Showing papers on "Dredging published in 1994"

Seagrasses, dredging and light in Laguna Madre, Texas, U.S.A.

Abstract: Light reduction resulting from maintenance dredging was the suspected cause of large-scale loss of seagrass cover in deep parts of Laguna Madre between surveys conducted in 1965 and 1974. Additional changes to 1988, together with an analysis of dredging frequency and intensity for different parts of the laguna, were consistent with this interpretation. Intensive monitoring of the underwater light regime and compilation of detailed environmental data for 3 months before and 15 months after a dredging project in 1988 revealed reduced light attributable to dredging in four of eight subdivisions of the study area, including the most extensive seagrass meadow in the study area. Dredging effects were strongest close to disposal areas used during this project but still were detectable on transects >1·2 km from the nearest dredge disposal area. In the subdivision of the study area where most of the dredge disposal occurred, light attenuation was increased throughout the 15 months of observation after dredging. In the seagrass meadow and the transition zone at the outer edge of the meadow, effects were evident up to 10 months after dredging. Resuspension and dispersion events caused by wind-generated waves are responsible for the propagation of dredge-related turbidity over space and time in this system.

Trace Metals as Tracers of Dredging Activity in Cleveland Bay - Field and Laboratory Studies

Abstract: This study has investigated in detail trace metal concentrations in Cleveland Bay in the central Great Barrier Reef and assessed the significant carrier phases of several metals during a simulated disturbance of sediments designed to investigate the effects of dredging. Organic, iron oxide and carbonate phases were shown to be important carrier phases for several trace metals. The application of an acid-leach technique to monitor labile or pollutant concentrations of copper, zinc, lead and nickel in sediments collected from coral reefs sampled before and after two dredging events in 1991 yielded useful information on the fate of dredged sediment. Trace metal contamination close inshore was attributed to port activities, sewage discharge and urbanization.

Dredging Impacts on Delaware Estuary Tides

Abstract: This study summarizes historic dredging activities and historic tidal trends in the Delaware Estuary. Since 1910, large-scale dredging and maintenance projects in the Delaware Estuary have increased navigation channel depths from approximately 20 ft (6.1 m) to 40 ft (12.2 m). Upper Delaware Estuary tidal ranges also increased approximately two-fold during this period. The potential influence of dredging activities on Delaware Estuary tides is examined through application of variable depth hydrodynamic models. The models suggest that historic increases in upper Estuary tidal ranges are associated with the deepening of middle/upper Estuary navigation channels. The models also suggest that competing effects of bottom friction and upstream `funneling' regulate tidal amplification in the Delaware Estuary and other similar systems.

Environmental Dredging on the Birmingham Canals: Water Quality and Sediment Treatment

Abstract: Sediments in sections of the central Birmingham canals are heavily contaminated with heavy metals and mineral oils. The sediments promote high levels of metals within the water column, which suppress the establishment of aquatic vegetation and fisheries. Sediment disturbance by boat traffic results in the release of mineral oils which cause unsightly surface oil sheens. Studies into a variety of treatment methods indicated that removal and ex-situ treatment of the contaminated sediments would significantly improve water quality and result in environmental benefits. Funding from Birmingham City Council and British Waterways enabled the award of a contract with a value of approximately £800000 and commencement in February 1993 for the removal and processing of about 24 000 m3 of sediments by soil washing prior to off-site disposal. The nature of the sediments and water are reviewed, and the results of the soils washings are discussed. The impact of the dredging operations on water quality was monitored during the works and generally observed to be localized. An environmental management plan is now being implemented and the objectives for this are discussed.

Environmental Impact of River Dredging in Arctic Alaska (1981-89)

Abstract: Economic and social development in northern Alaska during the past 25 years has created great demands for sand and gravel. Although this resource is available on some beaches, riverbeds, barrier islands, beach ridges, and alluvial fans, surficial mining often results in undesirable environmental modification in this permafrost-dominated environment and has led to restrictions on extraction. In 1981, for the first time, sand and gravel were transferred directly from the thalweg portion of a river to the tundra surface by a transportable suction dredge at Nuiqsut in the Colville River delta, Alaska. The 400 000 cubic meters of sand and gravel extracted were sufficient to construct a runway and create a stockpile for village use. This procedure has been used in two other rivers (the Meade and Kokolik), three lagoons (Wainwright, Kaktovik, and Barrow), and two lakes (Atqasuk and Barrow). Profiling through 1989 shows that infilling of the dredged channels is a variable process. Key words: Alaska, North Slope Borough, Colville River, Nuiqsut, permafrost, ice wedge, river dredging, gravel, construction pads

