Terra et Aqua

In the last decades and in the near future, large areas of land, especially in the Far East, were and will be reclaimed using trailing suction hopper dredgers. During the overflow phase of the loading stage of the dredging cycle, a part of the dredged volume of sand will not settle in the hopper but will be transported with the overflow discharge overboard. For production, sand quality and environmental reasons it is important to predict the amount of these so-called overflow losses. The total volume of losses as the influence on the loaded (and lost) particle size distribution (PSD) is important. In 1997 a research programme was started to get more understanding of the sedimentation process onboard a hopper dredger. The goal of the programme was to develop a numerical model that can be used to predict the influence of the relevant parameters as hopper geometry, sand, discharge and concentration on the overflow losses (and which fractions of the PSD will be lost). The research programme consists of three parts: laboratory experiments, development of numerical models and full-scale validation of the models. In this article the developed 1DV numerical model will be presented. The 1DV-model is one-dimensional, but in contrast to most existing models in the vertical direction. The influence of the PSD distribution is modelled using a coupled system of transport equations (convection-diffusion) for the different grain sizes. The numerical results will be compared with experiments. The financial support of the VBKO (Vereniging van Waterbouwers in Bagger-, Kust en Oeverwerken) for this research is gratefully acknowledged.

Several equations for calculating terminal velocity of sediment particles are presented. Each formula is teen used to determine friction loss in slurry pipelines. The study evaluates the effect of each equation on the value of friction loss. Results are presented first as a function of slurry velocity and then as a function of particle diameter. They are later compared with experimental data available in literature. It is found that there is a considerable difference between friction loss values yielded by those terminal velocity equations. This paper also discusses the advantages and disadvantages of applying these equations for friction loss calculation.

In this article the risks of dredging polluted sediments are shown to depend on the risk of dispersion and on the mode of occurrence of the pollutants. During dredging, shifts in mode of occurrence may occur which affect the availability for uptake by organisms (the bioavailability) and consequently the toxicity of the pollutant. General descriptions are given of the behaviour of the two main classes of pollutants: heavy metals and organic micropollutants. As a conclusion, the risks of dredging polluted sediments are related to the uncontrolled dispersion of pollutants, both in dissolved state and in particulate state, and to the incomplete removal of polluted sediment. -from Authors

Surface sediments from two ports affected by mining activities (Cartagena and Huelva were characterised following the traditional physicochemical characterisation based on contaminant concentrations together with laboratory toxicity tests. The toxicity tests included acute and chronic methodologies both on the whole sediment and on the sediment elutriates. As expected, sediments reported remarkable concentrations of metals, some failing the higher limit values for open-water disposal, and organic contamination in some areas affected by industrial and shipping activities. The toxicity assessment results showed differences amongst the two studied ones: the port of Huelva reported significant toxicities both for the whole sediment and the elutriate tests, while the sediments from the port of Cartagena reported significant toxicity only for some whole sediment bioassays. These sediments provoked little or no adverse effects for other benthic species and similar responses to controls for elutriate tests. These results show that SQGs are not always a good predictor for sediment toxicity, especially for evaluating the risks of elutriate waters. In this sense the advantages and disadvantages of laboratory toxicity tests for dredged material characterisation and its use in ecological risk assessment for decision-making is further discussed.

In recent years there has been a growing emphasis on characterisation and remediation of contaminated waterways. Often a long stretch of river and the asso-ciated terminal estuary has been contaminated by industries over a period of decades. The toxic substan-ces released by these industries become progressively buried in the sediments, such that profiles of these substances in the sediment become a record of the contamination process. These water systems may have only a fragmentary history of depth surveys, or none at all. Such water systems require another method to measure sedimentation rates and to deter-mine the calendar dates associated with buried toxic substances. Chronology information of this kind is useful in deter-mining which industries caused the contamination, for industrial production or release records can be compar-ed to the dates of buried materials. Chronology information is also useful for determining whether buried substances are migrating or degenerating. Abstract Laboratory analyses of sediment cores can determine sedimentation rates and the calendar dates associated with various depths within sediments. These chronolo-gy results can be used for characterising the deposition environment of a water system, which is pertinent to the planning of dredging operations. The methods are particularly useful for water systems which contain buried toxic substances. Measurements of different radioactive species give chronology information for time frames from a half year to 100 years before the present. Recent studies in the 1990s where this method has been applied include the Kalamazoo River in Michigan, the Housatonic River in Connecticut, the Passaic River in New Jersey, the Hudson River and Grasse River in New York. This paper presents the practical application of three geochronology methods which have been used.

This paper aims to describe in broad categories some aspects of the operation of dredging fleets. The ideas are presented from the dredging contractor's point of view. The article stresses aspects which have special relevance to dredging and port construction for the developing countries. The various types of dredging craft are outlined and dredging chains of equipment are then considered. The author discusses dredging economics, contracts and in particular fair conditions. The concept of an international ports policy is also considered n depth. (R.W.L.)

