Showing papers on "Dredging published in 1998"

The impact of dredging works in coastal waters : A review of the sensitivity to disturbance and subsequent recovery of biological resources on the sea bed

Abstract: The present review provides a framework within which the impact of dredging on biological resources that live on the sea bed ("Benthic" communities) can be understood, and places in perspective some of the recent studies that have been carried out in relation to aggregates dredging in European coastal waters. The impact of dredging works on fisheries and fish themselves, and on their spawning grounds is outside the scope of this review. We have, however, shown that empirical models for shelf waters such as the North Sea indicate that as much as 30% of total fisheries yield to man is derived from benthic resources, and that these become an increasingly important component of the food web in near-shore waters where primary production by seaweeds (macrophytes) and seagrasses living on the sea bed largely replaces that by the phytoplankton in the water column. Because dredging works are mainly carried out in near-shore coastal deposits, and these are the ones where benthic production processes are of importance in supporting demersal fish production, our review concentrates on the nature of ben thic communities, their sensitivity to disturbance by dredging and land reclamation works, and on the recovery times that are likely to be required for the re-establishment of community structure following cessation of dredging or spoils disposal. Essentially, the impact of dredging activities mainly relates to the physical removal of substratum and associated organisms from the seabed along the path of the dredge head, and partly on the impact of subsequent deposition of material rejected by screening and overspill from the hopper. Because sediment disturbance by wave action is limited to depths of less than 30m, it follows that pits and furrows from dredging activities are likely to be persistent features of the sea bed except in shallow waters where sands are mobile. Recent studies using Acoustic Doppler Current Profiling (ADCP) techniques suggest that the initial sedimentation of material discharged during outwash from dredgers does not, as had been widely assumed, disperse according to the Gaussian diffusion principles used in most simulation models, but behaves more like a density current where particles are held together during the initial phase of the sedimentation process. As a result, the principal area likely to be affected by sediment deposition is mainly confined to a zone of a few hundred metres from the discharge chute. Our review suggests that marine communities conform to well-established principles of ecological succession, and that these allow some realistic predictions on the likely recovery of benthic communities following cessation of dredging. In general, communities living in fine mobile deposits, such as occur in estuaries, are characterized by large populations of a restricted variety of species that are well adapted to rapid recolonization of deposits that are subject to frequent disturbance. Recolonization of dredged deposits is initially by these "opportunistic" species and the community is subsequently supplemented by an increased species variety of long-lived and slow-growing "equilibrium" species that characterize stable undisturbed deposits such as coarse gravels and reefs. Rates of recovery reported in the literature suggest that a recovery time of 6-8 months is charactristic

The direct and indirect effects of suction dredging on a razor clam (Ensis arcuatus) population

Abstract: Surveys were conducted in two shallow bays in the Orkney Islands, UK; Orphir Bay, an unexploited (control) site, and Bay of Ireland, a fished site, to investigate the eVects of suction dredging on the resident razor clam, Ensis arcuatus, populations. A lower density and significantly smaller mean length of razor clams were present at the dredged site compared with the control site. The age of individual razor clams was estimated using internal shell microgrowth patterns, visible in acetate peels of polished and etched shell cross-sections. Ensis arcuatus are relatively slow growing animals with the two study populations characterized by old individuals and an obvious lack of juveniles, indicating populations with little resilience to disturbance. An analysis of the shell sections of razor clams from the Bay of Ireland revealed the presence of shell margin breaks, consisting of deep clefts in which sand grains were embedded in the shell matrix, whilst those from Orphir Bay had fewer disturbances to shell growth. It is suggested the disturbances to shell growth are the result of repeated suction dredging operations in the Bay of Ireland. In situ reburrowing experiments were conducted to determine the survival rate of E. arcuatus (<160 mm shell length), returned to the sea after capture and to estimate the indirect eVect of dredging on the razor clam population. These individuals displayed a slow initiation of ‘‘escape-digging’’ which rendered them vulnerable to attack from predatory crabs and fish, indicating that there is likely to be a low survival rate of any returned undersized clams or ones that are disturbed and escape from the suction dredge. ? 1998 International Council for the Exploration of the Sea

