2013 Annual Report
In the area of upland erosion, the Laser Doppler Vibrometer- Multi-channel Analysis of Surface Wave (LDV-MASW) system has been improved greatly in terms of sampling speed, signal to noise ratio, and accuracy by adding one reinforcement beam and improving signal processing algorithm. The system was employed to study:
a. The “long term” weather effects on soil profiles up to 2.5 meters below the surface. The Shear-wave velocity profiles vary with temporal and spatial variations due to the changes in soil water content and water potential.
b. A field study was conducted using Laser Doppler Vibrometer- Multi-channel Analysis of Surface Wave in Mississippi Holly Springs Field Station with a vertical cross section image in terms of Shear-wave velocity. This result was compared with field observations from a soil pit face penetration test. The presence, depth, and extent of a fragipan identified in the Shear-wave cross-section were in good agreement with the other observations.
c. A laboratory acoustic experiment on artificial soil pipe flow was completed. The active acoustic measurements showed that the variation of the Pressure-wave velocity reflected the ongoing internal erosion processes such as the onset of soil pipeflow, the buildup of positive water pressures within the soil pipe, the saturation of soil adjacent to the pipe, the variation of water pressures within and adjacent to the soil pipe as the soil drained following removal of the constant head, and relaxation of the soil. The passive acoustic measurements showed that soil pipeflow can be identified and assessed from noise level measurements.
Research with respect to sediment transport has made progress in the development of acoustic technology for monitoring sediments across a wide range of grain sizes. These include:
a. A high frequency fines system was used in the field at Harris Bayou in Mississippi in conjunction with United States Geological Survey gauging stations. The system was also used at the Elwha River in Washington State in conjunction with the Bureau of Reclamation to measure sediments resulting from a dam removal. The unit operated well in both cases and indicated fine sediment concentrations in line with traditional measurements. Further data analysis is underway comparing the acoustic measurements to simultaneously-obtained traditional sampling measurements.
b. Data from the measurements on the combined use of attenuation and backscattered acoustic signals to monitor concentration and particles sizes noted in last year’s report was further analyzed. The data indicate that 20 Mhz is the preferred frequency to monitor concentration via attenuation but 10 Mhz is the preferred frequency to monitor size via backscatter. This suggests that either a second channel or second system is needed to transmit the alternate frequency. Field tests to utilize two of the prototype systems side by side (instead of redeveloping the entire hardware at a prohibitive cost) are in the planning stage.
c. Substantial progress has been made on the use of passive acoustic systems to monitor bedload (gravel). Hydrophones (underwater microphones) were installed at the United States Department of Agriculture flumes and various configurations of flow noise reduction systems were tested. Similar to the noise observed in outdoor sound systems, the underwater hydrophones have a difficult time discriminating the acoustic pressure variations from gravel collisions from the flow induced pressure variations from the high speed flows that make the gravel move. The hydrophones were placed inside of tubes and foam to isolate them from direct gravel collisions as well as to reduce the flow noise. A gravel plate with an accelerometer was also placed in the flume. Dragging gravel across the gravel bed provided data for the hydrophones studies and testing of the plate system. The use of various sizes and speeds of gravel provided varying mass flux. The data showed a correlation between mass flux and acoustic output indicating that the systems could be used in reverse. That is, the acoustic signal could be inverted to indicate bedload flux. d. A prototype system for bedload transport were used in both the Trinity River in California as well as the Elwha River in Washington State in conjunction with an allied Bureau of Reclamation project on bedload transport behind dammed rivers.
Rapid assessment of the potential failures in levee and earthen dam requires advanced screening tools to delineate, classify, and prioritize compromised locations. Progress of the research in this area includes:
a. A monitoring system has been designed for a United States Forestry Service earthen dam in Okissa, Mississippi, to monitor excessive pore pressures in a confined aquifer that extends below the dam. The system includes seven vibrating wire piezometers, a reservoir water level sensor and a weather station. All components communicate with the base station over a wireless network. Field installation will be in July 2013.
b. A series of laboratory tests on silty and sandy soils prepared over a range of pre-determined compaction specifications was conducted in order to better understand the relationship between measured geophysical parameters and the soil properties and conditions. The compressional and shear wave velocities and signal attenuation change dramatically as the sample is compacted past the optimum moisture content. The complex electrical resistivity varies with moisture and clay content.
c. Electrical, seismic and electromagnetic measurements were conducted on the landside of the mainline levees in the Mississippi delta where sand boils were observed. The data was used to delineate the geology controlling seepage associated with the piping. This data is also being used to test joint inversion algorithms as part of the National Science Foundation Partnerships for International Research and Education with Colorado School of Mines.