Location: Watershed Physical Processes Research
2019 Annual Report
Objectives
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.
Approach
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.
Progress Report
Progress was made on the two objectives and their subobjectives, all of which fall under National Program 211 (Water Availability and Watershed Management), Component 2 (Erosion, Sedimentation and Water Quality Protection).
Progress on Objective 1A relating to the development of acoustic methods to characterize sub-surface soil properties is as follows.
In 2018-2019, we applied our enhanced HF-MASW method to measure instantaneous variations in soil profiles induced by rainfall events. The testing site was located on the campus of the University of Mississippi. The HF-MASW measurements were conducted continuously before, during, and after rainfall events. The study demonstrated that the HF-MASW method has the capability to capture the temporal variations of the soil profile in response to rainfall events.
Using our LDV-based system, a surface sealing/crusting test is planned for the summer of 2019. The test site is located at the Mississippi Agriculture and Forestry Experiment Station (MAFES) at Holly Springs, Mississippi. Two plots were prepared: one was treated and exposed to the rainfall impacts to form surface crust/sealing, and the other was covered with plastic sheeting to avoid the formation of surface crust/sealing. The latter plot will be used as a control plot. The LDV-based HF-MASW method will be employed after the formation of surface crust/sealing.
A water tank (2.6 X 1.8 X 0.9 meter) was installed in the laboratory and was filled with unsaturated sand. The tank provides the capacity for controlling the water level and moisture measurements. A measurement system is under development which consists of a LDV-based HF-MASW method and a seismo-electric method. After the completion of the system, studies will be conducted under different soil moisture conditions.
Progress on Objective 1B relating to the development and application of geophysical methods to assess the potential for dam and levee failures are as follows.
Seismic refraction and electrical resistivity measurements were conducted at Carroll County Dam, a high hazard dam located in northwest Mississippi. The dam was affected by moderate seepage and sand boil formations. Sand boils developed despite the dam retaining very little water and it raised concern about the dam integrity and the existence of seepage through the dam structure. Results from the application of geophysical surveys indicated that seepage is taking place through a small lens of silty sand imbedded within a clay layer that was not properly sealed during construction of the base of the dam. A paper detailing the work at Carroll County dam was also published (Prediction of Possible Causes of Sand Boil Formations Using Multiple Geophysical Surveys: A Case Study at Carroll County Dam, EEGS FastTIMES, Volume 23, Number 3, pp 68-74, 2018).
In another case study, visual inspection of the concrete emergency spillway at Big Sand Watershed Structure Y-32-12, located in North Carrollton, Mississippi, indicated differential movement in the mid-section concrete slab. One suggested reason for slab movement was void formation associated with seepage and washing out of the fines-filter under the slab. Ground penetrating radar (GPR) measurements were conducted to investigate possible void formation. Multiple GPR survey lines were acquired across and down the slope of the concrete slab using 500MHz antennas. Preliminary results showed some small-scale amplitude anomalies observed near the bottom of one of the slab sections. However, an integrated interpretation of the entire survey does not support void formation.
Progress has been made on establishing relationships between geophysical observations and geotechnical properties. Geotechnical laboratory measurements were conducted on 11 sandy clay and 15 clay samples with different proportions of sand, clay, and silt mixtures. The clay consists of 80% kaolin and 20% bentonite. Samples were classified based on USDA soil texture classifications. Liquid limit, plastic limit, and specific gravity tests were conducted and other parameters, such as specific surface area and plasticity index, were determined using the liquid limit and plastic limit results. The geotechnical parameters found from the laboratory tests agree with the literature. Liquid limit, plastic limit and plasticity index increase with increasing clay content and decrease with increasing sand content. Specific gravity results indicated an inversely proportional relationship with clay content, while specific surface area results were directly proportional to liquid limit and plastic limit. Development of the ANN model has begun based on the collected laboratory results.
Due to the problems with the bender elements, the geophysical tests have not been completed. The plan is to perform electrical resistivity and seismic wave velocity experiments on the same compacted samples to maintain the uniformity of results. Once the bender element issue is resolved, the geophysics tests will be conducted
Measuring the mechanical properties of soils requires some type of source. Wind induced ground vibrations are being investigated as a possible source. A Master’s thesis was published describing wind induced ground vibrations which include: the dynamic response of the ground and the inhomogeneity and anisotropy of the ground. The dynamic response of the ground under the influence of a harmonic vertical surface load was obtained through both an analytical solution and finite element modeling. The two models are in good agreement. However, the use of these models for predicting the measured wind induced ground vibration was marginal. Research is continuing on the theoretical formulation of wind induced ground vibration models.
Progress was made on Objective 2 related to measurement methods for monitoring sediment flux. For Objective 2A, analysis of the data from multiple flume experiments has provided insight into the relation between water flow around the instrumentation and the sound recorded. These results are being incorporated into the design of upcoming experiments.
Experiments using the Benthowave BII-8030 underwater transmitter have been conducted in multiple natural environments. The sound level of tones generated by this source was measured at various locations within the river or stream. In addition, experiments were conducted with a set of moving hydrophones recording the generated tones as they moved with the flowing river. This helped reduce much of the flow noise present in some of the other tests. The data from these experiments is being analyzed and the results will help inform future experiments and deployment techniques.
Progress on Objective 2B relating to surrogate techniques to monitor suspended sediment transport include the following.
A prototype bracket has been modified to accept an orthogonal transducer that will allow for simultaneous backscatter measurements in concert with the attenuation measurements. Software was written to coordinate this data collection over the two measurement techniques. Laboratory tests were conducted in the NCPA recirculation tank to insure operation. A second long-term deployment was installed on the Rio Grande Floodway at San Acacia, New Mexico in conjunction with researchers from the Bureau of Reclamation and the USGS. An acoustic database of attenuated signals has been collected at San Acacia, NM and correlated with physical measurements collected by USGS.
Subsequent acoustic data sets from the San Acacia, New Mexico site have been analyzed to test the correlation between the attenuated signal and the suspended sediment concentration < 62 microns. Refinement of acoustic correlations is ongoing. Acoustic correlations will be expanded to incorporate a relationship between backscattered signals and physical samples once particle sizing has been completed by USGS. Following hardware modifications, a long-term deployment of a multi-frequency acoustic surrogate device is planned in the Goodwin Creek Watershed, MS in conjunction with USDA.
Accomplishments
1. Geophysical information in support of dam and levee inspections. The University of Mississippi, in collaboration with ARS researchers in Oxford, Mississippi, USDA-NRCS, and Mississippi Department of Environmental Quality (MDEQ), has been studying the added value of incorporating geophysical information into dam and levee assessments. In two case studies, geophysical investigations have provided valuable information about subsurface characteristics important to the assessments. In a seepage study of Carroll County Dam, a high hazard dam located in northwest Mississippi, the dam was affected by moderate seepage and sand boil formations. Results from geophysical surveys indicated that seepage was most likely taking place through a small lens of silty sand imbedded within a clay layer below the dam that was not properly sealed during construction. In a second case study, ground penetrating radar (GPR) measurements were conducted to investigate possible void formation in support of an assessment of the concrete emergency spillway at Big Sand Watershed Structure Y-32-12, located in North Carrollton, Mississippi. An integrated interpretation of the entire survey did not support the presence of voids below the concrete slabs. Geophysical surveys provide information about the subsurface and below structural components of dams that are not accessible during visual inspection. This added geophysical information is allowing for more confident assessments of watershed structures.
