Exhuming Munitions via Turbulence-Induced Bed Fluidization
Dr. Blair Johnson | University of Texas at Austin
The objective of this project is to determine the extent to which turbulence can fluidize a sediment bed and subsequently expose buried munitions or impact development of bedforms. The experimental campaign utilizes a laboratory facility in which turbulence is generated by a Randomly Actuated Synthetic Jet Array (RASJA – Variano & Cowen 2008) and mean flows are negligible. By design, turbulence generated in this facility is a close model for bore-advected turbulence, in which instantaneous local shear stresses significantly outweigh bed stresses due to mean or oscillatory flows. It has been observed that ripples can develop under turbulent forcing even in the absence of observable sediment suspension. Further, it has been shown in previous research that significant local pressure gradients may exist in the boundary layer of mean shear free turbulent flows. For these reasons, the researchers hypothesize that there is a great likelihood for sediment beds exposed to highly energetic turbulence to fluidize, leading to potential bed dilation and mobility of sand grains to form ripples. With this potential for significant mobility within the bed, it is further hypothesized that buried munitions may be exhumed during fluidization conditions.
The technical approach describes a detailed and systematic method of determining the requisite fluid and sediment conditions conducive to fluidization and munitions exposure. Emphasis is placed on establishing repeatable experiments with many synchronized measurement devices in order to obtain data linking instantaneous flow forcing with subsequent sediment dynamics. By performing dye studies where fluorescent dye is injected into the bed to highlight fluidization potential and experiments with cores of initially stratified sand layers, fluidization will be quantifiable in relation to the underlying flow conditions. In addition to exploring requisite turbulent forcing for exposure of munitions, the impact of buried or partially-exposed munitions on resulting bathymetry will be investigated as a means of noninvasively identifying submerged UXO in the field and their likelihood to be transported. Several model munitions of varying geometry and density will be tested in a range of flows scaled to mimic environmental conditions across an array of natural beaches and aquatic systems to evaluate mobility.
This project will provide guidance on how turbulent flows in low mean shear environments (e.g. nearshore, stratified lakes, estuaries) can impact mobility of buried munitions. Due to the highly controllable and optically accessible laboratory facility, there will be significant simultaneous datasets detailing links between flow velocity, instantaneous local pressure gradients, sediment suspension, bed porosity, morphological evolution, bed fluidization, and munitions mobility. Although such measurements are often prohibitively difficult in field conditions, the experimental facility can generate flows with a variety of turbulence levels, as measured via dissipation rates, integral scales, and energy spectra, among others, to ensure adequate scaling with a wide variety of environments. This dataset can be used to supplement experiments in field scenarios and/or in wave flumes where measurements relying upon full optical access may be restricted. By determining flow and sediment conditions conducive to exposure, as well as identifying morphological features indicative of buried munitions, detection processes may be significantly simplified and ultimately addressed via remote sensing techniques. Furthermore, if findings from the project work show thresholds of turbulence in which munitions across a range of geometries and densities are relatively easily exhumed, it may be possible to improve remediation technologies to more efficiently address buried munitions.