Acoustically-Derived Sediment-Index Methods in Large Rivers: Assessment of Two Acoustic Inversion Models

In large rivers across the world, measurement of sediment in transport can be challenging and expensive to collect, yet monitoring and measurement in these systems is critical to our management of these systems in the face of climate change and rising sea levels and because its movement with channels provides natural resilience of riverine, estuarine, and near shore environments. As such, developing programs for continuous monitoring of sediment is necessary. Here we present and assess acoustic methods used to estimate suspended sediment entering the Lower Fraser River. The channel is 550 m wide, which is greater than the range of acoustic instruments-a common challenge in large rivers. We demonstrate a sediment-index methodology that relates an acoustically-derived concentration estimate to channel-average concentration. We compare two different acoustic methods (multifrequency and single frequency) as acoustic index estimators. Between 2012 and 2014 we conducted 25 sampling campaigns where cross-section, point-integrated suspended sediment samples, and samples from within the acoustic range were collected. Annual flux was computed for the 3 years using both methods and compared to a more traditional sediment supply to discharge rating-curve. The most robust method used a single-frequency inversion where the sum of acoustically-derived, fractional sand and silt/clay estimates equated the total flux. Development of such monitoring programs will aid managers, engineers, and scientists in policy and practice, emboldening our ability to observe, adapt, and predict a changing environment.

Automated summary

Measurement of sediment in large rivers is difficult and expensive, but crucial for managing these systems in the face of climate change and rising sea levels. Acoustic methods are used to estimate suspended sediment, and a sediment-index methodology is presented and assessed for the Lower Fraser River.