Dredging, while earlier an art, is at present a scientific subject that successfully reflects human skills. This artificial process attempts to win material for beach nourishment, for making roads and railways or for removing settled sediments to facilitate marine transportation. The process, of course, brings about environmental problems in short and long time scales in marine and estuarine environments. This study mainly looks at the short-term impacts of dredging in the estuarine environment of Cochin harbour, India. As expected, the results indicate only transient changes, mainly during the time of dredging. On the other hand, precipitating or long-acting perpetual fluctu-ations are time bound reversible and are environmental-ly acceptable. The estuary being variant in hydrographic features, adds significance to this study in regulating the short-term impacts.

At a meeting held in Washington DC in January 2003 a number of organisations in the USA, including US Army Corps of Engineers (USACE) and Environmental Protection Agency (EPA), and Europe recognised the need for a more structured approach to conducting research into the generation and impact of sediment released by dredging. The authors were commissioned to produce a framework for research and presented some of the preliminary findings at WEDA XXIII. The aim of the research was to produce a framework of the steps and knowledge needed to properly assess dredge-generated plumes. This begins with improving knowledge of the source term and finishes with real impact assessment. For each item identified, the state of knowledge that already exists is being reviewed and this is leading to identifying what further research (if any) is needed. An attempt will eventually be made to prioritise what research is most needed and will achieve the greatest initial contribution to the assessment procedure. In this way it is to be hoped that future research funding may be well-targeted and that it will be possible to better protect the environment without the need to invoke the precautionary approach quite so often, which sometimes results in possibly unnecessary expense or restriction on development. The research presented here is still in progress and is to some extent an invitation to contact the authors with information that will help in setting priorities for research. The scope of the article has been focused primarily on the physical processes involved and their impacts. The research will also include contaminant release and impacts and it is hoped to present this in the future. The paper was presented at the WEDA XXIV in July 2004, Orlando, Florida and published in the Proceedings. It is reprinted here in a slightly revised form with permission.

A new design approach and offshore marine operations have been developed for the construction of the foundations for the first phase of the Thornton Bank Offshore Wind Farm, located some 30 km off the Belgian Coast. In contrast to the monopile foundations commonly applied to offshore wind farms, a novel Gravity Base Foundation (GBF) concept has been selected as the result of an extensive risk assessment and technical evaluation. Innovative dredging technologies play a key role in the realisation of these foundations. Concrete caisson foundations for offshore wind turbines have traditionally been applied in near-shore wind farm projects, in relatively shallow and sheltered waters as they were believed to become uneconomical and technically too difficult with increasing water depths.

Hurricanes pose a serious threat to existence along a large part of the coastline of the United States, including the whole Gulf coast and the southern part of the Atlantic coast. Almost every year several areas suffer from significant casualties and damage caused by hurricane winds, rain and storm surge. From an experienced coastal engineering viewpoint, it should be possible to mitigate the impact of the storm surge part of the problem. A typical coastal scenario is diminishing marshlands protected by shrinking barrier islands. Both re-creating marshlands and strengthening barrier islands can contribute to mitigating the impact of storm surges. But a relevant reduction of dangerous storm surge heights means re-creating immense surfaces of marshlands using vast quantities of sediment. Re-creating barrier islands into real barriers is far more promising. To demonstrate the feasibility a study area was selected: Terrebonne Bay with Port Fourchon, a stretch of coast of 65 km (40 mile) long, south of New Orleans. Typical cross sections are designed with narrowed channels between islands, to block the local storm surge from category 3 and 5 hurricanes, needing an estimated 40 million m 3 (53 M yd 3). Designed slopes depend on use of coarse sand (250 micron, 0-1 phi), and sufficient supplies were found at a distance of 200-250 km (120-150 Nm) in the coastal area south of Mobile, Alabama at depths from 30 to 80 m (100 to 250 ft). This would take a one-day cycle time for existing jumbo self-propelled trailing hopper dredgers. Access channels are foreseen to deliver the sand to pits with cutter suction dredgers that pump the sediment ashore over distances of 5 to 15 km (3 to 9 mile). This method of operation leads to a rough cost estimate for course sand of $11/m 3 ($15/yd 3); backfill and clay from local supplies will be less expensive. Maintenance costs are not estimated; they are mainly expected to arise from erosion of island heads owing to increased water velocities in narrowed channels. Estimated total costs of the whole 65 km stretch of Terrebonne Bay could amount to some $500 M. Though not all aspects could be taken into account in this feasibility study, nevertheless, examination of local circumstances in the light of previous dredging experience demonstrates that re-creating barrier islands into real barriers is a solution worth examining in greater detail. This paper was first presented at the WEDA Conference in San Diego, California in June 2006 and is published in a somewhat different version with permission.

