Michael Z. Li

SEDTRANS96: the upgraded and better calibrated sediment-transport model for continental shelves ☆

The sediment transport model SEDTRANS has been significantly upgraded based on new advances in both cohesive and non-cohesive sediment transport studies. For given input data of wave, current, and seabed conditions, the model applies the combined wave–current bottom boundary layer theories to derive the near-bed velocity profile and bed shear stresses, and then calculates sediment transport for currents only or combined waves and currents over either cohesive or non-cohesive sediments. Critical shear stresses for various sediment transport modes tested for combined waves and currents are adopted in SEDTRANS96. An explicit combined-flow ripple predictor is included in the model to provide time-dependent bed roughness prediction. SEDTRANS96 also predicts the vertical profiles of velocity and suspended sediment concentration and their product is integrated through depth to derive the suspended-load transport rate. More rigorous calibration of the model using measured sediment transport rates over fine and medium sands shows that the difference between the predicted and measured transport rates has been reduced from more than one order of magnitude to less than a factor of five. The proposed new cohesive sediment algorithm separates cohesive sediment transport into depositional, stable and erosional states. The applied shear stress, erosion/deposition time duration and the down-core profile of the critical shear stress for erosion are numerically integrated to predict the final erosion or deposition rate, suspension concentration and transport rate for cohesive sediment.

Predicting ripple geometry and bed roughness under combined waves and currents in a continental shelf environment

Waves, current and seabed response data collected by an instrumented tripod deployed on the Scotian Shelf during the winter of 1993/94 are analyzed to derive a ripple predictor for combined flows and to evaluate the applicability of existing ripple- and bedload-roughness algorithms under combined waves and current. Wave-dominant ripples developed during storms were generally higher and steeper than current-dominant ones. The ratio of the skin-friction wave shear velocity to that of the steady current, u∗ws/u∗cs, can be used to define the various types of ripples under combined flows. By comparing the measured ripple geometry and the predictions by existing ripple predictors, the wave-ripple predictors of Nielsen (1981), and Grant and Madsen (1982) are found to over-predict ripple height and ripple roughness for combined flows under the conditions of the present study. These methods also neglect the enhancement of shear stress at the ripple crest. A new empirical ripple predictor is proposed and it uses the combined shear velocity and the ratio u∗ws/u∗cs to predict the heights and wavelengths of ripples and their dynamic transition under combined flows. The effect of enhanced shear velocity at the ripple crest is also incorporated for the prediction of ripples in the weaktransport range. A simplified logarithmic profile method and the values of the bedload shear velocity due to the combined grain size and bedload roughnesses are used to evaluate the applicability of various ripple- and bedload-roughness height algorithms under combined flows. While the ripple roughness height algorithm of Grant and Madsen (1982) is found to give good predictions of the total current shear velocity u∗c and apparent bottom roughness z0c, the algorithm of Nielsen (1992), tends to underpredict both parameters. The bedload roughness algorithms of Nielsen (1992) and Li et al. (1997) are both found to give reasonable predictions under combined flows. The total bed roughness height under combined flows can be expressed as kb=2.5D+27.7η2/λ+170D(θcws−θcr)0.5.

BOUNDARY LAYER DYNAMICS AND SEDIMENT TRANSPORT UNDER STORM AND NON-STORM CONDITIONS ON THE SCOTIAN SHELF

Abstract Near-bed measurements of waves, currents, seabed responses and ripple migration rates were obtained using an instrumented tripod at a water depth of 39 m on the Scotian Shelf during the winter of 1992/93. These data and the Grant and Madsens (1986) [Grant W.D., Madsen O.S., 1986. The continental shelf bottom boundary layer. Annu. Rev. Fluid Mech. 18, 265–305.] combined-flow boundary layer model are used to examine wave-current interaction, various sediment dynamic thresholds and sediment transport on an exposed, high-energy continental shelf. The seabed was found to be rippled for most time of the deployment and thus the ripple-enhanced combined skin-friction shear velocity had to be used to determine the initiation of bedload transport under combined flows. This indirectly indicates adequate predictions of the skin-friction shear velocities inside the wave boundary layer by the Grant and Madsen model. At high transport stages, bedload roughness must be added to the grain size roughness to obtain a transportrelated combined shear velocity in order to predict correctly the onsets of saltation/suspension and sheet flow transports. Otherwise, the initiation of suspended load transport and the total sediment transport rates will be severely under-estimated. The comparison between the prediction by the Grant and Madsen (1986) model and the calculation of a quadratic stress law suggests that the total current shear velocity was enhanced by a factor of 2–3 due to the wave-current interaction during storms, while the apparent bottom roughness was increased by more than an order of magnitude. The non-linear coupling between waves and currents also causes a 20% increase of the combined skinfriction shear velocity inside the wave boundary layer. This non-linear coupling is most important when waves and currents are roughly equal in magnitude and the angles between them are less than 30 °. Four sediment transport formulae were tested. While the Engelund-Hansen total-load and Yalin bedload methods did not perform well, the Einstein-Brown and Bagnold formulae were found to, respectively, give reasonable predictions of the bedload and total-load sediment transport rates under the observed combined-flow conditions. The predicted sediment transport direction is also in good agreement with the observed ripple migration direction. The GSC sediment transport model SEDTRANS92 predicts that the net daily transport rates during the storms reached 822 kg m −1 day −1 and were 2–3 orders of magnitude higher than the non-storm transport, suggesting the dominance of storm processes in sediment transport on the Scotian Shelf. These values are in good agreement with the results from sand tracer experiments conducted in this region. We thus conclude that SEDTRANS92 can properly simulate boundary layer dynamics and sediment transport on continental shelves.

