Acoustic measurements on a shallow, sand-bed river: A case study from the Rio Grande

The Middle Rio Grande (MRG) is a dynamic and complex fluvial system where flow and sediment transported from the Upper Rio Grande and MRG tributaries influence the form of the river. How sediment is transported through the MRG is an important planning question as it addresses a wide range of concerns including flood control and river rehabilitation, thus continuous sediment measurements are needed to develop accurate sediment budgets.

Sediment measurement techniques have continued to improve and the advent of sediment surrogates, such as acoustic technology, have proven to be effective options at obtaining more complete spatial and temporal sediment data in larger fluvial systems. Measurements of sediment in shallow, sand bed rivers, like the Rio Grande, are more difficult because of the changing channel morphology and often limited water depth in which to install instrumentation. During the 2019 spring snow-melt runoff season two acoustic techniques were employed on the Rio Grande to evaluate sediment movement. Sediment movement near the bed was calculated by the Integrated Section Surface Difference Over Time version 2 (ISSDOTv2) using swath data collected from a multi-beam sonar. Measurements were made adjacent to U.S. Geological Survey (USGS) gaging stations where near simultaneous measurements were made by the USGS for streamflow, suspended-sediment concentration and gradation, and bed-material gradations. These measurements were conducted at two locations on the Rio Grande, one of the locations was co-located with two side-profiling suspended-sediment acoustic Doppler profilers that had been installed in the fall of 2016. Both a 1 MegaHertz (MHz) and 2 MHz side-profiling suspended-sediment acoustic Doppler instrument were installed on a fixed platform that was co-located with a USGS sediment gage.

The ISSDOTv2 method using multi-beam sonar and the side-profiling acoustic Doppler profilers proved successful in collecting sediment information and compared well with the more traditional sediment measurements, while providing insight into the sediment transport on the MRG because of the increase in spatial and temporal resolution. Overall, there are some limitations of these acoustical techniques, but the additional information gleaned is beneficial in understanding sediment transport in a shallow, sand-bed river, such as the Rio Grande.