Carl L. Amos

Sea Carousel—A benthic, annular flume

A benthic annular flume (Sea Carousel) has been developed and tested to measure in situ the erodibility of cohesive sediments. The flume is equipped with three optical backscatter sensors, a lid rotation switch, and an electromagnetic (EM) flow meter capable of detecting azimuthal and vertical components of flow. Data are logged at rates up to 10·66 Hz. Erodibility is inferred from the rate of change in suspended sediment concentration detected in the annulus. The energy-density/wave number spectrum of azimuthal flow showed peaks in the energy spectrum at paddle rotation wave numbers (k) of 14 and 7 m −1 (macroturbulent time scales) but were not significant. Friction velocity ( U *), measured (1) at 1 Hz using a flush-mounted hot-film sensor, and (2) derived from measured velocity profiles in the inner part of the logarithmic layer gave comparable results for Ū * −1 . At higher values of U*, method (2) underpredicted by up to 20%. Method (1) showed radial increases in Ū * in the annulus for Ū y > 0·32 m s −1 . Radial velocity gradients were proportional to ( Ū y − 0·32 m s −1 ). Maximum radial differences in U * were 10% for Ū y = 0·5 ms −1 . Suspended sediment mass concentration ( S ) in the annulus resulted in a significant decrease (10·5%) in Ū * derived by method (1) over the range 0 S −1 . These decreases were not evident in method (2). Method (1) may, therefore, be subject to changes in stress sensor calibration with changes in S . Subaerial deployments of Sea Carousel caused severe substrate disturbance, water losses, and aeration of the annulus. Submarine deployments produced stable results, though dispersion of turbid flume water took place. Results clearly demonstrated the existence of ‘Type I’ and ‘Type II’ erosion documented from laboratory studies.

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.

Bedforms of the Canadian eastern seaboard: A comparison with global occurrences

This paper presents a compilation of sedimentary bedforms identified from acoustic and seismic records collected over the southeastern Canadian continental shelf. In order to make the compilation it was found necessary to firstly review and synthesise the various terminologies used to describe the bedforms found, and thereafter to relate the subsequent terminology to any diagnostic, generic characteristic identifiable from the records. Sand waves are considered to be those forms that are composite in nature and are generated solely by the action of superimposed megaripples. These latter features are identifiable adopting the classification of Dalrymple et al. (1978) which proves adequate for the acoustic differentiation of 2-D and 3-D types; though we observed a greater coherence in bedform spacing of the 2-D types than reported hitherto. Sand ridges were adequately defined and recognised using the description of Swift et al. (1978) with respect to storm-generated ridges and that of Off (1963) and others with respect to tidal-current ridges. Such ridges appear not to be uniquely described by ratios of their metrics. The bedforms on the southeastern Canadian continental shelf are abundant and wide-spread. Virtually all types of bedforms, recognisable on the U.S. shelf are present to the north. The majority of these bedforms appear to be storm generated and are considered intermittently active. Many of the sand waves (s.s.) have no measureable relief though retaining their distinctive spatial geometry. Both active and moribund sand ridges were found. Of these ridges both tidal- and storm-generated types were identified. The storm-generated ridges were further broken down into shoreface-connected ridges and offshore ridges. A metrical analysis of the various bedforms show that a differentiation into stable bed phases can be made using height—length ratios provided the bedforms are active and fully developed. There is some overlap between megaripples and small sand waves and indeed between 2-D and 3-D megaripples, yet the distinction is good between sand waves and sand ridges (even moribund types). Sand ribbons in general can be confused with moribund sand waves and megaripples. A detailed morphological examination is required to distinguish each feature. Data from sand ridges of the eastern Canadian shelf, combined with data from similar features found globally, shows that ridge heights tend to cluster into three distinctive modes. These modes are at heights of 7, 13 and 27 m. The first mode is correlated with storm-generated ridges, the second with tidal-current ridges and the third possibly with submerged barrier islands.

