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19. Cross Shore Sediment Transport

Important because it is the mechanism by which the coastal morphology is modified. We distinguish cross shore from longshore sediment transport, though we acknowledge that both operate simultaneously. Basic notion: sediment is transported by a combination of waves (entrainment) and currents (move entrained seds).

Sediment Sizes in the Nearshore

Fine, cohesive sediment (clay and silt) tends to stay in suspension, when appreciable water movement is present. Fine sediments are prominent in lagoons, estuaries, marshes, etc., but not prevalent on open ocean coasts. Cobbles and boulders, although present in certain coastal environments, require exceptionally high shear stresses (from very energetic conditions) for transport. Our focus will be on sediment in the sand size range, because that is what is typically found on the beach.

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Sediment Transport Mechanisms

Irrespective of transport environment (rivers, beaches) the sediment can be classified as traveling via one of two dominant mechanisms: Suspended Load – Sediment that is held aloft from the action of turbulence; not in consistent contact with the bed. Bed Load – Rolling, trundling, saltating sediment that is in regular contact with the bed. But…material that is bed load during low energy conditions can be suspended load during high energy conditions, when shear stresses are elevated.

Show Youtube video: http://www.youtube.com/watch?NR=1&feature=endscreen&v=o3llzwvv1zc

Unidirectional Flow Boundary Layer Fluid motion over the sea bed generates friction and turbulence. The boundary layer is the portion of the flow directly above the bed, where flow velocity is rapidly changing. Within unidirectional flows, the BL can be divided into 3 layers. Lowest layer (Bed Layer) can exhibit an inner layer termed the viscous sublayer when smooth (Re<5), which disappears when the bed is rough (Re>70), meaning objects on the bed protrude into the logarithmic layer.

Bed roughness has two components: (1) Skin friction, which is proportional to the sediment diameter, and (2) Bedforms such as ripples and subaqueous dunes.

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Oscillatory Flow Boundary Layer

Boundary layers in unidirectional flow become relatively thick and well defined due to the steadiness of the flow – not so in oscillatory flow in the presence of waves. In oscillatory flow, the boundary layer develops and breaks down twice per wave period and the total height of the BL seldom exceeds 0.1m.

Shear Stress Under Waves and Sediment Motion Incipient sediment motion and transport is physically related to SHEAR STRESS, rather than flow velocity. But shear stress within a flow is related to the vertical velocity structure, thusly: Where µ is molecular viscosity, a property of the fluid, and ξ is eddy viscosity, a product of turbulent intensity, which obviously waxes and wanes rapidly during a single wave oscillation. If we would like to move (entrain) sediment, of interest to us, is the maximum bottom shear stress achieved during a wave oscillation, which is related to the max velocity. Sand will begin to move when the shear stress, exceeds the critical entrainment stress for a sand particle of given size.

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Bedforms

Bedform morphology is related to: sediment size, bed shear stress, and asymmetry of the flow. Photographs of bedforms on sandy coasts: (a) 2-D vortex ripples generated in a laboratory

oscillatory flume, (b) 2-D vortex ripples characteristic of shoaling

waves on a medium sand bed. Note the lateral extent of the ripple crests and the tuning fork junctions that are typical of these ripples in the field. The spacing between ripples is regular and it can be seen that this spacing adjusts to the junction (or splitting) of the crests;

(c) large vortex ripples developed in coarse sand

and granules, Veradero, Cuba at a depth of about 25 m;

More Bedforms

Much is still unknown about the development of bedforms under rapidly varying, combined oscillatory and unidirectional flows in the surf zone. This makes the prediction of bed roughness difficult.

(d) 3-D ripples on a bar crest; (e) cross-ripples or ladderback ripples (f) flow over quasi 2-D dunes just seaward of

the breakpoint. The photograph was taken just as the wave crest passed looking obliquely offshore. Plumes of sediment are visible seaward as vortices lift off during the offshore directed portion of the flow.

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Relating Bedforms to Depositional Environment

Understanding the mechanics of bedform generation has obvious implications for reconstructing depositional environments and the interpretation of the sedimentary geologic record. Certain sedimentary facies are diagnostic of environment of deposition.

Sedimentary Facies

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Bed Roughness and Suspension

Symmetric oscillatory flow produces two suspension events within each wave cycle. Vorticies generated by onshore and offshore directed strokes of the cycle, are shed off into the water column, carrying sediment plumes up to 3x the ripple height.

Suspended Sediment Concentration Profile

Net onshore-offshore transport can be computed by integrating the combination of 1. velocity profile (obtained with ADCP measurements) and 2. suspended sediment concentration profile (obtained with OBS measurements).

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Why is transport offshore during storms and onshore during milder conditions?

Consider the velocity profile in the surf zone – offshore directed undertow occupies the vertical positions from 10-20 cm from the bed to the wave trough. During low wave conditions, vortex generation is not large, so very little sediment is entrained high enough into the water column to reach the undertow layer. However, during storms, when large waves are present, vortices are stronger and are able to sweep sediment high into the water column – into the undertow portion of the velocity profile. Additionally, undertow velocity is greater during large wave events, fostering the offshore movement of entrained sediment. So during storms, sediment is stored in offshore bars at depths below the influence of storm waves – then slowly moves onshore during quiescent conditions.

Suspended Sediment Concentrations and Waves

Data of onshore-offshore flow components and sediment in the water column reveal the strong link between the passage of individual waves and suspended sediment concentration. Also visible in the data is the concept of wave group "pumping", which produces elevated episodes of suspension.

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Infragravity Waves and Cross Shore Sediment Transport

Numerical models (such as SBEACH) have been developed to predict behavior of cross shore morphology (beach profiles), based on calculations of cross shore sediment transport rates. Models must be tested with field observations, which can be based on measurements of processes (incident waves, currents, sediment transport), or measurements of profiles before and after some known wave conditions. Both testing methods have advantages and drawbacks. Bagnold energetics model is based on the notion that amount of sand movement (cross-shore) by waves is proportional to the local rate of energy dissipation.

Numerical Modeling of Cross Shore Sediment Transport

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Measurements of Cross Shore Sediment Flux