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(left) Cross-shore profile at the Advocate Beach study site on yearday 271, 2015. Pressure sensor positions are indicated by blue dots, with the vertical pressure sensor array identified near midbeach. Highest high and lowest low water during spring and neap tides during the experiment are indicated with dashed lines. (right) A schematic of the vertical array.

(left) Cross-shore profile at the Advocate Beach study site on yearday 271, 2015. Pressure sensor positions are indicated by blue dots, with the vertical pressure sensor array identified near midbeach. Highest high and lowest low water during spring and neap tides during the experiment are indicated with dashed lines. (right) A schematic of the vertical array.

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Article
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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 forcin...

Contexts in source publication

Context 1
... was measured using a vertical array of four MS5803-14BA high resolution (0.02 kPa) pressure sensors, at 0, 15, 30, and 50 cm sediment depth and near the midtide level on the beach face ( Figure 4). Each sensor was secured in an acrylic housing, the openings of which were covered in aluminum mesh to prevent the mechanical force of sand grains from impinging on the sensing surface. ...
Context 2
... AND HAY PORE PRESSURE ON A STEEP BEACHBeach surveys were carried out at each daylight low tide using a cart-mounted Hemisphere Model S320 RTK (real time kinematic) GPS. A typical beach profile is shown in Figure 4. ...

