A field experiment was conducted to examine stratified and unstratified curvature-generated lateral circulation and momentum balances in an estuarine tidal channel. Conductivity, temperature, depth, and current profiler data were collected vertically and laterally across the channel at a sharp bend over a fortnightly period to measure the terms of the lateral momentum budget. Well-mixed conditions allow the development of classic two-layer helical flow around a bend. Stratification strengthens curvature-induced lateral circulation, but the development of a lateral baroclinic pressure gradient opposes the resultant motions. The spatial and temporal response of this baroclinic pressure gradient is different than centrifugal acceleration, producing a three-layer profile. As the baroclinic term becomes stronger (or as centrifugal acceleration disappears as the flow exits the bend), two-layer flow with the opposite direction from curvature occurs. In both stratified and well-mixed conditions, downstream adjustment of lateral circulation (nonlinear advective acceleration) is of leading order in the lateral momentum budget; the depth-averaged term adjusts the streamline direction, while vertical deviations from the depth average account for changes in lateral circulation. The asymmetry of forcing mechanisms on flood and ebb, because of variations in stratification and strength of tidal flow, can strongly affect net lateral transport and generation of residual currents in regions of curvature.

[1] Tidal asymmetry in estuaries with mixed, mainly semidiurnal tides arises from both the interaction of principal tides and the higher harmonics generated by distortions within the estuary. The duration asymmetry in rise and fall of water level caused by principal tides on the west coast of the continental United States is ebb-dominant, and so the tide entering estuaries is also ebb-dominant, prior to any internal distortions within the estuary. The interaction of higher harmonics with principal constituents either augments or cancels the duration asymmetry in the principal tides. In estuaries where tidal elevation and velocity phase are near quadrature (90°out of phase), the duration asymmetry in tidal elevation leads to asymmetries in tidal current magnitude. Asymmetry can be conveniently quantified in terms of the sample skewness, g 1 , the normalized third sample moment about the mean. An analytic approximation to the skewness shows that traditional metrics of asymmetry, namely the ratio of constituent amplitudes and the relative constituent phase difference, arise from calculating the third sample moment. Observations from three California estuaries of different morphologies are presented as an illustration of how skewness can be used to quantify asymmetry in real systems. As in semidiurnal systems, morphology is a good predictor of whether higher harmonics engender ebb-dominance or flood-dominance, however asymmetry imposed by principal tides at the mouth must first be overcome and so there is a spatial evolution in the total asymmetry. Quantifying observations via skewness should be considered in addition to traditional metrics in estuaries with mixed tides.

[1] Anthropogenic and climatic forces have modified the geomorphology of tidal wetlands over a range of timescales. Changes in land use, sediment supply, river flow, storminess, and sea level alter the layout of tidal channels, intertidal flats, and marsh plains; these elements define wetland complexes. Diagnostically, measurements of net sediment fluxes through tidal channels are high-temporal resolution, spatially integrated quantities that indicate (1) whether a complex is stable over seasonal timescales and (2) what mechanisms are leading to that state. We estimated sediment fluxes through tidal channels draining wetland complexes on the Blackwater and Transquaking Rivers, Maryland, USA. While the Blackwater complex has experienced decades of degradation and been largely converted to open water, the Transquaking complex has persisted as an expansive, vegetated marsh. The measured net export at the Blackwater complex (1.0 kg/s or 0.56 kg/m 2 /yr over the landward marsh area) was caused by northwesterly winds, which exported water and sediment on the subtidal timescale; tidally forced net fluxes were weak and precluded landward transport of suspended sediment from potential seaward sources. Though wind forcing also exported sediment at the Transquaking complex, strong tidal forcing and proximity to a turbidity maximum led to an import of sediment (0.031 kg/s or 0.70 kg/m 2 /yr). This resulted in a spatially averaged accretion of 3.9 mm/yr, equaling the regional relative sea level rise. Our results suggest that in areas where seaward sediment supply is dominant, seaward wetlands may be more capable of withstanding sea level rise over the short term than landward wetlands. We propose a conceptual model to determine a complex's tendency toward stability or instability based on sediment source, wetland channel location, and transport mechanisms. Wetlands with a reliable portfolio of sources and transport mechanisms appear better suited to offset natural and anthropogenic loss.