The Waterways and Marine Affairs Administration of Flanders, Belgium initiated a research project, which aims at the environmental evaluation and monitoring of the dredging and relocation operations in the Belgian Coastal Harbours. The operations were required to meet the BATNEEC principle, that is, 'Best Available Technique Not Entailing Excessive Costs'. In order to test the monitoring methods, a small dredging site at Nieuwpoort Harbour was selected. Three types of studies were conducted: physical, chemical and ectoxicological. The authors acknowledge the Flemish Department of infrastructure and the Environment and the Waterway and Marine Affairs Administration for their support and for the opportunities offered by the ambitious project Mobag 2000.

In the contract of 1954 between the German Federal Waterways Administration and the newly formed Water and Land Federation Emden-Riepe, the Federation relinquished agricultural land within the Emden-Riepe Lowland to the Association of Port Operators to accommodate sediment from the Port of Emden which at that time was still classified as mud. This policy continued through the 1980s. During the privatisation of maintenance dredging in 1986, first concepts to reduce the, at the time, considerable amounts of sediment were drawn up. The costs of maintaining the nautical depths in the Port of Emden were significant, and in 1990 the provincial government issued an order to stop flushing in Riepe. Since 1990 the layer in the Outer Harbour of Emden has been kept in its natural balance in situ, by using a dredging system that was specially developed in Emden. The question of whether the technique developed in Emden can be adapted for other harbours with a higher sand content has triggered a series of tests which are also discussed.

Sediment transport predictions are critically dependent on the prediction of near-bed wave-induced velocities. Especially the asymmetry between the forward (onshore) and backward (offshore) velocities plays an important role in determining the magnitude and direction of the wave-induced sediment transport. Boussinesq wave models are amongst the most advanced wave models presently available to the coastal engineer. Moreover, they are highly efficient from a computational point of view. They are generally applied for wave propagation studies in which the focus is on the prediction of surface elevations. The knowledge about the capability of these models to predict the horizontal velocities under waves is limited. This work aims to explore the possibilities of using such a Boussinesq model for the prediction of the near-bed velocities. A spectral Boussinesq model is used in which wave breaking and dissipation in the surf zone are included. The model is tested against measurements of irregular (partially) breaking waves performed in WL I Delft Hydraulics' Delta flume. The comparison of measured and computed velocity asymmetry indicates that for moderately long waves the Boussinesq model can be successfully used for sediment transport purposes. For shorter waves the crest velocity values of the higher waves are underestimated and as a result the velocity asymmetry as well. The work was started as part of the MAST-2 G8 Coastal Morphodynamics Research Programme and finalised as part of the MAST-3 SAFE project. It was funded jointly by the Commission of the European Communities, Directorate General for Science, Research and Development under contract no. MAS2-CT92-0027 and MAS3-CT95-0004, and Delft Hydraulics and Delft University of Technology in the framework of the Netherlands Centre of Coastal Research (NCK). The laboratory data used was obtained during experiments in the framework of the 'Access to Large-scale Facilities and Installations Programme' (LIP), which were funded by the Commission of the European Communities, Directorate General for Science, Research and Development under contract no. GE1(*)-CT91-0032 (HSMU).

The Dutch Department of Water Management (Rijkswaterstaat) is developing an innovative concept: building mounds (terpen) of dredged material. Terpen or mounds were common in Holland centuries ago: man-made mounds a few metres high that were placed as a protection against the sea. Climate change, rising sea levels, and falling land levels mean that The Netherlands is still challenged to keep the country safe and dry. In addition modern water management and water safety must ensure that waterways are deep enough to keep the functionality (shipping, discharge of rain water) of the water systems intact. Maintenance through the removal of dredged sediment from ports, canals, and rivers is an ongoing necessity, with millions of cubic metres of often contaminated dredged sediment being removed annually. For this reason, the traditional "mound solution" may be ready to make a come back. Modern mounds made of (contaminated) dredged material create a beneficial use: on the one hand, mounds offer protection against flooding and, on the other, they provide a final destination for dredged material which allows dredging to be continued. At the initiative of the Dutch Department of Water Management several activities are in progress to involve stakeholders, and competitions have been organised to find possible placement sights for the mounds. The article describes the competitions conducted and the studies which have been done to develop the concept.