Citations

... Large upward vertical excess pore pressure gradients within the beach can result in the mobilization of sediments, commonly referred to as momentary liquefaction (Terzaghi 1943;Sakai et al. 1992;Mory et al. 2007;Yeh and Mason 2014;Guest and Hay 2017). Here, the risk of liquefaction of the sandy sediment is quantified in three ways: (1) exceedance of the sediment buoyant weight; (2) a critical hydraulic gradient exceeding a seepage factor of safety (Terzaghi 1943;Duncan et al. 2011); and (3) a theoretical liquefaction criterion (Mory et al. 2007) at which the soil particle-to-particle contact structure changes into suspension and sediment becomes available for transport with any applied flow (Sumer 2014). ...
... Although methods for calculating momentary liquefaction (both the in situ pressure gradients and the theoretical threshold criterion values) exist, determining the hydrodynamic or geotechnical conditions that facilitate these threshold exceeding values has proven difficult (Terzaghi 1943;Sakai et al. 1992 Guest and Hay 2017). The complexity of determining the hydrodynamic (e.g., wave height and period) and geotechnical (e.g., sediment bulk density and porosity) parameters arises from the difficulty in determining which parameters are relevant to momentary liquefaction and in measuring these temporally and spatially rapidly changing (i.e., significant changes on the order of seconds and centimeters for pore pressure and fluid velocities) parameters in situ without causing significant disturbance to the sediment (Yamamoto et al. 1978;Kirchner et al. 1990;Sakai et al. 1992;Mory et al. 2007;Guest and Hay 2017). ...
... Although methods for calculating momentary liquefaction (both the in situ pressure gradients and the theoretical threshold criterion values) exist, determining the hydrodynamic or geotechnical conditions that facilitate these threshold exceeding values has proven difficult (Terzaghi 1943;Sakai et al. 1992 Guest and Hay 2017). The complexity of determining the hydrodynamic (e.g., wave height and period) and geotechnical (e.g., sediment bulk density and porosity) parameters arises from the difficulty in determining which parameters are relevant to momentary liquefaction and in measuring these temporally and spatially rapidly changing (i.e., significant changes on the order of seconds and centimeters for pore pressure and fluid velocities) parameters in situ without causing significant disturbance to the sediment (Yamamoto et al. 1978;Kirchner et al. 1990;Sakai et al. 1992;Mory et al. 2007;Guest and Hay 2017). ...
... However, the KC number, lee-wake vortices, and horseshoe vortices may not be the only wave-related processes involved in scour mechanics. Wave-induced excess sediment porepressure gradients are known to occur in the uppermost (∼50 cm) layer of sediment in the nearshore zone (e.g., Sakai et al. 1992;Raubenheimer et al. 1998;Sumer and Fredsøe 2002;Mory et al. 2007; Guest and Hay 2017). These excess pore-pressure gradients have been observed to cause liquefaction of the soil, which may facilitate scour under a significantly smaller bed shear stress. ...
... Yamamoto et al. (1978) showed that excess sediment pore pressures in a porous media experience attenuation as a function of wave period and soil depth (among other geotechnical parameters). This excess pressure attenuation coupled with a sediment-depthdependent phase lag of a pressure signal (Raubenheimer et al. 1998;Mory et al. 2007;Stark and Quinn 2015;Guest and Hay 2017) can cause an upward-directed vertical excess pore-pressure gradient that is sufficiently large to overcome the effective stress of the soil. When the effective stress of the soil is zero, the soil has no contact force between the particles (Terzaghi et al. 1996;Holtz et al. 2017). ...
... As no pressure sensors were buried in the locations of interest in Mexico Beach, FL, before the hurricane, data were taken from two pore-pressure sensors deployed in the intertidal zone at Cannon Beach in Yakutat, AK, United States. These pressure sensors were attached to a vertical bar similar to previous experiments (e.g., Raubenheimer et al. 1998;Mory et al. 2007;Michallet et al. 2009;Guest and Hay 2017;Florence and Stark 2019). The intertidal zone was chosen at Cannon Beach, AK, as it becomes subaqueous during high tide and subaerial during low tide with water depths ranging from 0 m (at low tide, sensors are not submerged) to 1.3 m water depth. ...
... However, it increased slightly by 0.5-1 s with sediment depth. These observations are in line with observations and considerations by other researchers such as Raubenheimer et al. (1998) and Guest and Hay (2017). The phase lag between the uppermost sensor and the buried sensors was significant with 25-36 • , likely due to the tilting upwards position and resulting misalignment of the topmost sensor (Fig. 11 right). ...
... Therefore, a phase lag can be considered minor to negligible considering potential uncertainties in positioning of the sensors regarding the wave direction. The little observed phase lag however appeared to increase with decreasing water depth, being in line with observations by Guest and Hay (2017) for a mixed sand gravel beach. However, the magnitudes of the phase lag were significantly smaller in this study than in Guest and Hay (2017), likely associated to different saturation behavior of the different sediment types. ...
... The little observed phase lag however appeared to increase with decreasing water depth, being in line with observations by Guest and Hay (2017) for a mixed sand gravel beach. However, the magnitudes of the phase lag were significantly smaller in this study than in Guest and Hay (2017), likely associated to different saturation behavior of the different sediment types. Raubenheimer et al. (1998) conducted similar measurements at a sandy beach. ...
Article
Full-text available
