Showing papers on "Dredging published in 1995"

The impact of habitat disturbance by scallop dredging on marine benthic communities:what can be predicted from the results of experiments?

Abstract: Field experiments were conducted on 2 subtidal sandflats to identify the short-term impacts of commercial scallop dredging on macrobenthic communities. The 2 sites (1400 m2) were situated 14 km apart, both at about 24 m depth, with similar exposure aspects and were characterised by infaunal communities dominated by small and short-lived species. Prior to dredging, preliminary sampling failed to reveal significant differences in the density of common macrofauna within each site, although community composition was distinctly different between sites The experiment was initiated by using a commercial scallop dredge to dredge half of each study site. Macrofauna samples were collected in both the dredged and adjacent reference plot at each site irnmed~ately after dredging and again 3 mo later. The density of common macrofaunal populations at each site decreased as a result of dredging, with some populations still significantly different from the adjacent reference plot after 3 mo. Significant compositional differences in the assemblage structure between dredged and reference plots were also recorded at each site over the course of the experiment. The f ~ n d ~ n g s of this experiment are considered a conservative assessment of bottom disturbance by fishing because of the area of seabed used, the types of community present and the intensity of disturbance used in the experiment. The findings of this and similar short-term experiments are discussed in light of the need to predict and assess possible large-scale changes to benthic communities as a result of habitat disturbance by fishing. KEY WORDSFishing impacts . Habitat disturbance . Scallop dredging . Benthic communities Scaling-up

Dredging, remediation, and containment of contaminated sediments

Abstract: This conference was held June 23--24, 1994 in Montreal, Canada. One purpose of this conference was to provide a multidisciplinary forum for exchange of state-of-the-art information on identifying tests, methods, procedures, and materials, used in support of dredging, treatment, and containment of contaminated sediments that are in need of standardization. Another objective was to provide a forum for discussion of past dredging practices and future directions, including the effects of sediment properties and behavior, equipment requirements, and the impact of regulations. Individual papers have been processed separately for inclusion in the appropriate data bases.

Dredging-Induced Near-Field Resuspended Sediment Concentrations and Source Strengths.

Abstract: : Dredging in riverine, lacustrine, and estuarine environments introduces bottom sediments into overlying waters because of imperfect entrainment and incomplete capture of sediments resuspended during the dredging process and the spillage or leakage of sediments during subsequent transportation and disposal of the dredged sediments. Resuspension of bottom sediments and resulting dispersal may pose water quality problems in waters near dredging operations. Interest in this issue increases when the sediment being dredged is highly contaminated. Resuspension of sediments by dredging is affected by dredge characteristics, dredge operating conditions, properties of bottom and suspended sediments, and site-specific conditions such as bottom topography, ambient current, and depth. This report summarizes field studies conducted by the U.S. Army Corps of Engineers to assess the suspended sediment concentrations in the water column in the vicinity of various dredge types. These concentration data are combined with conceptual models for resuspended sediment source strength geometries and velocity patterns to estimate sediment source strengths for cutterhead and clamshell dredges. Although unverified, these models provide a starting point for a more thorough analytical evaluation of the entire resuspension, transport, and deposition process. (AN)

A mercury model used for assessment of dredging impacts

Abstract: The effects of dredging of contaminated sediments on the mercury (Hg) concentrations of prey and predatory fish were calculated for the Kokemaenjoki River and its estuary in Western Finland. The accumulation of Hg in fish is controlled by the Hg concentrations in water, zooplankton, zoobenthos and by suspended solids. Hg is accumulated into fish mainly through food web, eg. from perch (Perca fluviatilis) as prey and to pike (Esox lucius) as predator. In addition to dredging, temperature and flood situations have also increased the Hg accumulation and release from the bottom sediments.

Dredging Elutriate Test (DRET) development

Abstract: : The removal of contaminated sediments from waterways by dredging generates concern about the release of contaminants to the water column. The ability to predict the magnitude of these potential releases during the project planning process will improve decision making in regard to water quality impacts and controls or mitigation measures for the dredging project. This report describes the development of a simple laboratory test, the dredging elutriate test (DREl), to predict the concentration of contaminants in the water column at the point of dredging. The DRET is procedurally similar to the modified elutriate test developed by the Corps of Engineers to predict the contaminant concentrations in effluent from a confined disposal facility. The test involves mixing sediment and site water, allowing the heavier solid particles to settle, sampling and supernatant, and analyzing for dissolved and particulate bound contaminants. Results of the laboratory test compared well with field data collected while dredging New Bedford Harbor sediment, which was contaminated with polychlorinated biphenyls. Most of the contaminated loading was associated with the suspended particles. (AN)

