Geomorphic structure and function of hydraulic jumps in mountain river channels

High-resolution field surveying and 3-D digital elevation models (DEMs) were used to test the classical hydraulic jump (CHJ) analogue previously applied in geomorphic research. DEMs were constructed for submerged and sloping jump regions in an alpine step-pool channel. Since previous investigations of hydraulic jumps have been primarily conducted in 1-D, the first objective of the research was to develop a methodology for mapping and modeling bed and water surface topographic variations at lower and higher steady discharge conditions. Surveying was optimized by topographic setting and scaled to bed and water surface discontinuities. Traces through the jumps indicated a vertical precision comparable to previous laboratory measurements and dichotomous shifts in water surface topography that varied with hydraulic jet type. Systematic removal of survey points and DEM differencing indicated point densities of 10 points per sq. m, in conjunction with a survey structure targeting grade breaks of 0.3--0.5 m, were required to capture mesoscale form variations of the jump regions.; The second objective of this research tested for variations in form and hydraulics from CHJ and engineered conditions. Form variations included characteristics of the boulder-bed and pure-bedrock morphologies of the two jump regions relative to previous investigations in laboratory flumes. Hydraulic variations included free surface profiles of the jet and jump and air concentration in each region. The results indicated that while natural jump regions have some similarities to the CHJ and other engineered jumps, the boundary conditions of natural jump regions have systematic and random differences from engineered conditions. Form roughness elements driving supercritical flows had a characteristic lateral obliquity, and jet-induced bed scour was present near the jump toe for both jump regions. Roughness elements within the jump region were highly irregular in frequency, spacing, size, and orientation. Tests against simplified overfall profiles and roller surface profiles indicated similarities, and deviations, from 1-D investigations. Air concentration tests indicated significant spatial variations for both jump regions, with peak concentrations varying by a factor of two between the regions. Tests against CHJ conditions also indicated similarities and deviations from 1-D conditions. Revised conceptual models were subsequently developed for the natural jump regions.