Deep seabed rock dredging in areas with high relief

Abstract: Deep‐seabed dredging has been for more than 125 years and continues today to be the primary tool for sampling deep‐seabed, hard‐substrate environments. Much effort and innovation have been invested in this pursuit to accommodate the varying seabed types and the objectives of the investigators. Little progress has been made, however, during this period to improve the accuracy of locating the samples that are dredged and the efficiency of the technique. With the current availability of high‐resolution, Global Positioning System (GPS) navigation and accurate bathymetric maps in many deep‐seabed areas, considerable improvement in both accuracy and efficiency is possible. The purposes of this study are to examine the techniques used and results obtained using these relatively new tools during a recent expedition that dredged several seamounts at water depths of 1,300 to 2,500 m in the vicinity of Johnston Island in the north‐central Pacific Ocean, and also to test two different dredge designs for their perform...

Dispersion Analysis of Charleston, South Carolina, Ocean Dredged Material Disposal Site.

Abstract: : The dispersive characteristics of the Charleston, SC, Ocean Dredged Material Disposal Site were investigated to determine whether dredging operations could pose a threat to recently discovered live coral reef areas. Two levels of investigation were employed. A short-term analysis of the disposal operation was conducted to examine the immediate fate of material following release from the barge and subsequent descent to the ocean bottom. The second phase examines the long-term fate to determine whether local ocean currents are capable of eroding and transporting deposited material from the site to the reef area. Results of this study indicate the site to be dispersive and recommendations are made as to locations within the designated limits which will minimize the possibility of reef area effect as a result of the disposal operation. Disposal site classification, Sediment fate, Disposal site stability, Sediment transport, Dredged material.

Central San Francisco Bay suspended-sediment transport processes study and comparison of continuous and discrete measurements of suspended-solids concentrations

Abstract: Sediments are an important component of the San Francisco Bay estuarine system. Potentially toxic substances, such as metals and pesticides, adsorb to sediment particles (Kuwabara and others, 1989; Domagalski and Kuivila, 1993). Sediments on the bottom of the bay provide the habitat for benthic communities that can ingest these substances and introduce them into the food web (Luoma and others, 1985). Nutrients, metals, and other substances are stored in bottom sediments and pore water in which chemical reactions occur and which provide an important source andor sink to the water column (Hammond and others, 1985; Flegal and others, 199 1). The transport and fate of suspended sediment is an important factor in determining the transport and fate of the constituents adsorbed on the sediment. Seasonal changes in sediment erosion and deposition patterns contribute to seasonal changes in the abundance of benthic macroinvertebrates (Nichols and Thompson, 1985). Tidal marshes are an ecologically important habitat that were created and are maintained by sedimentation processes (Atwater and others, 1979). In Suisun Bay, the maximum suspended-sediment concentration marks the position of the turbidity maximum, which is a crucial ecological region in which suspended sediment, nutrients, phytoplankton, zooplankton, larvae, and juvenile fish accumulate (Peterson and others, 1975; Arthur and Ball, 1979; Kimmerer, 1992; Jassby and Powell, 1994). Suspended sediments confine the photic zone to the upper part of the water column, and this limitation on light availability is a major control on phytoplankton production in San Francisco Bay (Cloern, 1987; Cole and Cloern, 1987). Suspended sediments also deposit in ports and shipping channels, which must be dredged to maintain navigation (U.S. Environmental Protection Agency, 1992). The objectives of the Central San Francisco Bay suspended-sediment transport-processes study are to estimate which factors determine suspended-solids concentrations (SSC) in Central Bay and to collect time series of SSC data that are appropriate for (1) continuous monitoring of SSC and (2) calibration and validation of numerical models. Potentially important factors include semidiurnal and diurnal tides, the springheap cycle, delta discharge, dredging and dredged material disposal, and wind waves. SSC monitoring sites were established at Point San Pablo in December 1992 and at San Francisco Pier 24 in May 1993 (Figure 1) (Buchanan and Schoellhamer, 1995). At each site, optical backscatterance (OBS) sensors are positioned at mid-depth and near the bottom. The OBS sensors optically measure the amount of material in the water every 15 minutes, and the output of the sensors is converted to SSC with calibration curves developed from analysis of water samples. The sites are serviced every 1 to 5 Lveeks to clean the sensors, which are susceptible to biological fouling, and to collect water samples for sensor calibration. About half the data collected is invalid, primarily because of sensor fouling. SSC monitoring sites also are located in South San Francisco Bay and Suisun Bay (Figure 1). The sites are operated in cooperation with the U.S. Army Corps of Engineers (Central Bay): the California Regional Water Quality Control Board. San Francisco Bay Region, as part of the Bay Protection and Toxic Cleanup Program (South Bay); and the Interagency Ecological Program (Suisun Bay). Continuous SSC data can be used to help place the discrete water-quality data collected as part of the Regional Monitoring Program (RMP) into a proper context. Vertical profiles of SSC were collected with an

