Suspended Silt Concentrations Research Articles

The East River tidal strait, New York City, New York, a high-nutrient, low-chlorophyll coastal system

The East River tidal strait, located between New York Harbor and Western Long Island Sound, is characterized by high suspended silt concentrations with low organic content kept in suspension by intense tidal currents. Inorganic nutrients, including nitrate, nitrite, ammonia, and phosphate, were high even during the summer. Dissolved inorganic nitrogen (DIN) concentrations generally were above 20 µM and did not likely limit phytoplankton growth. Despite high nutrient concentrations, median chlorophyll a concentration was only 1.53 µg l−1, making the East River tidal strait a high-nutrient, low-chlorophyll (HNLC) area, likely a result of suspended silt blocking light penetration into the surface water. There were times at which the ratio of mixed layer to depth of the euphotic zone was generally greater than what has been suggested for phytoplankton to produce net primary production. The high-nutrient East River tidal strait is likely one of the sources of nutrients fueling summer phytoplankton production and consequent hypoxia in the Western Long Island Sound as silt settles from surface water in the lower turbulence conditions of the western narrows of Long Island Sound, thereby allowing light penetration and subsequent consumption of dissolved nutrients by phytoplankton.

Flow-velocity-dependent effects of turbid water on periphyton structure and function in flowing water

Land-use change, human activities, and dam management supply highly turbid water with high flow velocity; however, little is known about its effect on river ecosystems. Here, we studied the effects of flow velocity and suspended silt concentration on periphyton structure and function using experimental channels. The effects of flow velocity on algal biomass in the periphyton community depended on the suspended silt concentration; flow velocity decreased chlorophyll a in low concentrations of suspended silt (clear water), but not in moderate or high concentrations (turbid water). Filamentous cyanobacteria density was significantly influenced by flow velocity, whereas densities of non-filamentous cyanobacteria, green algae, and diatoms were not. Thus, flushing effects on attached algae, especially filamentous cyanobacteria, were exerted in clear but not in turbid waters, perhaps because algal assemblages were resistant to water current through the binding of large amounts of silt by extracellular polymeric substances. Inorganic matter and its ratio in the periphyton community decreased with flow velocity, but increased with suspended silt concentration, irrespective of flow velocity. Periphyton function, assessed by maximum photosynthesis rates and light-use efficiency, was influenced by flow velocity and suspended silt concentration through changes in periphyton structure. Flood events would be expected to refresh the periphyton community by removing senescent algae and deposited fine particles. However, we demonstrated that algae and silt in the periphyton community exposed to turbid waters with human-induced silt may accumulate, irrespective of flow velocity.

Sedimentation patterns caused by scallop dredging in a physically dynamic environment

Scallop dredging grounds in the Firth of Lorn, western Scotland, are juxtaposed with rocky reef habitats raising concerns that reef communities may be impacted by sediment disturbed by nearby scallop dredging. A particle-tracking model of sediment transport and settling is applied at two scales. In the near-field, a suspension of typical sand/gravel-dominated bed sediment is subjected to a steady current across the dredge track. In the far-field, silt particles, which may persist in suspension for multiple tidal cycles, are tracked in the context of a regional model of tidally-driven flow. The principal sedimentary risk to reef habitats is predicted to come from settling sand particles when dredge tracks approach within tens of metres of a reef. The cumulative effect of dredging at the relatively low intensities recorded in this region is not expected to have a significant long-term impact on suspended silt concentrations and settlement in this highly dispersive environment.

Effects of silt loading on the feeding and mortality of the mysid Mesopodopsis africana in the St. Lucia Estuary, South Africa

Mesopodopsis africana is an abundant zooplankton species in the currently drought stricken St. Lucia Estuary. The estuary is freshwater-starved, partly because its connection with the large Mfolozi catchment has been discontinued. This is due to the extremely high silt load carried by its waters. A series of in vitro experiments were conducted in May, July, August and December 2006, in order to determine the effect of silt loading on the feeding rate and mortality of M. africana. Mysids were subjected to different sediment concentrations in controlled experiments for a 12-hour period. The turbidity regimes were generated by adding pre-weighed silt concentrations, ranging from 0.03 g/L to 13 g/L, to natural estuarine water. Results in all cases showed a major effect of high silt levels upon food availability, as water chlorophyll losses increased with increased silt concentration. Hindrance in the food collecting abilities appears to have a compounding influence as on two occasions, May and August, ingestion rates decreased with an increase in silt concentration. However, in July, mysid ingestion and clearance rates were higher under increased suspended silt concentrations. In such cases, mysids may be clearing more water under conditions of low food concentration, or alternatively, they may switch to feeding on metazoan prey. In the December experiment, there was no relationship between ingestion rate and silt concentration, possibly because food supplies were sufficient to sustain constant ingestion rates or because mysids increased their clearance rates in the more turbid treatments to compensate for any negative effects of silt. Mysid health also declined, with the mean percentage of survivors decreasing consistently with an increase in silt concentration. Results indicate that the overall effect of increased silt load is negative, and subsequently, increased silt loading in the St. Lucia estuary is expected to have detrimental effects on communities of mysids within the estuary.

