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Transported large wood (LW) in rivers may lead to accumulations at natural or artificial obstructions. The hydraulic and geomorphic conditions change due to these accumulations. Backwater rise as well as scour can evolve in the vicinity of such an accumulation. In this first companion paper, results of hydraulic model tests are presented on backwater rise due to spanwise LW accumulations in combination with a movable bed. The findings are summarized in design equations and allow the estimation of (1) characteristic LW volume generating the primary backwater rise, (2) effect of LW accumulation shape and bed material on resulting backwater rise, and (3) effect of LW volume on backwater rise. Compared to a fixed bed, movable bed reduces backwater rise as the open cross-section area and thus discharge capacity increase. This work improves the understanding and predictability of the formation and impact of spanwise LW accumulations at natural or artificial obstructions (e.g., LW retention racks).

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In this second companion paper, hydraulic model tests were conducted to analyze local scour due to natural spanwise large wood (LW) accumulations. Spanwise accumulations were modeled using a vertical barrier, similar to a LW retention rack in prototype. The flume experiments were conducted according to Froude similitude in a scale of 1:30 for various approach flow conditions (subcritical and supercritical flow) and different uniform bed material (2.7-13.1-mm model dimensions). The findings allow the estimation of local scour depth due to spanwise LW accumulations as a function of unit discharge, sediment diameter, and wood volume. Higher unit discharge, finer bed material, and increasing wood volume lead to an increased scour depth. The scour length can be estimated based on the scour depth and a geometrical scaling factor. The longitudinal shape of the cross-sectional scour depth can be described with a Gaussian normal distribution. Based on the results of both scour depth and length, the design of LW retention structures can be significantly improved. At the same time, the results demonstrate that LW accumulations strongly affect the geomorphic conditions and may consequently create more heterogeneous morphological structures.

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