Vertical structure of pore pressure under surface gravity waves on a steep, megatidal, mixed sand‐gravel‐cobble beach. Issue 1 (11th January 2017)

• Record Type: Journal Article
• Authors: Guest, Tristan B.; Hay, Alex E.
• Is Part Of: Journal of geophysical research. Volume 122:Issue 1(2017)
• Page Start: 153
• Page End: 170
• Publication Date: 2017-01-11
• Subjects: pore pressure -- mixed‐sand‐gravel beach -- poro‐elastic model -- entrapped air -- attenuation -- phase lag; Oceanography -- Periodicals; 551.4605
• DOI: 10.1002/2016JC012257
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: The vertical structure of surface gravity wave‐induced pore pressure is investigated within the intertidal zone of a natural, steeply sloping, megatidal, mixed sand‐gravel‐cobble beach. Results from a coherent vertical array of buried pore pressure sensors are presented in terms of signal phase lag and attenuation as functions of oscillatory forcing frequency and burial depth. Comparison of the observations with the predictions of a theoretical poro‐elastic bed response model indicates that the large observed phase lags and attenuation are attributable to interstitial trapped air. In addition to the dependence on entrapped air volume, the pore pressure phase and attenuation are shown to be sensitive to the hydraulic conductivity of the sediment, to the changing mean water depth during the tidal cycle, and to the redistribution/rearrangement of beach face material by energetic wave action during storm events. The latter result indicates that the effects on pore pressure of sediment column disturbance during instrument burial can persist for days to weeks, depending upon wave forcing conditions. Taken together, these results raise serious questions as to the practicality of using pore pressure measurements to estimate the kinematic properties of surface gravity waves on steep, mixed sand‐gravel beaches. Key Points: First‐time measurements of wave‐induced pore pressure on a steep, mixed sand and gravel beach compare well with model predictions Pore pressure phase lags are proportional to mean water depth, and persist through the spring‐neap cycle Phase lag is due to entrapped air, and is sensitive to wave‐forced changes in hydraulic conductivity