Turbidity Events Research Articles

Numerical simulation on the evolution of sediment waves caused by turbidity currents

Interest in the forming mechanism of sediment waves increases recently because of its significance on submarine engineering, sedimentary dynamics and hydrocarbon reservoir prediction in deep water. In this paper, the time-averaged continuity equations and Reynolds-averaged Navier-Stokes equations are applied in the numerical simulation of fluid dynamics. The modeling results are used to illuminate the effects of topography on turbidity current and explore the origin of submarine sediment waves. The research results show that (1) deposition occurs firstly at the lower ramp due to the deceleration of fluid, increase of density, loss of flow capacity and longer duration of flow passage; (2) density increase at the upslope due to the local jam results in velocity decrease and pressure increase; (3) sediment waves begin to be formed and migrated toward upstream in an area far away from the source with increase of the turbidity events; (4) deposition becomes more slowly with decrease of grain sizes, but the shape and sequences of these deposits are controlled by topography, not grain size.

Particle Characterization and Settling Velocities for a Water Supply Reservoir during a Turbidity Event

Characterization of the particle population for a location in a water supply reservoir, Kensico Reservoir, N.Y., is documented for a high turbidity event, from its onset, through alum treatment and its waning. Supporting in situ measurements included the beam attenuation coefficient at 670nm(c670) and 660nm(c660) [surrogates of turbidity (Tn)], particle concentrations (N) and size distributions (PSDs), and size class specific settling velocities (SVs). Laboratory measurements included chemical and morphometric analyses of individual particles, and routine measurements of Tn. The turbidity is shown to be primarily derived from clay minerals, mostly in the size range of 1.5–6μm. An initial high c670 level (40m−1;Tn∼100NTU) decreased sevenfold in less than 1week in response to alum treatment that largely eliminated the particle size classes responsible for the elevated turbidity. Successful SV experiments, made using a laser in situ scattering and transmissometry (LISST) instrument, for seven particle size classes in the range of 1.25–129μm yielded SV values of 0.17–69.4mday−1. Size classes larger than ∼5μm settled much slower than Stokes law predictions, before alum treatment, indicating that these classes existed as porous flocs or aggregates. Decreases in SVs following treatment suggest changes in floc character consistent with increased porosity. In situ measurements of c670, N, PSDs, and SVs can contribute to the development and testing of a multiple particle size class model to simulate fate, transport, and impacts of suspended particles.

Remote sensing of particle backscattering in Chesapeake Bay: A 6-year SeaWiFS retrospective view

Traditional field techniques to monitor water quality in large estuaries, such as boat-based surveys and autonomous moored sensors, generally provide limited spatial coverage. Satellite imagery potentially can be used to address both of these limitations. Here, we show that satellite-based observations are useful for inferring total-suspended-solids (TSS) concentrations in estuarine areas. A spectra-matching optimization algorithm was used to estimate the particle backscattering coefficient at 400 nm, b bp(400), in Chesapeake Bay from Sea-viewing Wide-Field-of-view Sensor (SeaWiFS) satellite imagery. These estimated values of b bp(400) were compared to in situ measurements of TSS for the study period of September 1997–December 2003. Contemporaneous SeaWiFS b bp(400) values and TSS concentrations were positively correlated ( N = 340, r 2 = 0.4, P < 0.0005), and the satellite-derived b bp(400) values served as a reasonable first-order approximation for synoptically mapping TSS. Overall, large-scale patterns of SeaWiFS b bp(400) appeared to be consistent with expectations based on field observations and historical reports of TSS. Monthly averages indicated that SeaWiFS b bp(400) was typically largest in winter (>0.049 m −1, November–February) and smallest in summer (<0.031 m −1, June–August), regardless of the amount of riverine discharge to the bay. The study period also included Hurricanes Floyd and Isabel, which caused large-scale turbidity events and changes in the water quality of the bay. These results demonstrate that this technique can provide frequent synoptic assessments of suspended solids concentrations in Chesapeake Bay and other coastal regions.