Dredged material was transferred from the borrow site to the beach via a pipeline cutter suction dredger. Approximately 260,000 m3 of material was taken from the borrow area and placed on the beach. Survey transects were used to ensure complete sediment removal. The same transects continue being used to determine shoal evolution and migration. The project significantly widened and improved the severely eroded Emerald Beach shoreline by the placement of approximately 240,000 m3 of high quality sand. The immediate post-construction survey data for the beach profiles was scheduled to be resurveyed 6 months following the initial construction project, or in February 2005. This new information will be compared to the immediate post-construction survey data to determine the shoreline change rates and the volumetric change rates. A physical and biological monitoring plan for the Emerald Beach Sand Replenishment Project has been implemented to measure and evaluate the physical and biological systems within the nearshore coastal areas surrounding Leeward-Going-Through. The purpose of the post-Project monitoring studies are to evaluate the Project's overall performance and to determine the longterm beneficial and/or adverse impacts of the Project on the coastal environment. The results of the measured changes will be used to determine if erosion amelioration measures and mitigation is required. Water Cay, immediatel y north of Little Water Cay and connected by Donna Cut, was planned for development requiring an access channel be dredged to move construction equipment onshore. An environmental impact statement and construction drawings were developed to satisfy outline planning needs while in depth site investigations were conducted finalizing channel alignment and sediment containment location. Sediment analysis indicated beach compatible sand and beach nourishment was planned. Recognition of the local interests regarding the property owner rights, government planning interests, marine environment, dredge contractor, and client all were found to be essential to effective project management. The author wishes to thank Billy L. Edge of Texas A &M University and Karyn M. Erickson of Erickson Consulting Engineers of Sarasota, Florida for their collaboration and support. This paper was presented at WEDA XXV, New Orleans, Louisiana, USA in June 2005 and appears in the conference Proceedings. It is reprinted in a slightly revised form with permission.

Traditional methods for environmental management of marine reclamation works close to sensitive habitats have generally not provided the level of control necessary to ensure preservation of these habitats. Obtaining the level of control necessary to assure authorities and non-governmental organisations (NGOs) of compliance with environmental quality objectives, requires quantifiable compliance targets covering multiple temporal and spatial scales. Of equal importance to allow feedback of monitoring results into compliance targets and work methods are effective and rapid response mechanisms. This article describes the successful implementation of comprehensive Environmental Monitoring and Management Plans (EMMP), based upon such feedback principles, which allow reclamation activities to proceed in close proximity to Singapore's most important marine habitats under third party scrutiny. Specific focus is placed on describing the methods utilised to quantify compliance with daily spill budget targets and how such targets and compliances are assessed. To improve reliability, the spill budgets take into account specific habitat tolerance limits for varying magnitudes and durations of sediment loading. Refinements to sediment plume models were undertaken to enhance their ability to hindcast impacts from the contractors' complex reclamation schedules. Methods for segregation of impacts and assessment of cumulative impacts were also integrated into the hindcast procedures. Finally, the article describes the updating of tolerance limits and confirmation of spill budgets via targeted habitat monitoring. To date, the EMMPs have been able to document compliance of the works to all pre-project environmental quality objectives at a level of reliability that cannot be refuted by third parties. This has minimised the developers' and contractors' exposure to public complaints and liabilities associated with environmental impacts. The EMMPs have thus allowed the reclamation activities to proceed in an efficient manner, whilst ensuring protection of the environment.

Field experiments conducted to explore sediment resuspension and cross-shore cycling in nearshore environments are presented with special emphasis on the influence of wave groups. Field data presented are from City Beach, Mullaloo Beach and Leighton Beach of Western Australia and Chilaw, Sri Lanka. Measurements include simultaneous measurements of surface elevation, cross-shore current velocities and suspended sediment concentrations collected just offshore of the breaker zone. As it has been well established, wave groups appear more capable of resuspending sediments than incident waves. Results of cross-correlation and cross-spectral analysis show a considerable inconsistency especially in the direction of cross-shore sediment flux on the frequency domain at different locations and under different conditions. This lead to the hypothesis that there are additional factors such as local wave climate, grain size, beach slope and bed forms.

The limit between the land and the sea is an unstable and vulnerable area. It is exposed to multiple aggressions and undergoes changes generated by hydrodynamic factors. Such factors cause erosion and depositions that may be harmful to the natural balance of any site or seaport. Therefore, constant tracking is necessary to determine and possibly control the evolution of the sea bed. DRAPOR is a state-owned company in charge of navigation works maintenance in the seaports of the Kingdom of Morocco. It is seeking new possible ways to know in advance sea bottom levels in order to optimise dredging operations. Hence, the initiative to study the prospects of G.I.S. application in modelling sediment transport and erosion inside seaports. In various sections, this paper deals with sea phenomena that cause sediment transport. It also examines the mathematical models used for the purposes of sediment transport modelling, the general design of a prediction model for sand encroachments and erosions inside any seaport that is subjected to shoreline transit. Besides, in view of the necessity to give an example of integration of the various mathematical models in a Geographical Information System, an application has been developed with the Arcview G.I.S. 3.2 software using AVENUE language. Such application would make it possible to assess coastline evolution during the construction of a protection structure, namely a cross-shore jetty. The application hence made proved that the combination of mathematical models of sediment transport with a G.I.S. provides a tool that has management, analysis and processing capabilities and that is able to simulate coastline evolution after the construction of any coastal structure.