The calibration of optical backscatter sensors for suspended sediment of varying darkness levels

A series of calibration experiments were run in order to determine the effect of the colour of suspended sediment on the response of optical backscatter sensors (OBS). Munsell notation describes the properties of soil and sediment colour using the three parameters, hue, chroma, and value, which represent, respectively, the spectral position, lightness level, and amount of pure chromatic colour present. The exposure of the OBS to Munsell Soil Chart patches revealed that the OBS is sensitive to variations in Munsell value and not Munsell hue or chroma. Since OBS output decreased with darkness level (and increased with lightness level and Munsell value), it is suggested that the level of blackness of particles acts to absorb the near-infrared (IR) signal of the OBS, modifying its output. A strong correlation (r2=0.984) was found between the observed and predicted OBS outputs of varying concentrations and ratios of black and white suspended sediment. The OBS output was predicted using a linear mixture-model incorporating end-member calibrations of black and white sediment (similar to Green and Boon [Green, M.O., Boon, J.D., III, 1993. The measurement of constituent concentrations in nonhomogeneous sediment suspensions using optical backscatter sensors. Marine Geology, 110, pp. 73–81.]). A strong correlation (r2=0.907) was also observed between luminance factor and IR reflectance for various minerals, suggesting a link between Munsell notation and OBS output. Formulae were developed to predict the IR reflectance of sediment of known Munsell value, concentration, and size. Models predicting sediment transport should consider the effect of the level of blackness of particles on the OBS response.

Control of estuarine sediment dynamics by interactions between currents and waves at several scales

Abstract The effect of interactions between continuous (tidal currents) and intermittent (waves) processes on sediment dynamics and transport is addressed by presenting detailed field measurements of waves, boundary-layer currents and suspended sediment from an estuarine channel and an adjacent intertidal sandflat in Manukau Harbour, New Zealand. The aim is to determine in what ways it is necessary to couple waves and currents in numerical models, and thereby put limits on the fundamental structure of process-based estuarine sediment transport models. Waves were important on the intertidal flat: turbidity switched on and off with the appearance and disappearance of waves; wave groups dominated entrainment of bed sediment; a wave-current boundary-layer model explained measured bed shear stress and hydraulic roughness; and the measured near-bed time-averaged suspended-sediment concentration was mostly well predicted by a pure-wave model. Both the waves themselves and wave-related processes varied markedly over the tidal cycle. The variation in the former (principally changes in wave height) was related to changes in fetch caused by the harbour-wide emergence and submergence of intertidal regions. The variation in the latter was related to changes in water depth relative to the wavelength of the waves, which controlled the penetration to the bed of wave-orbital currents. In addition to the variation over the tidal cycle in the ‘intensity’ of wave processes, there was also a change in ‘kind’, which occurred with the arrival at the measurement site of the ‘turbid fringe’, which is the narrow, highly turbid edge of the estuarine water body. The relationship between suspended-sediment concentration and wave-orbital velocity in the turbid fringe was radically different to the relationship in the estuarine water body proper, which suggests a change in dynamics, perhaps related to breaking waves. A ‘hybrid’ modelling approach is required, i.e. one that treats discrete events but resolves tidal-cycle-scale variation within the event. There is a need to resolve the variation in the wave train over the tidal cycle and the penetration to the bed of wave-orbital motions, both of which could only be done adequately within an estuary tidal model. In contrast to the situation on the intertidal flat where waves intermittently entrained sediment, sediment transport in the channel was continuous, driven by tidal currents. To predict sediment flux in the channel we need to know the upstream sediment-transporting capacity of the flow (including that contributed by waves), the character of the bed sediment, and the sediment-settling time scale. These factors confounded even the simplest notion of flood and ebb dominance, which frequently has been applied to understand estuarine morphodynamics.

The Influence of Subaerial Exposure on the Bulk Properties of Fine-grained Intertidal Sediment from Minas Basin, Bay of Fundy

Measurements of the bulk properties of fine-grained, surface sediment were made bi-weekly across an intertidal mudflat in Minas Basin between June and September 1984. Spatial and temporal variations were substantial. The vane shear strength increased shoreward at a rate of 0·4 kPa km−1 and, at any point, showed a dramatic mid-summer increase from less than 4 to 5–42 kPa. Relationships between vane shear strength, sediment properties (grain size, bulk density, water content, organic content) and biological activity or abundance were often weak because of both spatial and temporal changes in exposure time, exposure index, evaporation index and precipitation. Shear strength at any site was most sensitive to changes in exposure index and evaporation, and least sensitive to organic content and biological activity. The bed level co-varied positively with shear strength, i.e. shear strength increased despite deposition. It is proposed that summertime, mudflat deposition is controlled by surface-sediment shear strength which is, in turn, controlled directly by atmospheric effects.