Dredging vessel, dredging assembly and method of dredging

Abstract: The present invention discloses a dredging assembly which includes a vessel having at least one opening therein. A dredging pole of a predetermined length is positioned in and extends through the opening and is positioned substantially perpendicular to the vessel when in the dredging position. A dredging wheel is positioned at the lower end of the dredging pole with a vertically movable tension wheel at an upper end of the dredging pole and a reversing wheel spaced horizontally from the dredging pole. An endless cable is mounted on the dredging pole extending around the tensioning wheel, dredging wheel and reversing wheel and a plurality of the scoops are mounted on the endless cable, wherein the scoops are turned upside down by the reversing wheel for dumping the material carried therein.

Dredge plant equipment and systems processes : summary report for technical area 3

Abstract: : Investigations conducted under the Dredging Research Program (DRP) Technical Area 3, 'Dredge Plant Equipment and Systems Processes,' resulted in new technologies and enhanced existing dredging systems to achieve economic load durmg dredging operations. Improvement included more effective eductors and a single-point mooring system for direct pumpout of hopper dredges onto beaches. Technology for dredge production and process monitoring was improved, and designs for hopper-dredge dragheads were modified to increase production. The following equipment, systems, or processes were improved: (a) eductors with more efficient sand-bypassing features and water-injection dredging to fluidize shoals and permit transport by density of natural currents; (b) dredging equipment for nearshore/onshore placement, including a direct pumpout capability for hopper dredges; (c) technology for monitoring and increasing dredge payloads for fine-grain sediments, including an automated load monitoring system and an electrical resistivity method; and (d) draghead design enhancements for more efficient production, including water jets and blades to loosen compacted sediments. (MM)

Influence of offshore banks on the adjacent coast

Abstract: A numerical model has been used to study the influence of offshore sand banks on the wave climate along a stretch of the Belgian coast. Results of the model have been interpreted to provide predictions of wave height under several scenarios, assuming that the banks do not respond quickly to changes in the local hydrodynamic environment. Results show that the sand banks afford substantial protection to the coast and that this effect may be reduced as a result of rising mean sea level and dredging.

Contaminated marine sediments dredging apparatus

Abstract: A marine dredging device comprised of scoop buckets hinge-connected to a soil receiver chamber with deployable silt curtains. The silt curtains are deployed to close off the aperture between the scoop buckets and prevent the extrusion of soil. Introduction of pressurized air lowers the water level in the soil receiver chamber to provide a void for dredged soil.

Effectiveness of Spur Jetties at Siuslaw River, Oregon. Report I: Prototype Monitoring Study.

Abstract: : In 1985, the rubble-mound jetties at the entrance to the Suislaw River, Florence, OR, were extended offshore. In addition, on the ocean side of each jetty, one long spur oriented 45 deg to the main structure was constructed shoreward of the seaward end of each of the twin jetties. The spur system was investigated as a cost reducing alternative to significant linear jetty length extension which would reduce sediment shoaling and dredging requirements in the channel and improve navigability. Cost reductions were expected in reduced maintenance dredging and in actual construction and material cost. Monitoring and evaluation of the jetty system were conducted through the U.S. Army Corps of Engineers, Monitoring of Completed Coastal Projects (MCCP) Program by the Coastal Engineering Research Center (CERC) in coordination with the U.S. Army Engineer District, Portland (NPP). Data collected during field monitoring of the area are related to incident wave conditions. Spur-induced current patterns and sediment deposition patterns, along with annual dredging records are analyzed to evaluate structure performance. The favorable results of this MCCP study substantiate physical model test findings and indicate potential application of spur jetties at other sites.

A Second Life for Dredged Material

Abstract: To ensure safe, navigable waters, rivers and waterways must be dredged. Dredging in the U.S. requires long-term alternatives for placement of more than 300 million cu yds of dredged material per year, according to the National Research Council. Dredged material has been used successfully throughout the U.S. for the development of wetlands and aquaculture; beach nourishment; shoreline stabilization and erosion control projects; the enhancement of agriculture, forest and horticulture; open cast mine reclamations’ solid waste management, construction and industrial projects (such as port development) and material transfer for fills, according to the U.S. Army Corps of Engineers. Most Corps wetland restoration and creation projects are aimed at stabilizing shoreline and sediment, controlling erosion and developing fish and wildlife habitats. Researchers at the Corps’ Waterways Experiment Station, Vicksburg, Miss., have developed and tested a number of engineering field methods, including the use of geotubes and bioengineering that work with dredged material to almost guarantee a thriving habitat. These techniques have been used successfully at many sites throughout the Chesapeake Bay area and as part of a demonstration marsh developed along the Houston Ship Channel.

