Riparian Forests and large woody Debris Supply in Mountain Torrents

log jam in a torrent reach

Introduction

Dead wood generally is recognized as an important ecological and structural component of torrents. However, during flood events, for example in August 2005 in Switzerland, driftwood often cause serious problems such as log jams at bridges or culverts which frequently have the effect of overbank flooding and sedimentation with a high damage potential.

Several mitigation measures exist to reduce driftwood problems: 1) engineering measures, e.g. retention of driftwood, 2) removal of large woody debris (LWD) or cutting LWD to smaller pieces which are less dangerous in connection with log jams, and 3) management of riparian forests with the objective to reduce LWD input in the torrent reaches.

For economical reasons, management and utilization of riparian forests are more and more extensified or completely given up. A research project was set up at the WSL with the aim to investigate certain aspects of the effects of riparian forest management on LWD supply in torrents.

Methods

A total of 10 torrents with different types of riparian forests are investigated in different regions of Switzerland. In each torrent a 1000 m long study reach is selected and a complete LWD inventory is conducted. LWD is defined as pieces of wood ≥ 0.1 m in diameter and ≥ 1.0 m in length located within the channel. In addition, topographical parameters, hillsolpe processes and comprehensive characteristics of the forest stands adjacent to the torrents are collected. The relations between certain stand characteristics of the riparian forests and the amount of LWD in the specific torrent reaches are expected be the basis for discussions concerning the optimal conditions of the forests stands and the management strategies to achieve the designated stand characteristics.

Results

The results of the investigation are described in the project-report Rickli and Bucher (2006).

Publications and Links

Rickli, C. und Bucher, H.U., 2006: Einfluss ufernaher Bestockungen auf das Schwemmholzvorkommen in Wildbächen. Projektbericht zuhanden des Bundesamtes für Umwelt BAFU. Eidg. Forschungsanstalt WSL. 94 S. (PDF-File)

Rickli, C. und Bucher, H.U., 2006: Schutzwald und Schwemmholz in Wildbacheinzugsgebieten. FAN-Agenda 1/06: 17-20. (PDF-File)

Waldner P., Rickli C., Köchli D., Usbeck T., Schmocker L., Sutter F., 2007. Schwemmholz. In: Bezzola G., Hegg C. (eds.), Ereignisanalyse Hochwasser 2005, Teil 1 - Prozesse, Schäden und erste Einordnung. Bundesamt für Umwelt BAFU, Bern und Eidg. Forschungsanstalt für Wald, Schnee und Landschaft WSL, Birmensdorf, p. 181-193. pdf: http://www.bafu.admin.ch/publikationen/index.html?action=show_publ&lang=de&id_thema=1&series=UW&nr_publ=0707

Zusammensetzung ausgewählter Schwemmholzablagerungen der Unwetter vom 19.-23. August 2005: http://www.wsl.ch/forschung/forschungsprojekte/schwemmholzablagerungen/index_DE

People involved

• Christian Rickli
• Albert Böll

Contact

• Christian Rickli