Structural and functional aspects of treated mine water and aquaculture effluent streams

Multiple effluent streams may flow into a single receiving stream making it difficult to understand the effects of a single effluent source or the interactions between effluents. In this study, we examined benthic community structure and function in an effluent stream formed by the release of treated mine water, in treated mine water that had been used as a water source for flow-through aquaculture and below the confluence of the two. This study demonstrated that macroinvertebrate communities, while taxonomically simple, developed in treated mine water. The addition of aquaculture effluent to the treated mine water allowed colonization by additional taxa, increased leaf decomposition rates and may have conferred resistance to a turbidity event. Ecosystem responses were mediated by the surrounding terrestrial environment. In shaded conditions macroinvertebrate densities could be as much as 10× higher and the taxa were dominated by simuliids and chironomids, while in open conditions filamentous algae flourished and the taxa were dominated by hydroptilids and chironomids. Consequently, consideration of stream processes that promote preferred ecological processes, such as macroinvertebrate production and organic matter processing, when siting effluent streams may reduce impacts on receiving streams.

Atmospheric aerosols during the 2003 heat wave in southeastern Spain I: Spectral optical depth

During the first half of August 2003 the Iberian Peninsula (IP) and a great part of Western Europe suffered a severe heat wave. This event was associated with a very robust and persistent high-pressure system over Western Europe that blocked the flow of rain-bearing low-pressure systems at 700 and 500 mb. Using sun-photometric data obtained during August 2003 we evidenced an exceptionally high turbidity event over Granada (37.16°N, 3.60°W, 680 m a.s.l.) in southeastern Spain from 1 to 15 August. Under heat wave conditions we found δ a(440 nm) in the range 0.12–0.70 with an average value of 0.35±0.10 whereas in the second half of the month δ a(440 nm) ranged between 0.04 and 0.30 with an average value of 0.15±0.06, with similar changes for other wavelengths. This long lasting turbidity event was also observed at other AERONET stations in the IP, El Arenosillo (37.11°N, 6.71°W, 17 m a.s.l.) and Évora (38.57°N, 7.91°W, 293 m a.s.l.), suggesting that this pronounced change in the atmospheric turbidity was not due to local causes. At a regional level the atmospheric aerosol is influenced by the synoptic situation and thus the special characteristics associated to the heat wave development can have strongly affected the regional atmospheric aerosol properties. The Ångström exponent, α, presented a rather large range of values, evidencing the high variety of aerosol particles that affected the area. Thus, during the heat wave α varied between 0.3 and 1.7 with an average value of 1.2±0.3 whereas after the heat wave it varied between 0.4 and 1.4 with an average value of 0.9±0.3. Considering the relationship between the Ångström exponent and the aerosol size distribution, it is evident that there was an increase in the contribution of fine particles during the heat wave period related to the typical summer conditions. This increase appears to be produced mainly by intense forest fires in southern Europe and by European-Mediterranean air masses influence. Trajectories ending at Granada (on 1–15 August) show that in all cases air masses overpass active forest fires in southern Europe, and that our study area was predominantly influenced by European-Mediterranean air masses. Also, in some cases air masses arrived from Africa. The occurrence of additional Saharan dust events after the heat wave highlighted changes in the features of these aerosol particles when combined with an important load of urban industrial particles and biomass burning aerosols. During dust events occurred in the heat wave period the average values of δ a(440 nm) and α ranged from 0.32 to 0.41 and from 0.8 to 1.0, respectively, while during those occurred after the heat wave event δ a(440 nm) and α ranged from 0.13 to 0.19 and from 0.4 to 0.5, respectively.

Hazard identification framework for water utilities with application to the city of Xalapa, Mexico

: There is a growing need for a comprehensive approach for assessing drinking water systems that enables managers and engineers to quickly address critical hazards that impact their water supply systems and plan for system improvements. A hazard identification framework for water utilities is developed and applied to the municipal water supply system at Xalapa, Veracruz, Mexico, operated by the Comisión Municipal de Agua Potable y Saneamiento de Xalapa (CMAS Xalapa). The framework is designed to aid in the analysis of a water supply system from catchment to tap and consists of four components: system characterization, identification of hazards, assessment of the impacts of the major hazards, and determination of priority recommendations for system improvements. When the framework is applied to the CMAS Xalapa system, a number of significant hazardous events are identified including elevated turbidity events and high pressure surcharges. Recommendations for system improvements include increased data collection and analysis and enhanced management of control measures for mitigating the impacts of the hazards.