Effects of Suction Dredging on Streams: A Review and an Evaluation Strategy

Abstract: Suction dredging for gold in river channels is a small-scale mining practice whereby streambed material is sucked up a pipe, passed over a sluice box to sort out the gold, and discarded as tailings over another area of bed Natural resource managers should be concerned about suction dredging because it is common in streams in western North America that contain populations of sensitive aquatic species It also is subject to both state and federal regulations, and has provided the basis for litigation The scientific literature contains few peer-reviewed studies of the effects of dredging, but knowledge of dredging practices, and the biology and physics of streams suggests a variety of mechanisms linking dredging to aquatic resources Effects of dredging commonly appear to be minor and local, but natural resource professionals should expect effects to vary widely among stream systems and reaches within systems Fishery managers should be especially concerned when dredging coincides with the incubat

The Persistence of Anthropogenic Turbidity Plumes in a Shallow Water Estuary

Abstract: Increased turbidity and light attenuation (Kd) in the suspended sediment plumes created by hydraulic clam dredging were examined in Chesapeake Bay, MD, U.S.A. Turbidity andKdvalues were measured along transects in areas with and without dredge plumes. The turbidity andKdof individual plumes were tracked as they returned to background levels using both Lagrangian and Eulerian techniques. Existing aerial photographs and a geographical information system (ARC/INFO) were used to examine plume sizes and dredge boat locations in relation to bathymetry. Hydraulic clam dredging produced plumes with significantly higher turbidity and light attenuation compared to background values. Plume characteristics were determined primarily by bottom sediment type and water depth. The greatest increase in turbidity and light attenuation occurred when dredges operated in shallow water (<1·0 m) where bottom sediments had increased amounts of silt and clay. Plume turbidity andKddissipated exponentially over time. The initial change in concentration of suspended sediments was rapid as the coarse sediments settled to the bottom. The rate of plume decay slowed as diffusion and resuspension acted on the finer sediments remaining in suspension. Some Lagrangian rates of plume dissipation were faster than Eulerian rates due to resuspension in shallow waters. Examination of aerial photographs indicated that 72% of the dredge boats digitized in the Chester River were operating in less than 2 m water. The area of a plume measured per boat in the Chester River was highly variable, ranging from 0·01 to 0·64 m2.

Environmental change over two decades since dredging and excavation of the lower Boro River, Okavango Delta, Botswana

Abstract: The Okavango Delta, southern Africa's largest wetland, is situated on the fringe of the semi-arid Kalahari Desert. It is a large alluvial fan, occupying a graben structure which is an extension of the East African Rift system. Of the 16 km 3 of water which enters the Delta each year, 96% is lost to the atmosphere by evapotranspiration, 2% to groundwater and only 2% leaves as surface flow. In order to increase surface outflow to meet human needs, the distal Boro channel-floodplain system was dredged, excavated and bunded between 1971 and 1974. The immediate impact of these measures was the destruction of in-channel flora. After 20 years, the aquatic flora has recovered in the excavated channel. However, little recovery has occurred along the channel reach which was dredged. Moreover, there has been significant encroachment of terrestrial species onto the floodplain in the region of the dredged channel. In addition, dredging created a nick point which has been migrating upstream by headward erosion since dredging ceased. The average rate of advance of the nick point has been about 500m per year. Incision associated with nick point migration has produced a channel which is indistinguishable in form from the dredged channel and, like the dredged channel, is almost completely devoid of inchannel aquatic flora. The adverse environmental impact of dredging has therefore continued to propagate in an upstream direction, although the height of the nick point has decreased, suggesting a natural attenuation process. This is likely to result in eventual elimination of the nick point.

Environmental Windows Associated with Dredging Operations

Abstract: : This technical note summarizes the types of concerns that lead to requests for environmental windows for Federal navigation dredging projects in both marine and freshwater systems, as well as the frequencies of occurrence of these concerns among U.S. Army Corps of Engineer (USACE) Districts. The information presented is derived from responses received from a survey sent to all USACE District offices that perform operations and maintenance (O&M) dredging in either marine or freshwater environments. This note serves to update earlier surveys by LaSalle et al. (1991) for dredging operations conducted in coastal and Great Lakes areas and by Sanders and Killgore (1989) for seasonal restrictions associated with dredging operations in freshwater systems.