Grab dredging method

Abstract: PURPOSE:To perform a dredging while preventing the diffusion of sludge in water, and also perform the dredging without causing a pollution problem even when the sludge contains harmful materials. CONSTITUTION:A dredging area A is surrounded by a water shielding pollution preventing frame 1, the polluted water is sucked and filtered by a pump 19, and the resulting water is released to the outside of the pollution preventing frame 1. The water bottom E of the dredging area A is excavated and dredged while the non-polluted water on the outside of the pollution preventing frame 1 is introduced into the pollution preventing frame 1.

Dredging Up the Past

Abstract: Disposal of contaminated dredged materials has become a national problem. The nation’s ports and harbors have postponed dredging because of the cost and difficulty in finding sites for these materials. In doing so, they jeopardize their economic health, since container ships need deeper channels to enter ports. Several engineers have devised methods to deal with these sediments. Some deal with containment, and innovation in new computer modeling that track flow paths and types of contamination to optimize the design of confined disposal facilities. Another method takes contaminated sediments and contains them in geosynthetic tubes that can be buried, or used for shoreline enhancement applications to create wetlands or fish farms. A method called LADS converts contaminated sediments to clean lightweight aggregate. Several kinds of equipment deal with dewatering the sediments, so less volume needs to be remediated or disposed of. A bio-remediation technique treats contamination in-situ and other methods are based on chemical extraction. Some consultants are using computer modelling to determine which soil types are likely to be contaminated with which pollutants. The idea is to pinpoint areas that need dredging and remediation, or disposal, and so minimize costs. Several case histories are described.

Geotechnical Site Investigations for Dredging Projects.

Abstract: : The objective of a geotechnical site investigation for a dredging project is to obtain the most complete and accurate estimate of the location, description, and dredgeability properties of the sediments to be dredged that is possible within the limits of available time and money and of practicality. This technical note offers guidance in the planning of a dredging site investigation and in the methods typically used for underwater geotechnical investigations. This guidance was developed as part of the U.S. Army Engineer Waterways Experiment Station's (WES) Dredging Research Program. (MM)

The success of elutriate tests in extended prediction of water quality after a dredging operation under freshwater and saline conditions.

Abstract: Dredging simulation by elutriate tests accurately predicted concentrations of Hg, Cu, Mn and Fe released to the water column from contaminated sediment to within 1 order of magnitude. Hg and Cu concentrations increased by up to 7-fold after dredging, but declined to background concentrations within 48 h. Maximum loadings of Hg and Cu coincided with Fe and total organic carbon (TOC) water column concentrations, suggesting Hg and Cu are adsorbed onto particulates of Fe oxides and organic material. Seasonal changes in redox potential and temperature did not significantly affect metal release from sediments. Saline water did not cause significant increases in contaminant release from sediments to the water column over that observed for freshwater. Water quality standards of 1 µg l−1 Hg and 28 µg l−1 Cu as annual averages were not breached by dredging operations. Long-term effects of dredging on Hg and Cu availability, due to deposition of contaminated material as surficial sediments, is, however, of concern.

REMOTS monitoring of dredging, disposal, sand mining, and organic enrichment on the Hong Kong shelf

Abstract: The territorial waters of Hong Kong encompass extensive areas of shallow continental shelf affected by many activities including sand dredging for reclamation fill, disposal of dredged mud, disposal of sewage sludge, commercial fishing, and mariculture. The REMOTS sediment profile imaging camera has been used successfully since 1993 for rapid seafloor assessment to define baseline conditions and impacts on sediment duality during both wet (typhoon) and dry seasons. Several hundred stations have been occupied in a two week survey. Analysis of sediment profile images has allowed assessment of dredging and disposal effects and organic enrichment "hot spots". It appears that tropical storms may have a large effect on mobilization of soft sediments during the wet season, while anthropogenic factors (dredging, disposal, and seabed fishing activities) may assume greater relative importance in some areas during the dry season when severe storms are less common. The soft bottom benthic community consists of a few species of small invertebrates adapted to physical disturbance.

Device for removing dredging material, and method for operation thereof

Abstract: A device for removing dredging material from a water bed, comprising a rotatable cutting head (2) for cutting away the dredging material, and also conveyance means (4) for discharging the dredging material cut away, which conveyance means comprise a pump (5) which is connected to a pressure line. For complete removal of the dredging material cut away, the suction side of the pump is connected to a bucket (1), in which bucket the cutting head is situated. The suction connection of the pump also opens into the bucket.