Turbidity dynamics during spring storm events in an urban headwater river system: The Upper Tame, West Midlands, UK

Turbidity is an important water quality variable, through its relation to light suppression, BOD impact, sediment-associated contaminant transport, and suspended sediment effects on organisms and habitats. Yet few published field investigations of wet-weather turbidity dynamics, through several individual and sequenced rainstorms in extremely urbanised headwater basins, have emerged. This paper aims to address this gap through a turbidity analysis of multiple storm events in spring 2001 in an urban headwater basin (57 km 2) of the River Tame, central England, the most urbanised basin for its size in the UK (∼ 42%). Data were collected at 15-min frequency at automated monitoring stations for rainfall, streamflow and six water quality variables (turbidity, EC, temperature, DO, pH, ammonia). Disturbance experiments also allowed estimates of bed sediment storage to be obtained. Six important and unusual features of the storm event turbidity response were apparent: (1) sluggish early turbidity response, followed by a turbidity ‘rush’; (2) quasi-coincident flow and turbidity peaks; (3) anti-clockwise hysteresis in the discharge–turbidity relationship on all but one event, resulting from Falling- LImb Turbidity Extensions (FLITEs); (4) increases in peak turbidity levels through storm sequences; (5) initial micro-pulses (IMP) in turbidity; and (6) secondary turbidity peaks (STP) or ‘turbidity shoulders’ (TS). These features provided very little evidence of a true ‘first-flush’ effect: instead, substantial suspended solids transport continued right through the flow recessions, and little storm-event sediment exhaustion was evident. A new, dimensionless, hysteresis index, HI mid, is developed to quantify the magnitude and direction of hysteresis in a simple, clear, direct and intuitive manner. This allowed the degree of departure from the classic ‘first-flush’, clockwise hysteresis models to be assessed. Of the 15 turbidity events considered, 10 coincided with ammonia spikes of up to 6.25 mg l − 1 at Water Orton (the downstream station): this suggests that spills from combined sewer overflows (CSO) or waste water treatment works (WwTWs) are significant in the throughput of turbid waters here. Substantial ammonia peaks related most strongly to total storm rainfall receipt, of four rainfall variables considered, and significant ammonia peaks were generated even from low-magnitude storms (rainfall totals < 4 mm), indicating that spills are a frequent occurrence. Local bed sediment stores appear to be limited, suggesting that other distal sediment sources, such as road networks and old mineworkings are possibly more important. Biofilms may also play a part in delaying sediment release until late in the hydrograph, and in suppressing late spring turbidity levels. Existing first-flush models appear to be an oversimplification here. Such urban headwater basin responses can provide useful insights into the generation of contaminant waves, and offer vital early-warning systems for pollution events propagating downstream.

Suspended solids transport: an analysis based on turbidity measurements and event based fully calibrated hydrodynamic models

Modelling suspended solids transport is a key issue for predicting the pollution load discharged by CSOs. Nonetheless, there is still much debate on the main drivers for suspended solids transport and on the modelling approach to be adopted. Current sewer models provide suspended solids transport models. These models, however, rely upon erosion-deposition criteria developed in fluvial environments, therewith oversimplifying the sewer sediment characteristics. Consequently, the performance of these models is poor from a theoretical point of view. To get an improved understanding of the temporal and spatial variations in suspended solids transport, a measuring network was installed in the sewer system of Loenen in conjunction with a hydraulic measuring network from June through December 2001. During the measuring period, 15 storm events rendered high-quality data on both the hydraulics and the turbidity. For each storm event, a hydrodynamic model was calibrated using the Clemens' method. The conclusion of the paper is that modelling of suspended solids transport has been and will be one of the challenges in the field of urban drainage modelling. A direct relation of either shear stress or flow velocity with turbidity could not be found, likely because of the time varying characteristics of the suspended solids.

