Showing papers on "Dredging published in 2008"

Evaluating the effectiveness of contaminated-sediment dredging.

Abstract: As the science' of environmental dredging and sediment management changes, adaptive management strategies can help long-term remediation projects keep pace.

Technical Guidelines for Environmental Dredging of Contaminated Sediments

Abstract: : This report provides technical guidelines for evaluating environmental dredging as a sediment remedy component. This document supports the Contaminated Sediment Remediation Guidance for Hazardous Waste Sites, released by the U.S. Environmental Protection Agency (USEPA) in 2005, by providing detailed information regarding evaluation of environmental dredging as a remedy component. This document is intended to be applicable to contaminated sediment sites evaluated under various environmental laws and regulatory programs. The intended audience for this report includes all stakeholders potentially involved in evaluating environmental dredging for purposes of a feasibility study, remedial design, and implementation. The scope of this document is limited to the technical aspects of the environmental dredging process itself, but it is important that environmental dredging be integrated with other components such as transport, dewatering, treatment, and rehandling and disposal options. This report covers initial evaluation, pertinent site conditions and sediment characteristics, environmental dredging performance standards, equipment capabilities and selection, evaluation of production, duration, and transport, methods for estimating resuspension, residuals and release, control measures, operating methods and strategies, and monitoring.

The Four Rs of Environmental Dredging: Resuspension, Release, Residual, and Risk

Abstract: A workshop sponsored by the U.S. Army Corps of Engineers and the U.S. Environmental Protection Agency was held at the U.S. Army Engineer Research and Development Center, Waterways Experiment Station, in Vicksburg, MS, in April of 2006. Fifty experts from government, the private sector, and academia participated in the workshop, which focused on four issues (the four Rs) relevant to environmental dredging: (1) sediment resuspension resulting from dredging operations, (2) release of contaminants from bedded and suspended sediments in connection with dredging, (3) residual contaminated sediment produced by and/or remaining after dredging, and (4) the environmental risks that are the target of and associated with dredging. Goals for the workshop were to: (1) promote consistency in the terms used to define the challenges represented by the four Rs, (2) develop consensus for a conceptual model that relates the relevant processes, (3) identify current resources and needs regarding data and methods/models to better describe and understand the processes, and (4) identify key uncertainties and make recommendations regarding future research to resolve those uncertainties. This technical report summarizes analysis and synthesis of the results of the workshop.

Sediment transport and morphodynamics of the Douro River estuary

Abstract: A combination of dredging data, hydrographic surveys and numerical modelling has been used to assess morphological change and sediment transport in the Douro River estuary. The system is dominated by sand- and gravel-sized sediments and confined by resistant rock types. The evolution of the bed in the last 20 years has been strongly influenced by the opening of a navigation channel. According to the data available to date, the average maintenance dredging volume has been of the order of 0.4 × 106 m3 year−1. Comparisons of hydrographic surveys reveal a rate of volume loss of the same magnitude. Apparently, maintenance dredging mainly involves local material, transported into the channel from shallower areas of the estuary. The results of numerical modelling indicate that the sediment transport capacity due to tidal currents is very limited. River flood events increase the transport capacity by several orders of magnitude, thus playing a critical role in sediment redistribution and supply to the coast. The average sediment transport capacity is estimated to be of the order of 0.1 × 106 m3 year−1 in most of the estuary and 0.5 × 106 m3 year−1 at the inlet, with a large uncertainty. It is concluded that, if morphological stability is set as an environmental objective, the dredged material should not be removed from the system but rather be used to nourish the estuarine beaches and the barrier spit.

Beach erosion and marine aggregate dredging: a question of evidence?

