Bagnold's energetics-based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed. In both the bedload and suspended load, the transport rate vectors are found to be composed of a velocity-induced component directed parallel to the instantaneous velocity vector and a gravity-induced component directed down slope. The model is applied to idealized surfzone conditions, leading to estimates of the local longshore and onshore-offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions. The model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions. The results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics.

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An energetics total load sediment transport model for a plane sloping beach

Over the past two decades the application of acoustics to the measurement of small-scale sediment processes has been gaining increasing acceptance within the sedimentological community. This has arisen because acoustics has the potential to measure non-intrusively, with high temporal and spatial resolution, profiles of suspended sediment size and concentration, profiles of flow, and the bedform morphology. In the present article we review the capability of acoustics to deliver on its potentiality to make a valuable and unique contribution to the measurement of small-scale sediment processes. The article introduces the reasons for using acoustics, the physics underlying the approach, a series of examples illustrating collected data, a discussion on some of the difficulties encountered when applying acoustics and finally a look to the future and possible new developments.

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A review of acoustic measurement of small-scale sediment processes

Onshore sediment transport and sandbar migration are important to the morphological evolution of beaches but are not well understood. Here, a model that accounts for fluid accelerations in waves predicts the onshore sandbar migration observed on an ocean beach. In both the observations and the model, the location of the maximum acceleration-induced transport moves shoreward with the sandbar, resulting in feedback between waves and morphology that drives the bar shoreward until conditions change. A model that combines the effects of transport by waves and mean currents simulated both onshore and offshore bar migration observed over a 45-day period.

/pdf/wave-induced-sediment-transport-and-sandbar-migration-2m7vms8a5i.pdf

Wave-Induced Sediment Transport and Sandbar Migration

Waves, currents, and the location of the seafloor were measured on a barred beach for about 2 months at nine locations along a cross-shore transect extending 255 m from 1 to 4 m water depth The seafloor location was measured nearly continuously, even in the surf zone during storms, with sonar altimeters mounted on fixed frames The crest of a sand bar initially located about 60 m from the shoreline moved 130 m offshore (primarily when the offshore significant wave height exceeded about 2 m), with 15 m of erosion near the initial location and 1 m of accretion at the final location An energetics-type sediment transport model driven by locally measured near-bottom currents predicts the observed offshore bar migration, but not the slow onshore migration observed during low-energy wave conditions The predicted offshore bar migration is driven primarily by cross-shore gradients in predicted suspended sediment transport associated with quasi-steady, near-bottom, offshore flows These strong (>50 cm/s) currents, intensified near the bar crest by wave breaking, are predicted to cause erosion on the shoreward slope of the bar and deposition on the seaward side The feedback amoung morphology, waves, circulation, and sediment transport thus forces offshore bar migration during storms

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JC02765

Observations of sand bar evolution on a natural beach

Transport of participate material across continental shelves is well demonstrated by the distributions on the seabed and in the water column of geological, chemical, or biological components, whose sources are found farther landward or farther seaward. This paper addresses passive (incapable of swimming) particles and their transport across (not necessarily off) continental shelves during high stands of sea level. Among the general factors that influence across-shelf transport are shelf geometry, latitudinal constraints, and the timescale of interest. Research studies have investigated the physical mechanisms of transport and have made quantitative estimates of mass flux across continental shelves. Important mechanisms include wind-driven flows, internal waves, wave-orbital flows, infragravity phenomena, buoyant plumes, and surf zone processes. Most particulate transport occurs in the portion of the water column closest to the seabed. Therefore physical processes are effective where and when they influence the bottom boundary layer, causing shear stresses sufficient to erode and transport particulate material. Biological and geological processes at the seabed play important roles within the boundary layer. The coupling of hydrodynamic forces from currents and surface gravity waves has a particularly strong influence on across-shelf transport; during storm events, the combined effect can transport particles tens of kilometers seaward. Several important mechanisms can cause bidirectional (seaward and landward) transport, and estimates of the net flux are difficult to obtain. Also, measurements of across-shelf transport are made difficult by the dominance of along-shelf transport. Geological parameters are often the best indicators of net across-shelf transport integrated over time scales longer than a month. For example, fluvially discharged particles with distinct composition commonly accumulate in the midshelf region. Across-shelf transport of particulate material has important implications for basic and applied oceanographic research (e.g., dispersal of planktonic larvae and particle-reactive pollutants). Continued research is needed to understand the salient mechanisms and to monitor them over a range of timescales.

/pdf/transport-of-particles-across-continental-shelves-4l0w7txf8x.pdf

Transport of particles across continental shelves

A miniature optical backscatter sensor (MOBS) and an electro-magnetic flowmeter were deployed seaward of the surf zone at Pte Sapin, New Brunswick during the first Canadian Coastal Sediment Study (C 2 S 2 ), autumn 1983. The MOBS had sensing elements at five vertical locations above the sea bed, and was sampled at 10 Hz. The suspension of sand is well correlated with the passage of individual waves and also with wave groups, with the influence of wave groups progressively more dominant at higher elevations above the bed. Suspension appears to be stronger during the onshore directed phase of the wave motion than during the offshore motion and there is evidence that initiation of suspension may be determined by fluid acceleration more than velocity. Time lags between suspension events at different heights suggest that vertical gradients in sediment flux are more important than horizontal gradients.

Continuous measurements of suspended sand concentration in a wave dominated nearshore environment

Direct Measurement of Suspended Sediment Transport

Acoustic measurements of suspended sand concentration in the C2S2 experiment at Stanhope Lane, Prince Edward Island

Ripple generation under the combined influences of waves and currents on the Canadian continental shelf

Bed state, ripple migration and bedform transport of fine sand (D50 = 0.23 mm) were recorded with time-lapse photography for 12 days in 22 m of water on Sable Island Bank, the Scotian Shelf. Canada. Near-bed wave and steady flows were recorded and correlated with observed bed state and ripple migration. A well-defined threshold for the traction of fine sand under largely orthogonal combined flows is apparent in our data. It is defined by the expression (-) cb ' = [(-) w ' + (-) c ’] = 0.04. The value is 20% less than the threshold criterion defined by GRANT and MADSEN (1979) for wave-dominant flows and extends it to the current dominant flow conditions when flows are orthogonal to each other. In addition, a threshold for saltation/suspension under combined flows is defined by the expression (-) cb ’= [(-) w ’ + 1.5(-) c ] + 0.17. This threshold marks the onset of saltation and grain bypassing of ripple lee faces and approximates closely the breakoff Shields parameter (-) br ’ = 0.16) of GRANT and MADSEN (1982). Bedform transport measurements are applicable only below this point; termed the equilibrium range by GRANT and MADSEN (1982). The sediment transport model SEDTRANS matched the duration and magnitude of sand transport over a wide range of conditions of combined flow. This was achieved by using the method of GRANT and MADSEN (1979), but minimizing the friction factor (f cw ’) to 0.006. This value equals the pure unidirectional flow drag coefficient of SOULSBY (1983).

/pdf/ripple-migration-and-sand-transport-under-quasi-orthogonal-46hygr9c9v.pdf

David A. Huntley

Papers

Continuous measurements of suspended sand concentration in a wave dominated nearshore environment

Direct Measurement of Suspended Sediment Transport

Acoustic measurements of suspended sand concentration in the C2S2 experiment at Stanhope Lane, Prince Edward Island

Ripple generation under the combined influences of waves and currents on the Canadian continental shelf

Ripple Migration and Sand Transport Under Quasi-Orthogonal Combined Flows on the Scotian Shelf