The Role of Dynamic Pressure Gradients on the Motion of Nearshore Sediments

Spatial gradients in the dynamic pressure associated with free surface gravity waves can impact the motion, transport, and scour of sediment or other objects in nearshore environments. As the characteristic length scale of the grain, object, or bedform increases, the vector force applied to these free seated objects must also include contributions provide by neighboring vortices. To date, resolving the role of the vertical and horizontal pressure gradients on the mobility of sediment or other objects has been limited by our ability to resolve the pressure field. This effort will detail a series of investigations that resolve the pressure gradients within a mobile sediment bed and on the surface pressure gradients around a short cylinder.

Observations of vertical pore water, water column, and surface pressure are obtained on a moderately sloping New Hampshire Beach. The beach has a 2.5 m tidal range with offshore wave conditions varying from 0.5 to 2 m high and with periods of 5-10 seconds. A self-recording pressure array composed of 0.3 mm diaphragm pressure sensors spanning from 0.5 m above the bed to 0.5 m below the bed were sampled at 16 Hz and able to measure pressure variability due to the tides, free surface gravity waves, and turbulence. The observations show a decay of the higher frequency energy with increasing distance beneath the bed.

The surface pressure surrounding a short cylinder designed to simulate a munition was accomplished using an array of 16 small diaphragm pressure sensors. Orientation changes of the untethered cylinder were recorded using an Inertial Measurement Unit. The instrument was able to resolve both the overlying progressive wave signal as well as the deviations due to vortex shedding around the cylinder. The instrument showed a negative pressure region in the lee of the vortex twice per wave period. When partially buried the cylinder showed the expected signal attenuation and a decrease of the pressure signatures associated with vortex shedding. These results suggest that vortex shedding may significantly contribute to the burial of free standing objects. Combined these observations demonstrate that spatial gradients in the local pressure field influence the motion and subsequent transport or burial of nearshore sediments and other free objects such as munitions.