Abstract: Coastal erosion is a global problem and a major concern for European Union Member States. In the UK, marine aggregate dredging is considered by many to be responsible for coastal loss and campaigns based on the ‘precautionary principle’ have been mounted to halt extraction. Two South Wales, UK coastal areas, where critical beach loss has been associated with dredging activities, were monitored to assess morphological change. In five years of beach monitoring along the Penarth coastline (September 1997 to September 2002) and six years monitoring Port Eynon and Horton beaches (January 2001 to October 2007), no qualitative or quantitative causal link was found between marine aggregate dredging and beach erosion. Conversely, many qualitative and quantitative relationships were established between beach erosion and forcing agents such as water level, wind and waves. Results from Port Eynon and Horton showed significant temporal variations in beach level and on-shore/offshore sediment movement was seen as significant in beach formation processes. At Penarth, changing wind direction and increased easterly storms were most significant. Furthermore, there were indications of recognised causes of coastal erosion such as increased water levels, storms and anthropogenic construction. Therefore, morphological changes on a relatively short timescale can be clearly attributed to influences other than marine aggregate dredging. Potential future work includes regular monitoring and analysis of shoreline changes in areas adjacent to dredging sites together with concurrent bathymetric surveys. As well as being a strategic approach, it would address stakeholder concern and reduce conflict.

Impact of Dredging on Phosphorus Transport in Agricultural Drainage Ditches of the Atlantic Coastal Plain1

Abstract: Drainage ditches can be a key conduit of phosphorus (P) between agricultural soils of the Atlantic Coastal Plain and local surface waters, including the Chesapeake Bay. This study sought to quantify the effect of a common ditch management practice, sediment dredging, on fate of P in drainage ditches. Sediments from two drainage ditches that had been monitored for seven years and had similar characteristics (flow, P loadings, sediment properties) were sampled (0-5 cm) after one of the ditches had been dredged, which removed fine textured sediments (clay = 41%) with high organic matter content (85 g/kg) and exposed coarse textured sediments (clay = 15%) with low organic matter content (2.2 g/kg). Sediments were subjected to a three-phase experiment (equilibrium, uptake, and release) in recirculating 10-m-long, 0.2-m-wide, and 5-cm-deep flumes to evaluate their role as sources and sinks of P. Under conditions of low initial P concentrations in flume water, sediments from the dredged ditch released 13 times less P to the water than did sediments from the ditch that had not been dredged, equivalent to 24 mg dissolved P. However, the sediments from the dredged ditch removed 19% less P (76 mg) from the flume water when it was spiked with dissolved P to approximate long-term runoff concentrations. Irradiation of sediments to destroy microorganisms revealed that biological processes accounted for up to 30% of P uptake in the coarse textured sediments of the dredged ditch and 18% in the fine textured sediments of the undredged ditch. Results indicate that dredging of coastal plain drainage ditches can potentially impact the P buffering capacity of ditches draining agricultural soils with a high potential for P runoff.

Recovering the Sand Deficit from a Century of Dredging and Jetties along Florida's Atlantic Coast: A Reevaluation of Beach Nourishment as an Essential Tool for Ecological Conservation

Abstract: A sand deficit on Florida's Atlantic coast affects sea turtle nesting, dune ecosystems, and storm protection. Ecological benefits of restoring very large deficits could exceed ecological costs. Dredging and beach nourishment databases revealed sand disposal dynamics and deficit size. Dredge-and-fill activities increased after 1950, peaked in the 1980s, then declined somewhat. Most sand disposal accompanied channel and harbor deepening; little was primarily for beach nourishment. Until the 1970s most dredged material was placed outside the coastal sand-sharing system (offshore and upland). After 1970, beach and nearshore disposal rapidly increased, but generally involved sand already within the system. Moreover, offshore and upland disposal did not immediately decline. To date, little sand has been returned. By 2003, net removal totaled ∼130 × 106 m3. Channels and harbors increased by ∼70 × 106 m3, leaving 60 × 106 m3 of standing sand deficit. Jetties could have redistributed another 70 × 106 m3 f...

