Location: Watershed Physical Processes Research
2021 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.
A laboratory measurement system that combines the high-frequency MASW (HF-MASW) method and the seismo-electric technique was constructed to explore soil profiles under controlled hydraulic conditions. Unfortunately, the seismoelectric signals were identified as cross-talk signals, and the actual signals for unsaturated soils were too weak to be detected. The HF-MASW system was updated by using a simultaneous data acquisition board, which greatly improved the measurement accuracy of dispersive curves at higher frequencies. A long term-survey was initiated which compares the soil profiles obtained from the HF-MASW and penetration tests under different field soil conditions. Preliminary data has been collected and more data, especially at dry soil conditions, are needed for data analysis.
Progress on Objective 1B relating to the development and application of geophysical methods to assess the potential for dam and levee failures is as follows.
A plan is underway to conduct geophysical measurements at Town Creek Watershed Structure #19, a flood control earthen dam completed in 1973. The dam, located in Union County, Mississippi, is currently classified as a high-hazard dam due to an imminent threat to human life in the event of a sudden failure. The dam has experienced multiple slide failures since 2016 when the first slide was observed on the downstream side of the dam. The dam was kept dry in 2017 while repair work was conducted. But in 2018, three slides were observed. After the most recent repairs in November 2020, a slide on the downstream side occurred in April 2021. Due to the low water level in the reservoir, it is not expected that water seepage through the dam is causing the slides. Geophysical measurements will be conducted on the dam's downstream side to locate and investigate the toe drain. Measurements down the slope will also be conducted very close but away from the current slide location to investigate if there are differences in the subsurface structure of the dam.
Laboratory measurements were expanded to include the cation exchange capacity (CEC) and unconfined compression tests. The CEC of a soil is the total amount of negative charge on soil surfaces that can hold positive cations such as calcium, magnesium, and potassium by electrostatic forces and contributes to the holding capacity of soil for nutrients and water. CEC depends on soil type, in particular the clay fraction. The CEC increases with an increase in clay content. The electrical conductivity increases with an increase in CEC however the sensitivity of seismic wave velocity on CEC requires further investigation. Unconfined compression tests are used determine the unconsolidated undrained shear strength of a clay soil. For this research unconfined compression strength of clay and sandy clay are determined according to ASTM D 2166 method. Standard proctor tests were conducted on one field soil. The behavior of this field soil is considerably more complex than synthetic samples. It is therefore very important that measurements be extended to include remolded field soils and undisturbed soils.
Published values for seismic wave velocity, liquid limit, plastic limit, water content, and dry density from field and laboratory measurements were used to develop ANN models. Due to the small number of data, models were developed with the validation step as well as omitting validation in order to use more data for training. Models incorporating the velocity information show better predictability of water content and dry density. Laboratory data provides better models than field data. Multilinear regression analyses were also performed and a comparison indicates that ANN performs better than multilinear regression analysis.
Progress on Objective 2 relating to measurement methods for monitoring sediment flux is as follows.
Propagation and flow noise data from previous experiments has been analyzed. Restrictions put in place due to the COVID-19 pandemic prevented further testing of flow velocity versus acoustic signal in the spring and summer of 2020. However, flow velocity was measured as part of some previous field deployments. A custom labview program has been written to collect raw acoustic data from hydrophones. By interfacing a laptop computer and a Measurement Computing USB-1602 data acquisition system, raw acoustic data can be collected and stored for long periods of time. The system can be used with an external trigger, or it can be configured to run at pre-determined intervals. Collecting continuous data is also possible, but requires separate data-management practices.
Combining all of the data collected from field and laboratory experiments is projected to be completed. Work has also been conducted investigating the use of frequency content to estimate the size of particles involved in collisions. This has been investigated both with laboratory experiments as well as different analysis of previously-collected field data.
Progress on Objective 2 relating to surrogate techniques to monitor suspended sediment transport is as follows.
It was determined that a better understanding must be reached of the diel patterns recorded from the San Acacia, New Mexico installation discussed in the peer reviewed Bureau of Reclamation report titled “Field Deployment of a Continuous Suspended Sediment Load Surrogate.” In partnership with Jason Taylor, research ecologist at USDA-ARS-NSL, it was hypothesized that the diurnal attenuated signal can partially be attributed to algal activity. In order to investigate the effects of algal activity on attenuation measurements, the Single Frequency Acoustic Attenuation Surrogate (SFAAS) was deployed in a series of limnocorrals in a pond at The University of Mississippi Field Station (UMFS) in Abbeville, Mississippi. Results suggest that as light intensity increases, attenuation and absorption increase due to greater algal metabolic activity.movements of algae in response to changes in light intensity. An empirical formula comparing the acoustic signal and sediment transport can also incorporate algal activity and make the corresponding corrections in calibration and improve the hardware. This work is still ongoing with new experiments launching during the last year.