Management of Sediments on the Red River Waterway Project

Abstract: The Red River is a large alluvial river system with the lower 280 miles currently being developed for shallow draft navigation. The navigation project entails construction of five locks and dams, channel realignment, and bank stabilization works. The project features will be completed to allow navigation to Shreveport, Louisiana by early 1995. The Red River is heavily laden with sediments which are generally fine grained sands. The Red River has one of the highest sediment concentrations of all major rivers in the United States. Its sediment concentration is comparable to large rivers of the world in sediment concentration. Due to the amount and nature of sediment, the project features have incorporated management techniques to minimize the adverse impacts. The first two locks and dams constructed have been modified to reduce the sediment maintenance problems. Three remaining locks and dams were modified in design to improve the sediment management capability. Channel structures have been utilized effectively to minimize the need for maintenance dredging.

Channel Dredging, Dredge Spoil Migration and Downdrift

Abstract: A major dredging programme to deepen the shipping channels at the Port of Tauranga has seen some 5 million m3 of mainly sandy sediment dredged from the shipping channels and dumped on the adjacent inner shelf. An Environmental Impact Assessment has identified major concerns and consent granting authorities have proposed a monitoring programme.

Cocks Eldorado Dredge

Abstract: A brief history of gold dredging in Victoria is outlined to provide context for consideration of the Cocks Eldorado Dredge, which is the last surviving bucket dredge in Victoria and probably Australia. It was constructed in 1936 during a new phase of dredging in Victoria. The dredges built in Victoria about this time had increased capacity and incorporated design features from the large dredges developed in Malaya and elsewhere. The dredge mined gold and tin until 1954. Control of the dredge eventually reverted to the Victorian Government and options for its conservation are considered.

Dredge device for sewage

Abstract: The utility model relates to a dredging device for a sewer pipe belonging to the field of dredging equipment for a pipe. The dredging device improves a supporting base, a valve plate, an air-filled bag and a seat pipe on the basis of the patent (90206053.8) of China. Therefore, the utility model has the advantages of good air tightness, convenient manufacture and installation, safety, low cost, etc. The utility model is suitable for dredging various sewer pipes with different pipe diameters.

Reservoir Sediment Removal: Hydrosuction Dredging

Abstract: Hydrosuction dredging is a relatively little-known method of removing sediment deposits from reservoirs. The method is similar to traditional hydraulic dredging except the difference between the upstream and downstream water levels provides the energy for the system instead of a mechanical pump, and the removed sediment is discharged into the receiving stream downstream from the dam. Removing sediment deposits from reservoirs entails technical and environmental issues. Fieldwork from Lake Atkinson, Nebraska, on the Elkhorn River, serve to illustrate the issues. The technical problem involves the design of a system with multiple unknowns. The system should be designed to remove sediment at the same rate that it enters the reservoir with a minimum head loss. Equations are presented for determining the required pipeline fluid velocity and the maximum sediment concentration in the pipe. Environmental issues include dealing with low dissolved-oxygen discharge into the river, increased turbidity, and potentially contaminated sediments. These issues are briefly addressed. Regulatory agencies play a role in determining the adequacy of any plan that changes river characteristics. The Nebraska experience is related as a positive example of how to proceed with full regulatory endorsement.