Ce Vincent

Storm sediment transport: observations from the British North Sea shelf

Measurements of wave heights, near-bed currents, bed shear stresses and suspended-sediment concentrations and fluxes from a severe storm are described. The data are from 25-m water depth on the British North Sea shelf. The current above the wave boundary layer was retarded by bed roughness that was dominated by wakes shed from saltating grains in the bedload layer and by wave-current interaction in the wave boundary layer. The bed roughness increased by two orders of magnitude immediately prior to overturning of the instrument installation by large waves; the cause for the increase is not clear but is consistent with the formation of large-scale bedforms at high transport stage. Backscatter data revealed wave resuspension of bed sediment, modulation of sediment concentration by wave groups, and advection of dense sediment clouds by the current. The vertical profiles of mean suspended-sediment concentration were, nevertheless, consistent with a simple turbulent-diffusion model that incorporates different diffusion scales within and above the wave boundary layer. A single measurement of the sediment reference concentration at the height of the storm yielded an estimate of 0.00025 for the entrainment parameter, γo, but at other times γo was O(10−3). The change in bed roughness might have caused the decrease in entrainment rate at the height of the storm. The erosion depth required to support the suspended-sediment load was a maximum of ∼1 cm, but was typically an order of magnitude smaller than that; suspension was the dominant transport mode. A wind-driven mean flow distorted the tidal ellipse and caused a net transport of sediment along- and off-shore; depth-integrated sediment transport during the storm was up to two orders of magnitude greater than prior to the storm under tidal currents. Wave-orbital currents directly transported a small, but not negligible, fraction of the total sediment load at 34 cm above the bed.

Beach response to shore-parallel breakwaters at Sea Palling, Norfolk, UK

Abstract The effects of four offshore breakwaters built between 1993 and 1995 at Sea Palling, Norfolk, UK, on beach morphology are discussed. The structures were built during the first phase of a multi-phase construction programme and are expected to provide long-term protection for Sea Palling and 6000xa0ha of low-lying land against tidal inundation through the provision of a wide beach in front of the existing seawall and dunes. The results presented here span all stages of breakwater construction during Phase 1. Beach level changes were monitored during the period from 1995 to 1999. A Geographical Information System (GIS) was used to calculate differential surfaces between surveys in order to investigate local patterns of accretion and erosion and to determine volume changes between consecutive surveys. Additional data collected by the Environment Agency since 1991 were analysed to investigate beach and bathymetry changes prior to the construction of the breakwaters. In association with periodic nourishment programmes the scheme has been locally successful in retaining recharged sediment on the beach but the extent of the salients prevents to a great extent the south-eastward littoral drift. As a result, effects on the beaches, both down- and up-drift of the breakwaters, have been considerable. The continuing increase in beach volume indicated that by January 1999 equilibrium had not been reached.

Phase relationships between fine sediment suspensions and tidal currents in coastal seas

[1]xa0In shelf and coastal waters, point measurements of fine suspended sediment concentrations commonly display phase and frequency differences with respect to local current forcing. Phase shifts are often attributed to tidal advection and slow-settling velocities of cohesive sediments. The phase relationship between tidal current speeds and near-bed mud concentrations obtained at a site off the east coast of England is investigated. A simple depth-averaged advection-diffusion model is introduced and used to explore the influence of relative changes in settling, advection, and local erosion on the temporal evolution of suspension events. The model includes two source terms: local resuspension in phase with current speed and advection of a horizontal gradient in phase with tidal displacement. Incorporating a settling (or sink) term introduces a negative or positive phase shift in the advection or resuspension term, respectively. The settling term accounts for the vertical structure of suspension and becomes important at slack water when the near-bed concentration, and thus deposition, rapidly increases. Model results are compared with suspended mud concentration data to help explain the observed phase relationships between concentration and current speed.

Examination of reference concentration under waves and currents on the inner shelf

[1]xa0We examine reference concentration using three different data sets of near-bed suspended sediment concentration observed under combined waves and currents. The data include observations made at 15 and 20 m depth off Dounreay, Scotland, UK, and observations obtained at 13 m depth off Duck, North Carolina, USA. These data accommodate different dynamic conditions (from wave-dominated conditions at Dounreay to wind-driven, current-dominated conditions at Duck) and sediment properties (median size of bed sediment ranging from 120 to 350 μm). Near-bed concentration profiles to elevations of about 80 cm were obtained using acoustic backscatter sensors with 1 cm resolution. The reference concentrations (Cr) at 1 cm were then evaluated by regressing the observed suspended sediment concentrations against a Rouse-type model. Bed shear stresses associated with each estimate of Cr were estimated using the wave-current interaction model of Grant and Madsen. Existing equations for reference concentration based on shear stress alone fail to accommodate all Cr estimates from different environments. We introduce a new empirical relationship between Cr and the product of Shields and inverse Rouse numbers. These dimensionless parameters represent the ratio of bed shear stress and submerged particle weight and the ratio of shear velocity and particle settling velocity, respectively. The new formula adjusts the amount of mobile sediment at the bed (related to the Shields number) to that available for suspension at the reference height (related to the inverse Rouse number). The new formula for reference concentration accommodates observations from different environments, suggesting that it may have wide applicability on sandy inner shelves.

