Potential secondary effects of in-stream wood structures installed for natural flood management: A conceptual model

The use of in-stream wood is one of the most commonly employed natural flood management (NFM) techniques. The effectiveness of NFM wood structures in reducing flood risks (i.e., their “primary” effect) has been relatively well documented. However, their additional or “secondary” effects on other natural processes have not been fully evaluated. These secondary effects can be inferred by reviewing previous studies that scrutinized natural wood accumulations or artificial wood structures constructed for purposes other than NFM. The degree of contact with base flows and the stream bed provides a broad classification of NFM wood structures. Having considered the similarities between NFM wood structures and other in-stream wood types, it is suggested that the following geomorphic effects are common to all types of NFM wood structures: pool formation; accumulation of clasts immediately upstream; buffering against stream bed coarsening; and bank erosion, causing channel widening and the formation of floodplain channels. These geomorphic changes contribute to stream bed heterogeneity, potentially creating new niches for aquatic organisms such as macroinvertebrates. Moreover, NFM wood structures may retain benthic organisms accidentally flushed away during flood events, serving as sources of colonists during phases of recovery. Geomorphic changes induced by NFM wood structures may also contribute to spatial variation in rates of biogeochemical processing. Accumulation of fine sediments in some areas may provide more surfaces for the attachment of organic matter and micro-organisms, hence increasing benthic metabolic rates. Stream bed scouring in other areas may lead to sediment instability, suppressing the growth of micro-organisms and benthic metabolic rates.