A temporal investigation of grazer dynamics, nutrients, seagrass leaf productivity, and epiphyte standing stock

An investigation of seagrass-epiphyte controlling factors was conducted within aThalassia testudinum meadow in Florida Bay from March 2000 to April 2001. Univariate and multivariate analyses were performed using water column nutrient concentrations, temperature, salinity, and turbidity, and gastropod grazer abundances, seagrass leaf area index, and leaf turnover rate data to explain the variation in total epiphyte standing stock, epiphyte chlorophylla, and epiphyte autotrophic index. Turbidity was positively correlated with total epiphyte standing stock and accounted for the most variation. Observations of adhered sediment onT. testudinum leaves and the combination, of increased total epiphyte standing stocks and low autotrophic indices observed in February and April 2001 suggest that the settling of resuspended sediments following turbidity events is one of the temporal mechanisms for increased epiphyte accumulation. Total epiphyte standing stock was also negatively correlated with the abundance of a robust gastropod grazer community dominated byTurbo castanea, Tegula fasciata, andModulus modulus. Distinct temporal size cohorts ofT. castanea andT. fasciata throughout the study period suggest recruitment in spring and an annual lifespan. Nutrient concentrations can also account for some of the temporal variation in total epiphyte standing stock, epiphyte chlorophylla, and autotrophic index. The low variation ofT. testudinum leaf turnover rates was unable to account for any of the variation in the epiphyte parameters.

Congo River signature and deep circulation in the eastern Guinea Basin

Cruises since 1976, have allowed identification of oxygen and silicate anomalies within the Guinea Basin at around 4000 m depth, particularly above the Congo River fan. The EQUALANT cruise in 2000 was an opportunity to revisit this area and to study such anomalies through hydrological and chemical analyses as well as current and turbidity measurements. The main goal was to determine the possible origins, the distribution and the fate of these signals and their spreading along the African continent. The shape of the oxygen, nutrient and turbidity profiles was the first evidence of this phenomenon and an observation of a silicate–oxygen anti-correlation indicates regeneration processes associated with recently deposited particulate material, carried to depth along the Congo channel. This is a special aspect of the morphology of this river. Turbidity events are observed to be dispersed offshore within the ocean and along the slope at around 4000 m depth. While previous studies have shown southward propagation of the Congo signature, this study shows it to occur as far as 7°W–5°N, off Ivory Coast, near the 4000 m isobath, therefore demonstrating a northward propagation in this depth range.

Impact of raw water turbidity fluctuations on drinking water quality in a distribution system

Turbidity is a widely used parameter around the world for describing drinking water quality. Sometimes, turbidity at water treatment plant outlets may reach high values during short periods of time, and this is acceptable according to some current drinking water regulations. In this study, the quantity and nature (chemical and microbiological) of suspended matter, which may travel throughout a distribution system (DS) during turbid events — affecting both raw water and water treatment — were evaluated. Treated water included filtration with no coagulant addition. During turbid events, the concentration of suspended particles increased in treated water, and a similar increase (quantity and nature) was observed throughout the DS. Bacterial indicators of contamination (total and fecal coliforms, enteroccocci, spores of Clostridium perfringens) were not found in either treated water nor in the DS during turbid events. Nevertheless, a higher bacterial aerobic spore concentration was associated with turbid events for raw, treated, and distributed water, therefore suggesting the potential passage of pathogens, if present in raw waters. Cultivable bacteria concentrations remained low in treated and distributed water regardless of the turbidity. These results emphasize the need to carefully monitor raw and treated water quality for utilities using "high quality" water resources with limited treatment barriers, especially when such water resources are affected by even slight turbidity variations. Key words: aerobic spore-forming bacteria, distribution system, drinking water, filtration, turbidity, suspended particles, water quality.

A decadal record of underflows from a coastal river into the deep sea

Four periods of extreme turbidity were measured in Monterey Submarine Canyon over the past 12 yr. These turbid events occurred simultaneously with the four largest flood events of the nearby Salinas River. They filled the canyon with fresher, warmer, and, apparently, buoyant water that extended to depths below 1 km. The low-salinity signature must reflect underflow of the Salinas River plume that was driven to depth by negative buoyancy contributed from the suspended sediment. A flux estimate for the 1995 event suggests that one-half, or more, of the river's suspended sediment load was carried down the canyon in the underflow. Underflows may contribute significantly to the carbon budget of the continental slope.

