribution, character and parameters of the coarse fluvial sediments in order to reconstruct the formation and significant changes in development of mid-mountain streams and their valleys.
 The studied area is situated to two small catchments in the Bohemian Forest Mts. Roklanský/Javoří brooks are observed in a 10 km long segment on the flat area of the Šumava Plateau, the Losenice R. was followed on a 7 km segment in a deeply incised, steep-sloped valley.
 First, the hydromorphological mapping of the floodplain and channel landforms took place to collect information on position of the fluvial landforms. The mapping was performed using the HEM methodics, developed by the authors. The Electric Resistivity Tomography was used in 20 profiles across the valleys. These cross-sections formed a backbone of the research aiming to observe the depth and distribution of the coarse sediment valley fill (above bedrock) both across the valleys and along. Next we employed digital optical granulometry using Sedimetrics software on the mapped gravel bodies, classic sieve granulometry on sandy accumulations, and on-site, in-channel boulder granulometry. Finally, magnetic susceptibility measurements were performed on suitable sediment profiles along the courses to assess the provenience of the material.
 There are several notions, indicated by the results of the abovementioned analyses. First, the valley floor sedimentary infill is surprisingly shallow (10-15 m), notably in the case of Javoří and Roklanský brook where the valley floor often exceeds 200 - 300 m. Also, the MS points to rather fast changes in the main sediment sources. Observations of the sediment character in the Losenice valley indicate significant role of the slope/channel coupling, namely persisting influence of large, slope-based sources of angular material (rockslides, debris flows), as well as strong impact of flood events on the valley floor morphology.