Polluted water body deposit environment-friendly dredging depth determination method

Abstract: The invention provides a determining method for polluted water bed mud environmental protection dredging depth, comprising analyzing and evaluating the releasing risk and ecological hazard risk of the pollutant at different layer side of the mud based on the vertical distribution of the pollutant in the bed mud by researching the relation of the releasing of bed mud pollutant nitrogen, phosphor or the like and the corresponding pollutant content and the ecological hazard evaluation corresponding with the content of the bed mud pollutant containing heavy metal; and determining the bed mud dredging depth based on the divided corresponding risk grade. After dredging, the anticipated release rate of the key pollutant such as nitrogen, phosphor at the new surface layer of bed mud-hydrosphere achieves or exceeds a release intensity threshold and the latent ecological hazard index of the heavy metal at the new surface layer of bed mud achieves or exceeds a hazard threshold, therefore the basis of the dredging depth is determined.

Ecological effects of the dredging in the West Lake, Hangzhou

Abstract: The West Lake, a famous scenic spot for tourists in China, is a small shallow eutrophic lake. However, the lake has been suffering from eutrophication from year to year and its beauty is marred by low transparency of the water. To control its eutrophication, a great deal funds had been invested and some projects had been organized by the government in the past twenty years. The effects of these projects were not satisfactory and the eutrophication is still progressing. Thus, the average depth 0.5m of silt on the bottom of the West Lake was dredged largely from 1999 to 2002. During the Jan. 1999 to Dec. 2000, the N and P of the sediment and water quality were surveyed, the phytoplankton, zooplankton and benthos communities (including species composition and density biomass) were investigated in the West Lake. To compare the investigated result with previous results and data (before the dredging), the succession of the communities was discussed. Further, the ecological effect of the dredging and the present trophic state of the lake were evaluated. The results showed that the N and P content in the sediment were decreased after the dredge, the water quality had been improved obviously, and the density in the phytoplankton community and the chlorophyll-a concentrations had decreased obviously comparing those in 1999, and the trophic level also had decreased in main lake areas.

Fate of Fine Sediment from Dredger-Based Mining in a Wave-Dominated Environment at Chameis Bay, Namibia

Abstract: Nearshore turbidity due to suspended fine sediment is a natural phenomenon around much of the western coast of southern Africa. However, there is concern about the potential impact on biotic communities of an increase in turbidity as a result of mining by means of marine dredgers. This paper discusses field measurements and the validation and application of mathematical models in investigations of the structure, composition, and decay of fine sediment plumes induced by dredging on a wave-dominated coast. In addition, the resuspension and mobility of the fine sediment deposited as a result of dredging are assessed. The expected trends of horizontal decay of concentrations with distance from the dredger and higher concentrations at greater depths were confirmed. The extent of plumes (i.e., where concentrations return to background levels) was predicted to be some 4 km at most, while a plume life span well in excess of 3 h was indicated. This extent and longevity of suspended sediment plumes were at...

Regulatory frameworks for sediment management

Abstract: Publisher Summary This chapter provides an overview of existing and developing sediment-relevant legislation and describes the influence of nonregulatory drivers on it. The chapter highlights a number of legislative and policy provisions that exist for the appropriate management of sediment quality and quantity issues. Waste regulation with respect to dredging issues also applies at very specific sites and principally applies in respect of disposal options rather than the need to dredge. The dredging of contaminated sediments only occurs where other legislation highlights this as a problem needing to be addressed or when sediments dredged for maintenance purposes are found to contain sufficiently high levels of specific substances that they have to be designated as contaminated. A sediment management framework needs to be developed to integrate, clarify, support, and further the legislation and regulations that currently exist for sediment management.

Effects of dredging on subtidal macrobenthic community structure in Mejillones Bay, Chile

Abstract: The construction of Port Angamos terminal 1 in Mejillones Bay, Chile, required the dredging of 1,200,000 m³ of sediment from depths of 2?20 m. The effects of dredging on macrobenthic faunal communities are due to chemical (organic material) and physical (granulometric) alterations of the environment and are variable depending on the specific conditions of the intervened area. This study uses descriptive statistics to determine the impacts of dredging on benthic macrofaunal communities. Dredging caused alterations of the physical environment which were reflected in the benthic communities which showed reduced richness, abundance, biomass and diversity, as well as diminished community complexity, in the study sites closest to the port.

