What Happens During a Minor Flood: Observations of Bedload Transport in a Gravel Bed River using New Methods

Abstract:

The question of “does the streambed change over a flood” does not have a clear answer due to lack of measurement methods during high flows. We seek to inform our understanding of bedload transport by linking field measurements using fiber optic distributed temperature sensing (DTS) cable, calculations of disentrainment over time and distance, and in situ measurements of streambed permeability with sediment transport theory and an existing explicit analytical solution to predict depth of sediment deposition and one-dimensional fluid velocity from amplitude and phase information. The method facilitates the study of gravel transport by using near-bed temperature time series to estimate rates of sediment deposition continuously over the duration of a minor flood coinciding with bar formation, including (1) a field method for measuring local rates of deposition and bed elevation change during a minor flood to compute rates of bedload transport, (2) use of an existing analytical solution to quantify the depth of sediment deposition over distance and time from temperature amplitude and phase information, (3) observational and theoretical evidence that incipient motion occurs during a minor flood, (4) observational evidence that suggests rates of sediment transport are not necessarily constant during a constant flow, and (5) field evidence for the persistence of armor layers in gravel bed rivers during a minor flood. These observations of partial bedload transport, taken along a 2 km gravel bed reach of the San Joaquin River, CA, USA during an experimental flow release, suggest that the discharge needed to create the boundary shear is lower than previous estimates, and that partial transport of grain sizes on the bed, including the median particle size, occurs during a minor flood with a current recurrence interval of approximately 1-2 years.