Showing papers on "Dredging published in 2015"

Temporal Patterns in Seawater Quality from Dredging in Tropical Environments.

Abstract: Maintenance and capital dredging represents a potential risk to tropical environments, especially in turbidity-sensitive environments such as coral reefs. There is little detailed, published observational time-series data that quantifies how dredging affects seawater quality conditions temporally and spatially. This information is needed to test realistic exposure scenarios to better understand the seawater-quality implications of dredging and ultimately to better predict and manage impacts of future projects. Using data from three recent major capital dredging programs in North Western Australia, the extent and duration of natural (baseline) and dredging-related turbidity events are described over periods ranging from hours to weeks. Very close to dredging i.e. <500 m distance, a characteristic features of these particular case studies was high temporal variability. Over several hours suspended sediment concentrations (SSCs) can range from 100–500 mg L-1. Less turbid conditions (10–80 mg L-1) can persist over several days but over longer periods (weeks to months) averages were <10 mg L-1. During turbidity events all benthic light was sometimes extinguished, even in the shallow reefal environment, however a much more common feature was very low light ‘caliginous’ or daytime twilight periods. Compared to pre-dredging conditions, dredging increased the intensity, duration and frequency of the turbidity events by 10-, 5- and 3-fold respectively (at sites <500 m from dredging). However, when averaged across the entire dredging period of 80–180 weeks, turbidity values only increased by 2–3 fold above pre-dredging levels. Similarly, the upper percentile values (e.g., P99, P95) of seawater quality parameters can be highly elevated over short periods, but converge to values only marginally above baseline states over longer periods. Dredging in these studies altered the overall probability density distribution, increasing the frequency of extreme values. As such, attempts to understand the potential biological impacts must consider impacts across telescoping-time frames and changes to extreme conditions in addition to comparing central tendency (mean/median). An analysis technique to capture the entire range of likely conditions over time-frames from hours to weeks is described using a running means/percentile approach.

Spatial Patterns in Water Quality Changes during Dredging in Tropical Environments

Abstract: Dredging poses a potential risk to tropical ecosystems, especially in turbidity-sensitive environments such as coral reefs, filter feeding communities and seagrasses. There is little detailed observational time-series data on the spatial effects of dredging on turbidity and light and defining likely footprints is a fundamental task for impact prediction, the EIA process, and for designing monitoring projects when dredging is underway. It is also important for public perception of risks associated with dredging. Using an extensive collection of in situ water quality data (73 sites) from three recent large scale capital dredging programs in Australia, and which included extensive pre-dredging baseline data, we describe relationships with distance from dredging for a range of water quality metrics. Using a criterion to define a zone of potential impact of where the water quality value exceeds the 80th percentile of the baseline value for turbidity-based metrics or the 20th percentile for the light based metrics, effects were observed predominantly up to three km from dredging, but in one instance up to nearly 20 km. This upper (~20 km) limit was unusual and caused by a local oceanographic feature of consistent unidirectional flow during the project. Water quality loggers were located along the principal axis of this flow (from 200 m to 30 km) and provided the opportunity to develop a matrix of exposure based on running means calculated across multiple time periods (from hours to one month) and distance from the dredging, and summarized across a broad range of percentile values. This information can be used to more formally develop water quality thresholds for benthic organisms, such as corals, filter-feeders (e.g. sponges) and seagrasses in future laboratory- and field-based studies using environmentally realistic and relevant exposure scenarios, that may be used to further refine distance based analyses of impact, potentially further reducing the size of the dredging footprint.

Use of multi-objective dredging for remediation of contaminated sediments: a case study of a typical heavily polluted confluence area in China

Abstract: Sediments in confluence areas are typically contaminated by various pollutants that have been transported there by inflowing rivers. In this study, we evaluated the pollution status of a confluence area in Lake Chaohu (China). Both the nutrients and hazardous pollutants were analysed. Most sediment cores showed large variations in nutrient concentrations at depths of 10 to 18 cm. Positive release rates of NH4 +-N and PO4 3−-P were detected in sediment cores. Hg and Cd were the most typical problematic metal contaminants encountered, and their contamination levels extended to depths of 20 and 25 cm, respectively. Polycyclic aromatic hydrocarbons (mostly acenaphthene and fluorine) were the primary persistent organic pollutants (POPs) present in sediments, and contamination levels frequently could be detected up to a depth of 16 cm. Simulated dredging operations were implemented in the laboratory, with a dredging depth of 15 cm found to be suitable for nutrient suppression. With the goal of suppressing nutrients release and removing high-risk metals and POPs, a multi-objective dredging plan was developed. This plan subdivides the confluence area into five parts that were treated with different dredging depths. A demonstration area was dredged in the most heavily polluted part, and the observed dredging effects were consistent with those expected on the basis of the plan. Such an approach to dredging might also be useful in other areas in the future.

