Evaluating post-fire surface mulch treatments: Assessing hillslope stability and mobilisation potential relative to debris-flow mitigation effects

Surface wood shred treatments are used widely for post-fire erosion mitigation, and recent modelling has shown they are highly effective for debris flow mitigation; however, little attention has been given to their potential mobilisation and displacement under high-intensity rainfall events. Such movement could reduce treatment effectiveness, contaminate downstream water bodies, or contribute additional material to debris flows, should they initiate. Despite this recognised risk, significant knowledge gaps limit our ability to evaluate when and where treatment mobilisation may occur, and consequently, whether alternative post-fire treatments may be more suitable under some conditions.  

The three key knowledge gaps that this study sought to address were: a) the stream power required to mobilise wood shred on hillslopes has not yet been quantified; b) there is no established method to determine the topographic and rainfall conditions under which sufficient stream power may be generated to mobilise wood shred in treated landscapes; and c) it is not yet possible to evaluate how the likelihood of shred mobilisation compares to the reduced likelihood of debris flow initiation resulting from treatment.  

We addressed these knowledge gaps through a combination of field experiments and catchment-scale modelling. Field experiments were used to parameterise the relationship between hillslope stream power and wood shred displacement under channelised flow. A simple methodology was then developed to apply the stream power-based relationship at a 1 m resolution across a debris flow-prone landscape comprising headwaters with varying morphometric characteristics and under varying rainfall scenarios, to quantify the expected proportion of wood shred treatment mobilised. Finally, we estimated the likelihood of debris flow initiation with and without treatment, and the likelihood of treatment mobilisation, across ~400 zero-order basins with high post-fire hydrogeomorphic sensitivity in Melbourne’s key water supply catchment to evaluate the risk of wood shred mobilisation against the expected benefits of treatment.  

Across the case-study water supply catchment, wood shred treatment is modelled to reduce the maximum likelihood (Annual Exceedance Probability [AEP], %) of debris flow initiation from 72% to 32%. However, we found that up to 40% of wood shred treatment would be mobilised before any treatment effect is realised, suggesting careful consideration of treatment risks and benefits is warranted. Nevertheless, even under a worst-case rainfall scenario (AEP < 0.01%), the mass of wood shred mobilised was 55 times lower than the modelled sediment load from debris flows triggered without treatment. This work provides critical insights for catchment managers evaluating potential post-fire erosion mitigation treatments and highlights the importance of considering the suitability of an area for treatment.