PREDICTING RIPPLE ROUGHNESS AND SAND RESUSPENSION UNDER COMBINED FLOWS IN A SHOREFACE ENVIRONMENT

Ripple measurements and flow and sediment dynamical data obtained from the shoreface of the Middle Atlantic Bight using instrumented tripods were analyzed to evaluate various predictors of ripple geometry and roughness. Ripple roughness controls on sand resuspension and suspended sediment concentration profiles under combined waves and currents were also evaluated. The limited observation of sand ripples in the field indicates that the Grant and Madsen (1982) method overestimates ripple roughness, while the Nielsen (1981) method tends to under-predict ripple roughness. A modified ripple predictor is thus proposed based on the Grant and Madsen method, and it is shown to give reasonable predictions under the present experiment conditions. This modified ripple prediction along with wave, current and suspended sediment concentration data recorded by the tripods were then brought into the combined-flow bottom boundary layer model of Grant and Madsen (1986) and the modified Rouse equation of Glenn and Grant (1987) to calculate sand resuspension coefficient γ0 and to predict suspended sediment concentration profiles. It was found that under low-energy fair-weather conditions, sand ripples are in the equilibrium range and ripple roughness increases with the bed shear stress. This causes strong vortex activity close to seabed and thus higher resuspension coefficient. Reference concentrations are moderate due to this high resuspension coefficient, even though bed shear stresses are low. Under moderate storm conditions, ripple break off occurs and ripple roughness will decrease with bed shear stress. This reduces the vortex activity and hence sand resuspension coefficient γ0. The combination of this moderately high bed shear stress and reduced but still moderate ripple roughness favours sand suspension and produces the highest reference concentration for the encountered experimental conditions. As bed shear stress is further increased, ripples are nearly washed out and sand resuspension coefficient is further decreased approaching the previously-suggested constant value of 1.3 × 10−4. This corresponds with the lowest reference concentration despite the high bed shear stress. Suspended sediment concentrations predicted by the modified Rouse equation using this time variable resuspension coefficient and properly calculated bottom boundary layer parameters are reasonable compared to the field measured concentration profiles.

Sedtrans05: An improved sediment-transport model for continental shelves and coastal waters with a new algorithm for cohesive sediments

The one-dimensional (vertical) sediment-transport model SEDTRANS96 has been upgraded to predict more accurately both cohesive and non-cohesive sediment transport. Sedtrans05 computes the bed shear stress for a given set of flow and seabed conditions using combined wave-current bottom boundary layer theory. Sediment transport (bedload and total load) is evaluated using one of five methods. The main modifications to the original version of the model are: (1) a reorganization of the code so that the computation routines can be easily accessed from different user interfaces, or may be called from other programs; (2) the addition of the Van Rijn method to the options for non-cohesive sediment transport; (3) the computation of density and viscosity of water from temperature and salinity inputs; and (4) the addition of a new cohesive sediment algorithm. This latter algorithm introduces variations of sediment properties with depth, represents the suspended sediment as a spectrum of settling velocities (i.e. size classes), includes the flocculation process, and models multiple erosion–deposition cycles. The new model matches slightly better the field measurements of non-cohesive sediment transport, than does the predictions by SEDTRANS96. The sand-transport calibration has been extended to high transport rates. The cohesive sediment algorithm reproduced well experimental data from annular flume experiments.

