2025 CSDMS meeting-021
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Modeling the influence of macro-roughness on gravel-bed river morphodynamics
Charles Shobe,
U.S. Forest Service Rocky Mountain Research Station Fort Collins Colorado, United States. shobe.charlie@gmail.com
Daniel Scott, Watershed Science and Engineering Seattle Washington, United States. dan@watershedse.com
The shape of gravel-bed rivers controls aquatic habitat, fluvial hazards, landscape evolution, and river response to human impacts. Gravel-bed river shape, on average, obeys the expectations of an equilibrium model in which the width-averaged bankfull shear stress is slightly greater than the critical shear stress required to move the median bed grain size. However, it is important to understand not just average equilibrium form but also responses of river shape to changes in formative conditions, both to elucidate process dynamics and to enable applied prediction. Many natural and human-driven changes to channels, and especially river restoration interventions, involve altering the flow resistance of gravel-bed rivers by changing macro-roughness. Despite rich bodies of literature on 1) macro-roughness effects on flow and sediment transport and 2) controls on gravel-bed river shape, understanding of macro-roughness effects on reach scale channel form evolution remains limited.
We ask how macro-roughness affects gravel-bed river geometry, and how rivers respond to changes in macro-roughness as might occur during river restoration. We develop a simple numerical model for gravel-bed river form in the presence of macro-roughness and use it to investigate trajectories and timescales of channel response to macro-roughnesss changes. Model sensitivity analysis reveals that greater macro-roughness drives channel widening, bed aggradation, and steepening, as the channel remains adjusted to maintain constant shear stresses on the bed and banks regardless of roughness. The presence of a floodplain reduces widening but increases steepening at high roughness values, because high roughness drives flow overbank and increases the width/depth ratio relative to a confined channel. Comparison against topobathymetric data and UAV imagery from the North Fork Snoqualmie River, WA shows that modeled trends in channel adjustment to macro-roughness are generally consistent with data from the field site, but that the field site exhibits significant variability due to processes and feedbacks not captured by our model. We expect our model to be useful for establishing a priori expectations for the effects of process-based restoration projects on river form.