Sediment transport on an estuarine intertidal flat: Measurements and conceptual model of waves, rainfall and exchanges with a tidal creek

Sediment (silt) transport on a wave-dominated estuarine intertidal flat dissected by a tidal creek that connects to the watershed freshwater network is investigated by analysing field data from Waikopua, New Zealand, and by applying simple models. The intention is to expand understanding beyond the case of the idealised, two-dimensional wave-dominated flat. During fairweather (no waves), there is a continuous exchange of silt between the bed of the tidal creek and the upper flat, and that exchange is controlled by the elevation of the flat with respect to the creek bed. Rainfall in the watershed does not fundamentally alter the way the intertidal flat and the creek interact, but it does increase silt loads in the creek, which in turn increases the amount of silt exchanged with the upper flat. Waves on the flat are fetch-limited, and frictional dissipation causes waves to reduce in height at the edge of the water body. Under some circumstances, a frictional-dissipation zone may occupy the entire middle-plus-upper flat. There is a maximum in wave-orbital speed at the bed ( U sigb) in the middle reaches of the flat, which arises from the particular balance between down-fetch wave growth, wave dissipation by bottom friction, and attenuation through the water column of wave-orbital motions under the short-period waves. There is a progressive decoupling of suspended-silt concentration (SSC) from U sigb moving from the bottom to the top of the flat, such that SSC is highest towards the top of the flat, where U sigb is virtually zero. We suggest that this is due to wave activity retarding the settling of suspended silt, and explore that idea with a simple model that is capable of reproducing the essential features of the data set. The results are assimilated in a conceptual model of the system, which shows the balances that control net silt transport in the creek and on the different parts of the flat, three different silt sources, and the role of waves and rainfall. The conceptual model also points at the feedbacks between sediment-transport processes and morphology that are inherent in the system. Implications of those feedbacks to long-term morphodynamics are essentially unexplored.

Temporal and spatial depositional patterns in the Severn Estuary, southwestern Britain: intertidal studies at spring–neap and seasonal scales, 1991–1993

The estuary is a high-energy and very variable system, with a hypertidal, semidiurnal tidal regime. The amount and coarseness of the sediment trapped on filter papers, and the suspended silt concentration in the flood tidal front, were measured daily from mudflat and marsh stations at each of six sites during eight spring–neap cycles spread over a two-year period. Four of the cycles were associated with vernal equinoxes and four with autumnal equinoxes. River discharge, wind speed, tidal height and the suspended sediment concentration all affect the amount and texture of the trapped mud. Complex tidal patterns are unlikely to be registered other than very imperfectly in these tidal deposits. At any site, the silts deposited on the mudflats are slightly coarser than those accreted on the marshes. Both marsh and mudflat deposits tend to become less sandy but more clayey toward the mouth of the estuary. The sediments trapped on the mudflats and marshes during vernal equinoxes tend at a site to be sandier and less clayey than their autumnal counterparts. As the vernal and autumnal, equinoctial tidal regimes are virtually identical, and river discharge has a negligible influence, winter storminess and the substantial annual change in water temperature, affecting viscosity, may largely be the explanation, with biological factors possibly augmenting the effect.

The influence of suspended silt on oxygen consumption by the redbreast tilapia, Tilapia rendalli (Boulenger,1896)

The routine oxygen consumption of redbreast tilapia, Tilapia rendalli exposed to silt from the Phalaborwa Barrage was investigated under controlled laboratory conditions. Exposure of adult fish to sublethal concentrations of silt did alter routine oxygen consumption by causing increased rates (53–77% from initial) of oxygen consumption. This indicates that sublethal levels of suspended silt concentrations results in an increase in oxygen consumption by the fish due to an increase in metabolic rate associated with increased activity and stress. From the laboratory experiments, it can therefore be predicted that sublethal releases from the Phalaborwa Barrage will stress the resident fish populations. However, the extent of the impact on the fish population will depend on factors such as frequency, duration and severity of exposure as well as the species in question. Long term monitoring of the fish population below the Phalaborwa Barrage is therefore essential.

Suspended silt concentrations in the lower Olifants River (Mpumalanga) and the impact of silt releases from the Phalaborwa Barrage on water quality and fish survival

Silt loads in the Olifants and Sabie river systems inside the Kruger National Park, were monitored by collecting water samples every consecutive week for a period of 20 months. The variation in silt concentration, changes in selected physico-chemical water quality variables and fish mortalities during flushing (i.e. release of silt, by sluicing) of the Phalaborwa Barrage, were also monitored. The Olifants River inside the Kruger National Park carried high silt loads in summer; in the dry season the suspensoid load was greatly reduced. A similar pattern was observed in the Sabie River, but the silt loads were generally lower. It was apparent that silt loads released from the Phalaborwa Barrage led to large variations in the natural silt loads of the Olifants River. These increased amounts of silt (25 000 mg/1 to >70 000 mg/1) caused drastic reductions in the dissolved oxygen concentration of the water, ranging from >6 mg/1 to 0 mg/1. Depending on the severity and duration of the flushing, fish succumb to such silt loads. These findings, as well as published information, indicate that the management strategy of flushing to improve storage capacity is ecological unacceptable. It is therefore suggested that the use of the Phalaborwa Barrage as a future reservoir should be critically re-evaluated.