Large-scale experiments on wave propagation over Posidonia oceanica

Posidonia oceanica, the most abundant seagrass species in the Mediterranean, supports a highly bio-diverse habitat and is crucial in protecting against coastal erosion. In this work, experiments in a large-scale facility have been performed, for the measurement of wave attenuation, transmission and energy dissipation over artificial Posidonia oceanica. The effects of submergence ratio corresponding to the seagrass height divided by water depth, and seagrass density as the number of stems per square metre on the above characteristics are investigated. Measurements of wave height at different locations along the vegetation meadow indicate the wave attenuation along the Posidonia oceanica for three different submergence ratios and two seagrass densities. Results are also analysed with regard to the wave-induced flow within the meadow, and the effects of the submergence ratio and the seagrass density on the mean flow characteristics, based on data of mean velocities taken at three locations within the seagrass.

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.

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

Abstract A multi-parameter instrument package (Ralph) was deployed for 15 days in 22 m of water on Sable Island Bank, Scotian Shelf. The instrument successfully recorded mean current velocity, statistics on near-bed wave motion, wave height, period and propagation direction, and time-lapse photographs of the seabed. The seabed at the deployment site was composed of well-sorted, fine sand (0.23 mm mean diameter) which moved during peaks in tidal flow and during periods of high wave activity. Eight distinct bed types were seen in time-lapse imagery: (1) wave ripples; (2) straight-crested current ripples; (3) linguoid current ripples; (4) wave and current ripples; (5) transitional wave ripples; (6) transitional current ripples; (7) poorly developed (biodegraded) ripples; and (8) flat bed. Each bed type occurred at well-defined combinations of near-bed wave motion, mean current speed and biodegradation. Wave-current ripples intermediate in form and orientation, were not seen. Bedform types were found to be well separated in plots of wave Reynolds number (Umb·d0/ν) and dimensionless mean flow (U100/Ws). A better separation was found if the wave Shields parameter (after Grant and Madsen , 1979 , Journal of Geophysical Research, 84, 1797–1808) and the current Shields parameter (after Sternberg , 1972 , Shelf sediment transport, process and pattern, pp. 61–83) were used. Bedform stability plots showed that wave ripple threshold is influenced by currents, and current ripple threshold is influenced by waves. Despite this, ripple type is defined by the partitioned, wave and current components of stress and not the total stress as no intermediate (wave-current) ripple type was observed. Thresholds for sand transport and the generation of well-developed bedforms were influenced by the combined, near-bed motions of waves and currents even when the bed exhibited wave ripples or current ripples only. The threshold for sand transport under combined flows is vague, but is adequately represented by a line drawn between the threshold for transport under unidirectional flow based on the modified Shields curve of Miller et al. (1977 , Sedimentology, 24, 507–527) (θc= 0.04) and the threshold specified for pure wave motion of Komar and Miller (1973 , Journal of Sedimentary etrology, 43, 1101–1110) (θw = 0.04). A first approximation to a threshold criterion for fine sand is proposed as [θw + θc]crit = 0.04.

Spatial and seasonal variation in wave attenuation over Zostera noltii

Wave attenuation is a recognized function of sea grass ecosystems which is believed to depend on plant characteristics. This paper presents field data on wave attenuance collected over a 13 month period in a Zostera noltii meadow. The meadow showed a strong seasonality with high shoot densities in summer (approximately 4,600 shoots/m2) and low densities in winter (approximately 600 shoots/m2). Wave heights and flow velocities were measured along a transect at regular intervals during which the site was exposed to wind waves and boat wakes that differ in wave period and steepness. This difference was used to investigate whether wave attenuation by sea grass changes with hydrodynamic conditions. A seasonal change in wave attenuation was observed from the data. Results suggest that a minimum shoot density is necessary to initiate wave attenuation by sea grass. Additionally, a dependence of wave attenuation on hydrodynamics was found. Results suggest that the threshold shoot density varies with wave period and a change in energy dissipation toward the shore was observed once this threshold was exceeded. An attempt was made to quantify the bed roughness of the meadow; the applicability of this roughness value in swaying vegetation is discussed. Finally, the drag coefficient for the meadow was computed: A relationship between wave attenuance and vegetation Reynolds number was found which allows comparing the wave attenuating effect of Zostera noltii to other plant species.