Maynard M. Nichols

Sediment accumulation rates and relative sea-level rise in lagoons

Abstract Sediment accumulation and relative sea-level rise rates are compiled for 22 United States lagoons to determine their accretionary status. The lagoons reviewed exhibit a range of accretionary differences (accumulation minus relative sea-level rise rate) varying between two end members: (1) a “surplus” lagoon in which accretion exceeds the relative sea-level rise rate, e.g., by 4.0 mm/yr for short-term (decades) differences in Atchafalaya Bay, and (2) a “deficit” lagoon in which relative sea-level rise exceeds the accretion rate, e.g., by −4.3 mm/yr for short-term differences in Matagorda Bay. The majority of lagoons have a near balance (±1.6 mm/yr) in which the rate of accretion nearly equals relative sea-level rise. The accretionary status reveals a lagoon continuum that emphasizes the way in which lagoons may be examined in a conceptual model as resultants of accretion and submergence. Short-term accretionary differences reflect the direction of long-term (millennia) differences, a trend suggesting that processes of the Holocene are continuing to the present day. Marked accretionary differences, as in Galveston Bay and Mobile Bay, indicate human intervention in the long-term trends. Although lagoons are commonly viewed as sediment sinks destined to be filled in a few millennia, the majority of lagoons reviewed have a near balance of accretion and relative sea-level rise rates and thus can persist.

Suspended-sediment transport in the Seine estuary, France: Effect of man-made modifications on estuary—shelf sedimentology

Abstract Man-made modifications of the Seine estuary have produced large geometric changes with important hydrographic and sedimentological consequences affecting estuary—shelf interrelationships. The estuary once had shallow, meandering ebb and flood tidal channels and broad tidal flats. Today, after 130 years of landfill, marsh reclamation, jettying and dredging, there is only a narrow, deep, jettied channel extending to the sea. Most of the tidal flats and marshes have been filled. These modifications have concentrated river discharge and ebb flow into a narrow jettied channel which, in turn, has produced a seaward migration of the salt intrusion and the turbidity maximum. As the estuary volume was reduced, river flow dilution was also reduced and its mean residence time in the estuary was shortened. At the same time, sedimentation and landfill have reduced the area available for sedimentation of fluvial sediments. This change has amplified seaward transport of fluvial suspended sediment and increased the size of shelf mud zones off the estuary mouth. Man has thus changed the geologic role of the estuary from a sink for fluvial and marine sediment, to a source of fluvial sediment for the shelf.

The Concept of an Equilibrium Surface Applied to Particle Sources and Contaminant Distributions in Estuarine Sediments

Studies have shown that many chemically-reactive contaminants become associated with fine particles in coastal waters and that the rate, patterns, and extent of contaminant accumulation within estuarine systems are extremely variable. In this paper, we briefly review our findings concerning the accumulation patterns of contaminants in several estuarine systems along the eastern coastline of the United States, and have applied a well-established concept in geology, that is “an equilibrium profile,” to explain the observed large variations in these patterns. We show that fine-particle deposition is the most important factor affecting contaminant accumulation in estuarine areas, and that accumulation patterns are governed by physical processes acting to establish or maintain a sediment surface in dynamic equilibrium with respect to sea level, river discharge, tidal currents, and wave activity. Net long-term particle and particle-associated contaminant accumulations are negliglible in areas where the sediment surface has attained “dynamic equilibrium” with the hydraulic regime. Contaminant, accumulation in these areas primarily occurs by the exchange of contaminant-poor sedimentary particles with contaminant-rich suspended particles during physical or biological mixing of the surface sediment. Virtually the entire estuarine particulate and contaminant load bypasses these “equilibrium” areas to accumulate at extremely rapid in relatively small areas that are temporally out of equilibriums as a result of natural processes or human activities. These relatively small areas serve as major sinks for particles from riverine and marine sources, and for biogenic carbon formed in situ within estuaries or on the inner shelf.

Effects of hopper dredging and sediment dispersion, chesapeake bay

Hopper dredging operations release suspended sediment into the environment by agitation of the bed and by discharge of overflow slurries. Monitoring of turbidity and suspended sediment concentrations in central Chesapeake Bay revealed two plumes: (1) an upper plume produced by overflow discharge and (2) a near-bottom plume produced by draghead agitation and rapid settling from the upper plume. The upper plume dispersed over 5.7 km2 extending 5,200 meters form the discharge point. Redeposited sediment accumulated on channel flanks covering an area of 6.4 km2 and reached a thickness of 19 cm. Altogether dredging redistributed into the environment an estimated 100,000 tons of sediment or 12 percent of the total material removed.Near-field concentrations of suspended sediment, less than 300 m from the dredge, reach 840 to 7,200 mg/L or 50 to 400 times the normal background level. Far-field concentrations (>300 m) are enriched 5 to 8 times background concentrations and persist 34 to 50 percent of the time during a dredging cycle (1.5 to 2.0 h). The overflow discharge plume evolves through three dispersion phases: (1) convective descent, (2) dynamic collapse, and (3) long-term passive diffusion (Clark and others 1971). The bulk of the material descends rapidly to the bottom during the convective descent phase, whereas the cloud that remains in suspension is dispersed partly by internal waves. Although suspended sediment concentrations in the water column exceed certain water quality standards, benthic communities survived the perturbation with little effect.

Fluid mud accumulation processes in an estuary

Fluid mud accumulates as pools and blanket deposits greater than 20 cm thick in channels of the James Estuary. It forms mainly in the turbidity maximum zone, a site of high near-bed concentrations (0.5 to 2 g/liter), intensive resuspension, and fast sedimentation (1 to 8 g/cm2/yr). Accumulation is promoted by stratification of interfacial fluid and pore water, by the pseudoplastic behavior of the mud with relatively high viscosity at low shear rates, by the high suspended sediment concentrations, and by resultant rapid-settling flux relative to the consolidation rate in the hindered state.

