Maintenance Dredging Research Articles

SEDIMENT DYNAMICS IN LAGOS HARBOUR - RECONNAISSANCE ON EFFECTS OF DREDGING

Due to economic growth, Lagos Harbour is expanding. Capital dredging is needed to allow larger vessels to call the port. As harbour siltation is already a problem, increase of maintenance dredging is a worry. In the past no data was available to understand the hydraulics and sediment transport to estimate maintenance dredging volumes. This paper presents the reconnaissance of the sediment dynamics in the harbour, based on field data and MIKE21 model simulations. Measurements of sediment concentration, salinity and flow velocity show that estuarine circulation occurs, transporting sediment from the ocean into the port. However, more upstream in the Commodore channel, at the location where dredging will be carried out, this circulation is small. Model simulations show that at that location, bed shear stresses are large, also after dredging. Therefore, fine sediment can not accumulate in the deepened areas and this will not result in large dredging volumes. Sand transport however can cause sedimentation, as bed forms can migrate. These mechanisms are also observed in initial sand transport simulations, but need to be studied further to fully understand the sediment dynamics in the harbour.

Impact on a fish assemblage of the maintenance dredging of a navigation channel in a tropical coastal ecosystem

Dredging and dredge-spoil disposal are among the major problems in coastal management. Many of the scientific contributions concerning the impacts of this practice are based on the study of sessile organisms and subtropical environments. We evaluated changes in the composition and abundance of a fish assemblage resulting from dredging and sediment disposal at the mouth and in the adjacent waters of the Caravelas River on the north-eastern coast of Brazil. Samples were collected in two directly impacted and three adjacent areas. Differences among stations were not significant, but the dredged site had the least diverse station, as expected. The stations farthest from the directly impacted areas apparently were not influenced by the coastal work, thus suggesting localised effects. The contribution of the present study is particularly important because of the study area's proximity to others that have high conservation value such as mangrove forests and coral reefs, and the relevance of the subject given the continuing dredging activity.

The Variation of Total Suspended Sediments Due to the Change in Sea Water Depth, Tidal Phase and Elevation of Sea Water Sample Collection in Khor Sabiya Inlet of the Arabian Gulf, Kuwait

ABSTRACT AL-HULAIL, F and NEELAMANI, S., 2011. The Variation of Total Suspended Sediments Due to the Change in Sea Water Depth, Tidal Phase and Elevation of Sea Water Sample Collection in Khor Sabiya Inlet of the Arabian Gulf, Kuwait. In: Micallef, A. (ed.), MCRR3-2010 Conference Proceedings, Journal of Coastal Research, Special Issue, No. 61, pp. 369–374. Grosseto, Tuscany, Italy, ISSN 0749-0208. The seawater intake channel in one of the power plans in Kuwait is attracting significant settlement of Total Suspended Solids (TSS). Maintenance dredging of about 400,000 m3/year of silt and clay is carried out with a total annual maintenance cost of about 1.4 million Kuwaiti Dinar. There is a proposal to increase the seawater intake for a proposed 2050 MW additional power plant and hence further increase in the TSS settlement is anticipated, if the present sea water intake system is used. Alternative types of seawater intake systems, which can reduce the entry of total suspended sediments, is explored. Before ...

Impacting Factors and Cumulative Impacts by Midcentury on Wetlands in the Louisiana Coastal Area

Natural and anthropogenic impacting factors simultaneously cause wetland loss in the Louisiana coastal area. Ongoing natural processes are integrated with anthropogenic impacting factors to rationalize the contribution of each impact type within a total system. From baseline conditions of today, two natural processes account for approximately 85% of the estimated cumulative land loss by 2050: (1) subsidence by sediment compaction and tectonism that is driven by the weight of the Mississippi Delta's sedimentary pile and (2) absolute sea-level rise. Hurricanes are sudden natural processes that act as impact accelerators. After the Mississippi River became hydrologically isolated from the delta it built, anthropogenic impacts coinciding with the ramp up to peak oil production in the Louisiana coastal area caused many direct impacts, such as the construction of access canals and pipelines, but indirect impacts are largely symptomatic of natural delta platform submergence. Formation extraction is the only significant anthropogenic activity influencing projected land losses: about 6%. Other anthropogenic activities, such as onshore development, maintenance dredging, and oil spills, are not significant impacting factors. Additive processes, such as delta outbuilding and coastal restoration, are also not significant. A growing body of Louisiana coastal area subsidence studies is proceeding in absence of insight as to how conclusions might be ordered by the subsurface structural fabric that has influenced coastal geomorphology and that continues to influence surface processes. Subsidence assessments have been heavily weighted with inputs based on surface and near-surface data sets that are easy to see and touch. Approaches using basin analysis techniques are a critical omission. Such outputs integrate subsurface geologic logs and seismic data sets into products able to inform decisions about locating coastal restoration or flood protection projects and avoid areas of geologically persistent subsidence.