During maintenance dredging operations in Dutch Harbours investigations have been carried out to assess the level of resuspension. Indicates the background to the research and the technical aspects of the turbidity phenomenon. -from Author

This case study is on a design and build turnkey project using a displacement filling method whereby the design methodology and construction technique of displacement filling in Lumut will be discussed to highlight the steps undertaken to ensure the effectiveness of this method and its overall economic advantages. A new method of extracting geotechnical parameters called Iternative Technique (IT) used to obtain critical parameters for slope stability analysis will be introduced. Some aspects and observations of rock bund construction on soft soil will also be explained. Apart from that, measures taken as part of the environmental management plan due to the usage of the displacement filling method will also be elaborated.

Open disposal of dredged matter in coastal areas is common practice all over the world. Especially if cohesive sediment is considered, it is a known fact that the location where the long-term deposition takes place may essentially differ from the disposal site. Knowledge of the transport and fate of disposed sediment improves the economic and environmental aspects of off-site disposal. Dredged matter which is pla ced at a disposal site usually differs from the local bottom-sediment with respect to grain sizes and grain size distribution. Moreover the bottom-topography at the disposal site changes as a result of the added sediment. The two aspects mentioned above usually lead to a "violation" of the local morphodynamic equilibrium state which is followed by a "reaction" of the topography (eg. increased erosion, changing sorting of the local sediment-mixture). Another important aspect related to unconfined disposals is the interaction between the disposed matter and the surrounding flow-field (e.g. density currents) which can become a dominant transport mechanism in the nearfield of the placement site. This contribution deals with th e development of a morphodynamic-numerical simulation model. SMOR3D consists of several modules which are fully coupled in a time-explicit mode. The model-approach consists of a 3D flow solver which is coupled with several transport modules for suspended sediments, bed-load sediment and salt. The movable bottom topography is balanced by a bed-model. All sediments (natural background and disposed matter) are represented by several fractions of different properties. The model allows the discrimination of disposed matter from the natural background sediment at every location and time. Results of numerical simulations of disposals by means of bottom doors and split barges are shown. The results are in good agreement with SPMC measurements carried out in the nearfield of different disposal locations.

PIANC Working Group Envicom 13 was established as a forum for the development of prudent guidance for the selection of management practices designed to provide environmental protection in dredging projects. The working group encompassed experts from around the world, working for the dredging industry, port administrations, consultancies and research institutions. The report has now been published as PIANC report Nr. 100 and provides guidance for the consideration and selection of management practices for environmental protection based on objective science-and engineering-based factors and specifications. It describes in brief the full process of a dredging project from conception to the operational phase and indicates where and how in this process the essential decisions on the implementation of management practices (MPs) should be taken. A distinction is made between MPs and best management practices (BMPs). The report also discusses the application of the "Precautionary Principle" as used in the assessment of effects on the environment and provides tools to conduct dredging projects in an environmentally sound manner. Properly applied the precautionary principle provides incentives to develop better solutions. Risks can be reduced by applying correctly selected MPs. A CD-ROM added to the report provides detailed descriptions of potential management practices and comprehensive information.

Over the past decade there has been increasing awareness of the environmental impact of dredging, and in particular, one of the side effects receiving attention was the extra turbidity of resuspended waterbed material occurring whilst dredging. This paper is the result of the work of the project group, Environmental Effects of Dredging, a joint effort of several organisations in dredging-related industries and the Netherlands Government. At present some 35 standardised turbidity measurements have been executed around various dredging techniques.

Summarises the optimization of maintenance dredging based on a knowledge of sediment behaviour, and dredging procedures and equipment. Mud and sand accumulation mechanisms, nautical bottom definition, induced sedimentation, and dredging system selection are discussed and illustrated by examples. (J.M.M.)

In many parts of the world, port development and economic activities regularly come into conflict with the desire to conserve valuable habitats. Europe is no different and in many cases this has led to wide-ranging European Union legislation to protect and conserve fauna, in particular birds, flora and certain habitats. A difficult situation arises when a port authority, trying to plan ahead, purchases land for future expansion. This land may not be used immediately and therefore lies fallow allowing habitats to evolve. These habitats are then, sometimes unbeknownst to the port authority, classified as "special area of conservation" and the port is not allowed to use the land for economic and commercial expansion, as was its original intention. Starting in 1979, EU habitats legislation expanded considerably, but not necessarily with clarity. After more than 25 years of evolving legislation, which includes designating certain sites as Special Protection Areas (SPAs) and others as Special Areas of Conservation or SACs, the European Dredging Association (EuDA) Environmental Committee has made an evaluation of the impact these directives are having on port expansion, including several important case studies. In particular Article 6 of the Council Directive 92/43/EEC of 21 may 1992 on the conservation of natural habitats and of wild fauna and flora (Habitats Directive) and its impacts are examined.

