Nearly 24% of formerly used Defense sites containing military munitions are in the nearshore coastal zone with approximately 30% of those sites containing heterogeneous sediment compositions of gravel/cobble and sand. Previous exploration in munition mobility was performed on beaches of sandy or muddy bottoms leaving the influence of heterogeneous sediment properties on coastal processes and munition mobility unstudied. The primary objective of this study was to classify the vulnerability of munition migration, exposure, and burial on mixed grain beaches within the swash zone.

Double-dam break flume experiments were performed on four permeable beach compositions consisting of pure sand, pure cobble, 30% cobble- and 50% cobble- to sand ratios. Beach slope was initially set to a 1:7 slope. For each sediment composition, three munition types, varying in size and density, were explored at two different energetic beach locations. Munition mobility was measured manually and tracked during the swash event, along with the beach hydrodynamics such as runup extent, flow velocities, water depth, internal sediment pore pressure, and beach profile morphological development.

Net munition migration distances were the least on the pure cobble beach and the greatest on the pure sand each composition, with the mixed grain beaches modifying munition mobility, likely due to bed friction. Incipient motion and migration for the least dense munition was the soonest and largest for all beach compositions, respectively; and for the two-densest munition types motion occurred during backwash only. Minimal correlation between the present formulation of the object mobility number and total distance traveled by the munition were observed.

The project team hypothesized that the object mobility number should be modified for slope, friction, and infiltration characteristics of the beach profile to better mimic observations of munition mobility on mixed grain beaches. Lastly, laboratory results guided non-hydrostatic wave model numerical experiments to determine the influence of beach state (intermediate, steep) on swash bore acceleration and corresponding impact to munition mobility and are used to predict munition mobility on 30% cobble and 50% cobble beach compositions.