Effects of salinity and sedimentation on the Gorgonian Coral, Leptogorgia virgulata (Lamarck 1815)

Construction of jetties along the Texas coast has provided hard substrate biota with suitable habitats for attachment that is otherwise scarce. Although similar to natural hard bottom environments such as coral reefs or offshore banks, jetties in temperate or subtropical areas are subjected to substantially more environmental instability including swift channelized currents, terrestrial runoff, maintenance dredging, fluctuating temperatures, and swells from ship and recreational boat traffic. The gorgonian coral Leptogorgia virgulata, found on south Texas jetties, is constantly exposed to the variable salinity and sediment loads associated with these jetty systems. In this study, L. virgulata was collected from the Aransas Pass jetties near Corpus Christi, Texas and experimentally tested to determine their ability to tolerate a range of salinities and sediment loads over 14 days. In response to salinity, L. virgulata showed significant tissue loss ( p < 0.05) at 10, 15, 45, 50 and 60 with 100% loss at 60, but showed little or no tissue loss and peak polyp activity at 25–40. When exposed to sedimentation treatments up to 20,000 mg l − 1 , L. virgulata maintained healthy tissue and polyp feeding activity and did not show any symptoms or significant differences in tissue loss. These results indicate that L. virgulata is well-adapted to euryhaline conditions and is very tolerant of high sediment loads, a factor in its success on structures along the Texas coast.

English

Harbor and ship channel maintenance dredging by the Maryland Port Administration has historically presented a dredge disposal and utilization challenge due to metals and the potential acidity of their sediments. Post-placement low pH (<4.0) has been problematic for vegetation establishment and also reduces the pH of water moving through or over the sediment disposal or utilization facility. To determine best management practices for one material (Cox Creek; H2O2 potential acidity - PPA = - 10 Mg CCE per 1000 Mg material; Total S = 1.31%; calcium carbonate equivalent - CCE = 7.13%) proposed for upland placement, we conducted lab and field experiments. In the lab, a series of liming rates (0.00, 0.50, 0.75, 1.00, and 1.25x of required lime additions as CCE) were evaluated through 32 wet-dry cycles. The pH of the 0.00x liming rate treatment remained between 4.10 and 4.20 throughout the experiment, while the effects of the lime rates on bulk sediment pH were not evident until after four wet-dry cycles were completed. The lack of development of pH lower than 4.10 was unexpected. Based on the PPA results, we predicted the pH of the unlimed treatment to drop below 4.0 over more extended periods of time. By the end of the experiment, the 1.25x liming rate sediment pH was 6.22 and the 0.50x pH was 5.11. Salinity of the material is clearly an additional issue. The EC of the 0.00x liming rate increased from 5.9 dS m -1 at the start of the experiment to 7.9 dS m -1 , while the 1.25x rate EC increased from 7.0 to 8.9 dS m -1 , presumably due to sulfate release. In the field, two lime application methods were tested (bulk blending and layering) against an unlimed control treatment. Three zero-tension lysimeters were installed under each plot to monitor pH and EC of leachates. First-year results indicate that the pH of both the bulk-blended lime and layered treatments remained between 4.4 and 5.5 after four sampling events. The unlimed control pH was slightly lower, between 4.2 and 4.5. The EC of the leachate samples (6 to 17 dS m -1 ) again indicated an issue with soluble salts across all treatments. The salts initially originate from entrained chlorides, but are enhanced by sulfates over time as sulfides react and are neutralized. Thus, managing these materials for plant growth and water quality in an upland environment will be limited by pH and the need to develop strategies for leaching soluble salts and predicting their ground- and surface water impacts. The PPA technique for estimating potential acidity was superior to a more conventional acid- base-accounting technique based on Total-S and CCE determinations.

