TDR measurements of hydraulic jump aeration in the South Fork of the American River, California

Turbulent pressure and velocity fluctuations associated with hydraulic jump air entrainment likely play an important role in local flow dynamics and morphologic changes of steep bedrock and alluvial rivers. Previous engineering research indicates that hydraulic jump air entrainment and energy dissipation locally intensify erosion of both bedrock and alluvial elements, enhance hydraulic quarrying, suppress cavitation, and decrease the friction factor of a flow. Despite studies in artificial settings, air content in natural hydraulic jumps remains largely unquantified. In this study, Time Domain Reflectometry (TDR), a method commonly used to determine the volumetric water content of porous media for the last two decades, was evaluated in the laboratory and then adapted to measure air contents of natural hydraulic jumps. Laboratory tests were conducted to determine the influence of turbulence, temperature, and sampling volume on the TDR signal over an air content range of 0–100%. Based on the results of the laboratory tests, an empirical calibration was developed that converts the TDR signal into air content. Local air contents were then measured in hydraulic jumps occurring along a mixed alluvial- and bedrock-dominated 8.4-km section of the South Fork American River. Two small jumps had a mean air content of 19% and 23%, a coefficient of variation (CV) of 27% and 28%, and a root-mean-square (RMS) of 19% and 24%, respectively. A large jump had a mean air content of 42%, a CV of 26%, and a RMS of 44%. TDR provides fast, accurate, repeatable, and nondestructive in situ measurements at one-tenth the cost of traditional methods and is far more robust for field applications than traditional lab instruments. Future studies can now expand upon the hydrogeomorphic and ecological significance of jump aeration.