Showing papers on "Dredging published in 2001"

Biological Effects of Suspended Sediments: A Review of Suspended Sediment Impacts on Fish and Shellfish with Relation to Dredging Activities in Estuaries

Abstract: Objective assessment of the effects of increased concentrations of suspended sediment caused by human activities, such as navigation dredging, on estuarine fish and shellfish requires an integration of findings from biological and engineering studies. Knowledge is needed of (1) the suspended sediment characteristics typical of both ambient and dredging-induced conditions, (2) the biological responses of aquatic organisms to these suspended sediment dosages, and (3) the likelihood that organisms of interest will encounter suspended sediment plumes. This paper synthesizes the results of studies that report biological responses to known suspended sediment concentrations and exposure durations and relates these findings to suspended sediment conditions associated with dredging projects. Biological responses of taxonomic groups and life history stages are graphed as a function of concentration and exposure duration. The quality and taxonomic breadth of studies on which resource managers must rely when...

Impact of scallop dredging on benthic megafauna: a comparison of damage levels in captured and non-captured organisms

Abstract: The impact of scallop dredging on benthic megafauna was assessed by direct observation of damage, both in the bycatch and in organisms encountering dredges but not captured. Damage was assessed using a simple 4- point scale adapted for different taxonomic groups. Experi- mental dredging was undertaken on a scallop fishing ground in the north Irish Sea, off the Isle of Man. Divers were deployed immediately after dredges had passed, to record levels of damage to megafauna left in the dredge tracks. Mean damage levels, and the proportions of the 4 damage scores in the bycatch and on the seabed, were the same in most species. Some common species did show differences. The edible crab Cancer pagurus was more severely damaged when not captured, while the starfish Asterias rubens and whelk Neptunea antiqua received greater damage within the bycatch. Capture efficiency for the megafauna was low, rang- ing from 2 to 25% among species. The results indicate that the majority of damage to large benthic invertebrates during scallop dredging occurs unobserved on the seabed, rather than in the bycatch.

Benthic community response to sand dredging and shoreface nourishment in Dutch coastal waters

Abstract: In the Netherlands marine sands are increasingly used for coastal protection and land infra-structural works. Both sand extraction and coastal nourishment cause disturbances of the seabed and its ecological functions. The results of three studies are used to discuss patterns in recovery of the benthic community. Survival, migration and recruitment may all contribute to the recovery of the benthic community after a disturbance. Following such an event a short-term opportunistic response was observed followed by an almost complete recovery of community composition and structure after four years.

Shinnecock Inlet, New York, Site Investigation Report 4, Evaluation of Flood and Ebb Shoal Sediment Source Alternatives for the West of Shinnecock Interim Project, New York

Abstract: : Shinnecock Inlet, New York, is a dual-jettied inlet located on the south shore of Long Island connecting Shinnecock Bay to the Atlantic Ocean. The down-drift beach, west of the inlet, experiences chronic erosion, and cost-effective and innovative measures for beach nourishment are being examined by the U.S. Army Engineer District, New York. The feasibility of mining of the flood and ebb shoals to serve as sources of material was examined in this report. Emphasis is on the concept of 'flood-shoal engineering' within an integrated inlet and beach system. Fifteen action alternatives were developed that involved dredging, modification of the jetties, and combined dredging and structural changes. The alternatives were evaluated by their potential changes to navigation conditions, availability of material for placement on the beach, changes to inlet and channel currents that would modify scour and deposition patterns, and changes in current strength near the beach that would modify erosion. The area of compatible material, established for the flood shoal from analysis of core samples, was the targeted mining area for the study and contains approximately 1.8 x 10(exp 6) cu yd of beach-compatible sand. Exploratory alternatives were also evaluated that involved dredging in other locations. Evaluation of alternatives was conducted through circulation, wave, and morphology modeling. A calibrated circulation model was applied to simulate each alternative and compare current strength and patterns to those for the existing condition. Wave modeling was conducted for one alternative that consisted of mining the attachment bar to determine changes in the wave patterns near the shore. Morphology modeling was conducted to calculate the long-term recovery rates of the system to mining of the flood shoal.