Influence of wave groups on SSC patterns over vortex ripples

Abstract Estimates of spatial and temporal variations in suspended sand concentrations (SSC) made with a multi-transducer Acoustic Backscatter Sensor (ABS) under a repeated wave group over a mobile rippled bed in the wave research flume at the National Hydraulics Laboratory in Ottawa, Canada, reveal an number of complex and intriguing patterns. Ensemble averages of 8 nearly identical wave groups provided much more robust estimates of SSC and allowed a detailed examination of the wave group effects. The largest SSC near the bed (

Measuring suspended sand concentration using acoustic backscatter: a critical look at the errors and uncertainties

Abstract Suspended sand concentrations in the near-bed layer of the sea can be estimated using acoustic backscatter systems (ABS) and hence, when combined with velocity, sand transport. ABS measurements are now relatively routine and widely regarded as one of the best tools to help to measure sand transport in coastal seas and estuaries. Potentially the ABS can be used to give estimates of both concentration and sediment size with a vertical resolution of 5–10 mm and a temporal resolution of fractions of a second, but in practice there are many practical limitations to what can be achieved in the field. Often these uncertainties are overlooked or ignored because of the difficulty in estimating their magnitude or of making measurements that are any better. The major limitations and sources of errors in the ABS are discussed, including the influence of sediment size and shape. In moderately to well-sorted sandy environments with appropriate calibration, acoustic estimates of mass concentration should be within ± 30% of the true value.

Designing offshore breakwaters using empirical relationships : a case study from Norfolk, United Kingdom

Abstract This paper examines the applicability of several empirical relationships for the performance of shore-parallel breakwaters at Sea Palling in Norfolk, UK. The research presented here compliments the work of Axe et al. (1996), who applied the models of Pope and Dean (1986), Suh and Dalrymple (1987), Ahrens and Cox (1990) and McCormick (1993) and compared their predictions with observations of beach response to the Elmer offshore breakwater scheme in West Sussex, UK. When these models were applied to the Sea Palling breakwaters, they generally revealed large inconsistencies in the predicted beach response. It is suspected that the inability of these methods to correctly predict beach response at this location is most likely caused by the overriding influence of factors such as wave transmission, longshore drift and a large tidal range, over geometrical relationships in the scheme design.

Spectral estimates of bed shear stress using suspended‐sediment concentrations in a wave‐current boundary layer

[1]xa0High-resolution time series of suspended-sediment profiles have been obtained using an acoustic backscatter system at an inner shelf site (North Carolina) where flows are dominated by wind-driven currents and waves. We analyzed the spatial and temporal structure of near-bed turbulence in particle-transporting flows and scalar-like fluctuations of suspended-sediment concentrations. An important element of our analysis is a new inertial dissipation method for passive tracers to estimate the shear stress acting on the seabed, using the spectral properties of suspended sediment concentrations observed by acoustic backscatter sensors. In flows that provide adequate separation of the scales of turbulence production and dissipation, a sufficiently thick constant stress wall layer, and significant sediment suspension, frequency (or associated wave number) spectra of near-bed sediment concentration exhibit a −5/3 slope in the inertial subrange that spans frequencies of order 1 Hz. This observation suggests that the suspended sediment is effectively a passive tracer of turbulent fluid motions. Inversion of the relevant, Kolmogorov scaling equations yields estimates of the shear velocity that agree reasonably well with other, independent and widely used measures. High- and low-frequency limits on application of the inertial dissipation method to sediment concentration are related to the inertial response time of sediment particles and the sediment settling timescale. We propose that, in future applications, the inertial dissipation method for passive tracers can be used to estimate either the shear velocity, effective settling velocity of suspended sediment (or equivalent particle size) or dynamic bed roughness if two of these three quantities are independently known.

Large-scale laboratory modelling of suspended sand concentration fluctuations under irregular waves

1. Coastal Research Centre FZK, Merkurstrasse 11, 30419 Hannover. Germany, gruene@fzk.uni-hannover.de 2. The Southern Branch of the P.P.Shirshov Institute of Oceanology, Russian Academy of Sciences. Gelendzhik-7, 353467 Russia. kosyan@coastdyn.ru 3. Coastal Research Centre FZK, Merkurstrasse 11, 30419 Hannover. Germany, office@fzk.uni-hannover.de 4. The Southern Branch of the P.P.Shirshov Institute of Oceanology, Russian Academy of Sciences. Gelendzhik-7, 353467 Russia. podymov@coastdyn.ru 5. Coastal Research Centre FZK, Merkurstrasse 11, 30419 Hannover. Germany, sk@fzk.uni-hannover.de 6. School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. c.vincent@uea.ac.uk

Modelling of the vertical distribution of suspended sediment concentration under waves with a group structure

Results of the modelling of the vertical distribution of suspended sediment concentration under the influence of waves with a pronounced group structure are presented in this paper. The reliability of the model has been checked against laboratory data from the SISTEX99 experiment. Fluctuations of suspended sediment concentration, calculated by the model, correlate very well with the experimental data and reproduce the form, number and duration of concentration peaks rather well.