The Effects of Dredging on Tidal Range and Saltwater Intrusion in the Pearl River Estuary

Abstract: Yuan, R. and Zhu, J., 2015. The effects of dredging on tidal range and saltwater intrusion in the Pearl River estuary. A dredging project in the Pearl River estuary would influence the topography, water dynamics, and mass transport process. The effects of dredging in the channel of the Pearl River estuary on tidal range and saltwater intrusion were studied using a hydrodynamic model (finite volume coastal ocean model) and historical water depth data. The measured data showed that the tidal range decreased and saltwater intrusion increased in the North River. The tidal range increased and saltwater intrusion decreased in the West River in recent decades from the 1980s. The simulations in this study showed that after dredging (1) the upstream flow split ratio in the river changed (i.e. more freshwater flowed through the Sanshui station), whereas less freshwater flowed through the Makou station; (2) the tidal range increased in the West River network and decreased in the downstream reaches of the No...

Interannual Variability of Two Offshore Sand Banks in a Region of Extreme Tidal Range

Abstract: Lewis, M.J.; Neill, S.P., and Elliott, A.J., 2015. Interannual variability of two offshore sand banks in a region of extreme tidal range. Offshore sand banks play important roles for coastal flood protection and fisheries, and they are sources of marine aggregates. An 11-year record (1991–2002) of annual bathymetric surveys from two sand banks (Nash and Helwick) in the Bristol Channel (U.K.) were analysed. Both sand banks have a history of commercial dredging, and have dimensions of the order 10 km by 1 km, with a crest height rising to around 20–25 m above the surrounding sea bed. The crest at Nash Bank is exposed on the lowest spring tides, while Helwick Bank is always covered to a depth of at least 3 m. The volume of Nash Bank decreased over 10 years, and dredging was estimated to be responsible for around one third of this reduction. The volume of Helwick Bank also decreased over an 8-year period, but at a rate six times greater than the loss due to dredging. Significant interannual variabili...

Wave Glider Monitoring of Sediment Transport and Dredge Plumes in a Shallow Marine Sandbank Environment

Abstract: As human pressure on the marine environment increases, safeguarding healthy and productive seas increasingly necessitates integrated, time- and cost-effective environmental monitoring. Employment of a Wave Glider proved very useful for the study of sediment transport in a shallow sandbank area in the Belgian part of the North Sea. During 22 days, data on surface and water-column currents and turbidity were recorded along 39 loops around an aggregate-extraction site. Correlation with wave and tidal-amplitude data allowed the quantification of current- and wave-induced advection and resuspension, important background information to assess dredging impacts. Important anomalies in suspended particulate matter concentrations in the water column suggested dredging-induced overflow of sediments in the near field (i.e., dynamic plume), and settling of finer-grained material in the far field (i.e., passive plume). Capturing the latter is a successful outcome to this experiment, since the location of dispersion and settling of a passive plume is highly dependent on the ruling hydro-meteorological conditions and thus difficult to predict. Deposition of the observed sediment plumes may cause habitat changes in the long-term.

The Effects of a Simulated Suspended Sediment Plume on Eastern Oyster (Crassostrea virginica) Survival, Growth, and Condition

Abstract: Bottom sediments are resuspended into the water column during dredging operations. These resuspended sediments are an often cited concern used to justify restrictions applied to dredging schedules in many areas of the USA. One example of a temporal restriction, commonly referred to as an environmental window, involves dredging schedules in the James River, Virginia, because of potential impacts on the eastern oyster (Crassostrea virginica Gmelin). Yet, effects’ data are lacking to understand the effects of suspended sediments to C. virginica. To address this data gap, we performed a laboratory study mimicking sediment resuspension during annual dredging operations in the James River. Field-collected oysters were exposed for 7 days under flow-through conditions to 0, 100, 250, and 500-mg/L total suspended solids (TSS) in a unique exposure system where oyster movements could be electronically monitored. Endpoints analyzed were survival, percent of time open, total number of shell movements, weight change, and condition index. Data indicated no significant effects of suspended sediment on these endpoints after 7 days of exposure. Weight change in oysters attached vertically to monitor their movements was significantly less than in oysters not monitored in every treatment. No significant differences in condition index, an indicator of oyster growth sensitive to environmental pollutants, were observed among treatments measured 30 days postexposure. Correlations performed for each treatment among monitored oyster endpoints found significant negative associations between number of movements and percent open in the 100, 250, and 500-mg/L TSS treatments and in all treatments combined. These data will help reduce the uncertainty surrounding the effects of suspended sediments on C. virginica.

