Sediment entrainment in bedrock rivers is a key process for river incision and landscape evolution. Bedrock riverbeds are typically comprised of both exposed bedrock and alluvial patches, meaning grains can be entrained from positions on both surfaces. The critical shear stress needed to entrain a grain will be affected by the topography that the grain is located on, as it determines grain pivot angle and exposure, and impacts the local flow profile. The aim of this pair of articles is to determine how the properties of bedrock surfaces with and without sediment cover affect the grain‐scale geometry of sediment grains, and consequently their critical shear stress. We report experiments using 3D‐printed scaled replicas of fluvial bedrock surfaces, with 0 to 100% additional sediment cover. For each surface, grain pivot angles were measured using a tilt table. In this first article, we report how surface roughness and grain pivot angle vary between surfaces and with different amounts of sediment cover, and we explore the relationship between pivot angles and different metrics for measuring surface roughness. We find that: (1) surface roughness is not necessarily a linear combination of the individual roughness of bedrock and alluvial areas, and the underlying bedrock topography can still influence surface roughness at 100% sediment cover; (2) pivot angles generally, but not always, decrease with increasing grain size relative to surface roughness; (3) changes in pivot angles with increasing sediment cover are best explained by changes in surface roughness at spatial scales comparable to the grain size; and (4) pivot angles are also best explained by roughness metrics that incorporate the direction of roughness with respect to the tilt direction, and the surface inclination. This work provides new insights into the processes behind grain entrainment in bedrock rivers that are critical for determining how landscapes may evolve.

<p>Sediment grains in bedrock-alluvial channels can be entrained from bedrock surfaces or from alluvial patches. Field tracer data has shown that grains entrained from different surfaces can have very different critical shear stresses, which will affect bedload transport rates, the stability of sediment cover and bedrock incision. We hypothesise that the topography of the bedrock surface affects the critical shear stress of a sediment grain in at least three ways: the pivot angle through which the grain must move to be mobilised; the extent to which the grain is sheltered by upstream bedrock protrusions; and the impact on the flow profile via the roughness length z<sub>0</sub>. Here we quantify how bedrock topography affects these three different components, and their overall impact on critical shear stress.</p><p>Our analysis is based around six samples of bedrock river topography, from rivers with different degrees of roughness and structural characteristics. Each surface was 3D printed at a reduced scale, and pivot angles were measured by dropping grains of different sizes at different locations, and tilting the surface until the grain moved. For the surface with bedrock ribs, experiments were repeated with the ribs parallel and perpendicular to the downslope direction. Further experiments were performed after incrementally covering 25% through to 100% of the surface with fixed sediment cover. Bedrock sheltering and z<sub>0</sub> were estimated from analysis of surface topography.</p><p>Overall, we find that measured pivot angles decrease with increasing surface roughness, similar to previous relationships from alluvial channels. However, we find that the pivot angle for a grain at any particular location cannot be predicted from the local surface topography, because of the complex interaction between grain shape and the different scales of roughness present on the surface. Rib direction also has a significant influence on mean pivot angle. The impact of sediment cover depends on the relative roughness of the cover and the bedrock surface.</p><p>We calculate critical shear stress using Kirchner’s force balance model, parameterised using our measurements of pivot angle, sheltering and z<sub>0</sub>. We find that z<sub>0</sub> has the largest impact on the predicted median values of critical shear stress. Including the measured pivot angles reduces the lowest values of critical shear stress, with implications for the onset of sediment transport. Overall, our data represent the first attempt to quantify fully how bedrock topography influences the critical shear stress of sediment grains in bedrock-alluvial channels.</p>