Entrainment by Hydraulic Dredges - A Review of Potential Impacts

Abstract: : Effects of entrainment by hydraulic dredging operations on aquatic organisms have been and continue to be an issue of environmental concern. Clearly, assessment of actual impacts due to entrainment pose severe technical challenges. Studies to date illustrate the difficulties in determining precise estimates of absolute entrainment rates and have seldom been able to determine population-level consequences with any degree of confidence. Placing mortalities due to entrainment by dredges into a fishery population dynamic framework, such that dredging can be viewed within the context of other anthropogenic and natural sources of mortality for a resource, would enhance the ability to balance needs for resource protection and dredging project schedules. The persistent lack of quantitative data characterizing effects of entrainment hampers efforts to resolve the issue as it applies to environmental windows. Much of the available evidence suggests that entrainment is not a significant problem for many species of fish and shellfish in many bodies of water that require periodic dredging. Also, many dredging restrictions are based upon limited scientific data, suggesting that well-designed studies and new assessment tools are needed to remove entrainment issues from the realm of subjectivity. In the same regard, however, certain types of dredging operations appear to pose sufficient risk to sensitive resources such that continued application of restrictions is justified. Notably, these exceptions include the conduct of dredging operations in narrow constricted river channels, particularly where sturgeon may be present in appreciable numbers or in channels seasonally occupied by sea turtles or other protected species. Additional studies are definitely warranted to establish the necessity of entrainment-related environmental windows for these resources of concern.

Dredged sediment disposal method

Abstract: PROBLEM TO BE SOLVED: To provide a dredged sediment disposal method in which salinity is removed from sediment dredged from gulf coasts, bleed rivers, or the like and also to effectively classify the dredged sediment into gravel, sand, particulate, and the like to enable recycling it. SOLUTION: Dredged material dredged from gulf coasts or rivers by a dredging robot 1 and sediment that a sediment filling is made particulate by a rotary crusher 10 are simultaneously fed into a liquid cyclone 3, and solid matter deposited in the bottom part of the liquid cyclone 3 is washed with an aqua screen 5 to separate sediment and sand from which salinity has been removed. Treated water containing particulate in the upper part of the liquid cyclone 3 is subjected to solid-liquid separation by a thickener tank 6 to make dehydrated cake of the particulate.

Benthic invertebrates and sediment characteristics in a shallow navigation channel of the Lower Columbia River, before and after dredging

Abstract: McCabe et al "Benthic invertebrates and sediment characteristics in a shallow navigation channel of the Lower Columbia River, before and after dredging." Northwest Science. 1998; 72(2): 116-126

Economic Impacts of Environmental Windows Associated with Dredging Operations

Abstract: : This technical note describes potential economic impacts that result from compliance with requests for environmental windows (i.e., temporal constraints) on dredging operations. The primary purpose of this effort was not to quantify the exact total cost attributable to environmental windows throughout the U.S. Army Corps of Engineers (USACE) dredging program, but rather to estimate incremental costs of windows compliance across a spectrum of dredging project scenarios. The information contained herein, when examined in tandem with characterizations of the frequencies of windows associated with Federal dredging projects and the technical issues used to justify requests for specific environmental windows (Reine, Dickerson, and Clarke 1998), is intended to support decisions regarding prioritization of directions of future research.

Characterization of anthropogenic and natural disturbance on vegetated and unvegetated bay bottom habitats in the Corpus Christi Bay National Estuary Program study area

Abstract: Benthic habitats can be broadly classified as either vegetated or unvegetated. Seagrasses are the dominant form of submerged vegetation. Vegetated bay bottoms are generally more productive than unvegetated bottoms. Over 85% of seagrasses in Texas occur in estuaries of the Corpus Christi Bay National Estuary Program (CCBNEP) study area. Seagrass ecosystems are noted for high commercial and sport fishery landings. This correlation is often attributed to the high primary and secondary productivity associated with large seagrass beds in the CCBNEP study area. Disturbances, operating at many different scales, however, can profoundly affect benthic processes in estuaries and threaten fishery resources. For example, upper Laguna Madre appears to be undergoing a dynamic transition from a seagrass-dominated to a phytoplankton-dominated system as a result of a brown tide algal bloom. This large scale disturbance has resulted in pronounced changes in the trophic dynamics and relative importance among key plant and animal species within the system. What is not known is if the disturbance in Upper Laguna Madre is natural or influenced by man's activities, or if there are interactive effects due to natural fluctuations in the environment and anthropogenic input. Therefore, there is a need to identify natural and anthropogenic disturbances and characterize effects on benthic habitats. The four most important benthic habitats in the CCBNEP study area are: seagrass beds, oyster reefs, open bay muddy bottoms and shoreline sandy bottoms. Disturbances to benthic habitats are well known. Seagrasses are stressed by nutrient enrichment, propeller scarring, and especially light reduction caused by brown tide, turbidity, and dredging. Oyster reefs are stressed by reduced freshwater inflows and concomitant higher salinities. Bay bottom habitats are a victim of their own bounty. The large economic benefit the CCBNEP region derives from shrimping and sport fishing, which is supported by bay bottom habitats, is at risk due to mechanized harvest. Trawling continually turns over sediment, keeping the benthos in a continual state of low abundance and biodiversity. Bycatch-removes potential food for recreationally and commercially important fish species. Numerous disturbances resulting from human activities and natural events that affect physical, chemical, and biological characteristics of estuarine benthic habitats have been identified within the CCBNEP study area. Examples of human activities include: shrimp harvesting with bottom trawls, commercial tug and barge operations, recreational boating, dredging, altered currents, altered inflow, nutrient and contaminant input, and hydrocarbon exploration and production. Each activity has variable effects on the different types of bay bottom habitats. Examples of natural disturbances include: wind-generated resuspension and deposition of fine sediments, abrupt salinity changes due to direct rainfall and runoff, erosion due to storms, fish mortality due to freezes, long-term climatic changes, and harmful algal blooms.

