Wave-current Bottom Boundary Layer Research Articles

Field observation and analysis of wave-current-sediment movement in Caofeidian Sea area in the Bohai Bay, China

In order to study the mechanism of flow-sediment movement, it is essential to obtain measured data of water hydrodynamic and sediment concentration process with high spatial and temporal resolution in the bottom boundary layer (BBL). Field observations were carried out in the northwest Caofeidian sea area in the Bohai Bay. Near 2 m isobath (under the lowest tidal level), a tripod system was installed with AWAC (Acoustic Wave And Current), ADCP (Acoustic Doppler Current Profilers), OBS-3A (Optical Backscatter Point Sensor), ADV (Acoustic Doppler Velocimeters), etc. The accurate measurement of the bottom boundary layer during a single tidal period was carried out, together with a long-term sediment concentration measurement under different hydrological conditions. All the measured data were used to analyze the characteristics of wave-current-sediment movement and the BBL. Analysis was performed on flow structure, shear stress, roughness, eddy viscosity and other parameters of the BBL. Two major findings were made. Firstly, from the measured data, the three-layer distribution model of the velocity profiles and eddy viscosities in the wave-current BBL are proposed in the observed sea area; secondly, the sediment movement is related closely to wind-waves in the muddy coast area where sediment is clayey silt: 1) The observed suspended sediment concentration under light wind conditions is very low, with the peak value generally smaller than 0.1 kg/m3 and the average value being 0.03 kg/m3; 2) The sediment concentration increases continuously under the gales over 6–7 in Beaufort scale, under a sustained wind action. The measured peak sediment concentration at 0.4 m above the seabed is 0.15–0.32 kg/m3, and the average sediment concentration during wind-wave action is 0.08–0.18 kg/m3, which is about 3–6 times the value under light wind conditions. The critical wave height signaling remarkable changes of sediment concentration is 0.5 m. The results show that the suspended load sediment concentration is mainly influenced by wave-induced sediment suspension.

A continental shelf sedimentary record of Little Ice Age to modern glacial dynamics: Bering Glacier, Alaska

The Bering Glacier System is the world's largest surging temperate glacier with seven events occurring over the past century under a range of north Pacific climatic conditions. Onshore records reveal changes in glacial termini positions and evidence of late Holocene glacial advances, but the Little Ice Age (LIA) record of potential glacial surging and associated flooding has not been examined. A 13.6m-long jumbo core collected on the adjacent continental shelf reveals a 600-yr-long record of sedimentation associated with changing glacifluvial discharge. The chronology is based on 210Pb geochronology and five radiocarbon dates, and the core can be separated into three distinct lithologic units based on the examination of X-radiographs and physical properties: (1) an uppermost unit dating from ∼125calyrBP to the present characterized by bioturbated mud interbedded with laminated, thick (5–20cm) low-bulk density clay-rich beds; (2) a middle unit dating from ∼120–400calyrBP that includes numerous interlaminated-to-interbedded low- and high-bulk density beds with infrequent evidence of bioturbation; thick laminated clay-rich beds are rare; (3) a lowermost unit that predates ∼400calyrBP and is composed of rare laminated beds grading down into mottled to massive mud. In each of these units, the laminated lithofacies from this mid-shelf location indicates both flood deposition and likely sediment transport in the wave-current bottom-boundary layer. The thick low-density, clay-rich beds in the uppermost unit correlate with historic outburst floods associated with known surge events. Based on previous terrestrial studies, the terminus was at its Holocene Neoglacial maximum extent close to the modern coastline at some point in the middle to late stages of the LIA in southern Alaska (100–350calyrBP). During the LIA, preservation of bioturbated intervals is rare while laminated intervals are common. This style of interbedding indicates frequent (<10yr recurrence interval) event-scale mud deposition, suggesting that frequent summer flooding and redistribution by winter storms were more prevalent during the LIA rather than the outburst flooding typical of the past century. Rare event-scale bedding indicative of outburst flooding and possible surge events is found within the middle unit, and may correspond to periods with similar climatic trends as in the 20th century. The infrequent deposition of event layers in the lowermost unit could be attributed to the less frequent flooding and/or enhanced diversion of glacial drainage to the eastern terminus instead of present day Seal River. The thickness and depositional frequency of event-scale bedding can be related to Gulf of Alaska tree-ring proxy temperature reconstructions, where more numerous event bed formation occurs when there are more frequent, higher-amplitude temperature excursions. These frequent fluctuations may have prevented the decadal-long periods of positive mass balance required to enable numerous surge events during this period.

A 3D SEDIMENT TRANSPORT MODEL FOR COMBINED WAVE-CURRENT FLOWS

Accurate prediction of current velocity and bottom shear stress, which both can be significantly influenced by wind waves, is essential for sediment transport predictions in the coastal environment. Consequently wind-wave effects must be taken into account in a numerical sediment transport model for application in coastal waters. In the present study, elements of a large-scale 3D numerical coastal circulation and sediment transport model are developed to predict net, i.e. the wave-period-averaged, sediment transport rates. The sediment transport components considered are (i) bed-load transport; (ii) mean suspended load sediment transport within the wave boundary layer, which is obtained from an analytical solution; and (iii) suspended load sediment transport above the wave bottom boundary layer, which is obtained from a numerical model. In all model components wind wave effects are accounted for through simple analytical models. Thus, the roughness prescribed for the hydrodynamic part of the numerical coastal circulation model is the apparent roughness, i.e. the roughness experienced by a slowly varying current in the presence of waves. Similarly, the reference concentration specified for the sediment transport part of the numerical model is obtained from analytical solutions for suspended sediment concentrations within the combined wave-current bottom boundary layer. Stratification effects caused by suspended sediment are included in the large-scale numerical sediment transport model. Results of idealized tests suggest that wind wave effects can be pronounced, e.g. in some typical coastal scenarios sediment can only be mobilized when wind waves are present and accounted for. It is also shown that stratification can significantly affect suspended sediment transport rates of fine sediments.

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.

Modelling of Suspended Sediment Transport in Coastal Areas Under Waves and Currents

A coupled hydrodynamic/suspended sediment transport model including waves and currents was developed. Four models, a nested grid circulation model (2-D and 3-D), a refraction-diffraction-dissipation wave propagation model, a wave-current bottom boundary layer model and a quasi-3-D suspended sediment concentration (SSC) model are integrated. The model system has been applied to Cleveland Bay, Australia to study SSC in January and March 1993 when two high SSC events (>100mg l−1) were observed during an intensive field monitoring programme. The swell has been found to be the dominant suspension mechanism and thus an important factor of high SSC in the bay. The model was able to predict the high SSC events in January and March 1993. A net deposition rate in Cleveland Bay was estimated to be about 0·1mm year−1, a figure consistent with geological evidence.

SEDTRANS92: A sediment transport model for continental shelves

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.

Bottom stress estimates and sand transport on northern California inner continental shelf

Measurements of velocities and light transmission in the bottom boundary layer on the continental shelf off northern California demonstrate the importance of storms in the transport of sediment along the coast and offshore in this region. Time-series estimates of bottom stress obtained from a combined wave-current bottom boundary layer model in which wave and current measurements from the Geoprobe tripod were used as input show high stress values of 10 dynes cm −2 during two distinct storm events in early February and early March, 1991. These stresses induce significant offshore sediment transport, achieving maximum values of about 0.5 g cm −1 s −1. The net transport over the entire measurement period from 30 January 1991 to 13 March 1991 was along the coast toward the north and offshore. This transport pattern explains slow migration of low amplitude, broad crescentic dunes along and across this portion of the inner continental shelf.