When considering pumping shells through a pipeline we have to consider that the shells are not spherical, but more discs shaped. When shells settle they will settle like leaves where the biggest cross section is exposed to the drag. But when they settle, they will settle in the same orientation, flat on the sediment, so the sides of the shells are exposed to the horizontal flow in the pipeline. Since the side cross section is much smaller than the horizontal cross section, a much higher velocity is required to make them erode and go back into suspension. The settling velocity is much smaller because of the large area of the cross section. Even when the slurry velocity exceeds the settling velocity, there will always be some shells that will reach the bottom of the pipe due to the combination of settling velocity and turbulence. Once these shells are on top of the sediment they are hard to remove by erosion, because they lay flat on the surface and have a small cross section that is exposed to the flow compared with the weight of the shell. So although their settling velocity is much lower than equivalent sand particles, the erosion velocity is much higher. If we look at the beach in an area with many shells, we can always see the shells on top of the sand, covering the sand. In fact the shells are shielding the sand from erosion, because they are hard to erode. The bigger shells will also shield the smaller pieces, because the smaller pieces settle faster. Compare this with leaves falling from a tree, the bigger leaves, although heavier, will fall slower, because they are exposed to higher drag. The same process will happen in the pipeline. Shells settle slower than sand grains, so they will be on top of the bed (if there is a bed), just like on the beach. Since they are hard to erode, in fact they protect the bed from being eroded, even if the line speed is increased. The combination of high erosion velocity and the shell 'protecting' the bed means that even a small amount of shells can lead to relatively thick bed in the pipeline. But there will always be velocities above the bed that will make the shells erode. The paper describes the settling and erosion process of shells and the consequences of this on the critical velocity when pumping a sand/shell mixture through a pipeline. A mathematical model of the processes involved will be presented.

This paper describes the development of the South Lake of Tunis which has recently been accomplished by the group LAC SUD 2000 (a consortium of five dredging contractors) at the request of the Tunisian Government. The project is within the framework of the national development programme of the coastal Tunisian lagoons, in an effort to improve the living conditions in this area and to protect the environment against the various forms of pollution which have affected it for more than half a century. It is amongst the rare projects which introduce viable solutions for limiting the extent of pollution in one of the most eutrophic lagoons in the world. Considering the location of the lake within the heart of the town of Tunis City, the project will offer to Tunis centre an opening onto the sea, giving it a whole different look. The history of the Lake Tunis, its location before the development works, the after-project results and the work volume executed during the project period will also be discussed here. The works were completed within the contractual period and a programme monitoring water quality will follow the project during a maintenance period of two years and a guarantee period of five years. For information about the remediation of the North Lake of Tunis see Terra, number 49, September 1992.

For the estimation of the sedimentation process in TSHD's a number of models have been developed. The oldest model used is the Camp (1946) model which was developed for sewage and water treatment tanks. Camp and Dobbins added the influence of turbulence based on the two-dimensional advection-diffusion equation, resulting in rather complicated equations. Groot (1981) added the effects of hindered settling. Miedema & Vlasblom (1996) simplified the Camp equations by means of regression and included a rising sediment zone, as well as hindered settling and erosion and an adjustable overflow. Van Rhee (2001) modified the implementation of erosion in the Camp model, but concluded that the influence is small due to the characteristics of the model. Ooijens added the time effect, since the previous models assume an instantaneous response of the settling efficiency on the inflow of mixture. Yagi (1970) developed a new model based on the concentration distribution in open channel flow. The models mentioned above are all black box approaches assuming simplified velocity distributions and an ideal basin. Van Rhee (2002) developed a sophisticated model, the 2DV model. This model is based on the 2D (horizontal and vertical) Reynolds Averaged Navier Stokes equations with a k-ε turbulence model and includes suspended sediment transport for multiple fractions. The Miedema & Vlasblom model is based on the Camp model but lacks a number of features. One can think of: • The layer thickness of the layer of water above overflow level. • The effect of scour during the final stage of the loading process. • The hindered settling effect using the correct average concentration above the bed. • The storage, time delay or buffer effect. This article describes the basic Miedema & Vlasblom model (1996) and shows how the above features should be implemented.

Research assessing the impacts of trade liberalization on poor rural populations can be divided into two categories: more quantitative research, assessing relationships between specific, measurable variables (such as changes in the macroeconomic environment and their impact on farmers’ income levels); and more qualitative research, which takes trade policy as a context and provides broad, descriptive data about dynamic livelihood strategies. In this paper, we outline a framework that could be used to integrate these two approaches by unravelling the macro-micro linkages between national policies and responses at a household level. Using the Mexican maize sector as an illustration, we trace the pathways through which trade liberalization (including the North American Free Trade Agreement) has interacted with changes in government institutions, and thereby impacted on farmers’ livelihood strategies. We identify three pathways through which trade policy affects households and individuals: via enterprises, distribution channels, and government, and we link these to a five-category typology of smallholders’ strategies for escaping rural poverty: intensification, diversification, expansion, increased off-farm income and exit from agriculture. Based on a case-study from Chiapas, Mexico, we report on farmers’ responses to post-liberalization agricultural policies. Data suggest that farmers have intensified maize production, sought more off-farm employment or have exited agriculture altogether. The potential for smallholders to escape poverty by diversifying farms or expanding their land-holdings or herd-size has been largely unrealized. We provide a conceptual framework for linking the impacts of liberalization to farmers’ livelihood strategies and suggest that this framework is useful in the context of agricultural modernisation initiatives that seek to increase agricultural production and productivity.