Predicting Surface Runoff Water Quality from Upland Disposal of Contaminated Dredged Material

Abstract: : The US Army Engineer Waterways Experiment Station (WES) Rainfall Simulator/Lysimeter System (RSLS) has been very effective in predicting surface runoff water quality from the upland placement of dredged material However, this procedure is expensive and lengthy, and requires specialized equipment The need for a simpler procedure has become evident In 1986, WES evaluated potential extraction procedures for use in predicting surface runoff water quality from dredged material These experiments were conducted in conjunction with RSLS experiments on Indiana Harbor dredged material as reported in Environmental Laboratory (1987) Results of these investigations indicated that a simple laboratory extraction procedure could be used as a tool to provide initial engineering estimates and to screen contaminated sediments prior to conducting the RSLS procedure In fiscal year 1996, funding became available from the Long-term Effects of Dredging Operations Program to develop a simplified laboratory runoff procedure (SLRP) using other contaminated sediments and to compare the results with the RSLS procedure The SLRP and RSLS procedures were conducted on five separate San Francisco Bay, CA, sediments for the US Army Engineer District, San Francisco, and on one sediment from Black Rock Harbor, CT-representing the US west and east coasts, respectively This technical note summarizes the results of this comparison

PCB in a river system during sediment remediation

Abstract: Remediation of 150 000 m3 of sediment containing400 kg PCB was performed by suction dredging in asmall lake in Sweden in the summers of 1993 and 1994.Upon project completion calculations indicated that97% of the PCB was deposited in a landfill withsubsequent low transport to the surroundings, andthat 2.9 kg was left in the lake sediment.Concentrations of PCB that were measured weekly in thewater leaving the lake were 12 ng/L (geometric mean)during dredging, not exceeding 30 ng/L and notsignificantly higher than in investigations preceedingthe remedial action. The concentration of PCB wasnegatively correlated to water discharge andpositively correlated to water temperature. PCBtransport in the river was correlated to waterdischarge but not to the dredging activity. Duringdredging of the most contaminated part of the lake,the dredging area was screened off by a geotextilesilt curtain that reduced leakage of PCB to the riverdownstream.

Sand Body Geometry of the Wax Lake Outlet Delta Atchafalaya Bay, Louisiana

Abstract: Deltas forming in Atchafalaya Bay, L-ouisiana, are the result of delta switching by the Mississippi River. The larger Lower Atchafalaya River delta has been heavily manipulated by dredging for navigation, but the Wax Lake Outlet delta is largely undisturbed and an excellent example of a 'bayhead' delta. Combining stratigraphy, aerial photography and digital terrain model data sets, the developmental history of this delta is presented. The Wax Lake Outlet delta is comprised of a typical upward-coarsening sequence, although its prodelta unit is extremely limited. Its plan-view form is typical of deltas developing in low-energy, unstratified, shallow basins. Early developmental processes were identified by Roberts and van Heerden (1992) Development through the 1980s involved the maturation of distributary channels. From 1989 to 1994, the majority of sediment was retained seaward of the delta proper, due to the efficiency of the distributary’ system. Greatest sand body thicknesses were found on the upstream portions of delta lobes, but not necessarily at points of bifurcation. Estimates of sand body volume range from 129 to 139 x 106 m3. A small area of the Atchafalaya River delta investigated for comparison also contains an upward-coarsening sequence but with upper and lower coarse­ grained bounding units generated by dredging activity. Comparison of the Wax Lake Outlet delta to other Mississippi deltas reveals some similar processes of development despite differences in settings. The Wax Lake Outlet delta has shown a lower rate of infilling compared to subdeltas of the Mississippi River Balize delta due to the relative immaturity of the Atchafalaya. Growth curves based on terrain model data predict an area of 111 knt (at and above 0.0 NGVD) by the year 2000, which falls within the range of values given by the Wells et al. (1982) generic model based on the Mississippi subdeltas.

