Aquatic cohesive sediments are often the final receptor of contaminants released by DoD activities. In estuaries, harbors, and channels, fine sediments can be mobilized and transported by the combined action of both relatively steady currents (forced by river flow, tides, and wind) and to an even greater extent by the larger, unsteady bottom shear stresses associated with higher-frequency oscillatory wave motions (either wind- or vessel-generated waves). Methods to measure contaminated sediment stability and erosion as influenced by wave processes are critical to assessing contaminant fate and associated risk as well as evaluating remediation options.
The research goal of this project was to develop laboratory methods to reliably measure and predict erosion of contaminated fine-grained sediments under bottom shear-stress conditions that represent combined wave and current action. The shear-stress conditions for erosion measurements replicate those expected in the field (nonbreaking wave, shallow water, estuarine, harbor, and coastal environments). The specific objective of this project was to calibrate and validate an innovative mobile erosion flume designed to measure cohesive sediment erosion under combined wave-current conditions.
This research focused on calibrating and verifying the applicability of a prototype flume designed to measure erosion of fine-grained cohesive sediment under combined unidirectional and superimposed oscillatory bottom shear stress. These are similar to the shear-stress conditions that exist in wave-current environments. The new flume is based on the design of a well-documented, frequently utilized flume for measuring cohesive sediment erosion under unidirectional bottom shear stress representing steady current. Preliminary results using the prototype flume (Sediment Erosion Actuated by Wave Oscillations and Linear Flow [SEAWOLF]) for the combined unidirectional and oscillatory flow have been published (Jepsen et al. 2002).
The SEAWOLF flume does not replicate the wave/current hydro-dynamic conditions but rather replicates only the bottom shear stress that causes erosion. Free surface waves creating high-bottom shear cannot be developed in a field-deployable device at reasonable cost. The interior of the flume channel must be instrumented to demonstrate that bottom shear stress over the sediment surface in the flume is equivalent to specified wave-current conditions. The first step in demonstrating flume validity to wave-current conditions is to demonstrate that bottom shear stress over the sediment surface in the SEAWOLF flume replicates shear stress due to combined wave-currents. To demonstrate this, two instruments have been incorporated into the flume, one for measuring both near-bed and full-field flow structure (including velocity profile) and the second that simply measures near-bed velocity profile from which shear stress can be derived. The first system is a novel high-magnification, particle-based velocimetry system that utilizes particle-image velocimetry (PIV) and particle-tracking velocimetry (PTV) algorithms to resolve micro- and macro-scale flow structure as well as the velocity profile to within 7 μm of the test section wall. Classical velocimetry systems are typically deployed in lower-energy, steady-flow, free-surface environments. Implementation within a closed-channel, high-shear, rapidly time-varying environment is a new application for these devices and one of the most challenging research aspects of this project. Issues related to time scale, turbulence, and secondary circulation under which these instruments have not been tested had to be analyzed.
The second system, the miniLDV, is a self-contained and permanently aligned laser Doppler velocimeter. The miniLDV probe contains a laser, miniature beam shaping optics, receiving optics, and a detection system. The size of the probe volume (measurement domain) is ~25 μm by 50 μm by 150 μm. The system is mated with a computer-controlled linear traversing system allowing for resolution of near-bed velocity profiles.
Based upon the very interesting results from the hydrodynamic data collection activities, the path forward changed in 2008 to focus exclusively on hydrodynamic calibration and validation. Verification is required to prove that the bottom shear stresses created in the flume represent bottom shear stresses under naturally occurring wave-current conditions. Demonstrating flume applicability to wave-current environments requires designing standard operating procedures for SEAWOLF flume operation and collecting sufficient comparative laboratory data to prove SEAWOLF shear-stress replication methodology.
Two major hydrodynamic studies are presented in this research. First, PIV was used to study the cyclic modulation of the wall shear-stress and turbulence properties of an oscillating channel flow. The PIV instrument employed utilized a dynamically adjusted delay between the laser pulses to accommodate the wide variations in velocity encountered in the oscillating flow. Both high- and low-magnification digital PIV recordings were obtained to reveal the near-wall boundary layer structure and wall shear stress, as well as the full-field turbulence throughout the channel. Presented are wall shear-stress and global turbulence data for Stokes-thickness Reynolds numbers of Reδ = 1220, 2033, and 2875. The results reveal a fully developed turbulent state, relaminarization, and an explosive transition back to turbulence. The flow was examined in detail for the case at Reδ = 1220, where instantaneous PIV realizations at low magnification reveal the structure of the flow during relaminarization and transition back to turbulence. High-magnification PIV results were used to reveal the phase modulation of the mean velocity profiles in the viscous sublayer and logarithmic layers through the half cycle and quantitative profiles of in-plane Reynolds stresses and turbulence production were presented. To the research team’s knowledge, this was the first PIV investigation of this canonical unsteady turbulent channel flow, and these results represent a needed contribution to the limited turbulence data that exists for unsteady wall flows.
Second, the structure of turbulence in an oscillating channel flow with near-sinusoidal fluctuations in bulk velocity was investigated. Phase-locked particle-image velocimetry data in the streamwise/wall-normal plane were interrogated to reveal the phase-modulation of two-point velocity correlation functions and of linear stochastic estimates of the velocity fluctuation field given the presence of a vortex in the logarithmic region of the boundary layer. The results revealed the periodic modulation of turbulence structure between large-scale residual disturbances, relaminarization during periods of strong acceleration, and a quasi-steady flow with evidence of hairpin vortices that was established late in the acceleration phase and persisted through much of the deceleration period. Again, to the best of the researcher team’s knowledge, this was the first structural information for an oscillating flow derived from direct velocity measurements.
Also in 2008, a newly developed commercial miniLDV sensor was identified for potential application within the SEAWOLF. While reliable, PIV-derived measurements of shear-stress time histories offer limited amounts of data. Characterization of a single wave-current frequency and amplitude operating condition generally requires a full day of testing and an additional 1–2 days for analysis of the PIV data for the extraction of wall shear-stress histories. For the purposes of obtaining large parametric data sets that vary in frequency, amplitude, and bed-roughness conditions, a more rapid shear-stress measurement must be incorporated. In previous years, a floating element mechanical shear stress sensor was tested that could be calibrated to a shear-stress value. These mechanical sensors have been applied for testing in SEAWOLF with mixed results; the sensors responded to time-varying shear stresses but displayed weak signal levels that were easily corrupted by electrical noise and suffered from time-dependent drift in sensor voltage, which corrupted the measurement as well.
As an alternative to mechanical shear-stress measurements, two newly developed commercial optical sensor technologies were tested, the microS and miniLDV, which (1) offer calibration-free measurements, (2) improved signal-to-noise characteristics, and (3) have been proven to work in harsh and dirty environments. The miniLDV significantly out-performed the microS in initial testing, and therefore it will be implemented within the SEAWOLF facility.
This research will provide more accurate methods for assessing contaminated sediment stability for many DoD and EPA managed contaminated sediment sites. The hydrodynamic validation exercises will produce stand-alone products that will benefit multiple other sediment transport projects. The final methods developed for measuring and predicting contaminated sediment erosion under wave-current conditions will be peer reviewed, cost effective, and standardized. Such methods do not presently exist, adding significant uncertainty in predicting contaminant fate and associated risk in shallow-water, coastal, estuarine, and harbor environments. This research from this project will develop methods to reduce uncertainty that presently complicates risk assessment and evaluation of remediation options.