A model for predicting waves and suspended silt concentration in a shallow lake

A model for predicting waves and suspended silt concentration in a shallow lake

The Thames barrier

This paper, presented at the International Symposium on El Paisaje del Agua organised in Madrid by Canal de Isabel II in October 1986, is based on the Verena Holmes Lecture of the Women's Engineering Society given by the author in 1983 in centres throughout the United Kingdom. It describes the background to tidal flooding in the estuary and the events leading to the commissioning in 1968 of hydraulic studies to predict how the construction and operation of a barrier at different sites would affect river levels, tidal currents and siltation in the Thames.Hydraulics Research, Wallingford, tackled the problem under six main headings: field measurements, continuous silt monitoring, laboratory tests on silt, computational modelling, physical modelling and desk studies. An account is given of a series of surveys undertaken at intervals between 1968 and 1981 to determine the present behaviour of the river under diurnal, bimonthly and seasonal differences in tidal range and river flow. The establishment in 1970 of stations for the continuous monitoring of suspended silt concentrations before, during and after barrier construction is described along with laboratory tests carried out to examine the properties of Thames mud. The silt monitoring still continues.Mention is made of the six physical hydraulic models used to examine the problems of flow and sediment redistribution associated with the Thames Barrier and flood-prevention investigation and of the first mathematical model ever developed to reproduce the movement of suspended silt in an estuary.The paper concludes with an account of site progress between 1971, when interim protection was provided for Londoners, and barrier completion 11 years later.

Sediment transport for desert watersheds in erodible materials

Sediment transport for five small watersheds in arid southern Israel, eroded into shales, chalks, anhydrites and alluvium is studied. Analysis includes data from three large watersheds in the southern United States. Results indicate that floodwater heavily laden with suspended load (10-60 per cent, by weight) is characteristic of watersheds cut in erodible materials. No correlation was found between environmental characteristics such as lithology, catchment area, angle of slope or rainfall intensity and suspended-sediment concentration or texture. The carrying capacity for sands, derived both from hillslopes and channel alluvium is greatly increased by high suspended silt concentration, the trend being (where R = ratio, % fines/% sand; Cs = suspended load concentration, per cent by weight). The high concentration of fine sediment effectively causes more sand transport than would normally occur. This is also true for transport of coarser clastics; higher velocities, typical of highly concentrated silt suspensions, increased competence for transporting pebbles and boulders. The action of a large variety of processes (splash, wash, rilling, gullying, sliding, mudflow) combine with low infiltration and high credibility to render every rainstorm erosionally effective, and high concentrations of washload increase both the capacity and competence of coarse clastic transport. Whole basins are thus subjected to high rates of denudation (2-10 mm/yr).

Siltation problems in relation to the Thames barrier

The problems confronting the Siltation Working Party of the Thames Flood. Prevention Investigation are stated, the need for an early solution necessitating rigorous limitation of the possible combinations of site, function and design of flood protection structure that could be studied. The reasons for excluding all schemes involving a perm anent barrage are given, the Hydraulics Research Station finally being asked to determine the effect on siltation of constructing a barrier at specified sites for use both as a simple surge excluding device and as a continuous half-tide control structure. The methods adopted by H .R.S. in tackling the problem are described. They include four large scale estuarine surveys; the establishment of stations for continuously monitoring the suspended solids content of the river; field and laboratory tests to determine the properties of Thames silt; the development of a mathematical model to study the effect of the tide control on the distribution of silt which is carried in suspension, and experiments on a physical model to establish the redistribution of bed sediments in the navigation channel likely to follow barrier construction and continuous half-tide control. The data from the field surveys demonstrate how current-velocity, suspended silt concentration, salinity and temperature at different depths along the estuary change throughout a spring and a neap tide during both high and low river flows. The results of the silt monitoring exercise supplement the survey data by indicating how the concentrations of suspended sediment in the estuary vary - with tidal range, with position in the bi-monthly spring-to-neap cycle and with seasonal variations in freshwater discharge. The tests to determine the properties of Thames silt provide values for the four parameters describing the processes of silt movement needed to develop the mathematical model. The indications from the physical model studies are that the construction of a well-designed barrier used only to exclude surges should produce no insuperable siltation problems. Both mathematical and physical model results show that continuous tide control leads to increased siltation, the zone of greatest deposition depending on the barrier site.