Short-term impact of blue mussel dredging (Mytilus edulis L.) on a benthic community

Abstract: The short-term effect of mussel dredging in a brackish Danish sound was studied. A commercial dredging track was identified and an analysis of the species composition inside the track and at an adjacent control area showed that dredging changed the community structure by reducing the density of polychaetes. In order to investigate the extent and the duration of the dredging impact experimental dredging was conducted. The experimental dredging removed 50% of the mussels in two dredged areas. Immediately after dredging, a significantly lower number of species was measured inside the mussel beds in dredged areas compared to control and boundary areas. This effect lasted for at least 40 days. The analysis of the species composition showed that the dredged area had a significantly lower density, particularly of polychaetes compared to the boundary area. An increased number of species was recorded outside the mussel beds just after dredging, but this effect lasted for less than 7 days. After dredging, brown shrimps, C. crangon invaded the dredged areas. This species is an important predator of smaller invertebrates, and it is suspected that it was feeding on small vulnerable polychaetes exposed at the sediment surface after dredging. The dredging process was observed to form 2–5-cm deep furrows in the seabed, but the sediment texture and the organic content of the sediment was not affected. The biomass accumulation of individual blue mussels was significantly lower in the dredged area compared to the boundary area. This indicates that the disturbance of the mussel bed structure reduced growth and that the lowering of intraspecific food competition caused by a reduced density of mussels did not increase the accumulation of biomass in the mussels which remained in the dredged area.

Dredging activities: marine issues

Abstract: This paper synthesizes scientific information on the effects of dredging activities on marine habitats. Direct and long-term effects, dredge methods, regulatory framework, contaminated sediment issues, and a separate bibliography of contaminated sediment-related reports are also presented. Direct behavioral effects include entrainment, increased turbidity, fish injury due to suspended sediment exposure, decreased dissolved oxygen levels, and the effects of noise. A turbidity threshold of 200 mg/L could reduce dredge-induced salmonid prey-predator reaction changes. High sediment load related fish injury deserves further analysis. Gill injury thresholds specific to marine environments have not been identified. Suspended sediment size, shape, and exposure duration are likely important risk assessment factors for salmonids and other fishes. The most relevant issue is likely the fish ability to avoid plumes and dredge areas. Benthic infauna, epibenthic and demersal organisms, such as borrowing shrimp, crabs, and fish, are subject to entrainment risks. A clearer understanding of dredging effects to biota requires further synthesis of physiology, life-history strategies, water column use, and timing. In Washington State, maintenance dredging conversion of shallower subtidal to deeper subtidal habitats is much more frequent than new construction dredging conversion of intertidal to subtidal habitats, which is rarely allowed. Loss of intertidal habitats represents potential reductions in coastal habitat carrying capacity. The fish effects of channel deepening alteration to estuarine-freshwater mixing are most evident during early life history stages. Productivity recovery rates range from three months to many years, depending on the species. Lack of long-term and dredge pre- and post-project monitoring makes it difficult to conclusively identify effects.

Macrobenthic community characterisation of an estuary from the western coast of Portugal (Sado estuary) prior to dredging operations

Abstract: The present work is part of an environmental assessment undertaken in the Sado estuary (Southern Channel and Mitrena Peninsula), western coast of Portugal, on February 1999 prior to maintenance dredging works. The macrofauna communities in the study area are generally rich and abundant. A single exception was found: a sampling site in the Mitrena area, with extreme impoverishment, probably related to sediment characteristics, i.e., fluid mud, unfavourable to the establishment of individuals. A comparative analysis of these macrofauna results to previous data from 1986 showed that this biological component had suffered no significant changes, especially as far as the most characteristic species. Although the Southern Channel had been dredged in 1995, no clear signs of such operation were apparent in the present survey. The main differences between both periods (1986-1999) was an increase in the abundance and presence of some common species of organic enriched areas, such as Tharyx sp., Corbula gibba, Spiochaetopterus costarum and Ampelisca spp., which might be related to organic enrichment in the Southern Channel. The joint consideration of our results on benthic macrofauna and those regarding sediment contamination and sediment bioassays, performed at the same time by other researchers, does not indicate the necessity of any particular constraints on the dredging operations.

Optimization of mechanical dredging operations for sediment remediation

Abstract: Sediment remediation involves restricted activities in the dredging process in order to minimize environmental risk. Operational constraints add significant capital cost to the overall project. This paper presents an example of the special conditions for a typical environmental dredging operation. REMSIM software, which simulates the sediment remediation processes, aids in planning for environmental dredging by reducing uncertainties in cost and performance estimations. The scow and bucket capacities, number of scows, and effective working time are parameters that improve productivity significantly as they are increased. The purpose of this paper is to help dredging contractors use their navigational dredging knowledge base to estimate costs, performance, and equipment selection for future environmental dredging projects where contaminant dispersion is a concern. REMSIM is used to obtain cost and productivity comparisons between environmental dredging and typical navigational dredging situations. Equipment and performance parameters can be adjusted in order to compensate for environmental dredging limitations while improving productivity and minimizing costs.