Assessment of the relative importance of major sediment‐transport mechanisms in the central Great Barrier Reef lagoon

The delivery, flux and fate of terrigenous sediment entering the Great Barrier Reef lagoon has been a focus of recent studies and represents an ongoing environmental concern. Wave‐induced bed stress is the most significant mechanism of sediment resuspension in the Great Barrier Reef, and field data and mathematical modelling indicates that the combined effects of short‐period wind waves, longer period swell waves, and tidal and wind‐driven currents can often exceed the critical bed stress for resuspension. Suspended‐sediment concentrations at 20 m water depth indicate resuspension seldom occurs on the middle shelf under normal wave conditions. Non‐cyclonic turbidity events are generally confined to the inner shelf. The wave climate in the southern sector of the central Great Barrier Reef lagoon is the most erosive, and resuspension of outer shelf sediments was hindcast for recorded cyclones. Wind‐driven, longshore currents are fundamental to the northward movement of sediment, and the annual northward mass flux from embayments undergoing resuspension in the Burdekin region is estimated to be one order of magnitude larger than the mass of sediment introduced by a moderate flood plume. Strong onshore winds are estimated to generate significant three‐dimensional bottom return currents on approximately 30–70 days per year, forming a potentially significant offshore‐directed sediment flux during high suspended‐sediment concentration events on the inner shelf.

Seagrass survival during pulsed turbidity events: the effects of light deprivation on the seagrasses Halodule pinifolia and Halophila ovalis

Pulsed turbidity events caused by factors such as flooding rivers have the potential to seriously impact seagrass communities by depriving the plants of all available light. The effects of light deprivation was investigated on the survival, morphology and physiology of the tropical seagrasses Halodule pinifolia and Halophila ovalis growing in the South-East Gulf of Carpentaria, Australia, a region where pulsed flood events are common. Additionally, physiological and morphological responses to light availability along natural gradients were examined. Responses to both experimental and natural light gradients were investigated for their potential use as indicators of impending seagrass loss during pulsed turbidity events. H. pinifolia was deprived of light for 80 days using in situ shade screens and the following parameters measured at three depths and under the shade screens: biomass, shoot density, canopy height, amino acid content, chlorophyll content, δ 13C signature, %C and sugar concentration. The quantity of light was extremely variable, with mean daily irradiances between 9–12 mol photons m −2 day −1, and a range of 0.05–42 mol photons m −2 day −1. H. pinifolia leaf amino acid content increased with increased water depth (from 8 to 18 μmol g fresh wt.), chlorophyll a to b ratio decreased (from 2.4 to 2.1) and δ 13C values became more negative (from −9 to −12). H. ovalis displayed little tolerance to light deprivation, with plant death occurring after 38 days in the dark. H. pinifolia showed a high degree of tolerance to light deprivation with no biomass loss before day 38 and complete die-off predicted after 100 days. Shoot density, biomass and canopy height all declined after 38 days. Physiological parameters that responded significantly to the light deprivation were the amino acids which increased (from 20 to 80 μmol g fresh wt.), the chlorophyll a to b ratio which decreased (from 2.5 to 2.1) and the values which became more negative (from −9 to −10). Changes in leaf physiology (e.g. amino acid content, chlorophyll content and δ 13C) occurred before morphological changes (e.g. biomass, shoot density, canopy height) or die-off, and were thus considered to be potential indicators of impending seagrass die-off during light deprivation. In conclusion, only long duration (>38 days) pulsed turbidity events would have a detrimental impact on H. pinifolia growing in the Gulf of Carpentaria and that by assessing specific physiological responses, seagrass loss during pulsed turbidity events can predicted.

