Observations of the nearbed velocity field over a rippled sediment bed under asymmetric wave forcing conditions were collected using a submersible particle image velocimetry (PIV) system. To examine the role of bed form-induced dynamics in the total momentum transfer, a double-averaging technique was implemented on the two-dimensional time-dependent velocity field by means of the full momentum equation. This approach allows for direct determination of the bed form-induced stresses, i.e., stresses that arise due to the presence of bed forms, which are zero in flat bed conditions. This analysis suggests that bed form-induced stresses are closely related to the presence of coherent motions and may be partitioned from the turbulent stresses. Inferences of stress provided by a bed load transport model suggest that total momentum transfer obtained from the double-averaging technique is capable of reproducing bed form mobilization. Comparisons between the total momentum transfer and stress estimates obtained from local velocity profiles show significant variability across the ripple and suggest that an array of sensors is necessary to reproduce bed form evolution. The imbalance of momentum obtained by resolving the different terms constituting the near-bed momentum balance (i.e., acceleration deficit, stress gradient, and bed form-induced skin friction) provides an estimate of the bed form-induced pressure that is consistent with flow separation. This analysis reveals three regions in the flow: the free-stream, where all terms are relatively balanced; the near-bed, where momentum imbalance is significant during flow weakening; and below ripple crests, where bed form-induced pressure is the leading order mechanism.

[1] Observations of the spatially dependent velocity field over movable bed forms subjected to slightly skewed and asymmetric regular wave forcing were collected. The dynamics between the ripple elements is dominated by coherent vortices, characterized by the swirling strength, and evidenced in the temporal and spectral characterization. Within the boundary layer, spectral energy in the second harmonic (3f 0 ) is amplified at the ripple slopes and is consistent with the location of the expected strongest pressure gradients. Firstmoment and second-moment velocity statistics were used to address the spatial variability of the intra-ripple hydrodynamics. Estimates of displacement and momentum thicknesses ( Ã and mom ) are smaller than suggested by the higher harmonics, but consistently highlight areas of adverse and favorable pressure gradients. Shear stress and roughness estimates were inferred by fitting a logarithmic model to first-moment and second-moment statistics of the velocity field. The maximum Shields parameter was observed to peak at the stoss slope of asymmetric ripples during the strongest and shorter half-wave period (onshore). First-moment roughness estimates are similar in magnitude to bed load parameterizations provided by Li et al. (1997), and about a factor of 3 larger than second-moment estimates. Assessment of the vertical transfer of horizontal momentum derived using a Reynolds decomposition suggests that stresses inferred from the logarithmic law using first-moment velocity statistics appropriately reproduce the mean momentum transfer for the longer and weaker offshore half-wave period.Citation: Rodr ıguez-Abudo, S., D. L. Foster, and M. Henriquez (2013), Spatial variability of the wave bottom boundary layer over movable rippled beds,

New friction factor estimates are computed from the total momentum transfer applied to a rippled sediment bed. The total time‐dependent momentum flux is achieved by implementing the double‐averaged horizontal momentum equation on the nearbed flow field collected with PIV. Time‐independent friction factors are obtained by regressing the total momentum flux to the common quadratic stress law given by 12ρu∞|u∞|. The resulting friction factors compare favorably with available analysis techniques including energy dissipation, vertical turbulence intensity, and maximum shear stress, but can be 2‐6 times smaller than estimates determined with the model by Madsen (1994) and the formula of Swart (1974) using the ripple roughness.

[1] A three-dimensional mixture theory model for flow and sediment transport in the seafloor boundary layer, SedMix3D, simulated the flow over and the resulting sediment entrainment and evolution of rippled beds. SedMix3D treats the fluid-sediment mixture as a continuum of varying density and viscosity with the concentration of sediment and velocity of the mixture simulated by the Navier-Stokes equations coupled with a sediment flux equation for the mixture. Model validation was performed by comparing simulated time-dependent flow quantities and bulk flow statistics with measurements obtained in the laboratory under scaled forcing conditions. Two-dimensional planes extracted from a three-dimensional simulation were compared to observations made using planar Particle Image Velocimetry (PIV) in a laboratory flume. The simulated results of time-averaged velocities and time-dependent quantities of vorticity and swirling strength were in good agreement with the observations. The model was used to analyze the three-dimensionality of vortex formation and ejection produced by oscillatory flow over vortex ripples, a process that cannot be observed in the laboratory with planar PIV measurements. The three-dimensional simulated results showed that the swirling strength varied significantly in the cross-flow direction, indicating that the vortices formed and dissipated non-uniformly due to random fluctuations. Subsequently, an order of magnitude difference in offshore sediment flux was observed using two different methods to calculate sediment fluxes (spatially averaging and at a point). The results suggest that while a two-dimensional plane may be sufficient to examine the general hydrodynamics over ripples, three-dimensional analysis is necessary for a complete understanding of sediment transport.

Time series from open ocean fixed stations have robustly documented secular changes in carbonate chemistry and long‐term ocean acidification (OA) trends as a direct response to increases in atmospheric carbon dioxide (CO2). However, few high‐frequency coastal carbon time series are available in reef systems, where most affected tropical marine organisms reside. Seasonal variations in carbonate chemistry at Cheeca Rocks (CR), Florida, and La Parguera (LP), Puerto Rico, are presented based on 8 and 10 years of continuous, high‐quality measurements, respectively. We synthesized and modeled carbonate chemistry to understand how physical and biological processes affect seasonal carbonate chemistry at both locations. The results showed that differences in biology and thermodynamic cycles between the two systems caused higher amplitudes at CR despite the shorter residence times relative to LP. Analyses based on oxygen and temperature‐normalized pCO2sw showed that temperature effects on pCO2sw at CR were largely counteracted by primary productivity, while thermodynamics alone explained a majority of the pCO2sw dynamics at LP. Heterotrophy dominated from late spring to fall, and autotrophy dominated from winter to early spring. Observations suggested that organic respiration decreased the carbonate mineral saturation state (Ω) during late summer/fall. The interactive effects between the inorganic and organic carbon cycles and the assumed effects of benthic metabolism on the water chemistry at both sites appeared to cause seasonal hysteresis with the carbonate chemistry. Improved integration of observational data to modeling approaches will help better forecast how physical and biogeochemical processes will affect Ω and carbonate chemistry in coastal areas.