Optimizing Dredge-and-Dump Activities for River Navigability Using a Hydro-Morphodynamic Model

Abstract: Worldwide, significant dredging activities of riverbed sediment are employed to ensure that freight transportation on rivers can continue year-round. Imbalances of sediment budget may produce relevant impacts regarding river morphology and related environmental services. This study shows that hydro-morphodynamic modeling tools can be used to optimize dredge-and-dump activities and, at the same time, mitigate problems deriving from these activities in rivers. As a case study, we focused on dredging activities on the Lower Parana River, Argentina. Navigation on this river is of crucial importance to the economies of the bordering countries, hence, each year significant dredging activities are employed. To estimate dredging loads under different strategies, a 25 km river reach of the Parana River was modeled using the Delft3D-modelling suite by Deltares. The Netherlands, to simulate flow-sediment interactions in a quasi-steady and uncoupled approach. Impacts of dredging activities were explicitly included. Different dredge-and-dump strategies included variations in dredging over-depth (clearance) and variations in dumping locations. Our results indicate that dredge-and-dump strategies can be targeted to stimulate natural processes that improve the depth and stability of the navigation channel and to counteract unwanted bed level responses in the long-medium term. A ~40% reduction in dredging effort could be achieved by moving the dredged material to distant locations in the secondary channel rather than dumping to the side of the waterway in the main channel.

Synthesis of current knowledge of the biophysical impacts of dredging and disposal on the Great Barrier Reef: report of an independent panel of experts

Abstract: [Extract] This report provides an independent synthesis of the current knowledge of the effects of dredging and sediment disposal on the physico-chemical environment and the biological values of the Great Barrier Reef World Heritage Area (World Heritage Area), as assessed by an Expert Panel. Dredging and sediment disposal can change the physical and chemical environment and affect the biological values of the World Heritage Area. Many of these effects will be context dependent and will differ between locations, types and extent of dredging and sediment disposal activities. The Expert Panel's evaluation identified the following key direct and indirect effects:

Benthic biodiversity changes in response to dredging activities during the construction of a deep-water port

Abstract: During the construction of a deep-water port (JadeWeserPort), bathymetry, sediment distribution, and macrofauna community structure were studied in the Inner Jade, a highly anthropogenically impacted tidal channel located in the southern North Sea. In order to assess the effects of additional disturbance by dredging activities, macrofaunal community compositions between 2002 (before the construction work had begun) and 2010 (during the final construction phase) were compared. The sand extraction for land reclamation and the redirection of the fairway changed the bathymetry markedly. While the old fairway in the centre of the study area remained mud dominated, a general increase in coarser sediments was detected in 2010. The dynamic nature of the study area in combination with the direct and indirect effects of dredging increased bathymetric heterogeneity [measured by singlebeam (2002) and multibeam (2010) echo-sounder]. In 2010, the macrofauna community structure roughly resembled the different categories of dredging activities. The most recently dredged northwestern area was inhabited by a community, which was different from the community in the regularly dredged old fairway. Both were different from the community in the northeastern, non-dredged area. In the southern area, and in the transition areas between the other three communities, a fourth community was found. A general increase of macrofaunal abundance and taxa number was observed in 2010, with the exception of the recently dredged area. The structure of the macrofauna community during the port construction phase seemed to be determined by secondary dispersal of the dominant taxa and recolonisation by highly mobile and opportunistic species.

Changes in the vegetation of a small lowland river valley (Krąpiel, NW Poland) after dredging

Abstract: The paper presented the impact of dredging on the vegetation of a regulated section of a lowland river based on the example of the Krąpiel River (NW Poland). The field research with pre-investment monitoring was conducted over three vegetation seasons in the years 2008–2010. Mechanical dredging and alignment of the river bed caused changes in vegetation structure. Reducing the diversity of habitat contributed to observed impoverishment of plant communities and species that build them. In the first year after dredging, communities from class Bidentetea tripartitae (Chenopodietum rubri) were dominant along the flood terrace covered with a layer of dredging material. In the second year after dredging, terophytes were again replaced by rush communities, dominated by Phalaridetum, Phragmitetumand locally Riparian tall herb communities.