Turbidity and Particle Signatures Imparted by Runoff Events in Ashokan Reservoir, NY

ABSTRACT The occurrence, temporal and spatial patterns, and origins of turbidity events, and their linkage to runoff events, are documented for a water supply impoundment with two separated basins, Ashokan Reservoir, NY. The analysis is supported by a comprehensive 6-week study of the major inflow and the reservoir during the summer of 1996, that captured the effects of a single runoff event, and turbidity (Tn) measurements made in the reservoir and the water supply intakes for the entire year. Measurements supporting the short-term study include: temperature, specific conductance, beam attenuation coefficient, electronic particle counts, Secchi disc transparency, Tn, up- and downwelling cosine irradiance, chlorophyll, microscopy-based individual particle size and chemistry, and total suspended solids (TSS) on sediment trap collections. The external load of mostly quartz and clay particles delivered by die principal tributary, as an interflow, during the summer runoff event imparted distinct signatures in Tn and deposition within the epilimnion of the receiving basin; the other basin remained unaffected by comparison. The deposition rate of TSS and Tn increased in the receiving basin in response to die runoff-based loading; decreasing gradients in both parameters were observed within this basin downstream of the entry point of die tributary. More than 85% of Tn in die receiving basin during die 6-week study is attributed to inorganic tripton particles of terrigenous origins. Analysis of the longer-term data indicates elevated Tn values (maximum of 150 NTU) occur routinely in die receiving basin following runoff events, and diat diis effect extends to die other basin, including the water supply intake(s), during intervals other than summer stratification.

Effects of suspended, diesel-contaminated sediment on feeding rate in the darter goby, Gobionellus boleosoma (Teleostei: Gobiidae)

The darter goby, Gobionellus boleosoma, a bottom-feeding gobiid fish, was used as a meiofaunal predator in laboratory experiments designed to determine the effect of suspended diesel-contaminated sediment on feeding rate. Estuarine sediments are sinks for hydrophobic contaminants, and contaminant exposure from suspended sediment seems likely in estuarine mudflats. The darter goby is a small estuarine fish that lives in shallow mudflats surrounding Spartina alterniflora marshes, feeding primarily on meio- and small macrofauna. We first showed that darter gobies are effective feeders on suspended meiobenthic copepods and that this feeding was not affected by suspended sediment loads as high as 2000 mg l −1. Harpacticoid copepods then were offered as ad libitum prey to gobies after exposure to either contaminated or non-contaminated sediments via a single turbidity event 24 h prior to feeding. Based on gut content analysis, goby standard length influenced feeding rate as larger gobies ingested more meiobenthic copepods in the 1-h experiment. Diesel-fuel contamination reduced feeding rate by 50–100% in all but the lowest dosage examined; ANCOVA (with length as a co-variate) indicated that feeding rate after exposure to suspended sediments above 200 mg PAH kg dry sediment −1 was significantly reduced relative to uncontaminated controls. No feeding occurred at 687 mg PAH kg dry sediment −1 (the greatest concentration tested). The proportion of non-feeding fishes increased with increasing levels of contamination, and an additional experiment suggested that the source of contamination was associated with direct contact with contaminated sediment rather than exposure to water previously associated with contaminated sediment.

Seafloor morphology and sedimentary processes, Knight Inlet, British Columbia

Seafloor morphology and sediment distribution in Knight Inlet are discussed as they relate to turbidity currents and associated gravity resedimentation phenomena. The study is based on acoustic imagery, high-resolution seismic profiling, echosounding, sediment sampling and seabed monitoring. Seafloor morphology shows that while the upper part of Knight Inlet resembles other British Columbia fjords, it differs in that a great many channels emanate from fjord-head deltas to coalesce ultimately into a few and finally one channel farther down-inlet. This is in contrast to the adjacent fjord, ButeInlet, where only one channel emerges opposite each of the two fjord-head rivers, joining a short distance downslope to form a single incised channel. In both fjords sand-transporting turbidity currents are responsible for creating these extensive channel systems; in Knight Inlet these channels are up to 40 m deep and up to 300 m wide continuing 41 km down-fjord. The upper fjord can be divided into four morphological zones which reflect not only the seabed character but also the flow characteristics of turbidity currents. Using a variety of theoretical techniques, based on channel morphology, coarsest grain sizes and bedform characteristics, turbidity current dynamics were modelled for the seabed morphological zones. Turbidity current densities are estimated to range from about 1.024 g cm −3 in distal areas to a maximum of 1.048 g cm −3 in the proximal region of greatest erosion and highest velocities. Average flow velocities are as high as 7.00 m s −1 diminishing to less than 0.50 m s −1 at the terminus of the channel. Based on estimates of velocity and turbidity current density, sediment fluxes were computed for various channel segments; changes in flux rates clearly delineated areas of erosion and deposition and were well correlated with the morphological character of the channel and nearby seafloor. Estimates of turbidity current velocity based on bedform characteristics tend to be lower than those calculated from coarsest grain sizes and channel morphology; preserved bedforms may reflect later waning phases of turbidity current flow whereas channel wall heights and coarsest grain size reflect maximum velocities. A close relationship was found between turbidity current events, as monitored by turbidity event detectors and river discharge peaks on the Klinaklini River at the head of the inlet. Fluvial suspended matter concentrations associated with such floods are not believed to be sufficient to produce turbidity currents directly. Rather, it is postulated that river mouth bars, which accumulate during periods of low river flow, are destroyed by floods and the coarse sediments, along with flood-related bedload, are swept onto the steep delta front where they accumulate as temporary unstable sediment masses, failing occasionally, or continue directly downslope as debris flows/turbidity currents.