Contaminated sediment excavator for subsurface sediment removal

Abstract: A system and method removes contaminated sediments during dredging operations to minimize the dispersion of particulate matter that normally occurs during conventional mechanical or hydraulic dredging operations. The system uses a specially designed pressure controlled enclosure or enclosed excavator that houses a mechanical bucket. The enclosure is designed to capture suspended particles that are dispersed into the water column during the mechanical dredging process. The system also includes the use of a self-contained membrane filtration treatment vessel to remove fine particulate matter from dredge drainage waters collected in the process.

The Effectiveness of Environmental Dredging: A Study of Three Sites

Abstract: This manuscript is the result of a research project on the effectiveness of environmental dredging. The project consisted of technical evaluations of dredging performed at sites in Louisiana, New York, and Michigan. Models and measurements were used to examine increased water concentrations, post-dredging sediment concentrations, effects on biota (primarily fish), contaminant losses, etc., for dredging primarily, but including the natural-recovery and capping remediation options. The effectiveness was evaluated by quantifying 1) enhanced contaminant release during dredging, 2) short-term on the aquatic environment and 3) long-term impacts on the aquatic environment. It was found that existing technologies provide the means to extract large volumes of contaminated bed-sediment. Targeted removals of greater than 95 percent are achievable. However, 100 percent removal was impossible, so that significant contaminants remained. The surficial-sediment concentrations remained high, with reduction in the 50 to 75 percent range being typical. The impact on fish in the short term was nearly always negative. During dredging, significant contaminants enter the water column in both soluble and particulate form. The long-term impact of dredging was indeterminate because of the limited observational database. Complete chemical and biota data sets necessary to quantify the efficiency of the dredge and residual losses were generally absent at all three sites. 2001 © John Wiley & Sons, Inc.

Environmental studies on impacts of dredging

Abstract: The coastal zone acts as a major interface between the ocean and continents. Harbours located in this zone face grave problems from sedimentation, which is a global issue for most of the harbours of the world. Dredging which counter acts sedimentation, brings about innumerable environmental impacts — both positive and negative. As a case study, this paper reports the impacts of dredging conducted at Cochin harbour which is the second largest port along the west coast of India; this port is a nodal center in trade and commerce connecting south east Asia to European sector. The harbour operation mainly depends on maintenance form of dredging bringing about impacts which have both direct and indirect effects in the long term. The important parameters selected for this study are salinity, current, extinction coefficient, turbidity, nutrients, chlorophyll and bottom fauna. An assessment is made on both positive and negative impacts which indicate rise in turbidity, changes in nutrient content and biota. The cu...

Executive summary: dredging activities: marine issues

Abstract: This paper synthesizes scientific information on the effects of dredging activities on marine habitats. Direct and long-term effects, dredge methods, regulatory framework, contaminated sediment issues, and a separate bibliography of contaminated sediment-related reports are also presented. Direct behavioral effects include entrainment, increased turbidity, fish injury due to suspended sediment exposure, decreased dissolved oxygen levels, and the effects of noise. A turbidity threshold of 200 mg/L could reduce dredge-induced salmonid prey-predator reaction changes. High sediment load-related fish injury deserves further analysis. Gill injury thresholds specific to marine environments have not been identified. Suspended sediment size, shape, and exposure duration are likely important risk assessment factors for salmonids and other fishes. The most relevant issue is likely the fish ability to avoid plumes and dredge areas. Benthic infauna, epibenthic and demersal organisms, such as burrowing shrimp, crabs, and fish, are subject to entrainment risks. A clearer understanding of dredging effects to biota requires further synthesis of physiology, life-history strategies, water column use, and timing. In Washington State, maintenance dredging conversion of shallower subtidal to deeper subtidal habitats is much more frequent than new construction dredging conversion of intertidal to subtidal habitats, which is rarely allowed. Loss of intertidal habitats represents potential reductions in coastal habitat carrying capacity. The fish effects of channel deepening alteration to estuarine-freshwater mixing are most evident during early life history stages. Productivity recovery rates range from three months to many years, depending on the species. Lack of long-term and dredge pre- and post-project monitoring makes it difficult to conclusively identify effects.

