Direction Of Sediment Flux Research Articles

Suspended sediment fluxes in a shallow macrotidal estuary

Residual suspended sediment flux in estuaries is dependent on water level, velocity, and suspended sediment concentration (SSC), but complex interactions between these variables and other forcing mechanisms can lead to drastic differences in the magnitude and direction of sediment flux. The goal of this study was to quantify residual suspended sediment flux in a shallow, macrotidal estuary, and to determine its most important forcing mechanisms, using the Dyer flux decomposition equation and a simplified analytical model. Water level, velocity, and acoustic backscatter were measured in the Aulne River estuary in Brittany, France, and acoustic backscatter converted to SSC. The vertical tide was slightly flood dominant near the mouth, but strongly flood dominant upstream. Velocity was ebb dominant throughout the estuary. The magnitude and direction of total residual suspended sediment flux changed with position in the estuary and seasonally. The Eulerian flux was dominant at the mouth, but the tidal pumping and Stokes drift components increased in importance landward. Residual suspended sediment flux in the Aulne is dependent on several processes in addition to those included in the simplified model. The strong spring-neap control and tidal resuspension of sediments in the Aulne and the presence of higher-order tidal velocity terms contribute in a non-negligible way to residual suspended sediment flux. Finally, all of the first five components of the Dyer flux decomposition equation are needed to accurately represent residual suspended sediment flux in the Aulne.

Waves, currents, sediment flux and morphological response in a barred nearshore system

A shore-normal array of seven, bi-directional electromagnetic flowmeters and nine surface piercing, continuous resistance wave staffs were deployed across a multiple barred nearshore at Wendake Beach, Georgian Bay, Canada, and monitored for a complete storm cycle. Time-integrated estimates of total (ITVF) and net (INVF) sediment volume flux together with bed elevation changes were determined using depth-of-activity rods. The three bars, ranging in height from 0.10 to 0.40 m accreted during the storm (0.03 m), and the troughs were scoured (0.05 m). Sediment reactivation depths reached 0.14 m and 12% of the nearshore control volume was mobilized. However, the INVF value for the storm was less than 1% of the control volume revealing a near balance in sediment volume in the bar system. Landward migration of the inner, crescentic and second, sinuous bars occurred in association with an alongshore migration of the bar form itself; the outermost, straight, shore-parallel bar remained fixed in location. The surf zone was highly dissipative throughout the storm ( ϵ = 3.8 × 10 2–192 × 10 2) and the wave spectrum was dominated by energy at the incident frequency. Spectral peaks at frequencies of the first harmonic and at one quarter that of the incident wave were associated with secondary wave generation just prior to breaking and a standing edge wave, respectively. The former spectral peak was within the 95% confidence band for the spectrum while the latter contributed not more than 10% to the total energy in the surface elevation spectrum even near the shoreline. During the storm wave height exceeded 2 m ( H s) and periods reached 5 s ( T p k): orbital velocities exceeded 0.5 m s −1 ( u rm s) and were above the threshold of motion for the medium-to-fine sands throughout the storm. Shore-parallel flows in excess of 0.4 m s −1 were recorded with maxima in the troughs and minima just landward of the bar crest. The rate and direction of sediment flux is best explained by the interaction of antecedent bed slopes with spatial gradients in the mean and asymmetry of the shore-normal velocity field. These hydrodynamic parameters represent “steady” flows superimposed on the dominantly oscillatory motion and assumed a characteristic spatial pattern from the storm peak through the decay period. Increases spatially in the magnitudes of both the mean flows and flow asymmetries cause an increasing net transport potential (erosion); decreases in these values spatially cause a decreasing net transport potential and thus deposition. These transport potentials are increased or decreased through the gravity potential induced by the local bed slope. Shore-parallel flow was important in explaining sediment flux and morphological change where orbital velocities, mean flows and flow asymmetries were at a minimum.

Shallow Carbonate-Bank-Margin Growth and Structure, Little Bahama Bank, Bahamas

Extensive high-resolution seismic profiling and bottom sampling of the shallow northern margin of Little Bahama Bank have revealed a complexity and diversity in structure, facies, and growth history that cannot be encompassed within any single model. Cores, aerial photographs, LANDSAT imagery, and SCUBA diving observations provided supplemental data. Five bank-margin types have been identified. Counterclockwise from Walker's Cay on the northeast to Grand Bahama Island on the south they are: a windward, reef/oolite shoal complex; a terraced, rock shoulder; and three leeward margins that exhibit discontinuous reefs in various stages of development. On leeward margins unprotected from the rear the reefs now are buried; on ridge- and island-protected margins the reefs, although small, are still growing. The dominant processes responsible for the variability of these bank-margin types are: (1) physical processes, i.e., the direction, magnitude, and duration of tidal currents, waves, and storms; (2) the nature and history of sea-level fluctuations; (3) the antecedent (usually bed rock) topography; (4) biogenic barriers (both reefs and sea-grass zones); and (5) sediment production, type, and texture. Physical processes are the most important, as they in turn control biologic and chemical responses and set up feedback relations with both the antecedent and modern topography. The original pre-Triassic continental(?) basement of the Bahamas has subsided approximately 10 km. All of the margin-forming processes act within a larger tectonic framework which imparts an important long-term influ nce on growth and development of the entire bank. However, the present shallow margin of the Bahamian-type bank seems to be determined mainly by environmental processes acting within the most recent cycles of sea-level change. Relations mapped on this bank-margin area contradict some commonly held generalizations. Oolite shoals and reefs are present together and not at the expense of one another. Talus debris can be thin or absent seaward of active reefs and instead can develop as thick wedges on the bank side in the lee of deep, drowned reefs. Distance as well as topography can act as an energy barrier. Also, the site and history of reef development can depend on the direction of sediment flux. Reefs not only require wave energy from offshore, but protection against burial from behind by offbank sediment transport. Complexities such as these, coupled with significant variations in structure and sediment facies along a relatively short segment of the total Bahamian bank margin, show that more factors need to be considered in any attempt at bank-margin modeling. No one generalization as yet should be held up against particular ancient paleogeographic and paleoecologic settings.