Logs Wanted – Dead or Alive

Abstract:

Rivers cover only a small part of the Earth`s surface, yet they transfer sediment in globally significant quantities. In mountainous regions, the majority of the total channel length occurs in headwater streams. Those mountain channels are influenced in terms of sediment connectivity by processes on the slopes. For example in such a sediment routing system, sediment originating from debris flows on the slopes is delivered along sediment pathways to the channel system and can be transported further downstream as solid load. Interruption of instream coarse sediment connectivity is closely related to the existence of channel blocking barriers which also can be formed by biota. By storing sediment large wood (LW) log jams disrupt in-channel sediment connectivity. We present a study design in order to decipher the short to long term effects (c. 10^-2-10² years) of sediment (dis)connectivity effects of large wood. The study areas are two basins in mountain ranges in Germany and Austria. In Austria the drainage area of the river Fugnitz was chosen which is located in the National Park Thayatal. The other drainage area of the river Sieber in Saxony-Anhalt, Germany, is located in the Harz National Park.

Since studies on LW and its geomorphological effects in Central European rivers are still rare the main goals of the project are:

•to identify important triggers for LW transport from slopes into the channels

•to examine the spatial distribution and characterization of LW in main and slope channels by mapping and dGPS measurements

•to determine the effects of LW on channel hydraulic parameters (e.g. slope, width, grains size composition, roughness) by field measurements of channel long profiles and cross section with dGPS and Wolman particle counts

•to quantify the direct effects of LW on discharge and bed load transport by measuring flow velocity with an Ott-Nautilus current meter and to measure bed load up- and downstream of log jams using a portable Helley-Smith bed load sampler

•to quantify the sediment volume stored by LW and monitoring of those accumulations using terrestrial laserscanning or structure from motion

•to quantify the in-channel stored and deposited LW and to date at least the key logs by dendrochronology in order to get information on the maximum longevity of sediment storage

Large wood in the Fugnitz River: