Location: Watershed Physical Processes Research2013 Annual Report
1a. Objectives (from AD-416):
1. Develop, adapt and evaluate, and integrate non-invasive geophysical methods and complementary modeling efforts, in support of a comprehensive earthen dam interrogation and monitoring program (2.4.1). Sub-Objective 1a: Develop acoustic and orthogonal geophysical methods for rapid non-invasive assessment of internal structure of earthen dams and levees in agricultural watersheds. Sub-Objective 1b: Develop an intelligent sensor based system for continuous monitoring for dam and levee structures in agricultural watersheds. Sub-Objective 1c: Develop an acoustic based method for the non-invasive rapid estimation of soil surface erodibility with and without grass cover. 2. Develop acoustic hardware and measurement techniques for non-experts to improve the monitoring of suspended and bedload sediment transport. (2.2.1, 2.2.2). Sub-Objective 2a: Utilize acoustic attenuation and backscatter measurements as a surrogate technique for monitoring suspended sediments. Sub-Objective 2b: Investigate the use of various passive and/or active acoustic techniques to characterize bedload transport. 3. Develop non-invasive acoustic/seismic techniques and orthogonal geophysical methods to characterize surface and sub-surface soils as well as visualize and monitor subsurface pedological features affecting erosion processes (2.1.1, 2.1.3). Sub-Objective 3a: Conduct a systematic lab study of the measurements of P-wave and S-wave velocities, nonlinear acoustic parameter, and electrical resistivity as a function of density, water content, water potential, compaction, and soil textures. Sub-Objective 3b: Develop and use in-situ non-invasive acoustic/seismic techniques to characterize and visualize the properties and processes of surface and subsurface soils. Sub-Objective 3b-1: Develop a Laser Doppler Vibrometer (LDV) based, in- situ, noninvasive, acoustic/seismic surface wave technique to characterize and visualize the properties of surface and subsurface soils. Sub-Objective 3b-2: Using high frequency Multi-channel Analysis of Surface Wave (MASW) technique to characterize soil surface properties such as sealing and crusting effects. Sub-Objective 3b-3: Measuring and characterizing soil sub-surface properties with the MASW method. Applications include layer delineation, and high velocity zones (plow-pan/fragipan) localization, and compaction effects study. Sub-Objective 3b-4: Measuring and characterizing soil sub-surface hydraulic properties in terms of soil water potential, water content, and infiltration rate with the MASW method. Sub-Objective 3b-5: Develop acoustic techniques for the detection and characterization of soil pipes and preferential flow pathway in associated with internal erosion.
1b. Approach (from AD-416):
1. Developing methodology and protocols for the rapid assessment of earthen embankments using non-invasive geophysical techniques will extend the application of geophysical methods by investigating the simultaneous use of seismic and electrical resistivity tomography to detect compromised zones within earthen embankments. Design and install a remote monitoring system of geophysical and geotechnical sensors in an earth embankment dam. The goal is to design and install the first remote real time continuous monitoring system in an earthen dam. The acoustic interaction with the soil medium depends upon mechanical and hydraulic properties of the soil surface that also influence soil surface erodibility. Therefore, acoustically measured properties can be used as a proxy for estimating soil surface erodibility. The first step in testing the hypothesis of this objective is to determine the experimental configuration and optimal range of acoustic frequencies. A numerical model based on existing theories of sound interaction with soil will be used to predict the acoustic response. The parameter space for the model will be constrained based upon typical soil characteristic (porosity, shear modulus, permeability, bulk density, elasticity) for a realistic range of erodibilities. 2. A design effort to advance the state of the art in the use of ultrasonic backscatter and attenuation to measure suspended sediment flux, developing a technique to evaluate the concentration and particle sizing of fines (0.1–100 µm) suspended in water, could be achieved by using either a single frequency acoustic system utilizing attenuation from forward propagation in conjunction with back-scattered signals or utilizing backscatter from multiple high (2-20 MHz) frequencies. Also, a design effort will be made to advance the state of the art in the use of kinetic energy impact plates instrumented with geophones or accelerometers to monitor bedload movement of gravel or sand. Under a parallel research effort, the NCPA has been working with the BOR to instrument a set of kinetic energy plate impact sensors to monitor bedload gravel movement as part of the Elwha River dam removal in the Olympic National Park in Washington State. The effort provides an opportunity to expand the state of the art for monitoring gravel bedload movement across other watersheds that are of interest to the USDA. 3. A seismic technique known as the multi-channel analysis of the surface wave (MASW) method has been developed at NCPA to obtain soil profile data at depths from a few centimeters to a couple of meters. A lab-scale test will be conducted. The MASW procedure consists of an acoustic/seismic excitation, a stepper motor controlled LDV for multi-points surface vibration measurement, a spectrum analysis to convert collected time-domain signals into the frequency-phase velocity domain, and an inversion process. Applying the MASW method to field soil, time/climate-related variations will be used for calculating the infiltration rate. Additional testing will consist of two parts: (1) lab tests with an acoustic transmission method and (2) field tests using MASW setup and dynamic move-out technique.
3. Progress Report:
This project has progressed on the development/evaluation of innovative acoustic/seismic techniques as well as understanding physical principles for soil erosion, sedimentation, and structural integrity of earthen dams. In the area of upland erosion, the Laser Doppler Vibrometer-Surface Wave system has been improved. Results indicate that long term weathering effects on soil profiles can be monitored using the dependence of the shear-wave velocity on temporal and spatial variations in soil water content and potential. Field studies show the depth and extent of a fragipan identified in a shear-wave image was in agreement with observations from a soil pit and penetrometer test. Laboratory acoustic experiments showed that the variation of the pressure-wave velocity correlated with different stages of an internal erosion process. Passive acoustic measurements showed that soil pipeflow can be identified and assessed from noise level measurements. Research in sediment transport has made progress on acoustic monitoring of sediments across a wide range of grain sizes. A high frequency system was utilized in the field to measure fines content. Measurements 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. Progress was made on the use of passive acoustic systems to monitor bedload (gravel). Hydrophones were installed at the United States Department of Agriculture flumes. The data showed a correlation between mass flux and acoustic output indicating the acoustic data could be inverted for bedload flux. A prototype system for bedload transport was used in the Trinity River in California and the Elwha River in Washington State in conjunction with a Bureau of Reclamation project. Rapid assessment of the potential failures in earthen dam requires advanced screening tools to delineate, classify, and prioritize compromised locations. A system has been designed for an earthen dam in Okissa, Mississippi, to monitor pore pressures in a confined aquifer that extends below the dam. The system includes vibrating wire piezometers, a reservoir water level sensor and a weather station communicating over a wireless network. A series of laboratory tests were conducted in order to better understand the relationship between measured geophysical parameters and the soil properties and conditions. The seismic velocities and attenuation change dramatically as the sample is compacted past the optimum moisture. Geophysical 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.
1. A monitoring system was 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 and allow for remote monitoring of the dam.
2. Application of the Laser Doppler Vibrometer- Multi-channel Analysis of Surface Wave system to study weather effects on shallow soil and detecting the presence of a fragipan has been completed. The results revealed that shear-wave velocity profiles vary with temporal and spatial variations due to the changes in soil water content and water potential and this system can be used for non-invasive measurements of weather effects on soil profiles up to 2.5 meters below the surface.
3. Field testing of the acoustic based system for bedload transport 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 has been conducted. These measurements were made in conjunction with impact gravel measurements as well as physical samples taken by Graham Mathews and Associates. Once the physical sample data is available, it will be used in the calibration of the acoustic system.
4. The high frequency fines system was used at the Elwha River in Washington to assist the Bureau of Reclamation in measuring the 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.