Showing papers on "Dredging published in 2018"

Management strategies to minimize the dredging impacts of coastal development on fish and fisheries

Abstract: Accelerating coastal development and shipping activities dictate that dredging operations will intensify, increasing potential impacts to fishes. Coastal fishes have high economic, ecological, and conservation significance and there is a need for evidence-based, quantitative guidelines on how to mitigate the impacts of dredging activities. We assess the potential risk from dredging to coastal fish and fisheries on a global scale. We then develop quantitative guidelines for two management strategies: threshold reference values and seasonal restrictions. Globally, threatened species and nearshore fisheries occur within close proximity to ports. We find that maintaining suspended sediment concentrations below 44 mg/L (15–121 bootstrapped CI) and for less than 24 hours would protect 95% of fishes from dredging-induced mortality. Implementation of seasonal restrictions during peak periods of reproduction and recruitment could further protect species from dredging impacts. This study details the first evidence-based defensible approach to minimize impacts to coastal fishes from dredging activities.

Assessing the Hydrodynamic Response of the Mar Menor Lagoon to Dredging Inlets Interventions through Numerical Modelling

Abstract: The Mar Menor lagoon has been subjected to high anthropogenic pressures. Among them, in the early 1970s, dredging and enlargement of one of the inlets to make a navigable channel has had strong consequences on the hydrology, ecology, and fisheries of the lagoon. In recent years, changes in agricultural practices have induced an eutrophication process, leading to loss of water quality. As a solution, some management proposals have included dredging of the inlets in order to increase the water renewal. However, these proposals did not take into account the negative effects of previous experiences nor the consequences on environmental conditions of the lagoon and therefore on biological processes. The purpose of this work is to assess the effect that proposed mitigation measures, could have on the hydrodynamic conditions and discuss its possible ecological consequences. A three-dimensional (3D) numerical model has been used to simulate the lagoon under different dredging scenarios, covering different dredging depths and extensions. The simulated current fields and fluxes through the inlets, as well as water renewal times have been compared for the different scenarios. It is found that some of the considered scenarios take the system beyond the threshold of sustainability, where the modified current dynamics could affect sediment transport, beach dynamics and fishing capacities in a significant way. Water exchange with the Mediterranean is also strongly affected, with consequences for species connectivity, and a homogenization of the water renewal times that could lead to loss of ecosystem heterogeneity and structural complexity. The study demonstrates the utility of numerical models as effective tools for the management of coastal areas.

Monitoring and modelling concept for ecological optimized harbour dredging and fine sediment disposal in large rivers

Abstract: In this paper, a monitoring and modelling concept for ecological optimized harbour dredging and fine sediment disposal in large rivers is presented. According to the concept, first a preliminary assessment should be performed previous to the dredging and dumping procedure to derive knowledge about the current status in hydrodynamics, morphology and instream habitat quality. During the performance of the maintenance work, a high-resolution monitoring program has to be organized to measure flow velocities, the suspended sediment concentrations and the extent of the occurring plume. These data can then be compared with natural suspended sediment conditions and serve as input data for numerical sediment transport modelling. Furthermore, bathymetric surveys and biotic sampling enable the detection of possible effects of dredging and disposal in the post-dumping stage. Based on sediment transport modelling approaches, short- to mid-term developments of the sediment plume can be predicted with an additional and final habitat evaluation at the end of the project. This concept was applied and optimized during the maintenance work at the case study winter harbour Linz at the Danube River. The findings of the presented study highlight the necessity of integrated monitoring and modelling approaches for harbour dredging especially in large river systems.

Coastal erosion in Shandong of China: status and protection challenges

Abstract: Shandong has more than 70% of natural coasts are under erosion. Coastal erosion started from the 1970’s and became a very serious problem at 1990’s. The dramatic decrease of sediment supplies from rivers caused rapid erosion at the delta and estuary areas, especially in the abandoned Yellow River Delta. Most sandy coasts along the Peninsula were eroded due to lack of sand supply and interruption of alongshore sediment drift, sand dredging from the beach or the offshore area caused serious erosion during short time. Sea-level rise causes slow but constant shoreline retreats and became a more serious threat. Different types of hard solutions for coastal protection against erosion were used in Shandong. Seawalls are most widely used, especially at the Yellow River Delta and city center waterfront. Groynes, jetties and breakwater are used on the north and east sandy coast of the Peninsula. Hard approaches are effective to protect the coast erosion but not change the erosion causes and led secondary impact on the coast. Soft engineering solution or the combined solutions are taken into acts. Beach nourishment is mostly considered as the better soft solution, especially to those tourists attracting sandy beaches along the Shandong coast. Long term monitoring and continuous lessons learning from the coastal erosion management will be adaptive for better coast solution in the future.