Temporal dynamics of deep infralittoral macrobenthic fauna in a subtropical estuarine environment

Seasonal and inter-annual patterns of hydrological variation determine different salinity regimes inside the Patos Lagoon estuarine region, southern Brazil. Throughout two environmentally distinct years (2000 and 2006), macrobenthic samples were taken seasonally from a 14 m deep main channel region, at nine monitoring sites, using a van Veen grab (0.078 m2). In order to keep the Patos Lagoon main access channel at constant navigable depths, the Rio Grande Harbour Superintendence periodically executes maintenance dredging operations on this infralittoral area, but our multivariate analyses evidenced no significant difference between dredged and non-dredged sites regardless the year. Among the abiotic parameters analyzed here (temperature, percentage of fine sediments and salinity), the later was evidenced to be highly correlated to the biological data. Reductions in density and/or changes in the species composition could both be attributed to a natural macrobenthic response related to environmental variations. Along the year 2000 high salinity values decreased constantly through seasons to almost limnetic condition in spring, reducing diversity and abundance of marine species and densities of the dominant gastropod species Heleobia australis. On the other hand, salinity values along the year 2006 revealed the maintenance of brackish waters throughout seasons, favoring the input and survival of marine species from coastal adjacent areas inside the estuary, raising its macrobenthic diversity mainly by mollusks and polychaete worms.

New Ebb-Tidal Delta at an Old Inlet, Shark River Inlet, New Jersey

Abstract Shark River Inlet, located on the north New Jersey coast, is served by a federal navigation channel that has until recently required little maintenance dredging. Although possessing a small estuary, the inlet is hydraulically efficient because of the small width to depth ratio of its entrance that is stabilized by parallel jetties. After placement of approximately 4.8 million m3 of beach nourishment to the north and south of the inlet as part of an erosion-control project conducted in the late 1990s, inlet maintenance increased beyond that anticipated. Analysis of channel and nearshore surveys indicates that an ebb-tidal delta is forming where none had existed previously, attributed to the recent availability of sand from the beach nourishment and a lack of sand prior to that construction. Jetty tip shoals also encroach on the channel, dependent on season, with longshore transport directed primarily to the north during summer (the predominant direction of transport) and to the south during winter...

TECHNICAL FEASIBILITY ANALYSIS OF SUBMARINE PIPELINES FOR INTAKE OF SEA WATER WITH HIGH TOTAL SUSPENDED SEDIMENT LOADS

ABSTRACT Submarine pipeline is used as a seawater intake system since it is cost effective compared to canal and jetty type intake structures. Open channel type intakes attract large deposition of sediments and hence periodic maintenance dredging is essential. Jetty type seawater intake is generally expensive. Much coastal water around the world hold high concentration of Total Suspended Sediments (TSS) like the coastal waters of Sabiya area, Kuwait in the north western part of Arabian Gulf. The existing seawater intake channel of a power plant is attracting significant settlement of TSS. Maintenance dredging of about 400,000 m25/year is carried out. It is proposed to increase the seawater intake for a proposed 2050 MW additional power plant and hence further increase in the TSS settlement is anticipated, if the present sea water intake system is used. Alternative type of seawater intake system, which can reduce the TSS is explored. Feasibility study is carried out to reduce the sediment entry into the power plant utility. The TSS levels were assessed, and its variations with respect to tidal phase, water depth and elevation along the depth are assessed. The settling velocity of the sediments is assessed. A sea water intake system is designed and the modality of taking water from the inlet and the configurations of the settling tanks are provided. It is advisable to draw water closer to the free water surface using a vertically moving intake well. It is found that the TSS level reduces 40% to 60% when the ambient flow velocity of sea water is minimum. Hence it is beneficial to avoid taking seawater when the sediment carrying capacity of the seawater is high during high currents. This study will be useful for feasibility design of submarine pipeline type intake for similar locations around the world.

Monitoring the effects of disposal of fine sediments from maintenance dredging on suspended particulate matter concentration in the Belgian nearshore area (southern North Sea)

The impact of continuous disposal of fine-grained sediments from maintenance dredging works on the suspended particulate matter concentration in a shallow nearshore turbidity maximum was investigated during dredging experiment (port of Zeebrugge, southern North Sea). Before, during and after the experiment monitoring of SPM concentration using OBS and ADV altimetry was carried out at a location 5km west of the disposal site. A statistical analysis, based on the concept of populations and sub-sampling, was applied to evaluate the effect. The data revealed that the SPM concentration near the bed was on average more than two times higher during the dredging experiment. The disposed material was mainly transported in the benthic layer and resulted in a long-term increase of SPM concentration and formation of fluid mud layers. The study shows that SPM concentration can be used as an indicator of environmental changes if representative time series are available.