The UK marine sand and gravel dredging industry: an application of Quaternary geology

Abstract: Marine sands and gravels currently contribute 24% (over 20 million tonnes/year) of the total sand and gravel aggregate consumption of Great Britain. To maintain or increase this contribution into the future, the identification, assessment and licensing of additional sand and gravel resource areas is of fundamental importance. Research into the Quaternary history of the continental shelf surrounding the UK assists in the prediction of sand and gravel resource locations. Similarly, resource assessment is significantly improved through an understanding of the origin and formation of these Quaternary deposits. Geological considerations also feature strongly in the management of existing dredging licence areas and in the acquisition of future licences from the Crown Estate. Precise resource assessment, coupled with accurate dredger positioning and track recording systems, minimizes the extent of dredged sea bed, thereby limiting environmental impact and improving the consistency of dredged cargoes. Also important is the need to overcome marine aggregate prejudice which arises from the perception by some customers that marine dredged sands and gravels differ markedly from those obtained onshore. Central to this issue is the argument that some of the most substantial marine deposits originated in subaerial environments at similar times and by the same processes as their present-day terrestrial equivalents, having been deposited in Quaternary cold climate fluvial environments.

Risk-cost trade off considerations for contaminated sediment disposal

Abstract: Delays in dredging and inability to dredge the nation's harbors, due to the presence of contaminated sediments and the lack of environmentally acceptable disposal sites are interfering with shipping activities and hampering trade growth. The United States Government is committed to provide continuing support to the port industry's goals for enhancing economic growth while protecting, conserving and restoring natural resources within coastal aquatic lands. The government's commitment has resulted in the articulation of a national dredging policy in the Action Plan for Improvement of the Dredging Process in the United States. This national challenge calls for a systematic and consistent decision making approach to dredging and disposal including contaminated sediment management. In building an effective decision mak ing framework for costs, risk reduction and potential beneficial uses of the disposal material must be considered in identifying and evaluating environmentally acceptable and cost-effective disp...

Dredging as Remediation for White Phosphorus Contamination at Eagle River Flats, Alaska.

Abstract: The Eagle River Flats impact area is a Ft. Richardson Superfund site. It is a salt marsh that is contaminated with white phosphorus (WP), and remediation of sediments in permanently ponded areas may require dredging. A remotely piloted dredging system was designed, constructed, and deployed at the Flats as part of the overall site remediation feasibility study. Experience gained over two years of engineering study and contract operation indicates that, although feasible and effective, this alternative is slow, difficult, and very expensive.

The HIDROVIA Project: Should We Be Concerned for Fish Resources?

Abstract: HIDROVIA is a waterway project that, if completed, will require dredging and deepening of select reaches along the 3400 km De la Plata River basin. Fish diversity in the basin is great and dominated by detritivorous and bottom feeders. Large-sized, migratory species support valuable subsistence, commercial, and sport fisheries. In the upper Paraguay, dredging and anticipated increases in navigation traffic will severely impact shore-line habitats and likely affect fish fauna and mobilize shoreline sediments. In the middle and lower Paraguay Rivers and in the Parana River, dredging should not affect upstream fish migrations and therefore we anticipate only minor effects on the fishery. Dredging and sediment disposal will not release significant levels of contaminants because sediments are dominated by medium and fine sands. However in De la Plata River, dominance of silt and clay elevates concentrations of contaminants that may be released by dredging thus increasing bioaccumulation in bottom-feeding fishes and benthic organisms. These effects could be more severe in estuarine spawning habitats used by several eurihaline fish species.

Toussaint River Unexploded Ordnance (UXO) Demonstration Dredging Project.