Effects of Dredging Season on Sediment Properties and Nutrient Fluxes across the Sediment–Water Interface in Meiliang Bay of Lake Taihu, China

Abstract: The influence of dredging season on sediment properties and nutrient fluxes across the sediment–water interface remains unknown. This study collected sediment cores from two sites with different pollution levels in Meiliang Bay, Taihu Lake (China). The samples were used in simulation experiments designed to elucidated the effects of dredging on internal loading in different seasons. The results showed that dredging the upper 30 cm of sediment could effectively reduce the contents of organic matter, total nitrogen, and total phosphorus in the sediments. Total biological activity in the dredged sediment was weaker (p < 0.05) than in the undredged sediment in all seasons for both the Inner Bay and Outer Bay, but the effect of 30-cm dredging on sediment oxygen demand was negligible. Dredging had a significant controlling effect on phosphorus release in both the Inner Bay and Outer Bay, and soluble reactive phosphorus (SRP) fluxes from the dredged cores were generally lower (p < 0.05) than from the undredged cores. In contrast, NH4+-N fluxes from the dredged cores were significantly higher (p < 0.05) than from the undredged cores in all seasons for both sites, this indicates short-term risk of NH4+-N release after dredging, and this risk is greatest in seasons with higher temperatures, especially for the Inner Bay. Dredging had a limited effect on NO2−-N and NO3−-N fluxes at both sites. These results suggest that dredging could be a useful approach for decreasing internal loading in Taihu Lake, and that the seasons with low temperature (non-growing season) are suitable for performing dredging projects.

Managing seagrass resilience under cumulative dredging affecting light: Predicting risk using dynamic Bayesian networks

Abstract: Coastal development is contributing to ongoing declines of ecosystems globally. Consequently, understanding the risks posed to these systems, and how they respond to successive disturbances, is paramount for their improved management. We study the cumulative impacts of maintenance dredging on seagrass ecosystems as a canonical example. Maintenance dredging causes disturbances lasting weeks to months, often repeated at yearly intervals. We present a risk-based modelling framework for time varying complex systems centred around a dynamic Bayesian network (DBN). Our approach estimates the impact of a hazard on a system's response in terms of resistance, recovery and persistence, commonly used to characterise the resilience of a system. We consider whole-of-system interactions including light reduction due to dredging (the hazard), the duration, frequency and start time of dredging, and ecosystem characteristics such as the life-history traits expressed by genera and local environmental conditions. The impact on resilience of dredging disturbances is evaluated using a validated seagrass ecosystem DBN for meadows of the genera Amphibolis (Jurien Bay, WA, Australia), Halophila (Hay Point, Qld, Australia) and Zostera (Gladstone, Qld, Australia). Although impacts varied by combinations of dredging parameters and the seagrass meadows being studied, in general, 3 months of duration or more, or repeat dredging every 3 or more years, were key thresholds beyond which resilience can be compromised. Additionally, managing light reduction to less than 50% can significantly decrease one or more of loss, recovery time and risk of local extinction, especially in the presence of cumulative stressors. Synthesis and applications. Our risk-based approach enables managers to develop thresholds by predicting the impact of different configurations of anthropogenic disturbances being managed. Many real-world maintenance dredging requirements fall within these parameters, and our results show that such dredging can be successfully managed to maintain healthy seagrass meadows in the absence of other disturbances. We evaluated opportunities for risk mitigation using time windows; periods during which the impact of dredging stress did not impair resilience.

Effects of harbour excavating and dredged sediment disposal on the benthic invertebrate fauna of River Danube (Austria)

Abstract: Dredging Austrian Danube harbours becomes necessary to keep the navigation channel free from bed sediment and to insure safe navigation, especially after major floods. In most cases, the excavated substrate is deposited in the free-flowing river section in the vicinity of the mouth of a harbour. The effects of harbour dredging on the biota need to be studied to minimize possible outcomes that may endanger the ecological status of the river system in order to implement the aims of the European Water Framework Directive. This study presents the first quantitative information on the colonisation of benthic invertebrates with respect to Danube harbours. One sampling at Linz Harbour was conducted before dredging and four samplings were carried out after dredging. Four transects were documented in the affected river section. The study used an air-lift sampler and a grab sampler to investigate if and to what extent the benthic invertebrate community is disturbed. It found that the sediment dredging heavily affected the benthic invertebrates (decline of 82% of biomass), while the dumping of the material had no effects on the benthic Danube biota. As a rough estimate (conferring to a simple trend line analysis) the harbour bottom fauna would recover after 235 days.

Environmental dredging residual generation and management.

Abstract: The presence and magnitude of sediment contamination remaining in a completed dredge area can often dictate the success of an environmental dredging project. The need to better understand and manage this remaining contamination, referred to as "postdredging residuals," has increasingly been recognized by practitioners and investigators. Based on recent dredging projects with robust characterization programs, it is now understood that the residual contamination layer in the postdredging sediment comprises a mixture of contaminated sediments that originate from throughout the dredge cut. This mixture of contaminated sediments initially exhibits fluid mud properties that can contribute to sediment transport and contamination risk outside of the dredge area. This article reviews robust dredging residual evaluations recently performed in the United States and Canada, including the Hudson River, Lower Fox River, Ashtabula River, and Esquimalt Harbour, along with other projects. These data better inform the understanding of residuals generation, leading to improved models of dredging residual formation to inform remedy evaluation, selection, design, and implementation. Data from these projects confirm that the magnitude of dredging residuals is largely determined by site conditions, primarily in situ sediment fluidity or liquidity as measured by dry bulk density. While the generation of dredging residuals cannot be avoided, residuals can be successfully and efficiently managed through careful development and implementation of site-specific management plans. Integr Environ Assess Manag 2018;14:335-343. © 2018 The Authors. Integrated Environmental Assessment and Management Published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Protecting natural landforms and habitats by nourishing an eroding estuarine beach