Reducing the impact of blue mussel (Mytilus edulis) dredging on the ecosystem in shallow water soft bottom areas

Abstract: Dredging blue mussels (Mytilus edulis) and thus removing structural elements, inducing resuspension of sediment as well as reducing filtration capacity, will inevitably affect the ecosystem. The study demonstrates that the impacts of fishing can be reduced through gear developments. A new light dredge was tested on commercial vessels using two different experimental setups. First, a twin haul experiment tested the standard gear (i.e., a Dutch dredge) against the light dredge by fishing the two gears side by side onboard the same vessel. Second, a single dredge experiment tested the absolute performance of the two gears by fishing in areas with a known blue mussel density. Results from the twin haul experiment demonstrate that the weight of sediment retained in the gear per square metre fished is 49% less in the light dredge compared with the Dutch dredge which will reduce resuspension of sediment at the surface. Also, the drag resistance of the light dredge was significantly less (177.1 vs. 202.7 kg m-1). In the twin haul experiment no significant difference was found in the catch per unit effort (CPUE) of the two gears. The single dredge experiment, on the other hand, demonstrated a significant increase in CPUE exceeding 200% when using the light dredge. Seafloor tracks made by the two dredges could not be distinguished by use of side-scan sonar and the tracks were still detectable 2 months after fishing. It was concluded that replacement of the Dutch dredge with the light dredge would reduce the impact of the fishery on the ecosystem by (i) reducing resuspension of sediment, (ii) reducing fuel consumption, and (iii) potentially reducing energy transfer to the sediment through a reduced gear drag resistance. A potential increase in catch efficiency may reduce the area affected. Fishing with the light dredge is discussed in relation to management of Natura 2000 sites. Copyright © 2014 John Wiley & Sons, Ltd.

Using Information Technologies In Dredging

Abstract: The article deals with various aspects of using information technologies in dredging. The possibilities of modern information technology in the framework of improving the efficiency of dredging.

Agricultural Rivers at Risk: Dredging Results in a Loss of Macroinvertebrates. Preliminary Observations from the Narew Catchment, Poland

Abstract: Ecosystem deterioration in small lowland agricultural rivers that results from river dredging entails a significant threat to the appropriate ecohydrological conditions of these water bodies, expressed as homogenization of habitats and loss of biodiversity. Our study was aimed at a comparison of abundance and taxonomic structure of bottom-dwelling macroinvertebrates in dredged and non-dredged stretches of small lowland rivers and tributaries of the middle Narew River, namely: Czaplinianka, Turośnianka, Dąb, and Ślina. The experimental setup was (1) to collect samples of the bottom material from the river stretches that either persisted in a non-modified state (dredging was not done there in the last few years) or had been subjected to river dredging in the year of sampling; and (2) to analyze the abundance and taxonomic structure of macroinvertebrates in the collected samples. The study revealed that at the high level of statistical significance (from p = 0.025 to p = 0.001), the total abundance of riverbed macroinvertebrates in the dredged stretches of the rivers analyzed was approximately 70% lower than in non-dredged areas. We state that the dredging of small rivers in agricultural landscapes seriously affects their ecological status by negatively influencing the concentrations and species richness of benthic macroinvertebrates.

Assessing Lead Contamination in Buffalo River Sediments

Abstract: The Great Lakes Water Quality Agreement between Canada and the United States has identified the Buffalo River as an Area of Concern. The watershed has a long history of heavy industrial activity that contributed to its overall pollution. Sediment core data collected by the New York State Department of Environmental Conservation in 2005 were used to determine lead sediment contamination in a section of the Buffalo River. The ordinary kriging spatial interpolation technique was used to generate surface and subsurface sediment contamination estimates. Due to the meandering nature of the river, two kriging models were used to analyze surface contamination: a global kriging model and a regional kriging model, consisting of three separate sections. The results show that both the global and regional kriging models display similar interpolated surfaces and do not vary significantly. Within the sediment, lead contamination in the surface layer is lower than at the various subsurface depths. In 2011, habitat restoration efforts commenced to remediate environmental damage due to years of pollution inputs from various sources. Sediment dredging operations were initiated that are expected to be completed in 2015. The goal of these operations is to remove heavily contaminated sediments and rehabilitate the Buffalo River. The kriging results provide area-wide estimates of contamination. When compared to the dredging plan, the results indicate that additional removal of contaminated sediments may need to be considered where no dredging has occurred or is not currently planned.