Characterization of Underwater Sounds Produced by Bucket Dredging Operations

Abstract: : This technical note presents a characterization of underwater sounds produced by bucket dredging operations in Cook Inlet, Alaska. Both continuous sounds, such as those produced by propellers, pumps, and generators, and repetitive sounds produced by particular dredging events (e.g., dredge bucket striking the channel bottom,jaws of bucket clamping shut) are potential sources of underwater noise. This note examines the intensity, periodicity, and attenuation of emitted sounds from bucket dredging operations and how they are influenced by factors including substrate type, hydrodynamic conditions, equipment maintenance, and skill of the dredge plant operator.

High-resolution magnetic mapping of contaminated sediments in urbanized environments

Abstract: The remediation of contaminated sediments in industrialized harbors and waterways is a growing environmental concern in North America. More than 100 marine ports and 42 inland harbors in the Great Lakes basins are currently listed by U.S. and Canadian regulatory agencies as having severe habitat and water-quality impairments due to bottom-sediment toxicity. Current approaches to remediation include sediment dredging and removal, capping with clean sediment, and in-situ treatment with oxidants.

Method and apparatus for dredging and transporting dredged solids

Abstract: A system and method of the type for quickly mobilizing a unit for dredging a layer of material from a water bottom is provided. The dredging system includes: a motorized, substantially self-contained vessel positioned above a borrow site, at least one dredging shoe extended from the vessel to a bottom of a water adapted for dredging a material from the bottom to a pre-selected depth, a power source positioned on the vessel in operational connection with the dredging shoe for drawing the material into the shoe and transporting the material through a conduit to a deposit site, and a dredging motivation system connected to the vessel for moving the vessel and the dredging shoe along a dredging path.

