Monitoring Streambed Scour/Deposition Under Nonideal Temperature Signal and Flood Conditions

Abstract

Streambed erosion and deposition are fundamental geomorphic processes in riverbeds, and monitoring their evolution is important for ecological system management and in‐stream infrastructure stability. Previous research showed proof of concept that analysis of paired temperature signals of stream and pore waters can simultaneously provide monitoring scour and deposition, stream sediment thermal regime, and seepage velocity information. However, it did not address challenges often associated with natural systems, including nonideal temperature variations (low‐amplitude, nonsinusoidal signal, and vertical thermal gradients) and natural flooding conditions on monitoring scour and deposition processes over time. Here we addressed this knowledge gap by testing the proposed thermal scour‐deposition chain (TSDC) methodology, with laboratory experiments to test the impact of nonideal temperature signals under a range of seepage velocities and with a field application during a pulse flood. Both analyses showed excellent match between surveyed and temperature‐derived bed elevation changes even under very low temperature signal amplitudes (less than 1°C), nonideal signal shape (sawtooth shape), and strong and changing vertical thermal gradients (4°C/m). Root‐mean‐square errors on predicting the change in streambed elevations were comparable with the median grain size of the streambed sediment. Future research should focus on improved techniques for temperature signal phase and amplitude extractions, as well as TSDC applications over long periods spanning entire hydrographs.

Was That Assumption Necessary? Reconsidering Boundary Conditions for Analytical Solutions to Estimate Streambed Fluxes

“…When comparing the clean sine wave information at depth to more spiked and asymmetric information at the surface, substantial error can result. In our application here, and in others before using the amplitude‐phase solution (e.g., DeWeese et al, ; Gariglio et al, ; Tonina et al, ), in particular those where we looked at depth which can be directly validated, there were no obvious issues when using a discrete Fourier transform. Investment in research on signal processing to get the best Fourier coefficient information for these signals is likely worthwhile, and the research here suggests that we take advantage of the flexibility afforded by frequency independent solutions to select frequencies that offer the best error for a given depth and input signal.…”

“…Note that Lautz () also noted little difference in flow estimates when nonsinusoidal forcing was used. Additionally DeWeese et al () show no difference in error of depth estimates between using sine and sawtooth signals.…”

“…An area of recent discussion is the nonstationarity of signal filtering (the transfer functions) because of changes in the vertical flux, which is important to monitor over time (e.g., Rau et al, ). Bed scour and sediment transport are important geomorphological and ecological processes (e.g., Goode et al, ; Tonina et al, ), and these equations have been applied for monitoring such processes as well (Bray & Dunne, ; DeWeese et al, ; Luce et al, ; Sebök et al, ; Tonina et al, ). Such changes in the bed also induce nonstationarity in the transfer functions (Tonina et al, ).…”

“…If changes are nearly linear over time, the average may seem to track well. However if the change in flux rate or elevation is sudden or nonlinear within the window span, there will be apparent “smearing” of flux and depth estimates across the window (e.g., DeWeese et al, ; Rau et al, ; Tonina et al, ). A concern noted for tracking bed scour is that such changes often occur as head in the river changes and fluid fluxes may shift simultaneously (Rau et al, 2015), furthermore nonsinusoidal, even sudden, surface temperature changes may accompany rapid hydrograph changes.…”

“…We informally tested the independence of solution in the lab and further explored information from multiple frequencies using a small container filled with sand and apparatus to allow vertical flow through the sand with varying input temperature patterns (DeWeese et al, ). The experiment was done incidental to other experiments in the sediment tank but offered an opportunity to apply spectral analysis of temperature patterns and nominally compare parameters estimated from a sinusoidal and sawtooth input signal.…”