Concurrent to the UMFS algae study, the NCPA acoustic system has continued to be deployed in the Goodwin Creek Watershed, MS in conjunction with USDA Agricultural Research Service National Sedimentation Laboratory. Pump samples from events were shared with NCPA to provide a site-specific calibration for the SFAAS. The acoustic measurement shows that the SFAAS data tracked well with increased concentrations, which are clearly correlated with changes in stage in Goodwin Creek. The agreement between SFAAS-derived concentration data and pump-sample data breaks down after the passage of the storm hydrograph. Additional pump samples are needed to calibrate the acoustic device for greater concentrations.
The modified system has been deployed at Goodwin Creek, which allows users to select a calibration fit and generate a suspended sediment concentration measurement using the acoustic surrogate technique. The calibration fits cover all data shared with NCPA to date.
The results suggest that the SFAAS may be used to track algal movements and possibly population density. In a stream such as Goodwin Creek, where hydrographs are short in response to runoff events, the high temporal resolution of the SFAAS can provide valuable sediment transport data. The low base flow of Goodwin Creek, coupled with rapidly rising flows and short hydrographs that only last for a few hours, are likely reasons for the successful use of the SFAAS. The SFAAS system remains at Goodwin Creek to capture events and flow conditions with the use of a Brinno time-lapse camera on-site.
Accomplishments
1. Detection of algal activity using acoustics. Diel patterns in measured underwater ultrasonic acoustic signals can partially be attributed to algal movements and possibly population density in response to light intensity. An understanding of algal activity’s influence on ultrasonic acoustics is necessary to improve the algorithm for converting attenuated acoustic signal to sediment concentration as well as to provide a novel approach to track algal activity in-situ. Acoustic monitoring provides a technique to reduce the cost of studying algae growth. Using ultrasonic acoustics as a technique to monitor algal movements and density could be useful for ARS researchers in Oxford, Mississippi, who need to track the movement of algae in water.
2. Installation of suspended sediment transport measurement algorithm using an acoustic surrogate. With the installation of the new algorithm, ARS researchers in Oxford, Mississippi, managing the Goodwin Creek watershed in Panola County, Mississippi, can make sediment concentration measurements using an acoustic surrogate device. Pump samples were compared to acoustic signal loss relative to clear water to establish a calibration fit. Once calibrated to a site, this acoustic measurement device will allow for greater spatial and temporal resolution than pump sample measurements. In turn, this device could be applied to other watersheds containing greater than 100 mg/L suspended sediment concentrations, which some pump samplers struggle to measure. These sediment measurements using an acoustic surrogate will provide a better representation of the storm event peak than pump sample methods.
3. Mapping soil pipes. Goodwin Creek Experimental Watershed is located in Panola County, Mississippi, east of the Mississippi River valley. Due to the high rate of rainfall and the highly erodible loess cap, which contains a fragipan layer that fosters lateral subsurface flow, the area has established soil pipes and ephemeral gullies. ARS researchers in Oxford, Mississippi, used Ground penetrating radar (GPR) and electromagnetic induction (EMI), for delineating soil pipes within a 1.36 ha catchment, which contained 40 pipe collapse features for a density of 29.4 per hectare. Results from the study showed that GPR diffractions are an effective indicator of soil pipe formations. Soil pipe locations, indicated with low penetration resistance (PR) values (< 0.5 MPa), acted as singular discontinuities and produced strong diffractions on GPR cross-sections. The locations of diffraction apexes on GPR cross-sections can be used to determine the depth of soil pipes. GPR depth slices can be used to determine the depth and pathway of soil pipes. The EM38 ground conductivity meter can be used to quickly cover large agricultural areas. The method lacks the resolution to delineate individual soil pipes but allows for mapping fields with respect to varying degrees of soil pipe susceptibility. This study suggests that soils susceptible to soil piping are associated with low ECa (apparent electrical conductivity) zones. Overall, agrogeophysical methods such as GPR and EMI provide mapping of the subsurface of agricultural fields.
Review Publications
Bakhtiari Rad, P., Hickey, C.J. 2021. Seismic diffraction separation in the near surface: Detection of high-contrast voids in unconsolidated soils. Geophysics. 86(3):WB13-WE23. https://library.seg.org/doi/10.1190/geo2020-0366.1.
Lu, Z. 2021. A review of the high-frequency multi-channel analysis of surface wave method for proximal soil sensing. The International Journal of Earth & Environmental Sciences. 6:180. https://doi.org/10.15344/2456-351X/2021/180.
Wodajo, L.T., Rad, P.B., Sharif, S.I., Abas, M.A., Mamud, M.L., Hickey, C.J., Wilson, G.V. 2021. Agrogeophysical methods for identifying soil pipes. Journal of Applied Geophysics. 192:104383. https://doi.org/10.1016/j.jappgeo.2021.104383.
Wodajo, L.T., Hickey, C.J., Brackett, T.C. 2019. Application of seismic refraction and electrical resistivity cross-plot analysis: A case study at Francis Levee Site. Book Chapter. p. 23-40. https://doi.org/10.1007/978-3-030-27367-5_2.
