AbstractMomentary liquefaction (ML), an instantaneous loss of effective stress between individual sand grains, is caused by the upward vertical pressure gradient of the excess pore‐water pressure in the sediment during the passage of a wave. Few field experiments have provided detailed observations of vertical pressure gradients within a sediment bed. Here, a novel autonomous pressure‐profiling instrument, the Pressure Stick, was deployed in the surf zone of an ocean beach in Rye, New Hampshire. The Pressure Stick has eight time‐synced absolute pressure sensors distributed over a 70 cm distance to measure absolute pressure in the water column and in the sediment bed. The absence of direct measurements of the sediment‐water interface location and sediment porosity limit direct observations of ML but allow for an examination of the hydrostatic pressure deviations. Observed upward vertical pressure head gradients within the sediment exceed the assumed effective weight of the sediment. Exceedances of two ML thresholds are observed within 16 cm of the sediment‐water interface and consistently occur at the onset of steeper and larger wave crests. Individual waves are characterized with an approximation of the Sleath parameter and the depth normalized wave height. The ML thresholds are exceeded 1%–40% of the time when the Sleath parameter is greater than 0.1 and 1%–33% of the time when the depth normalized wave height is greater than 0.4. These results suggest that wave steepness and nonlinearity may be necessary to induce large vertical pressure gradients that substantially deviate from hydrostatic pressure.
