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

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Abstract 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. This article is categorized under: Water and Life > Conservation, Management, and Awareness Science of Water > Water Extremes Engineering Water > Sustainable Engineering of Water
(a) NFM woody dam and (b) natural woody debris in the upper river cover catchment, North Yorkshire, UK
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Conceptual model describing the potential secondary effects of NFM wood structures (red: Geomorphic effects; green: Ecological effects; blue: Biogeochemical effects)
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Impact of NFM wood structures on carbon and nutrient cycling (brown: Wood features; dark blue: Biogeochemical consequences; yellow: Geomorphic factors; green: Processes on the surface; light blue: Processes in the subsurface); pathways considered most relevant to Type 1 structures enclosed by dotted box
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Mechanisms through which NFM wood structures diversify macroinvertebrate communities (yellow: Mechanisms associated with geomorphic changes; green: Mechanisms associated with movements of organisms; blue: Mechanisms associated with biogeochemical changes); pathways most relevant to Type 1 structures enclosed by dotted box
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Factors influencing the geomorphic significance of NFM wood structures (yellow: Frequently observed geomorphic effects; blue: Wood structure characteristics; red: Stream characteristics; green: Temporal factors; positive sign: Accelerating; negative sign: Inhibiting)
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Frequently reported (a) geomorphic, (b) ecological, and (c) biogeochemical effects of in‐stream wood in the 527 research articles reviewed
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(a–d) Different types of NFM wood structures currently in use
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Engineering Water > Sustainable Engineering of Water
Science of Water > Water Extremes
Water and Life > Conservation, Management, and Awareness

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