The day after we stop dredging : a world without sediment plumes?

Abstract: Dredging activities are a pre-requisite for the development of human welfare, coastal safety and economic profit, yet the dredging industry is often criticised for having an adverse environmental impact, particularly through generation of sediment plumes during project implementation. Would the day after we stop dredging mark the onset of a world without sediment plumes? To answer this question a wider range of natural and humaninduced drivers of sediment plumes in delta areas should be considered. Would shipping activities cease the day after we stop dredging? Would natural rivers stop discharging large quantities of fine sediment during periods of high water run-off? To assess the environmental benefits of an “idyllic” world without dredging, the impact of maintenance dredging activities as compared to the impact of other, ongoing drivers of sediment plumes must be evaluated. The research presented here reflects recent progress in the framework of the TASS (Turbidity Assessment Software) programme, which involves a series of largescale field trials to collect high-quality data that can be used for model validation purposes. Recent field trials in Bremerhaven (2006) and Rotterdam (2007) resulted in valuable insight in optimal means to collect overflow samples for the quantification of overflow losses over a range of soil types, overflow configurations and environmental conditions. Moreover, the Rotterdam (2007) field trial is expected to help to assess the relevance of draghead plumes and propeller wash in view of dredging-induced turbidity, as well as the benefits of using a green valve. Both data sets will be used for TASS model validation and the identification of future model developments and research needs. Although the TASS programme focusses on dredging-induced turbidity increases, it should be noted that dredging is just one out of a series of processes that drive sediment plumes. These processes include natural events, shipping operations and fishing activities. An inventory of these processes suggests, at least qualitatively, that the annual impact of these processes is of the same order of magnitude as dredging. The author wishes to acknowledge the important contributions to this research by W.F. Rosenbrand of Royal Boskalis Westminster nv, Dredging Development Department, C. van Rhee of Van Oord Dredging and Marine Contractors BV and T.N. Burt, recently retired from HR Wallingford Ltd., UK as well as the funding by Stichting Speurwerk Baggertechniek (SSB) and Fonds Collectief Onderzoek as part of CROW. The paper was originally presented at the CEDA Dredging Days in November 2007 and was published in the conference proceedings. It is reprinted with permission in a slightly revised and updated version.

Modelling Impact of Dredging and Dumping in Ebb-Flood Channel Systems

Abstract: For a channel-shoal system in a funnel-shaped basin the impact of dredging and dumping is investigated using a complex process-based model. First, the residual flow and sediment transport circulations are analysed for the channel-shoal pattern, which has emerged after a long-term model simulation. Results are compared to the Western Scheldt estuary, which forms the inspiration for this study. Subsequently, different dredge and dump scenarios are modelled, according to a conceptual model, in which ebb-and flood-channels and enclosed shoals form morphodynamic units (cells) with their own sediment circulation. Model results show that dumping sediment in a channel further reduces the channel depth and induces erosion in the opposite channel, which enhances tilting of the cross-section of the cell and eventually can lead to the degeneration of a multiple channel system into a single channel. The impact of different dredging and dumping cases agrees with results from a stability analysis. This means that this type of model applied to a realistic geometry can potentially be used for better prediction of the impact of human interventions.

Offshore Sand and Gravel Mining

Abstract: This is a general overview of marine sand and gravel extraction. The origin of the relict or modern resource and the nature of the deposits are described, together with the related hydrodynamic and sedimentological regimes. Some important variations between countries are stressed in the marine aggregates production and usage. Details are provided on the prospecting methods for the identification of new resources, with emphasis on the ‘state-of-the-art’ sea bottom survey instrumentation. The article also focuses upon the principal techniques of extraction, that is, anchor and trailer suction dredging, and the associated features which are generated on the seabed (i.e., pits and furrows, respectively). The creation of such features, together with turbid plumes resulting from the extraction process, may induce significant environmental (physical and biological) impacts. These impacts are described in detail, while the recovery time of the changes is addressed. The article concludes with an overview of the supply and future of marine aggregate dredging.