Flood-tide Delta Morphological Change at the Gippsland Lakes Artificial Entrance, Australia (1889–2009)

Analysis of historical survey and navigation charts, aerial imagery, and digital hydro-data (representing a time-series of 1889–2009) allows the morphological evolution of a flood-tide delta at the artificial entrance to the Gippsland Lakes (Victoria, Australia) to be visualised and quantified. Analysis shows that flood-tide delta growth has imposed progressively greater demands upon port managers for publicly funded maintenance dredging. This flood-tide delta growth corresponds with a progressive diminution of Gippsland Lakes catchment river discharge volumes, and dampening of flow variability, due to both regional precipitation pattern changes and changes in catchment water resource allocation. Future climate change predictions for the Gippsland Lakes catchment and coastal area suggest both further decreases in catchment river discharge, and changing sediment flux along this sector of the Ninety Mile Beach. Thus, scope for flood-tide delta nourishment will probably increase, as will the demand for mitigation of the inevitable effects upon entrance channel navigability. Information derived from the analysis of time-series bathymetry used for this study offers baseline information in support of stakeholder consensus building regarding options for maintaining navigability. Analysis points to the merit of considering (and testing via morphological modelling) alternative options for navigation channel maintenance than those in present practice. We argue that the circumstances prevailing at the artificial entrance call for consideration of engineered configuration changes practised elsewhere by entrance managers faced with similar issues.

大阪港湾域における底泥からの汚濁発生源除去効果からみた航路浚渫の評価

大阪港湾域における底泥からの汚濁発生源除去効果からみた航路浚渫の評価

Impact and recovery associated with the deposition of capital dredgings at UK disposal sites: Lessons for future licensing and monitoring

The majority of studies relating to impacts and recovery at dredgings disposal sites have concentrated on areas subject to regular and frequent disposals of maintenance dredgings over relatively long time periods. In comparison less is known regarding the significance of impacts and the recovery processes associated with the disposal of capital dredgings that commonly involves the infrequent deposition of heterogenous material over relatively restricted time periods. Impacts and recovery processes are likely to be different to those associated with the disposal of maintenance dredgings. For example, findings suggest that capital dredgings deposited at both the Roughs Tower and Barrow-in-Furness result in the occurrence of persistent changes to seafloor substrata within the license area and this subsequently effects the composition of associated faunal communities present. Moreover, whilst the two disposal sites are geographically distinct similar species are identified as being particularly sensitive to capital disposal activities in both areas.

Impacts of maintenance channel dredging in a northern Adriatic coastal lagoon. II: Effects on macrobenthic assemblages in channels and ponds

Coastal lagoons are ephemeral habitats whose conservation requires human intervention, such as maintenance dredging of inner channels. Dredging can reduce the abundance of benthic species due to the removal of individuals with the sediment, modify sediment properties, and resuspend fine sediment, nutrients and pollutants, which can lead to eutrophication, hypoxic events and increasing toxicity. Both direct effects in the dredged channel and possible indirect effects in surrounding shallow areas could be expected. This study assesses the effects of the channel maintenance dredging, performed between October 2004 and August 2005, on the invertebrate assemblages both in channels and adjacent ponds in the northern Adriatic coastal lagoon of Pialassa Baiona. The lagoon is affected by eutrophication, chemical and thermal pollution from wastewater treatment and power plants. Three impacted sites were located in the dredged channel and three in the adjacent interconnected shallow water ponds, while three non-impacted sites were located in a channel and in a pond far from the dredged area. Replicate samples were collected from each site one time before and one time after the dredging operations. Despite the extent of the intervention, effects of the dredging on macrobenthic assemblages were detected only within the dredged channel, while in the surrounding ponds no clear and unequivocal effects were found. In particular the dredging could have promoted the increase of the abundance of the polychaete Streblospio shrubsolii in the southern and central parts of the dredged channel and the increase in abundance of the amphipod Corophium insidiosum in the northern side, compared to the controls. Instead, species diversity was reduced in the central and northern parts of the dredged channel. These effects on the macrobenthic invertebrate assemblages could be related to the observed changes of sediment characteristics, contamination and toxicity. Overall, direct effects on benthic assemblages in the dredged channels were more detectable than the possible secondary effects in the surrounding shallow ponds, where the higher spatial heterogeneity can mask any relevant effects.