Abstract The use of dredgers to work at sea is strongly dependent upon the ability to work in waves of various heights and lengths. As soon ,as the critical wave ,height is reached for a particular vessel, work ceases, so there is general interest in an improved performance with a consequent,reduction in down ,time. In order to extend ,the possibility of using ,floating dredgers as opposed to semi-submersibles or fixed platforms, the application of swell compensators for the excavating element, for example, cutter heads, is being attempted. This isnot so simple,as it might appear. In order to design,swell compensators,for the excavating element of dredgers, it is essential to have some understanding of the interactions such as those between ,cutter head and soil when ,cutter suction dredgers are working at sea. Both qualitative and quantitative predictions relating to this iteration must be made. ,This article describes an empirical, physical cutting model ofthe cutting forces and gives some results of model tests with a cutter headin densely compacted,sand. Pore Water, Shear Stress and Cutting Tooptimize,the dredging process it is important to understand the cutting process. This will

TASS is a software programme that enables the user to predict the spatial development and concentration of turbidity plumes arising from dredging activities by trailer suction hopper dredgers. TASS has been developed because of a recognised need by the dredging industry to improve the quality of predictions of the effects of dredging in Environmental Impact Assessments. This article describes the TASS model and the validation of TASS predictions against validation measurements off the Dutch and German coasts.

A novel transient one-dimensional horizontal simulation model that predicts the multi-fractional transport and sedimentation of sand along a hopper of a Trailing Suction Hopper Dredger (TSHD) has been developed. The model includes an external current model and an internal current model mutually coupled by vertical exchange processes. It solves cross-section averaged mass and momentum equations to predict sediment transport by barotropic and baroclinic currents. Relevant vertical physical processes are modelled using closure relations for hindered settling, entrainment, erosion and continuity-based vertical transport of water and sediment. Deposition and erosion closures dynamically couple both the external current model and the internal current model with a sediment bed model. A practical simulation tool is provided which balances the need of sufficient predictive capability through simulation of time and along hopper variation of model quantities with the requirement of low computational effort and complexity compared to transient two-dimensional vertical or three-dimensional models through the use of closure assumptions to model vertical processes. The model verification against a number of analytical solutions for idealised test cases indicates that the discretised model is mass, momentum and mechanical energy conservative. Moreover, it demonstrates that the model can cope with drying and flooding phenomena and with transitions from subcritical flow to supercritical flow and vice versa. The model validation against laboratory and prototype measurements shows that our model predicts the total cumulative overflow losses for a wide range of conditions on laboratory and prototype scale well at low user complexity level and at low computational time.

摘要：人们对于珍贵金属需求的日益增长，推动了深海采矿行业的发展。目前，竖直方向上的运输、海底开挖过 程以及竖管系统的稳定性等都是在这项新兴行业中最主要的技术挑战。 由于深海采矿的开挖活动通常在3000~6000 m 的水深处进行，海水产生的高压会极大地影响到切削过程。尤其 当切刀的切削速度很高时，“剪胀强化作用”(Brace and Martin, 1968) 可能会增加海底岩石开挖的难度。上述因素 都会导致深海岩石开挖过程不同于普通疏浚工程中的浅水开挖过程。本文将首先介绍深水开挖过程的物理原理。 由于开展高压岩石切削实验难度较大，用数值方法模拟切削过程具备更高的可行性，因此数值方法最终代替了实 验方法。数值方法的主要难点在于如何模拟岩石的破坏以及岩石与孔隙水之间的相互作用。基于尺度以及材料特性的 考量，离散元方法（DEM）将作为最佳方法用于模拟岩石的表现。此外，为描述由海水引起的高压影响，本文推导了 液相的控制方程，并使用有限体积法（FVM）求解了其相应方程。基于这个特定问题，本文将详细介绍该数值方法以 及二者之间的相互影响，并给出一部分关于粘土类材料切削过程的初步测试结果。

摘要：过去的文献中存在用于在斜管中输送泥浆的不同方法。多数的模型都假定所谓的“固体效 应”必须以特定的幂乘以倾斜角的余弦。 本文概述了一些现有一些模型的优缺点。这里为每个流态导出了各自的基本模型，这种方法的优 势是其使得流动状态之间的过渡的转变变得可见。我们基于这些流态构造了水力梯度曲线，在水平 管线中发生的某些流态可能不会在斜管中发生。 实验在位于中国上海的中交疏浚技术装备国家工程研究中心进行。这些实验使用大管径的管线和 d50 为 0.77 mm 的砂进行，浓度达16%，倾角达44°，管线速度达7 m/s。 将新导出的模型于新获得的独特实验数据进行比较，得出了很好的相关性。应当注意的是势能一 项始终主导着测量出的水力梯度。

