Simple Parameterized Models for Predicting Mobility, Burial and Re-Exposure of Underwater Munitions
Dr. Carl Friedrichs | Virginia Institute of Marine Science
The objectives of project MR-2224 were: (i) to identify and compile existing quantitative data from the scientific literature and from the coastal engineering, geology and Department of Defense (DoD) communities regarding the mobility, burial and re-exposure of unexploded ordnance (UXO) and UXO-like objects; (ii) to utilize these data to further develop and constrain simple, rational, parameterized models for UXO mobility, burial and re-exposure; and (iii) to provide these data and improved parameterized model formulations to the Strategic Environmental Research and Development Program (SERDP) investigators for use within a more sophisticated Underwater Munitions Expert System (UnMES) as well as providing them to the larger SERDP, DoD, coastal engineering, and scientific communities.
Relevant data were identified and compiled through searches of academic journals, thesis and dissertation databases, DoD reports, interlibrary loan, and the internet. SERDP colleagues also provided new data through targeted laboratory experiments designed to fill crucial gaps in parameter space. Development of parameterized models for (a) initiation of motion and (b) scour-induced burial were guided most directly by (a) geological literature on the motion of gravel and cobbles in streams and (b) coastal engineering literature addressing seabed pipelines. Data and improved parameterizations were provided to SERDP investigators through an iterative approach, such that gaps in the data supporting parameterized model development were used to guide new lab experiments. Results were provided to the larger community via two peer-reviewed articles: Friedrichs et al. (2016), which concentrated on object burial by scour, and Rennie et al. (2017), which focused additionally on the initiation of object motion. The findings of Friedrichs et al. (2016) and Rennie et al. (2017) that are most relevant to the objectives of MR-2224.
A compilation of 761 observations of scour-induced burial and 406 observations of initiation of motion of UXO-like objects are presented. The main factors that increase scour-induced burialto- diameter ratios (B/D) under steady currents without waves are found to be an increased Shields parameter (θ) and small D (< 3 cm). For larger D, greater cylinder density also increases B/D under steady currents. The main factor that increases scour-induced B/D under wavedominated conditions is an increased Keulegan-Carpenter number (KC). B/D additionally increases as the mean current component parallel to wave orbitals decreases. For cylinders under waves, B/D also increases as θ increases and as the angle between wave orbitals and a cylinder’s axis increases. All else being equal, cylinders bury most, then spheres, and conical frustums bury least. Parameterized models dependent on the above variables explain 85% of observed variance in B/D. The force balance for onset of motion is parameterized by the mobility criteria, fI ΘUcrit where ΘUcrit is the critical object mobility parameter, and fI accounts for the possible effect of a time-varying horizontal pressure gradient. The threshold mobility parameter for an object on a 2 rigid seabed is observed to systematically decrease as D/k increases, where k is the bed roughness. Theory combined with observations suggested a power law relationship of the form fI ΘUcrit = a1(D/k)b1. Using all complied data with D > 1 cm, the best-fit power-law coefficients at 95% confidence is determined here to be a1 = 1.75 ± 0.16 and b1 = - 0.72 ± 0.03, which explains 89% of the observed variance in fI ΘUcrit.
MR-2224 has led to a highly successful collaboration between the Virginia Institute of Marine Science (VIMS) and the Johns Hopkins University Applied Physics Laboratory (JHU/APL) (via SERDP MR-2227, led by S. Rennie and A. Brandt) in support of the Underwater Munitions Expert System (UnMES). Via this project and its collaboration with MR-2227, simple parameterized process models for initiation of motion and scour-induced burial of seabed UXO by waves and currents were incorporated into the initial version of UnMES by Rennie and Brandt. Before this effort, existing data on the initiation of motion and scour-induced burial of UXO-like objects had not been as thoroughly compiled and synthesized. The lack of simple, robust parameterizations based on a sufficiently wide range of lab and field data may have limited the ability of DoD to efficiently determine the potential for underwater UXO burial and/or migration. Through the results of this project, a better understanding and predictive ability regarding the initiation of motion and scour-induced burial of UXO has been made possible, potentially enhancing the ability of DoD to productively detect, characterize and remediate UXO-related safety hazards. The scientific and engineering communities have also benefited from a better and more unified understanding of fundamental controls on the interaction of sediment with UXO-like objects, based on a synthesis encompassing a wide range of object sizes, shapes and densities.