POSSIBLE EFFECTS OF SEDIMENT TRANSPORT UPON THE DECAY OF AN IMPACT PRODUCED TSUNAMI

Glimsdal et al. (2007) modeled the tsunami associated with the Mjølnir impact crater. Mjølnir is a 36-kilometer diameter Jurassic/Cretaceous crater lying in the Barents Sea. The impact occurred in 400 m depth seas with a bottom of unconsolidated clays. Glimsdal et al.’s (2007) showed that at ≈150 km from the impact the original undular-bore had organized itself into a series of solitary waves, and the ratio of wave-height to water-depth is ≈0.5. Depth averaged water velocities in their models range from 8 to 25 m/s. Glimsdal et al. (2007) hypothesized that sediment transport causes significant-wave-amplitude-decay as the wave moves away from the impact.

This study investigates this hypothesis by combining the equations for a solitary wave and the transport equations for ‘wash load’. Munk (1949) presents equations describing how velocity varies with water depth, the shape of the wave, and total mechanical energy per unit width. Rijn (2007) gives two equations: one that gives the power per unit area to keep the wash load suspended and the other the power dissipated per unit area to transport the sediments. At equilibrium, these two equations are equal and sediment concentration can be calculated. Using the sediment concentration, the power dissipated per unit area to transport the sediments is integrated over a distance accounting for >99% of the volume of the wave and with respect to time. This value for the energy density is subtracted from Munk’s total mechanical energy density and a wave-height is calculated. Calculations show that sediment transport results in a ≈10% decay of wave amplitude in 1000 km, whereas decay due to geometric spreading produces between 75 (Glimsdal et al., 2007) and 90% (Wunnen, 2010) decay in 1000 km.