SEDTRANS92: a sediment transport model for continental shelves

Abstract The Atlantic Geoscience Centre sediment transport model (SEDTRANS92) is an ANSI standard FORTRAN-77 numerical model that has been under development for the last 9 years. For given input data of wave, current, and seabed conditions, SEDTRANS92 applies combined wave-current bottom boundary layer theories to derive enhanced bottom shear stresses. Then it calculates sediment transport rates for a given grain size using one of seven algorithms: Engelund and Hansen, total load equation; Einstein-Brown, bedload equation; Bagnold, total load equation; Yalin, bedload equation; Ackers and White, total load equation; Smith, suspended load method; and the cohesive sediment transport algorithm of Amos and Greenberg. This model adopts new advances of bottom boundary layer theory and recently available field measurements of sediment transport rates on the Scotian Shelf. The model gives reasonable predictions compared to the field measurements.

FIELD OBSERVATIONS OF BEDFORMS AND SEDIMENT TRANSPORT THRESHOLDS OF FINE SAND UNDER COMBINED WAVES AND CURRENTS

Abstract Seabed video images and S4 wave–current meter data, collected during the build-up of a moderate storm on the Scotian Shelf, are analysed for bedform development and sediment transport threshold of fine sand under combined waves and currents. As the storm built up, the following sequence of bedforms was observed: (1) relict wave-dominant ripples with worm tubes and animal tracks during the preceding fairweather period; (2) irregular, sinuous, asymmetrical current-dominant and intermediate wave–current ripples under bedload transport; (3) regular, nearly straight or sinuous asymmetrical to slightly asymmetrical wave-dominant ripples under saltation/suspension; (4) upper-plane bed under sheet-flow conditions; (5) small, crest-reversing, transitory ripples at the peak of the storm; and (6) large-scale lunate megaripples which developed when the storm decayed. These data also show that only single sets of asymmetrical intermediate wave–current ripples will form when waves and currents are co-linear. The development of the crest-reversing transitory ripples indicates a high-energy transition stage under quasi-sheet-flow conditions. A direct comparison of the skin-friction combined shear velocity and the critical shear velocities for bedload, suspension and sheet-flow transport under-estimated the onset of these sediment transport modes. As the presence of ripples causes the shear stress to increase from ripple trough to ripple crest, the ripple-enhanced skin-friction shear velocity must be used to determine properly the initiation of bedload transport. At high transport stages, the boundary layer dynamics is controlled mainly by the thickness of the bedload transport layer. Thus a transport-related bedload shear velocity, predicted based upon the sum of the grain roughness and bedload roughness, has to be compared against the conventional threshold criterion to properly define the onset of suspension and sheet-flow transport modes.

Storm-generated, hummocky stratification on the outer-Scotian Shelf

Hummocky megaripples occur on Sable Island Bank, Scotian Shelf. Submersible observations show that the megaripples form during winter storms and are subsequently obliterated through bioturbation and fair-weather reworking. The megaripples of this study were underlain by a storm bed composed of: (A) a basal scoured and infilled gravel lag facies; (B) low-angle tangential crossbedding in gravel to coarse sand; (C) anisotropic hummocky stratification in medium sand; and (D) wave ripple cross-lamination in medium/fine sand. This sequence forms a tempestite bed created by a winter storm during our sampling program. Numerical simulation of bed conditions during the storm suggests that the hummocky megaripples and hummocky stratification formed together during late stages of storm decay from conditions of sheet flow. Near-bed wave motion during deposition exceeded steady currents by an order of magnitude.

Chapter 2 Continental shelves of Atlantic Canada

Abstract The wide continental shelves of Atlantic Canada are characterized by a series of banks separated by transverse troughs. These shelves have been imprinted by repeated Quaternary glaciations, so that fluvial valleys have been deepened into fjords and shelf-crossing troughs, and a suite of glacigenic sediments has been deposited. In shallow areas the seafloor is shaped by waves and currents, including the strong tidal currents of the macrotidal Bay of Fundy. Glacigenic sediments have been reworked by modern processes to yield thick muds in basins, and thinner deposits of sand and gravel on wave-dominated banks and the littoral zone. As a result of a cold climate and the Labrador Current, seasonal sea ice occurs to varying degrees across the region, and iceberg impact continues on much of the Newfoundland and Labrador shelves. For the purpose of description, we divide Atlantic continental shelves into four regions and focus on advances in understanding over the past several decades relating to: (1) processes on upper continental slopes; (2) glacial history in the last glacial cycle; (3) glacial land systems; (4) geographical changes caused by glacio-isostasy; and (5) sediment mobility on the offshore banks. We conclude with a brief overview of the biota.