The primary objective of this project is to classify the vulnerability of munition migration, exposure, and burial on mixed grain (sand with gravel and cobble) beaches within the surf-swash transition zone during dam-break forcing conditions via small-scale physical modeling. The project team has three sub-objectives:

1. Test munition mobility, exposure, and burial response to dam-break wave impact on laboratory scale mixed-grain beaches composed of three variable mixtures of sand and gravel/cobble (25%, 40%, 80% gravel/cobble to sand) and compare results to purely sand tests in same dam-break scenario (in collaboration with Puleo SERDP project MR20-1094).

2. Test two munition densities and one shape (40 mm) in the dam-break scenario to discern the influence of munition density on burial in mixed grain beaches (following Calantoni SERDP project MR-2320).

3. Determine the influence of beach state (intermediate, steep) on swash bore acceleration and corresponding impact to munition mobility through non-hydrostatic wave model modeling guided by the laboratory results.

There have been SERDP funded laboratory and field experiments preformed on sandy beaches and over muddy bottoms - but none in mixed grain (sand + gravel) environments. This project will preform 1) dam-break wave bore experiments over a range of mixed grain sediments and 2) perform non-hydrostatic depth dependent simulations of intermediate and steep sloping beaches on a numerical flume based on the dam-break flume facility. The model runs will both inform physical experiment beach configurations and fill in parameter space for conditions that cannot be tested in the experimental facility. The hypothesis is that mixed grain sediment substrates will change munition exposure, migration, and burial behavior compared to sandy substrates alone. The measurements will complement those of Puleo [MR20-1094]; the project team will test the same wave conditions and same munitions, but using mixed grains. The difference between mixed grains and sandy bottoms may or may not be significant to munition mobility, exposure, and burial characteristics.

Mixed grain sediments are present on 24% of Formerly Used Defense Sites. This project will provide needed data for ongoing probabilistic modeling efforts on munition mobility (e.g. Underwater Munitions Expert System, Rennie [MR19-1126]; Probabilistic Environmental Modeling System for Munitions Mobility, Palmsten [MR-2733]) and process based modeling efforts of munition mobility (e.g. two-phase simulations for burial dynamics of munitions, Hsu [MR20-1478]; three-dimensional computational modeling of turbulence and bed morpho dynamics adjacent to munitions, Liu [MR-2732]). Munition detection applications where mixed grain sediments are present may also benefit from incorporating this dataset. If mixed grains are considered important to munition exposure, mobility, and burial, results from this project will be used to narrow the parameter space and inform subsequent prototype-scale and or field scale testing.