Requirements for sediment plumes caused by dredging

Abstract: In many dredging contracts, environmental restrictions on sediment plumes must be met. Sediment plumes cause reduced light penetration, increased concentration of suspended solids and sedimentation which all can affect benthic ecosystems. Contractors confronted with these requirements get the impression that these regulations are not well founded. Because the costs of dredging increase with stricter limits, it is important to know why restrictions are necessary. Goal of this investigation was to review and clarify some aspects of the situation. This report about requirements for sediment plumes caused by dredging is divided in three parts. In the first part consists of an inventory of the standards for sediment plumes caused by dredging as applied in different countries and limits set at dredging projects for sediment plumes with their backgrounds. The possible effects of sediment plumes on different ecological environments are listed and the extent to which limits are based on well documented environmental effects is discussed. There is no international regulation or legislation about uncontaminated sediment plumes caused by dredging. It seems impossible to make limits universal because the effects of sediment plumes are site and time specific. Site specific because ecosystems and natural conditions vary from site to site and time specific because they depend on tide, day or night, seasons and hydrodynamic conditions (like monsoons, breeding and hatching seasons, currents). Per project has to be decided if the environment is sensitive to sediment plumes and whether limits are required. It is concluded that requirements for sediment plumes in dredging contracts are often not well founded. Limits usually are copied from other projects or reports without much investigation and/or based on water criteria standards or community perception. The second study focuses on the effects of sediment plumes on one specific ecosystem: coral reefs. Probably this is the most sensitive marine ecosystem. The literature on the effects of suspended sediment and turbidity on corals is reviewed and restated in the context of dredging. The effects are evaluated in terms of acceptable damage, for which a recovery time of one year is used, considering the long-term equilibrium situations, or natural conditions, of coral reefs. Most published experiments were focussed on the sublethal effects. It appears that, taking in account the observed species and sediment sizes, corals can clear themselves from 100 mg/cm2/day and can handle concentrations of suspended solids up to 100mg/l for a few days. Drastic lethal effects on corals were experimentally observed only when burial was complete. There must be an intermediate dose of sediment, but no data exist in this range. Corals can not handle burial caused by dredging activities for more than, depending on the natural conditions, 1 day to 1 week. To put the data in perspective and to give direction to future discussions about norms and limits, it is proposed to work with dose-effect curves as used in toxicology. This approach also indicates that experiments with higher sediment doses where partial but significant damage occurs and where recovery times are registered must be done to achieve higher precision in the conclusions. The impact of sediment plumes probably can be minimised by modifying dredging techniques. Dredging also can be confined to specific time windows from which sensitive seasons of the ecosystem and conditions that adversely affect the distribution and impact of the plume can be excluded. Another possibility to reduce the impact of sediment plumes caused by dredging is to remove the sediment with a flowdredge. Further an approach is given to calculate the extent of possible effects caused by dredging plumes. The third part is a contribution to the methodology of how to implement and control a set limit in real time during a dredging operation. A probabilistic description of sediment plume behaviour in the Øresund Fixed Link Dredging project is given. Very many measurements were done during this project, which gives the opportunity to investigate the minimum but required frequency of measurements to check if the actual turbidity or concentration of suspended solids complies with the limits. When measurements are taken continuously the probability that the limits are exceeded without being noted is small, but continuous measurement methods are expensive. The objective of this study is to optimise the number of measurements at a location, i.e. the number of ship crossings through a sediment plume, given the costs and the accuracy of the turbidity measurements, by a statistical data analysis. Turbidity is assumed to be a random variable and the inherent uncertainty of turbidity is described by a Binomial-Exponential (BE) distribution function. The data contain natural background values as well and the BE distribution separates the background values from the turbidity due to dredging. The BE distribution has two distribution parameters, which are calculated. as well as their uncertainty in dependence on the number of ship crossings. The statistical uncertainty of the probability distribution function of the parameters and of the turbidity depends on the number of ship crossings. The maximum allowable turbidity is probabilistically determined as function of the number of ship crossings. With this approach the number of ship crossings can be determined for a required reliability. A method is proposed and illustrated with the data to optimise the number of ship crossings by optimising total costs, consisting of the costs of ship crossings and the gains of having a smaller uncertainty. Also a sensitivity analysis of the cost parameters is given. With this method, when the costs (disadvantages) of having a smaller accuracy of the turbidity probabilistic distribution function are assessed in relation to the Øresund case and assuming the same plume behaviour, the number of ship crossings can be optimised for other cases. List of relevant terminology Background conditions Natural turbidity and concentration of suspended solids Benthic organisms, benthos Organisms that spend all or most of their life at the bottom of the water column Bleaching A phenomenon in which Zooxanthellae microalgae leave a coral, resulting in coral whitening, under the stress of high water temperature, low salinity or high turbidity. If such a stress is removed the coral will recover but if bleaching continues for an extended period, coral will die. Coral reef A coral reef is defined as the topography formed by hermatypic organisms that are mainly composed of coral. In a coral reef, animals called coral live in a colony. The reef includes the area where corals grow and the seaweed and seagrass habitats and sandy or muddy places around the coral. Cnidaria A phylum of animals including corals and jellyfish Hermatypic The property of corals that produce a skeleton of calcium carbonate in a coral reef. Most of the corals that coexistent with Zooxanthellae are hermatypic. Mitigation The policy of trying to eliminate the effects of development on the environment, through avoidance, minimization, correction, reduction, and compensation. Natural conditions Climate-, background-, and hydrodynamic conditions Phylum A taxonomic group of similar classes having common properties Planula A coral or other cnidarian larva Zooxanthellae Single-cell algae, which are symbiotic with hermatypic corals and other animals List of notation and abbreviations

Secure dredging system and water treatment vessel

Abstract: A system and method for removing contaminated sediments during dredging operations. A crane barge (116) operates a pressure controlled enclosure housing an excavator bucket. A self contained membrane treatment vessel (115) removes fine particulate matter from dredge drainage water, as well as treating other contaminated water and shipboard wastes. A control zone is formed below a floating vessel (102) by a continuous wall formed by sheet piles and covered by an impermeable flexible drape.

Method and device for processing dredging slurry

Abstract: PROBLEM TO BE SOLVED: To provide a dredging slurry processing device capable of separating sand to be effectively utilized from the dredging slurry at a low cost during the time for carrying the dredging slurry to a slurry pond or the like, and to provide a dredging slurry processing method using the same. SOLUTION: Sand separating parts 1, 2, 3, 4 forming a part of a dredging slurry carrying passage and for separating the sand are arranged in plural series along a carrying route in a dredging slurry processing device. The sand separating parts 1, 2, 3 are provided with flow passages 15, 25, 35 for flowing the slourry from inflow ports 13, 23, 33 to outflow ports 14, 24, 34 and sand discharge ports 171, 271, 371 for discharging the sand settled on bottom parts 161, 261, 361 of the flow passages 15, 26, 35 and to be freely opened and closed. Flow speed of the dredging slurry flowing inside of the upstream side sand separating parts 1, 2 is set higher than the flow speed of the dredging slurry flowing inside of the downstream side sand separating parts 2, 3.