Abstract: : In 1991, the U.S. Army Engineer District, Buffalo, initiated a Civil Works dredging operation on the Toussaint River where it enters into Lake Erie. The Toussaint River is adjacent to the former Erie Army Depot (near Port Clinton, Ohio), a previous Department of Defense facility that used a section of Lake Erie from 1918 through 1965 as an impact area for testing and proof firing Army ordnance. A hydraulic dredge had nearly completed the 42,OOO-m3 (55,000-yd3) project when a 106-mm artillery projectile was found jammed in the cutterhead and the Buffalo District immediately halted dredging operations. Subsequent unexploded ordnance (UXO) investigation and removal actions along the beach of the former Erie Army Depot (from the mouth of the river to approximately 3 miles east) located over 5,000 items of ordnance ranging in size from 20 mm to 165 nun. The Buffalo District, under the auspices of the Defense Environmental Restoration Program (DERP) - Formerly Used Defense Sites (FUDS), conducted a demonstration dredging project on the Toussaint River from 10 July through 26 October 1995, that was specifically designed to address the UXO presence. The purpose of this demonstration project was to evaluate the operational effectiveness of a modified clamshell bucket dredging process designed to safely separate and retrieve UXO from the sediment prior to its disposal. During the demonstration, 14,757 m3 (19,300 yd3) of sediment were dredged, 31 inert pieces of ordnance were located and removed, and 6 live UXOs were destroyed (totaling 258 kg (568 lb) of scrap). This paper describes the demonstration project with regard to dredging system design, operation, and associated costs.

Method for development of deep placer deposits

Abstract: FIELD: mining technology. SUBSTANCE: this relates to dredging of thick productive sediments in development of placer deposits at improved ecology of dredging operations. In process of preparatory work before start of dredging operations, additionally carried out is extraction of barren rock beyond perimeter of workable reserves and within limits of dumping boundaries. Sand is piled in dumps at maximal possible relation of depth and width for dredge in given placer deposit. More reach sands are placed in lower part of dump, and leaner sands are placed atop. After that, placed to surface of dump is layer of barren rock. Sands are flooded down to ground water level in valley. During dredging of sands, after regular advancement, upper edge of work face is caved down. This procedure reduces loss of mineral being extracted and cuts cost of nature reclamation expenditure. EFFECT: higher efficiency. 3 dwgu

Sources of sand and gravel on the Northern European Continental Shelf

Abstract: The extraction of marine sand and gravel has taken place in a number of countries around the North Sea, the Baltic Sea and English Channel for several centuries, but large-scale dredging for aggregates only began in earnest in the 1960s. Today, marine sands and gravels have an increasing role to play in maintaining European supplies of concreting aggregates as well as material for beach nourishment and constructional fill. The distribution of sand and gravel resources offshore is uneven. They vary in their thickness, their composition and grading, and their proximity to the shore. Many deposits lie in places that are currently inaccessible to the dredging industry. This paper outlines the production of marine sand and gravel in northern Europe and describes the distribution, composition and Quaternary origins of the most important marine sand and gravel resources in northern Europe. Examples are given for the UK, the Netherlands and Denmark, and in summary form for France, Belgium, Norway, Sweden, Ireland and Germany. Most marine sand and gravel deposits are of fluvial or glacial origin and have been reworked to varying degrees by marine and coastal hydrodynamic processes. They represent a range of former depositional environments, including fluvial channel-fill or terrace deposits, glacial meltwater plain deposits, seabed lag gravels and degraded shingle beach or spit deposits, as well as modern marine tidal sandbanks and sandwave deposits.

Method for continuously operating filtration apparatus for dredging and cleaning work ship

Abstract: PROBLEM TO BE SOLVED: To provide a method for automatically cleaning screen without requiring the man power and without interrupting the dredging work even if a screen of a filtration apparatus is clogged, and also a method for heightening the refuse matter density sucked in the hatch of a work ship at the time when dredging or water surface cleaning work is carried out by a dredging work ship. SOLUTION: A plurality of filtration apparatuses 11 , 12 are arranged in parallel on the suction side of a dredging pump 3 of a dredging work ship and during the time when one or more filtration apparatuses are operated as a cleaning route for dredging or removing refuse matter, the another filtration apparatus is driven as a back washing route to back-wash the screen of the filtration apparatus with water stored in ship storehouse 4 and the cleaning route operation and the back washing route operation are reciprocally carried out, so that dredging and cleaning work can continuously be carried out and at the same time the density of the refuse matter sucked to the ship storehouse can be heightened.