1,721,043 research outputs found
Review of: Coastal Defenses Processes, Problems and Solutions (by P.W. French, London, Routledge, 2001, 389pp
No full textStructure, stability, and transformation of contaminated lacustrine surface fine-grained laminae
No full textThe effect of sand movement on a cohesive substrate
No full textFlume experiments investigated the effect of mobile sand on the erosion of cohesive beds. The fluid-induced stress alone was not enough to cause erosion, and sand motion as bed load was needed. Erosion rates and suspended sediment concentration were found to increase with increasing sand transport and to decrease with increasing median grain size. The erosion rate was found to be at a maximum during saltation, intermediate during creep, and lowest during suspension
Natural coastal mechanisms - flume and field experiments on links between biology, sediments and flow. Preface
No full textThe impact of mobile disarticulated shells of Cerastoderma edulis on the abrasion of a cohesive substrate
No full textAn annular laboratory flume was used to investigate the effect of mobile cockle shells on the erosion of a cohesive sediment bed. A standard clay bed was created and shells of differing sizes placed upon it. Flow in the flume was increased in increments and the onset of motion and transport pattern of the cockles was monitored. The release of bed material to the water column was monitored and compared to releases in the absence of shells (due only to the flow). The shells moved in traction; firstly as surface load (dragging) and then by rolling. The motion of the shells was found to be directly related to their motion settling rate in still water. The fluid induced stresses were unable to cause any detectable erosion of the bed. The addition of even single shells induced significant erosion. The erosion was found to be the result of abrasion rather than corrosion, as the shells never entered into saltation. There was a linear increase in erosion rate with increasing shell size, and an exponential increase in the suspended sediment concentration with time. The presence of large numbers of cockle shells in areas such as Southampton water and Lymington have suggested that the processes investigated here may be responsible for the erosion regimes in these areas
The influence of vegetation on turbulence and flow velocities in European salt-marshes
No full textFlow hindrance by salt-marsh vegetation is manifested in the structure of the tidal current; it has a significant impact on sediment transport and it has been related to increased sediment accretion. The flow characteristics in three different vegetation types (Spartina maritima, Sp. anglica and Salicornia sp./Suaeda maritima) were measured on three salt-marshes in Portugal and England. These in situ measurements differ from laboratory flume experiments with 'clean' vegetation by the complexity of natural canopies. Skimming flow develops above the Spartina canopy when the vegetation is fully submerged. In this situation, a low turbulence zone with nearly constant velocity in the denser canopy is separated from the skimming flow above by an interface characterized by high Reynolds stresses. In the low turbulence zone, a positive relationship exists between turbulence intensity and shoot density, which is due to wake turbulence generated locally in the canopy. The rate of particle settling should be increased in that zone. The lower limit of skimming flow is best predicted by the height within the canopy that includes 85% of the biomass. For emergent Spartina canopies and the short Salicornia/Suaeda marsh, the maximal velocity-gradient is shifted upwards compared to a standard boundary layer over bare sediment and the turbulence is attenuated near the bed, but to a lesser extent than for fully submerged Spartina canopies. A turbulence reduction near the bed was observed in all measured profiles; that should enhance sediment deposition and protects the bed against subsequent erosion
A comparison between fluid shear stress reduction by halophytic plants in Venice Lagoon, Italy and Rustico Bay, Canada - analyses of in situ measurements
No full textA series of in situ experiments on bed stability were carried out at three sites across Venice Lagoon using the benthic annular flume—Sea Carousel. Turbulence measurements were made at a range of flow speeds over different vegetated beds as well as ‘smooth’ muddy beds. The drag induced by the various bed types was estimated using flow deceleration. Bed shear stress was also estimated using three methods, and the results were compared with the bed shear stress as determined over a smooth bed in a laboratory equivalent of Sea Carousel—Lab Carousel. The stress was found to increase with increasing bed roughness and with the addition of vegetation in the form of the sea grasses Cymodocea nodosa and Zostera noltii. The stress was also found to be affected by the bending of the sea grass blades under flow velocities exceeding 0.4 m s?1, the sea grasses became flattened and the shear stress was found to decrease to produce skimming flow. It was concluded that the presence of sea grasses decreases erosion due to (1) stress reduction and (2) stabilization of the bed, thus reduction of the distribution of sea grass beds in Venice Lagoon will likely enhance bed erosion and hence habitat destruction. Stress was also reduced by an increase in levels of turbidity level in the water column
