Location: Watershed Physical Processes Research2017 Annual Report
1a. Objectives (from AD-416):
1. Develop acoustic and orthogonal geophysical methods to characterize and monitor surface and sub-surface soil properties and processes that contribute to water driven erosion and transport of soil and to assess the potential for dam and levee failures. 1.A-1. Develop seismic instrumentation and methods for characterizing subsurface soil mechanical and hydraulic properties in the vadose zone. 1.A-2. Develop a combined seismoelectric technique and high frequency-MASW (HF-MASW) method to measure subsurface soil hydraulic properties. 1.B-1. Geophysical monitoring and surveying of dams, levees and streambanks within the agricultural watershed. 1.B-2. Conduct laboratory studies to investigate the correlation between geophysical properties and the physical state of a soil. 1.B-3. Investigate wind induced ground surface vibration as a source for measuring the mechanical properties of the ground. 2. Develop and deploy acoustic measurement systems across a watershed to provide improved data collection of sediment flux for decision makers. 2.A. Interpreting the acoustic environment of natural fresh-water gravel-bed channels for use in monitoring bedload flux. 2.B. Advance the application of multiple acoustic surrogate techniques to monitor suspended sediment transport.
1b. Approach (from AD-416):
There is a continuing need for better methods to non-invasively measure sediment transport and soil properties in situ. Furthermore, the Nation’s aging dams need to be assessed for structural integrity. Acoustic and orthogonal geophysical techniques will be developed for measuring the mechanical response of soil to remedial measures for upland erosion, autonomous monitoring of sediment transport in streams, and imaging the internal structure of earthen dam and levees. Shear wave propagation can be used to map spatial distributions of subsurface soil mechanical and hydraulic properties, and field experiments will be used evaluate their use for detecting compaction and the extent of plow-pans. A modified shear wave acquisition system will be developed to measure temporal changes in the shear wave velocity profile to infer variations in water potential and water content. The results will be correlated with information from time domain reflectometers (TDR) buried in the test site at different depths to measure water content, a tensiometer to measure water potential, and a rain gauge to measure precipitation. In exploratory work, a laboratory study will be conducted under controlled conditions to establish a relationship between seismoelectric signals and soil hydraulic properties. We will investigate the use of wind-induced vibrations to determine mechanical properties of soil. The method does not need high-energy acoustic/seismic signals, making it suitable for remote field sites. We will perform geophysical site characterization at dams or levees showing signs of internal erosion or seepage during visual inspection. The same procedures will be applied to groundwater recharge zones and streambanks. In order to facilitate the integration of geophysical and geotechnical information, laboratory measurements of compressional and shear wave velocities and electrical resistivity will be conducted on synthetic, remolded soils and field cores. Acoustic methods can be used to improve the accuracy and effectiveness of sediment monitoring programs, but they are in need of continued development. Multiple acoustic methods will be deployed across a watershed to improve the integration of technologies and interpretation of acoustic data. The movement of coarse particles along the stream bed is particularly difficult to measure. Sound generated by coarse particle movement in streams will be used to improve the measurement of bed load transport. The focus will be on separating the sound made by moving particles from other sounds, such as bubbles and other extraneous environmental noise. Through collaborative efforts with soil scientists, hydrologists, and agricultural engineers, the new measurement technology will facilitate more comprehensive studies on sources of sediment, sediment transport and deposition in streams and lakes, and stability analysis of earthen dams and stream embankments.
3. Progress Report:
A simultaneous multi-channel data acquisition system consisting of a geophone array (40 geophones inserted into the ground in a variable space configuration) and an electrodynamic shaker was developed and installed at a test site near the National Center for Physical Acoustics building. The system can automatically measure soil profile every 5 mins and is used to capture instantaneous variation of soil properties during rainfall events. The system also includes a rainfall gauge and several moisture sensors to measure precipitation and moisture content. A long-term survey is underway. The research is currently in a phase of collecting data at different seasons and soil conditions. Surveys have been conducted on an earthen dam of interest to the Mississippi Department of Environmental Quality (MDEQ) to complement their dam integrity assessments. The lab instrumentation has been configured to do shear and compressional wave velocity measurements. We are currently deciding on the parameter space (i.e. clay content, compaction effort, and soil type) that will be investigated. Finite element models of wind-induced ground surface vibration have been developed, and data has been collected at several field sites. The first of the flume experiments is expected to begin by July 20th. The results from the initial experiments will inform future experiments and iterations with regard to available flow parameters and hardware configurations. Multiple configurations of the hydrophone system will be deployed in Goodwin Creek to conduct propagation tests in August or September. A scientific acoustic source will be used to generate known signals that will be recorded by the hydrophone system at various locations within the test area. Researchers from the University of Mississippi are currently installing a long-term monitoring system in San Acacia, New Mexico in conjunction with researchers from the Bureau of Reclamation and the United States Geological Survey.