The role of geotechnical properties and soil behavior for beach dynamics has been recognized before, but geotechnical field measurements in energetic beach environments are still rare. This study focused on two days of field measurements along a cross-shore transect reaching from the foot of the dunes to the upper subtidal zone at the western sandy beach of the island of Sylt, Germany, just south of the city of Westerland. Sediment properties and geotechnical parameters were obtained from sediment sampling and limited in-situ testing. Pore pressure measurements were conducted along a vertical array in the upper 55 cm of the beach surface in the lower intertidal zone. Pore pressure recordings were then analyzed using a one-dimensional-vertical (1DV) model based on Biot (1956) and Mei and Foda (1981). Laboratory testing results demonstrated slight trends of increasing grain size and friction angles from the subaerial to the lower intertidal zone. In-situ sediment strength testing using a portable free fall penetrometer supported the trends in friction angles for the subaerial and intertidal zone. Additionally, a significant increase in sediment resistance was observed in the swash zone and upper subtidal zone. Pore pressure recordings showed a consistent trend associated with the tidal water elevations. However, data collected during low tide suggested a decoupling of surface water effects and groundwater, possibly associated with gas content and negative pore pressures in the vadose zone. Pore pressure recordings also suggested a more pronounced wave damping in the upper sediment layers and a minor phase lag. The 1DV pore pressure model succeeded to simulate the observed pressures at all sediment depths well, and suggested no liquefaction events during the measurement period, but a reduction of effective weight that may affect sediment dynamics.
... During periods of weak local wind forcing, low-amplitude swell waves propagating from outside the Bay of Fundy may dominate the wave energy spectra (e.g. Guest and Hay, 2017). The combination of a steep beach slope and typically short period, wind-generated incident waves result in a narrow surf zone and energetic shore break for offshore significant wave heights of ca. ...
... The pressure array observations are not treated in this paper. Results from the vertical array are presented in Guest and Hay (2017). ...
... Accretion of coarse material on cusp horns would increase the thickness of the permeable layer, decreasing the strength of the backwash and thus the erodibility of the horns. Measurements of the hydraulic conductivity of Advocate Beach sediment reported by Guest and Hay (2017) are consistent with the conceptual framework from Longuet-Higgins and Parkin (1962): the maximum value of nearly 12 × 10 −4 m s −1 was in the upper 5 cm of the sediment column, compared to values of less than 8 × 10 −4 m s −1 in the underlying 45 cm. ...
Article
Field observations are presented of the morphological evolution of beach cusps on a 1:10 slope, megatidal (8–12 m range), mixed sand-gravel beach at the head of the Bay of Fundy, Nova Scotia. Cusps had mean wavelengths of 3–6 m and displayed pronounced horn/bay sediment size segregation, with sand-sized material in the bays and gravel-sized material in the horns. Cusp occurrence was limited to the upper third of the beach face. Shoreline position during the tidal cycle was estimated at three minute intervals from time-averaged video imagery. Three cusp events are examined in detail, two exhibiting pronounced topographic relief, and the third demonstrating sensitivity of the rate of cusp evolution to the beach surface grain size distribution. Forcing conditions were weak, with significant offshore wave heights of 10–20 cm and peak periods of 4–7 s. Relict cusp morphology was inundated with the rising tide and destroyed or reworked during high tide. New cusps formed during the falling tide through a combination of accretion at the horns and erosion in the embayments. Timescales of growth and decay were short, ranging from 10 to 30 min. The location and dynamics of cusp horns appeared to depend on the high water line and its location relative to any pre-existing cusp morphology. The apparent sensitivity of cusp formation timescales to the local grain size distribution suggests that size segregation is intrinsic to the process of mixed sand-gravel cusp evolution.
... Temperature measurements suggested to provide complementary information about sediment air ventilation, sediment motion, and groundwater flow paths if temperatures between air and water differ noticeably. This may contribute to answer questions raised by Stark and Hay (2014) and Guest and Hay (2017) investigating wave attenuation and potential sediment liquefaction due to ocean wave forcing at beaches. This data set only represents a small insight into a larger data set during the experiment at Advocate Beach. ...
Article
Full-text available
The increasing urbanization of coastal regions makes beach erosion and coastline protection an important field of research (Elko et al., 2014). Excess pore pressures and pore pressure gradients in the soil matrix can impact sediment mobilization and erosion in terms of liquefaction (Sumer, 2014). Despite previous studies, there are still unsolved questions regarding coastal liquefaction due to wave action. Particularly, the role of groundwater dynamics, the impact of wave breaking, sediment reorganization, and potential air content represent unsolved problems. Furthermore, open questions still exist regarding the interaction and roles of excess pore pressure built-up, vertical pressure gradients and horizontal pressure gradients (Foster et al., 2006; Yeh and Mason, 2014; Sumer, 2014; Stark, 2017). We hypothesize that temperature variations may reveal complementary information with regard to pore water fluid behavior, such as pore space saturation, groundwater flows, exfiltration and infiltration processes, and impact of wave forcing. The study presented here shows some preliminary data sets of combined pore pressure and temperature recordings.