From literature on this subject it is known that the process of cutting clay is different from that of water saturated sand. Sand is often modeled as a continuum with an internal friction angle and a sand/steel friction angle but without cohesion and adhesion. Clay is considered to be a continuum with cohesion and adhesion, but with an internal friction angle and a clay/steel friction angle equal to zero. In this paper clay will be considered this way. It has been noticed by many researchers that the cohesion and adhesion of clay increase with an increasing deformation rate. It has also been noticed that the failure mechanism of clay can be of the "flow type" or the "tear type", similar to the mechanisms that occur in steel cutting. Previous researchers, especially Mitchell 1976 [5], have derived equations for the strain rate dependency of the cohesion based on the "rate process theory". However the resulting equations did not allow pure cohesion and adhesion. In many cases the equations derived resulted in a yield stress of zero or minus infinity for a material at rest. Also empirical equations have been derived giving the same problems. Based on the "rate process theory" with an adapted Boltzman probability distribution, the Mohr-Coulomb failure criteria will be derived in a form containing the influence of the deformation rate on the parameters involved. The equation derived allows a yield stress for a material at rest and does not contradict the existing equations, but confirms measurements of previous researchers. The equation derived can be used for silt and for clay, giving both materials the same physical background Based on the equilibrium of forces on the chip of soil cut, as derived by Miedema 1987 [4] for soil in general, criteria are formulated to predict the failure mechanism when cutting clay. A third failure mechanism can be distinguished, the "curling type". Combining the equation for the deformation rate dependency of cohesion and adhesion with the derived cutting equations, allows the prediction of the failure mechanism and the cutting forces involved. The theory developed has been verified by using data obtained by Hatamura and Chijiiwa 1975-1977 [2] with respect to the adapted rate process theory and data obtained by Stam 1983 [8] with respect to the cutting forces. However since the theory developed confirms the work carried out by previous researchers its validity has been proven in advance. In this paper simplifications have been applied to allow a clear description of the phenomena involved.

In deep sea mining, the valuable materials will often be transported to the surface by means of slurry transport through pipelines, using centrifugal pumps to generate the pressure. The slurry transport pipeline has vertical, but also inclined trajectories. It is thus of interest what are the dominating factors in slurry transport in inclined pipes. Here this is investigated for Newtonian settling slurries. Experiments to investigate the dominating factors in slurry transport in inclined pipes were carried out in the CCCC National Engineering Research Center of Dredging Technology and Equipment, Shanghai, China. These experiments were carried out in a D p =0.3 m pipe with sand with a d 50 of 0.77 mm, concentrations up to 16%, inclination angles up to 44° and line speeds up to 7 m/s. The physics of slurry transport can be divided into 5 main flow regimes. Each flow regime has its dominating physics. The stationary bed regime is based on bed friction, the sliding bed flow regime on sliding friction, the heterogeneous flow regime on collisions and collision intensity, the homogeneous flow regime on wall friction and the sliding flow regime on sliding friction. So, each flow regime requires its own approach how to deal with inclined pipes. Models in literature most often multiply the so-called solids effect with the cosine of the inclination angle, without considering different flow regimes, which is considered here as incorrect. Because different flow regimes respond differently, also the transitions between the flow regimes will depend on the inclination angle. It should be noted however that the potential energy term always dominates the hydraulic gradients measured.

Clay is one of the three most commonly encountered soil types in riverbed and seabed, other than sand and silt. A major mechanical property of clay is the adhesion factor that reflects the ratio between its cohesive and adhesive strength under different water content. Currently, this adhesion factor is not exactly known. It is important however, to get a better understanding of the relationship between cohesive and adhesive forces, since large surfaces on dredging tools can generate a lot of resistance, therefore slowing production. In this study, experiments were undertaken to determine the actual relation between adhesion and cohesion. The results can help the implementation of analytical cutting models, in turn, allowing the optimal cutting angle in dredging practice to be calculated.

Depending on its size and installed power, a Cutter Suction Dredger (CSD) is capable of cutting a wide range of soil types from silts and clays to fractured or solid rocks. Its high precision allows for utilization in a variety of dredge operations including navigational channel deepening, port construction and pipeline trenching. In spite of being considered relatively efficient, a CSD can spill significantly. This article proposes a classification of the concurrent sources of CSD spillage as well as a analytical model for a priori computation of spillage due to high rotational velocity-induced flow. As of yet, in literature, no analytical models exist that describe spillage due to centrifugal advection.