1 2Horseshoe waterfalls are a common feature of steep bedrock rivers. As a first step toward 3 understanding their geomorphology, a detailed study of the fluid mechanics at a 0.91-m vertical-4 drop, horseshoe waterfall was performed in a 2.75-m wide flume. Five non-dimensional 5 upstream energy levels, each with 3-5 non-dimensional downstream tailwater depths (21 runs 6 total), were assessed for water surface topography via digital elevation modeling, flow dynamics 7 via digital videography, and overall energy dissipation via an energy and momentum 8 conservation model. Regardless of tail depth, the horseshoe waterfall was found to have three 9 distinct zones beyond the step brink-1) a nappe whose degree of convergence depends on 10 upstream energy and brink configuration, 2) a convergence zone whose features vary strongly 11 with upstream energy, brink configuration, and tail depth, and 3) a downstream tailwater region 12 whose dynamics primarily depend on tail depth. The centerline nappe profile and brink velocity 13 were reasonably predicted using Rouse's jet trajectory equations when (H+P)/H>2. Peripheral 14 profiles were not predictable using existing equations. For any arbitrary broad-crested step brink 15 configuration, maximum energy dissipation was found to occur when no jump was present and 16 downstream tail depth was exactly critical. Rather than providing maximal energy dissipation, 17 hydraulic jumps below steps provide efficient conversion of kinetic energy to potential energy. 18 19

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High-resolution tacheometric field surveying was integrated with computer-assisted drafting to visualize and contrast three-dimensional bed and water surface digital elevation models (DEMs) for submerged and unsubmerged hydraulic jump regions in a bedrock step-pool channel. Measurements were conducted for two discharge conditions. Since previous applications of three-dimensional field mapping and digital elevation modelling of stream channels have been limited to smoothly contiguous gravel-bedded systems, surveying was optimized by topographic setting and scaled to localized bed and water surface discontinuities. Traces and visualizations of the jump regions indicated that dichotomous decimetre shifts in water surface topography occurred for both jump regions from lower to higher discharges. Systematic removal of the survey points and DEM differencing indicated that point densities of ten points per square metre, in conjunction with a survey structure targeting grade breaks of 0·3-0·5 m, were required to capture decimetre form variations of the natural jump regions. The DEMs highlight the importance of recognizing the relationship between transcritical flow structures and localized topographic heterogeneities in bedrock channels.

[1] The classical hydraulic jump, which is a hydraulic jump formed in a horizontal, smooth, and prismatic laboratory flume, has often been used for estimating hydraulic and morphologic characteristics of mountain river channels. However, most studies utilizing classical hydraulic jump relationships have neglected the role of aerated flows and jump types prevalent in natural systems. By developing a methodology to survey, digitize, and visualize air concentrations for jumps in bedrock rivers, we present new data for submerged jumps and sloping jumps in a bedrock step-pool channel. As with laboratory research, our results indicated peak, mean, and local air concentrations of natural jumps were skewed by submergence of the jump roller beneath the free surface. However, unlike laboratory research, cross-stream variations were significant for both jumps, varying up to 20% across the jump width. Traces through each jump thalweg also indicated the classical hydraulic jump was a poor analogue for estimating surficial air concentrations.

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