Failure modes in cedar tree revetments: Observations on rivers and streams in eastern Kansas, USA

A cedar tree revetment is a bioengineering technique intended to stabilize eroding stream banks using longitudinally placed cedar trees. This technique, which has been implemented on many rivers and streams across the United States, has been proposed as a less expensive, ecologically compatible bank stabilization method. The limited documentation of these types of bioengineering techniques indicates high failure rates. River engineers need to understand the potential failure modes of cedar tree revetments, so they can take appropriate countermeasures when applying this technique. This article documents four common failure modes observed during post-project site assessments on 12 streams in eastern Kansas, USA that took place in 2019 and 2020. These modes are (1) bed degradation with structure perching, (2) failure in flexion, (3) loose cables, and (4) lack of sediment infilling. Computed factors of safety for top of bank discharge range from 0.3 to 6.0 in flexion (bending stress vs. strength) and range from 1.7 to 40.1 for anchor forces vs. anchor strength. These factors of safety suggest that failure in flexion is an important failure mechanism that should be considered and mitigated during design of cedar tree revetments. Moreover, failure rate varies directly with project age. The authors hypothesize that progressive processes such as breaking of bankside branches may cause loose cables and cyclical loading and wetting/drying may lower the bending strength of the trees over time. Avoiding degrading streams, additional anchoring, and trimming the bank-side branches of the cedar trees are suggested as means to reduce these types of failures.

Automated summary

This article documents four common failure modes observed during post-project site assessments on 12 streams in eastern Kansas, USA that took place in 2019 and 2020. These modes are bed degradation with structure perching, failure in flexion, loose cables, and lack of sediment infilling. It also suggests that failure in flexion is an important failure mechanism that should be considered and mitigated during design of cedar tree revetments. The authors hypothesize that progressive processes such as breaking of bankside branches may cause loose cables and cyclical loading and wetting/drying may lower the bending strength of the trees over time. Avoiding degrading streams, additional anchoring, and trimming the bank-side branches of the cedar trees are suggested as means to reduce these types of failures.