A numerical model for the transport of salinity in estuaries

Abstract: In this paper, a numerical model for the simulation of the hydrodynamics and of the evolution of the salinity in shallow water estuaries is presented. This tool is intended to predict the possible effects of Civil Engineering public works and other human actions (dredging, building of docks, spillages, etc.) on the marine habitat, and to evaluate their environmental impact in areas with high productivity of fish and of seafood. The prediction of these effects is essential in the decision making about the different options that could be implemented. The mathematical model consists of two coupled systems of differential equations: the shallow water hydrodynamic equations (that describe the evolution of the depth and of the velocity field) and the shallow water advective–diffusive transport equation (that describes the evolution of the salinity level). Some important issues that must be taken into account are the effects of the tides (including that the seabed could be exposed), the volume of fresh water provided by the rivers and the effects of the winds. Thus, different types of boundary conditions are considered. The numerical model proposed for solving this problem is a second-order Taylor–Galerkin finite element formulation. The proposed approach is applied to a real case: the analysis of the possible effects of dredging Los Lombos del Ulla, a formation of sandbanks in the Arousa Estuary (Galicia, Spain). A number of simulations have been carried out to compare the actual salinity level with the predicted situation if the different dredging options were executed. Some of the obtained results are presented and discussed. Copyright © 2007 John Wiley & Sons, Ltd.

Dredging in sediments containing munitions and explosives of concern (MEC)

Abstract: : This document provides guidance to personnel (e.g., planners, cost estimators, specification writers, engineers, managers, and dredging contractors) involved in dredging projects with sediment containing Munitions and Explosives of Concern (MEC). The guidance is primarily in the form of compiled information gained from experiences on past dredging projects involving MEC and was compiled from a variety of sources. This report describes the different types of dredges and dredging projects that can encounter MEC, describes how these dredges' operational methodologies can be impacted by MEC, and summarizes past project methodology modifications that have been used to deal with MEC. Technical aspects of past MEC/dredging projects are presented with regard to engineering controls to mitigate detonation hazards, underwater MEC detection and discrimination technologies, contracting, public awareness, safety requirements, and MEC separation techniques and (where available) subsequent impacts on production rates and costs.

A synthesis on the assessment of an alternative disposal strategy to serve sustainability in the Scheldt estuary

Abstract: The Scheldt estuary is characterised by a valuable multiple-channel system consisting of sandbars in between primary and secondary channels. Within the framework of the Long Term Vision (LTV) for the Scheldt estuary the conservation of this multiple-channel system is defined as the key goal to achieve morphological sustainability. Dredging is one of the human activities having an impact on morphology, hence dredging and disposal should be optimised by minimising the negative impact on morphology. The current disposal strategy for maintenance dredging consists mainly of disposing sediment into the secondary channels. As a consequence of the sustainability goal a maximum dumping capacity is assigned to each secondary channel in order not to exceed the natural bearing capacity of the multiple-channel Scheldt estuary. The assigned yearly disposal capacity for all secondary channels together is sufficient to counter the yearly sediment volume arising from maintenance dredging operations. However, if the total sediment volume that is produced by the deepening of the navigation channel towards Antwerp would be disposed into the secondary channels, the conservation of the multiple-channel system would not be guaranteed. To anticipate on this expectation, shallow water sandbar slopes have been proposed as an alternative disposal location. The use of these alternative disposal locations provides an increase in the estuary’s disposal capacity without increasing the disposal intensity in the secondary channels and is expected to enlarge the area of shallow water habitats, creating opportunities for ecosystem development. The feasibility of this proposal has been investigated by desk studies on historical maps, by field measurements, by physical scale model tests and by numerical simulations and has been confirmed by two in situ experiments. Consequently, the alternative disposal locations will be used during the deepening of the navigation channel towards the port of Antwerp. As a result of the adaptation of the disposal strategy, the conservation of the valuable multiple-channel system in the Scheldt estuary is not endangered by dredging and disposal operations.

