Horizontal and vertical pressure gradients may be important physical mechanisms contributing to onshore sediment transport beneath steep, near‐breaking waves in the surf zone. A barred beach was constructed in a large‐scale laboratory wave flume with a fixed profile containing a mobile sediment layer on the crest of the sandbar. Horizontal and vertical pore pressure gradients were obtained by finite differences of measurements from an array of pressure transducers buried within the upper several centimeters of the bed. Colocated observations of erosion depth were made during asymmetric wave trials with wave heights between 0.10 and 0.98 m, consistently resulting in onshore sheet flow sediment transport. The pore pressure gradient vector within the bed exhibited temporal rotations during each wave cycle, directed predominantly upward under the trough and then rapidly rotating onshore and downward as the wavefront passed. The magnitude of the pore pressure gradient during each phase of rotation was correlated with local wave steepness and relative depth. Momentary bed failures as deep as 20 grain diameters were coincident with sharp increases in the onshore‐directed pore pressure gradients, but occurred at horizontal pressure gradients less than theoretical critical values for initiation of the motion for compact beds. An expression combining the effects of both horizontal and vertical pore pressure gradients with bed shear stress and soil stability is used to determine that failure of the bed is initiated at nonnegligible values of both forces.

A large‐scale laboratory experiment was conducted to evaluate cross‐shore sediment transport and bed response on a sandbar under erosive and accretive field‐scale wave conditions (total of 11 cases). Unprecedented vertical resolution of sediment concentration was achieved through the use of conductivity concentration profilers alongside miniature fiber optic backscatter profilers. Observations were made of intrawave (phase‐averaged) and wave‐averaged cross‐shore sediment flux profiles and transport rates in the lower half of the water column on the crest of a sandbar. The net sediment transport rate was partitioned into suspended sediment (SS) and bed load (BL) components to quantify the relative contributions of SS and BL to the total sediment transport rate. Net SS transport rates were greater than net BL transport rates for the positive (wave crest) half‐cycle in 6 of 11 cases, compared to 100% (11 of 11) for the negative (wave trough) half‐cycle. Net (wave‐averaged) BL transport rates were greater, in magnitude, than net SS transport rates for 7 of the 11 cases. The dominant mode of transport was determined from the ratio of net BL to net SS transport rate magnitudes. The net transport rate was negative (offshore‐directed) when SS dominated and positive (onshore‐directed) when BL dominated. Net BL transport rate correlated well with third moments of free‐stream velocity (r² = 0.72), suggesting that energetics‐type quasi‐steady formulae may be suitable for predicting BL transport under the range of test conditions.

Intense sediment transport experiments were performed in a gravity‐driven open‐channel flow with two sizes of uniformly distributed nonspherical acrylic particles having median diameters of 1.0 and 3.0 mm and a maximum packing volumetric concentration (ϕ) of 0.55. The flow conditions were adapted to each particle size to ensure similar sediment transport flow regimes as sheet flow corresponding to Shields numbers slightly above unity and a suspension number, ratio of settling velocity to friction velocity, near unity for the two experiments. An acoustic scattering‐based system (Acoustic Concentration and Velocity Profiler) and conductivity probes (Conductivity Concentration Profiler [CCP]) with two different vertical resolutions, 1 mm (CCP_1mm) and 2 mm (CCP_2mm) were used to measure instantaneous concentration profiles across the bedload and suspension layers. Measured concentration profiles, bed interface position, and sheet flow layer thicknesses are compared between the two techniques. The capabilities and limitations of both technologies are outlined. Average volumetric sediment concentration profiles were overestimated by 10% with the Acoustic Concentration and Velocity Profiler in the dense sheet layer when ⟨ϕ(z)⟩≳ 0.35, and by 100% with the CCP in the more diluted region when ⟨ϕ(z)⟩≲ 0.015 and ⟨ϕ(z)⟩≲ 0.20 for CCP_1mm and CCP_2mm, respectively. Good agreement is found between the three systems in terms of average and time‐resolved bed level position and sheet layer thickness, validating the different bed interface detection methods on data from the two sensors.