NWDTW 2024

The LISST-ABS sensor, developed by Sequoia Scientific, was designed as an alternative to turbidimeters which rely on optical back scatter (OBS) for estimating suspended-sediment concentrations (SSCs) in surface waters. The acoustic backscatter sensor (ABS) emits an 8-megahertz (MHz) acoustic signal that returns to the sensor as backscatter, the intensity of which is directly translated to a sediment concentration. The relation between the signal and backscatter intensity is nearly constant beyond the Raleigh limit at 8 MHz for grain sizes larger than about 60 micrometers (µm; Agrawal and others, 2019). However, the ABS fails to accurately quantify concentrations containing large amounts of fine grains (< 60 µm). Therefore, a pairing factor is needed to predict SSC more accurately for systems transporting large amounts of grains

Acoustic instrumentation can be used to accurately and cost-effectively provide time-series and discrete estimates of suspended-sediment concentration, load, and sediment particle sizes, which are essential for creating informed solutions to many sediment-related environmental, engineering, and land management concerns. Historically, scientists have developed relations between suspended sediment characteristics and other parameters, most commonly measured streamflow, to estimate sediment information when physical sediment samples can’t be collected. Approaches using streamflow can have substantial accuracy limitations because of hysteresis effects, however. As a result, the use of more direct surrogate methods such as acoustic methods have become increasingly important. Interagency efforts in recent years have advanced the testing, methods development, operational guidelines, and training on acoustic methods for measuring suspended sediment. Scientists and technicians interested in using these methods are faced with many decisions on type of application and deployment: horizontal profiling, vertical profiling, or point acoustic instruments; single or multi-frequency instruments; continuous or discrete sediment measurements; and fixed or mobile instrument deployments. To promote cost effective, accurate, and high-resolution fluvial sediment data for the Nation, the interagency Sediment Acoustic Leadership Team (SALT) develops technical guidance and training for using acoustic instruments to measure aquatic sediment. Even though acoustic instrumentation has been used successfully to measure suspended-sediment characteristics through the world, some deployments have been unsuccessful because of limited technical guidance and selection of an inappropriate method. To guide decisions on method selection, the SALT has compiled the state of the science for the main types of acoustics-based suspended-sediment measurement methods in development, testing, and use, and has created a flowchart to guide method selection.

Concerns over maintaining antiquated samplers and manufacturing techniques led to the determination that a new sampler design and production would be in the best interest of Federal Interagency Sedimentation Project (FISP) participating agencies (USGS, Bureau of Reclamation, Corps of Engineers, USDA). Applying current 3-D printing technology presents the possibility of producing one new generation sampler that could serve as both a depth- and point-integrating sampler. Components that would change the function of the sampler would be more economical to produce and reduce the cost of purchasing an entirely new sampler. 3-D printing materials will also allow many components of the sampler to be buoyant. This feature would improve recovery chances if the sampler head or parts were dropped in the water while working from a boat, therefore reducing replacement costs. Also, with sediment surrogate technology on the rise, a sampler designed to incorporate surrogate instrumentation would enhance sediment data-collection efforts.

Utilizing the newest sonar technology allows river sampling teams to greatly enhance their capability to view a sampler track during deployment. This essential tool allows the sampler operator to visualize in real-time, the ascending and descending location of the sampler throughout the water column. The active sonar screen information provides a clear side perspective of not only the position of the moving sampler, but upstream and downstream moving bed forms. During high water events which produce higher velocities, and increased depths, sampler cable payouts tend to differ from actual water depths due to drag on the tether cables and samplers. This unwanted effect can misinform operators of the sampler’s actual depth. Using Live Scope sonar technology greatly enhances an operator’s capability to deploy a sampler to precise depths and therefore avoiding unwanted bed contact or hazards.