The underwater environment presents significant challenges for the detection, characterization, and recovery of military munitions. Relatively little is known about the location and condition of the munitions, the manner in which the munitions move over time, the distribution of scrap and other sources of clutter, and specific site conditions such as bottom composition and water turbidity. To address the location of unexploded ordnance (UXO) in reef environments, ESTCP investigators have expanded the capabilities of the UXO Mobility Model to capture reef geomorphology. This feature will enable prediction of UXO migration and burial in reef environments, informing risk assessments and munitions response actions at underwater munitions sites.
Dr. Gerald D'Spain and his team from the Marine Physical Laboratory at Scripps Institution of Oceanography leveraged the UXO Mobility Model, which previously was developed with ESTCP support by Scripps in cooperation with the Naval Facilities Engineering Command – Engineering Service Center. The Mobility Model is a process-based model that uses vortex lattice computational methods to generate 3-dimensional simulations of subsequent burial, exposure, and migration specific to UXO shapes. In these simulations, the model accounts for effects of large-scale erosion of the seabed, fine-scale vortex shedding, scour, and bedform evolution around the UXO shape. Under the ESTCP project Vortex Lattice UXO Mobility Model for Reef-Type Range Environments (MR-201003), Dr. D’Spain and his team refined the Mobility Model to increase its computational efficiency in complex reef environments by building on the concept of interconnected geomorphic control cells consisting of a reef platform bound by sand channels, also known as “awa” channels.
As with any model, the quality of the output is only as good as the quality of the input data. The four main data inputs are bathymetric or general underwater conditions, climatologic or wave/current history, sediment properties and material composition, and UXO initial population and condition. Most of this input information can be mined from National Oceanic and Atmospheric Administration (NOAA) databases, with the exception of the initial UXO population distribution data. These data are usually the least detailed and least reliable, except in cases where a detailed UXO survey has been performed.
For the advanced model demonstration, the project team used field data acquired from the original demonstration of the UXO Mobility Model at the Pacific Missile Range Facility (PMRF) in Kauai, Hawaii. Twenty-four UXO surrogates were emplaced and tracked over the course of approximately 5 months to supply sufficient data to validate the model. The PMRF site is situated in a narrow, meandering sand channel, which bisects a limestone and coral reef bottom. The sediment properties are a composite of coarse-grained carbonate and fine-grained volcanic sediments. To validate the model, the project team collected data on the movement of the UXO surrogates and documented the environmental conditions that caused those movements.
The Mobility Model accurately predicted the movement and distribution of UXO at the PMRF site. The level of validation achieved has resulted in widespread use of the expanded Mobility Model at other reef sites such as the Vieques Island South Impact Area. The predicted and demonstrated natural sorting mechanism of the UXO into the awa channels over time occurs in response to the long-term wave climate interacting with the unique features of the awa channel bathymetry. Based on the model predictions for this type of environment, conventional sand dredging methods can be applied to efficiently remove the UXO concentrated in the awa channels.
The analysis provided by the expanded Mobility Model can be used by site managers as part of a comprehensive underwater munitions response at coastal sites with reef environments. The results of this ESTCP demonstration show that such analyses can be used to reduce the areas of interest requiring remediation, predict the percentage of time that UXO are exposed to human contact or other hazardous processes, and determine the rate and direction of net movement as a function of weather and other local conditions, all of which lead to more efficient site management and remediation efforts.