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 observed form of the river. How sediment moves through the MRG is an important planning question as it addresses a wide range of concerns including flood control and river rehabilitation, thus periodic or continuous sediment measurements are needed. 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. Shallow, sand bed rivers, like the Rio Grande, have their own set of issues, however, and measurements are more difficult because of the changing morphology. During the 2019 spring snow-melt runoff two acoustic techniques were employed on the Rio Grande to evaluate bed load and suspended sediment load. Bed load was calculated by the ISSDOTv2 method (Integrated-Section Surface Distance Over Time) using swath data collected from an Applanix INS integrated Norbit WBMS multi-beam sonar mounted on a 14-foot jet-boat. Measurements were made adjacent to USGS gaging stations where near simultaneous measurements were made by the USGS for discharge, suspended sediment using a D-95 or DH-95 suspended sediment sampler, and bed material using a BMH-60 sampler. These were conducted at two different locations on the Rio Grande. One of these locations was where a second acoustic technique has been installed since the fall of 2016. At this location two AquaDopp’s (1 MHz and 2 MHz) were located on a fixed platform that was co-located with a USGS sediment gage. The fixed platform was situated in a way that placed both AquaDopp’s close to the main thalweg, enabling them to measure a horizontal slice of the cross section continuously. Both methods proved successful in collecting sediment information and compared well with the more traditional sediment measurements, while providing some insight into the sediment transport on the Rio Grande. While sand was observed to move in the system, there was an observed flow dependency. In the northern test section (around Albuquerque, NM) the flux of sand was greater overall, and more was carried in suspension then further south (around San Acacia, NM) where the flux of sand was less, and a greater percentage was carried as bed load. Overall, there are some limitations of these techniques, but the additional information gleaned is beneficial in understanding sediment transport in a shallow, sand-bed river, such as the Rio Grande.