The stability of a remediated bed in Hamilton Harbour, Lake Ontario, Canada
No full textIn situ measurements of lakebed sediment erodibility were made on three sites in Hamilton Harbour, Lake Ontario, using the benthic flume Sea Carousel. Three methods of estimating the surface erosion threshold (&tgr;c(0)) from a Carousel time series were evaluated: the first method fits measures of bed strength to eroded depth (the failure envelope) and evaluates threshold as the surface intercept; the second method regresses mean erosion rate (Em) with bed shear stress and solves for the floc erosion rate (Ef) to derive the threshold for Em = Ef = 1 × 10-5kg m-2 s-1; the third method extrapolates a regression of suspended sediment concentration (S) and fluid transmitted bed shear stress (&tgr;0) to ambient concentrations. The first field site was undisturbed (C) and acted as a control; the second (W) was disturbed through ploughing and water injection as part of lakebed treatment, whereas the third site (OIP) was disturbed and injected with an oxidant used for remediation of contaminated sediment. The main objectives of this study were: (1) to evaluate the three different methods of deriving erosion threshold; (2) to compare the physical behaviour of lacustrine sediments with their marine estuarine counterparts; and (3) to examine the effects of ploughing and chemical treatment of contaminated sediment on bed stability. Five deployments of Sea Carousel were carried out at the control site. Mean erosion thresholds for the three methods were: &tgr;c(0) = 0·5 (±0·06), 0·27 (±0·01) and 0·34 (±0·03) Pa respectively. Method 1 overpredicted bed strength as it was insensitive to effects in the surface 1-2 mm, and the fit of the failure envelope was also highly subjective. Method 2 exhibited a wide scatter in the data (low correlation coefficients), and definition of the baseline erosion rate (Ef) is largely arbitrary in the literature. Method 3 yielded stable (high correlation coefficients), reproducible and objective results and is thus recommended for evaluation of the erosion threshold. The results of this method correlated well with sediment bulk density and followed the same trend as marine counterparts from widely varying sites. Mass settling rates, expressed as a decay constant, k, of S(t), were strongly related to the maximum turbidity at the onset of settling (Smax) and were also in continuity with marine counterparts. Thus, it appears that differences in salinity had little effect on mass settling rates in the examples presented, and that biological activity dominated any effects normally attributable to changes in salinity. Bedload transport of eroded aggregates (2-4 mm in diameter) took place by rolling below a mean tangential flow velocity (Uy) of 0·32 ms-1and by saltation at higher velocities. Mass transport as bedload was a maximum at Uy = 0·4 ms-1, although bedload never exceeded 1% of the suspended load. The proportion of material moving as bedload was greatest at the onset of erosion but decreased as flow competence increased. Given the low bulk density and strength of the lakebed sediment, the presence of a bedload component is notable. Bedload transport over eroding cohesive substrates should be greater in estuaries, where both sediment density and strength are usually higher. Significant differences between the ploughed and control sites were apparent in both the erosion rate and the friction coefficient (&phgr;), and suggest that bed recovery after disruption is rapid (< 24 h). &tgr;c(0) increased linearly with time after ploughing and recovered to the control mean value within 3 days. The friction coefficient was reduced to zero by ploughing (diagnostic of fluidization), but increased linearly with time, regaining control values within 6 days. No long-term reduction in bed strength due to remediation was apparent
Dewatering effects on the erodibility of newly deposited cohesive beds by unidirectional currents
Full text linkA series of laboratory experiments on cohesive sediments under inorganic conditions was undertaken in order to evaluate the impact of fluid bed shear stress on the build-up of bed resistance to erosion with time. The importance of small pressures due to flowing water to increase bed strength is presented. It is also shown that the susceptibility of a cohesive bed to changes in its erodibility is related to deposited bed thickness due to sediment disturbance caused by dewatering from the consolidating bed. Laboratory experiments that use beds deposited from suspension should thus report the thickness of the bed prior to resuspension.Fil: Gomez, Eduardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; ArgentinaFil: Amos, Carl L.. Southampton Oceanography Centre; Reino Unid
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