Abstract
The effects of large wood (LW) presence in streams on river ecology and morphology are becoming widely researched and nowadays their ecological benefits are undisputed. Yet LW presence in most Swiss plateau streams is poor mainly due to anthropological pressure on river ecosystems. The use of anchored, engineered LW structures under various forms in stream restoration projects is now state of the art. However, binding benchmarks for the equivalent naturally occurring instream LW quantities and complex LW structures do not yet exist. Therefore, hydraulic engineers often find themselves in a conflict between acceptable instream LW quantities for flood protection, quantities desirable from an ecological point of view and, last but not least, quantities accepted by the public based on the current ideologies of landscape design. In the first section, this paper treats the complexity of defining benchmarks for LW quantities in restoration projects. In the second section, we provide a qualitative practical insight into relevant questions when planning engineered LW structures, such as placement, anchoring, naturalness, and effectiveness from a hydraulic engineer’s point of view. The third part presents three examples of restoration projects with different dimensions where various engineered LW structures with different outcomes were built and introduced into active streams. Finally, the conclusion provides further possible measures to retain LW in streams and to restore more natural LW dynamics in rivers.
Highlights
Between the 18th and mid-20th century, Swiss plateau streams were widely modified for agricultural land reclamation and to augment flood protection
Due to the restrictions regarding flood protection and an almost negligible natural large wood (LW) supply based on the storage of driftwood at a broader catchment scale, large wood structures (LWS) usually need to be anchored and kept in place when mechanically introduced into stream beds (“hard engineering”)
Deep embedding: Wherever the riverbed substrate allows it, anchoring can be achieved by a sufficient embedding of the LWS in the riverbed. It can be combined with additional weighting by using large boulders that are covered with substrate; Piling: If deep embedding is not possible or when piles themselves are foreseen as structural elements, piling into the river substrate is an alternative; Alien means of anchoring: Especially where streams flow over a rocky riverbed with an insufficient riverbed substrate for embedding or piling, LWS can be attached to large boulders by using rock anchors, anchored directly into the bedrock or attached to riparian trees or boulders with ropes
Summary
Between the 18th and mid-20th century, Swiss plateau streams were widely modified for agricultural land reclamation and to augment flood protection. LW showed signs of bark bugs, implying that this fraction was most probably not recruited in stream beds This assumption was underlined by more recent empirical GIS-based research on driftwood recruitment conducted in various streams in Switzerland by the Federal Office for the Environment, stating that instream wood accounts for a fraction of less than 10% of total driftwood [9]. (1996) [11]) and merely affects stream hydraulics at higher discharges Under these circumstances, the definition of the ecologically appropriate but tolerable quantity of (engineered) LWS in a restoration project in terms of increased flood risk, is a sensitive yet ecologically crucial subject with various stakeholders to consider
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