Resonant internal waves and their role in transport and accumulation of fine sediment in Eckernförde Bay, Baltic Sea

In spring 1993, an instrumented tetrapod was deployed in Eckernförde Bay, Baltic Sea, with the purpose of characterizing physical processes most relevant to sediment resuspension, transport and deposition. Results suggest that sediment transport events in Eckernforde Bay are associated with resonant internal waves. Observed turbidity events were associated with marked along-bay current oscillations and spectral analyses of these currents are suggestive of baroclinic resonance. Furthermore, the peak in the current spectra is close to the previously reported 26–28 h Baltic-wide seich. A one-dimensional, two-layer analytic model is applied which explains the generation of resonant internal waves within Eckernförde Bay by periodic, barotropic flows across a sill near the mouth of the bay. The analytic solution accounts for velocities much larger than those otherwise predicted by barotropic processes or by progressive internal waves, and also explains an observed sign reversal in the correlation between barotropic forcing and the internal wave response. Despite this enhancement of near-bottom currents, estimates of shear velocity suggest that bottom stress never reached the critical magnitude necessary to locally resuspend sediment and was only rarely sufficient to prevent deposition of sediment that may have been suspended elsewhere. During turbidity events, observed suspended sediment concentration did not increase with proximity to the bottom, suggesting sediment advection rather than local resuspension. Bottom photographs support this inference. Although internal wave resonance may commonly produce velocities sufficient to advect fine sediment into Eckernförde Bay, maximum currents are constrained by the wave's phase velocity, which is only in the order of 30 cm s −1.

High-frequency turbidity currents in British Columbia fjords

The frequency of turbidity currents in Bute Inlet and Knight Inlet (British Columbia, Canada) was monitored. A prototype instrument (turbidity event detector) was deployed adjacent to prominent incised sea-floor channels. Approximately 25–30 turbidity currents occur annually. They appear closely correlated to periods of higher river discharge into the heads of the fjords. Two peaks in both discharge and turbidity current fequency occur, one in response to snow melt in late June–early July, the other to glacier melt in August. Virtually no turbidity currents were observed in winter. River mouth bars, channel deposits, and other deltaic sediments build up during lower discharge periods and are swept onto the steep delta front and into subaqueous channels, along with bedload, during floods.

Influence of sediment transport events upon bottom backscattering, Eckernfoerde Bay

In Spring 1993, oceanographic and acoustic apparatus were deployed in Eckernfoerde Bay, Baltic Sea. An instrumented tetrapod recorded physical data, revealing turbidity events associated with resonant internal waves. Acoustic bottom backscattering data at 40 kHz were acquired simultaneously and show features that correlate with the turbidity events. Such a correlation is surprising as estimates of shear velocity suggest that bottom stress never reached the critical magnitude necessary to resuspend sediment; rather the turbidity events appear to involve advection from a distant location. Correlations are evident in comparisons of acoustically derived time series for change in temperature and change in echo character with time series from current meters, optical backscattering sensors, and thermistors. [Work supported by the NRL CBBL Special Research Program.]