Impacts of maintenance channel dredging in a northern Adriatic coastal lagoon. I: Effects on sediment properties, contamination and toxicity

Conservation and management of coastal lagoons envisage direct human intervention. To prevent siltation and to preserve the hydrodynamics features of the lagoon system, the inner channels undergo regular maintenance dredging. Sediment properties (RDP, organic matter, grain size), trace metals (Cd, Cu, Cr, Hg, Ni, and Pb), and toxicity vs. the amphipod Corophium insidiosum and the luminescent bacterium Vibrio fischeri, were analysed before and after dredging operations in a coastal lagoon (Pialassa Baiona, Italy). To detect the actual impacts, disturbed sites were contrasted with multiple controls in two distinct times, i.e. before and after disturbance, according to a sampling design based on Beyond BACI principles. The integrated methodology here adopted suggests that dredging operations carried out are not likely to pose dramatic effects on environmental quality of the lagoon.

Modelling navigation in muddy areas through captive model tests

Many access channels and harbours suffer from sedimentation and formation of mud layers. To keep navigation safe and economic, extensive maintenance dredging is needed. Until now, the required level of dredging has always been determined by making use of the physical characteristics of the mud layer, which does not necessarily result in efficient dredging. Ship behaviour also matters, and knowledge of ship manoeuvrability can help to redefine the level of dredging required. A mathematical model of manoeuvring for simulation purposes, based on a comprehensive series of captive model tests, is presented, along with a selection of results.

Ebb-Jet Dynamics and Transient Eddy Formation at Tauranga Harbour: Implications for Entrance Channel Shoaling

Abstract In 1992 the entrance channel through the tidal inlet to Tauranga Harbour, which is located along the Bay of Plenty littoral drift system, was deepened from 10 m to 14 m. The deepened channel has become a sediment trap for littoral drift bypassing and tidal current driven sediment transport through the inlet. Since 1992, there has been an increase in maintenance dredging requirements at the inlet, because of sand accumulation along the southeastern border of the entrance channel. Previous studies have identified an ebb tide–induced eddy operating on the eastern side of the ebb-jet as it exits the tidal gorge. In this article, the eddy system has been simulated with a validated two-dimensional hydrodynamic model, detailing time-varying current patterns over the ebb-tidal delta. Particular emphasis is placed on defining the trajectory of the eddy and evaluating its influence on the observed sedimentation patterns. The model results indicate the formation of opposing eddies on either side of the entr...

Plan-view photos, benthic grabs, and sediment-profile images: Using complementary techniques to assess response to seafloor disturbance

A monitoring survey was conducted in July 2005 at the Rhode Island Sound Disposal Site (RISDS) as part of the Disposal Area Monitoring System (DAMOS) program. The survey included the collection of sediment-profile and plan-view images, and benthic biology grabs. Each of these techniques provides a different, yet complementary perspective on benthic community conditions. These complementary techniques aided in the assessment of the benthic recovery process within RISDS following the placement of dredged material from the Providence River and Harbor Maintenance Dredging Project (PRHMDP). Based on observed patterns of physical, chemical, and biological responses of seafloor environments to dredged material disposal activity it was expected that the benthic community within RISDS would be in an intermediate phase of recolonization (Stage II). Results of the 2005 RISDS survey indicated that in the six months since disposal activities at RISDS had concluded, the biological community at RISDS was recovering relatively rapidly and Stages II and III infauna were present throughout the region.

L'envasement dans les ports

ABSTRACT The distributions of mud deposits and turbidities are studied in the estuary of the river Loire and in the coastal area of Loire-Atlantique (France). A description of the sedimentary dynamics is given in regard to tidal streams and water agitation due to waves. The silting up of the harbour sites is investigated in relation with the environmental conditions and local hydrodynamics associated with the harbour infrastructures. Some configurations are suggested for the implantation of fishing ports and marinas, taking into account economical and environmental conditions. A possible solution for the long term harbour planning in the estuary of the river Loire is suggested; it would induce an important decrease of the maintenance dredgings.

Validation of a Deterministic Hydrodynamic Model in the ICZM Framework for Gulf of Kachchh, India

Abstract The Gulf of Kachchh (Gulf), India, an ecologically rich habitat, has a wide variety of activities, such as extensive salt farming along the coast, brine release from salt farms, maintenance dredging by ports and disposal of dredged material, release of hot water from thermal power plants, and navigation. Management of coastal and marine areas by maintaining the balance between the developmental activities and protection of marine environment is a challenging task for coastal managers and requires a complete understanding of the physical, chemical, and biological aspects of the system. In this study, a two-dimensional hydrodynamic model (MIKE-21) was developed for Gulf of Kachchh with rectangular grid resolution of 450 by 450 m to understand the role of circulation pattern for mixing capacity of the Gulf. The model was calibrated with tide and current at predetermined locations. The model predictions show good agreement with the measured tide and current data. The model-predicted amplitudes and ph...