BOX MODEL APPLICATION TO A STUDY OF SUSPENDED SEDIMENT DISTRIBUTIONS AND FLUXES IN PARTIALLY MIXED ESTUARIES

Abstract Box model theory provides a simple method for the investigation of the behavior of nonconservative quantities in estuaries. It permits, within its limits, a quantitative examination of biological, chemical and geological distributions, transformations and other effects which depend, in part, on estuarine hydrodynamics for their explanation. The model inputs are salinity distribution, geometry of the estuary, and river flow, in addition to the distribution of the nonconservative quantity being investigated. The theory is applied here to a study of suspended sediment distributions and fluxes and to net deposition and erosion as a function of longitudinal distance along the estuary. Results are given for two partially mixed estuaries and for various river flow conditions within each estuary.

Chapter 7 Sediment Transport Processes in Coastal Lagoons

A review of sediment transport in lagoons provides a better understanding of how processes act to modify, retain and accumulate sediment. The lagoon transport system is examined as a series of processes that distribute fine-grained sediment between sources and sinks. The processes cycle sediment from one part of a lagoon to another with small amounts being added intermittently from diverse sources to balance amounts removed from the system or that go into storage. Residual transport of fine suspended sediment is regulated by tidal pumping, shear transport or time-flow asymmetry. During transport and recycling, fine particles are modified by aggregation, break-up and reforming. After deposition, benthic fauna further modify the sediment by changing its stability, geotechnical properties, and erosion resistance. Additionally, wind waves winnow fines from shoals thus modifying textural characteristics, while tidal mechanisms have selective effects on the particle composition and size distributions. Lagoon sinks incorporate a number of fill components reflecting multiple sources and fluctuations in energy dissipation interacting on the sediment supply. Climate mainly influences the source material and the sediment character of intertidal zones. Although sediments are extensively modified, recycled and reworked, especially by storms, lagoons primarily function as net sediment sinks in which the accumulation rates adjust to submergence. Sediment processes are a crucial link to understanding the fate of materials in lagoons since they modulate the chemical reactivity and biological productivity of lagoons. Our knowledge, however, is still imperfect and sediment processes therefore warrant increased study and scrutiny.

Sedimentologic fate and cycling of Kepone in an estuarine system: Example from the James river estuary

Chemical wastes supplied to an estuary from rivers are either transported to sea or, alternately, trapped in the system. The fate is a complex problem because estuaries are heterogeneous, chemically reactive, hydrodynamically variable and sedimentologically dynamic. This paper traces the fate of Kepone, a polychlorinated hydrocarbon, through the fine-sediment dispersal system to its sediment sinks. Evidence is drawn from a review of field observations in the 120 km long estuary and from hydrodynamic model analyses that address contaminant input, partitioning, particle transport and cycling mechanisms. Kepone escaped undetected from a manufacturing site for more than 9 years. It spread through the aquatic food chain and contaminated 37 × 106 metric tonnes (t) of sediment extending 120 km seaward from its source, and 80 cm into the bed. Highest concentrations (>200 ppb) occur in near-source sediments, while the greatest Kepone mass resides in far-field sinks where sedimentation is relatively fast. Kepone enters the fine-sediment dispersal system mainly through sorption to particles (Kd ∼= 5 × 103). Pathways approximately follow the estuarine circulation: (i) seaward through freshwater reaches of the upper estuary; (ii) an efflux route seaward through the upper estuarine layer, for semi-permanently suspended particles, or a reflux route downward by settling for high-density suspended material; (iii) landward return through the lower layer to the inner salt limit. A marked seasonal migration, or refluxing, occurs whereby sediment accumulated in the upper estuary during summer at low river inflow, is scoured by river currents during spring and redispersed seaward into the middle and lower estuary. Refluxing distributes Kepone over the middle estuary and increases retention and residence time. Despite strong seaward transport by river floods an estimated 3.2–4.1 t of Kepone, or 42–90% of the input, accumulated in the estuary during 21 years. After production ceased, natural sedimentation provided the best means to decontaminate the estuary.

Foraminiferal populations in a coastal plain estuary

Abstract Populations of benthonic Foraminifera in the James River estuary, Chesapeake Bay region, comprise seventeen species; two species constitute 90% of the fauna. An arenaceous Ammobaculites fauna inhabits freshened upper reaches (0.5–14‰ salinity) and a calcareous Elphidium fauna inhabits the lower estuary where bottom salinity exceeds an average of 14 ‰ in spring and summer. The faunas meet at a sharp boundary which extends upstream in the channel and downstream over marginal shoals. The distributions are broadly related to salinity and shift with time and changing river inflow. Large populations of few species occur in the middle estuary, a reach of weakly saline water with marked salinity fluctuations. Live-total ratios are not related to sedimentation rates but reflect varied foraminiferal production, local transport or other processes. The James estuary fauna is similar to other nearshore faunas reported from many parts of the world. Estuarine faunas are distinguished by sharp boundaries, scattered fossil and marsh dwelling species, and distinct distributional patterns.

Distribution of recent Ostracoda in the Rappahannock Estuary, Virginia

The distribution of recent Ostracoda as determined from 69 bottom samples in the Rappahannock Estuary, Virginia, is related to the pattern of estuarine circulation and different types of fresh and salty water. Three ostracode biofacies are established on the basis of dominant species: (1) a river biofacies, (2) a shoal biofacies, and (3) a basin biofacies.