[1] It is often assumed that, in short, shallow estuaries, at leading order velocity and elevation are exactly out of phase, so that duration asymmetries in the rise and fall of the tide should be manifest as skewed velocities. We observed alternating ebb-and flood-dominant velocity skew in response to the spring-neap modulation of incident asymmetry generated by the mixed, mainly semidiurnal astronomical tides within the macrotidal Skagit River delta in Puget Sound, Washington. We describe three factors that may contribute to local asymmetries: (1) ebb dominance caused by phase lags between the surface gradient and local depth, (2) ebb dominance due to fluvial discharge, and (3) near-bed flood dominance due to baroclinicity. Large spring tides led to greater frictionally generated phase lags and resulted in ebb-dominant velocity skew. This ebb dominance caused by tidal drainage was reinforced by fluvial discharge across the tidal flat at lower-low water. The baroclinic component of this discharge, however, produced flood-dominant near-bed velocity skew that countered the ebb dominance of the frictional effects. The balance of these processes depends strongly on the spring-neap cycle, magnitude of river discharge, and position within the tidal flat and channel system. Our observations are notable in the context of previous studies describing these processes because our analyses indicate that these mechanisms are relevant over very short spatial scales of just a few kilometers and in very shallow systems.Citation: Nidzieko, N. J., and D. K. Ralston (2012), Tidal asymmetry and velocity skew over tidal flats and shallow channels within a macrotidal river delta,

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Tidal flats at a river mouth feature estuarine and fluvial processes that distinguish them from tidal flats without river discharge. We combine field observations and a numerical model to investigate hydrodynamics and sediment transport on deltaic tidal flats at the mouth of the Skagit River, in Puget Sound, WA during the spring freshet. River discharge over tidal flats supplies a mean volume flux, freshwater buoyancy, and suspended sediment. Despite the shallow water depths, strong horizontal density fronts and stratification develop, resulting in a baroclinic pressure gradient and tidal variability in stratification that favor flood-directed bottom stresses. In addition to these estuarine processes, the river discharge during periods of low tide drains through a network of distributary channels on the exposed tidal flats, with strongly ebb-directed stresses. The net sediment transport depends on the balance between estuarine and fluvial processes, and is modulated on a spring-neap time scale by the tides of Puget Sound.We find that the baroclinic pressure gradient and periodic stratification enhance trapping of sediment delivered by the river on the tidal flats, particularly during neap tides, and that sediment trapping also depends on settling and scour lags, particularly for finer particles. The primary means of moving sediment off of the tidal flats are the high velocities and stresses in the distributary channels during late stages of ebbs and around low tides, with sediment export predominantly occurring during spring low tides that expose a greater portion of the flats. The 3-d finite volume numerical model was evaluated against observations and had good skill overall, particularly for velocity and salinity. The model performed poorly at simulating the shallow flows around low tides as the flats drained and river discharge was confined to distributary channels, due in part to limitations in grid resolution, seabed sediment and bathymetric data, and the wetting-and-drying scheme. Consequently, the model predicted greater sediment retention on the flats than was observed.3

[1] Measured turbulence power spectra, cospectra, and ogive curves from a shallow tidal flow were scaled using Monin-Obukhov similarity theory to test the applicability to a generic tidal flow of universal curves found from a uniform, neutrally stable atmospheric boundary layer (ABL). While curves from individual 10 min data bursts deviate significantly from similarity theory, averages over large numbers of sufficiently energetic bursts follow the general shape. However, there are several differences: (1) Variance in the measured curves was shifted toward higher frequencies, (2) at low frequencies, velocity spectra were significantly more energetic than theory while cospectra were weaker, and (3) spectral ratios of momentum flux normalized by turbulent kinetic energy (TKE) indicate decreased fluxes and/or elevated TKE levels. Several features of the turbulence structure may explain these differences. First, turbulent dissipation exceeded production, indicating nonequilibrium turbulence, possibly from advection of TKE. Indeed, using the production rate rather than dissipation markedly improves agreement in the inertial subrange. Second, spectral lag of the largest eddies due to inhomogeneous boundary conditions and decaying turbulence could explain spectral deviations from theory at low frequencies. Finally, since the largest eddies dominate momentum transfer, the consequence of the cospectra difference is that calculated ogive curves produced smaller total momentum fluxes compared to theory, partly because of countergradient fluxes. While ABL similarity scaling applied to marine bottom boundary layers (MBBLs) will produce curves with the general shape of the universal curves, care should be taken in determining details of turbulent energy and stress estimates, particularly in shallow and inhomogeneous MBBLs.

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