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Representing natural and artificial in‐channel large wood in numerical hydraulic and hydrological models

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Abstract The influence of naturally occurring in‐channel large wood (LW) on the hydraulics, hydrology and geomorphology of rivers is well documented. To inform management and better understand naturally occurring or artificially placed LW, hydraulic and hydrological models are applied to predict the possible benefits and drawbacks for habitat, sediment management and flood risk mitigation. However, knowledge and guidance on appropriate representation in models, needed to underpin realistic predictions, is lacking. This could lead to unrealistic expectations of the effectiveness of LW for different river management goals. To date, seven types of LW representation in hydraulic and hydrological models have been applied, the range partly reflecting the variety of LW, model types, scales and purposes. The most common approach is by altering channel roughness to represent flow resistance. Although qualitatively the effects of LW have been captured using models, to date quantitative validation, as well as transferable knowledge to help a priori parameterization of LW representations, remain limited. Therefore, additional empirical investigations and robust model validation are required to inform defensible LW representations for specific purposes and scales in numerical models coupled with better accounting of input uncertainty to improve confidence in predictions. Future studies should also consider a greater range of artificial and natural LW features, settings, larger spatial scales and better account for temporal variability of flow, morphology and LW configuration. This article is categorized under: Water and Life > Methods Science of Water > Methods Water and Life > Nature of Freshwater Ecosystems
Examples of naturally formed and artificial in‐channel large wood. (a) A cluster of individual fallen trees (Bowmont Water, UK), (b) a channel spanning log jam (Logie Burn, UK; Carol Taylor, James Hutton Institute), (c) a mid‐channel tree with its rootwad facing upstream (River Feshie, UK), (d) a dense accumulation of large wood at the head of a tree covered island (River Dee, UK), (e) a flow restrictor built of wooden planks added to a ditch and designed to reduce flow for flood risk management (Belford Burn, UK), (f) a bar apex engineered log jam on a gravel bar designed to trap coarse sediment (Bowmont Water, UK), (g) added trees with intact root‐wads for restoring river habitat (Allt Lorgy, UK; Spey Catchment Initiative) and (h) an individual bank attached tree trunk designed to improve fish habitat (Gelder Burn, UK; Dee District Salmon Fishery Board)
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Manning's n values of naturally occurring large wood features versus channel reach slope normalized by drainage area. Further details given in Table 2
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Water and Life > Nature of Freshwater Ecosystems
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