Complex satellite monitoring of the nord stream gas pipeline construction

Abstract: Shipping The Nord Stream is a more than 1200-km long gas pipeline that will link Russia to Europe via the Baltic Sea from Vyborg in Russia to Greifswald in Germany. The construction process of the pipeline may cause, in particular, the following impact on the marine environment: (i) oil pollution due to the operation of ships, pipelay vessel, dredge ships and mechanisms in the sea; (ii) increase of suspended matter concentration due to dumping of sand and gravel, and dredging operations; (iii) provoking of local algal bloom events in summertime due to vertical mixing resulted from dumping and dredging works. Thus, there are two very important and interrelated tasks: (i) to monitor in the operational regime the ecological state of the sea at the site of the pipeline construction, and (ii) to discriminate between natural effects and anthropogenic impacts, related to the construction itself.

Numerical Modeling for Sedimentation Characteristics of the Lower Nakong River and Sediment Dredging Effects at the Nakdong River Estuary Barrage

Abstract: The Nakdong River Estuary Barrage (NREB) was constructed in 1987 to prevent saltwater intrusion and to provide the sustainable water supply in the upstream channel. Sediment dredging has been conducted to eliminate deposited sediments in the approached upstream channel of the NREB. Fluvial changes and sedimentation problems have been continued due to urbanization and development in the watershed as well as construction of the NREB. However, the sufficient field monitoring and researches for sedimentation characteristics and bed changes have not been performed after construction of the NREB. Therefore, bed elevation changes and seasonal sediment concentration distribution were analyzed using the quasi-steady state model with historical field data in this study. The water surface elevation changes with and without sediment dredging operation were calculated using the developed quasi-steady state model and finally the sediment dredging effects were evaluated.

Real-Time Coral Stress Observations Before, During, and After Beach Nourishment Dredging Offshore SE Florida

Abstract: Beach nourishment in Southeast Florida involves dredging sand source borrow areas located between offshore reefs. From May 2005 to February 2006 Broward County, FL. nourished 10.9 km of beach with 1.5 ×10 6 m 3 of sand. As part of a program to monitor potential reef community impacts, a visual stress index was developed from laboratory experiments and histological analyses for three stony coral species (Montastrea cavernosa, Solenastrea bournoni, and Siderastrea siderea). Scoring involved healthy = 0; moderately stressed = 1 (polyp swelling, increased mucus); markedly stressed = 2 (coloration changes, increased mucus secretion, tissue thinning); and severely stressed = 3 (severe swelling/thinning tissue erosion/necrosis). Colonies were scored weekly at sites adjacent to borrow areas and control sites pre-, during, and post-dredging. Permit conditions were established which would suspend dredging based on mean stress index values above 1.5 at 50% of monitored sites adjacent to borrow areas. This condition was never met. However, three hurricanes, passing the region during dredging, contributed to an elevated mean stress level above 1.0. Post-dredging observations documented recovery to pre-dredging stress levels. This program was effectively used to monitor stress on a sensitive marine habitat adjacent to sediment dredging activities.

Method for delivering large quantities of under water soil to a reclamation area

Abstract: The invention relates to a method for delivering large quantities of under water soil and/or ore bearing soil from long-distance borrow areas to a reclamation site. The method comprises the steps of mooring at least one seagoing transport vessel (1) with a size considerably larger than the size of a dredging vessel (3) and adapted to receive a large quantity of soil in the vicinity of the at least one borrow area, providing at least one dredging vessel that dredges soil at the at least one borrow area and transports it to the transport vessel, loading the soil into a buffering vessel (4) or into the transport vessel (1) directly, transporting the soil in a transportable state to the long distance reclamation site with the aid of the transport vessel, mooring the transport vessel in the reclamation site, and unloading the soil from the transport vessel to the reclamation site.