Abstract: An important issue facing coastal managers is how natural coastal habitats can be protected against wave erosion by human action. An assessment is made of attempts to protect a rare maritime forest on the bay side of a barrier island at Fire Island, New York, USA using artificial beach fill. The sediment deficit is caused by human attempts to stabilize the island by restricting inlet formation, overwash, and dune migration, and it is exacerbated by a nearby marina bulkhead. A total of 1099 m3 of sand dredged from the navigation channel was placed near the bulkhead. Loss of fill was 5.54–7.87 m3 m−1 in the first 6 months. The fill was gone 18 months after placement. Sediment moving out of the fill area caused almost 4 m of shoreline advance 40 m downdrift after 6 months (gain of 3.28 m3 m−1), followed by retreat of about 5 m at that location over the next 12 months (loss of 5.51 m3 m−1). These results reveal how small changes in volume of microtidal estuarine beaches can cause great shoreline displacement rates. The amount of fill from maintenance dredging is insufficient to replenish erosion losses. Placing additional fill sediment on the bayside to create artificial washover fans can mimic natural landforms and overcome loss of sediment inputs caused by stabilizing the ocean shore. Mechanical placement may be preferable to natural processes, which would deliver sediment across the island and through pre-existing stable habitat, eliminating some of the features that take long to form.

Benthic Community Response to Dredging Borrow Pits, Panama City Beach, Florida

Abstract: : This report gives biological and physical oceanographic data from baseline work, and studies of dredged and undredged sediments before and after dredging (9-meter contour) for beach nourishment at Panama City Beach, Florida. These studies were designed to show major short-term environmental effects of offshore dredging and included analyses of hydrology, sediments, and benthos. (Author)

Impacts of projected climate change on sediment yield and dredging costs

Abstract: Hydrological Processes. 2018;32:1223–1234. Abstract Changes in climate may significantly affect how sediment moves through watersheds into harbours and channels that are dredged for navigation or flood control. Here, we applied a hydrologic model driven by a large suite of climate change scenarios to simulate both historical and future sediment yield and transport in two large, adjacent watersheds in the Great Lakes region. Using historical dredging expenditure data from the U.S. Army Corps of Engineers, we then developed a pair of statistical models that link sediment discharge from each river to dredging costs at the watershed outlet. Although both watersheds show similar slight decreases in streamflow and sediment yield in the near‐term, by Mid‐Century, they diverge substantially. Dredging costs are projected to change in opposite directions for the two watersheds; we estimate that future dredging costs will decline in the St. Joseph River by 8–16% by Mid‐Century but increase by 1–6% in the Maumee River. Our results show that the impacts of climate change on sediment yield and dredging may vary significantly by watershed even within a region and that agricultural practices will play a large role in determining future streamflow and sediment loads. We also show that there are large variations in responses across climate projections that cause significant uncertainty in sediment and dredging projections.

Environmental Dredging to Remove Fine-Grained, Organic-Rich Sediments and Reduce Inputs of Nitrogen and Phosphorus to a Subtropical Estuary

Abstract: AbstractEnvironmental dredging of fine-grained, organic-rich sediments, locally referred to as Indian River Lagoon (IRL) muck, have been promoted as an integral part of restoring the IRL, Florida, to a healthy ecosystem. In Turkey Creek, a tributary to the IRL, ~300 metric tons of N and ~70 metric tons of P were removed with 160,000 m3 of wet muck and sand via environmental dredging during 2016 and 2017. Within the established dredged area, muck removal efficiency was ~63%; some areas were not dredged deep enough to remove all the muck. An additional 24,000 m3 of muck located outside the dredged area were not removed due to the presence of docks and seawalls. Prior to dredging, benthic fluxes of dissolved N (as ammonium) and P (as phosphate) from sediments to the overlying water, adjusted to 25°C, averaged 11 mg N/m2/h and 2.5 mg P/m2/h, respectively. Where IRL muck was removed to expose the underlying sand or mixed sand and muck, benthic fluxes of N and P were 20- to 30-fold lower after dredging. Subsequent disturbances, including Hurricane Matthew in October 2016, redistributed residual muck, leaving the dredged area 26% muck-free. Where muck was incompletely dredged or reintroduced by slumping, fluxes returned to predredging values within 6 months as equilibrium was reestablished between sediments and interstitial water. Dredging produced a 50% increase in water depth and basin volume with positive increases in salinity and the total inventory of dissolved oxygen. This deeper basin also serves as a sediment trap that will sequester future inputs of muck and mitigate future benthic fluxes of N and P by reducing the transport of muck into the IRL.

Geotextile tubes and beneficial reuse of dredged soil: applications near ports and harbours

Abstract: Geotextile tubes can offer a sustainable solution for beneficial reuse of dredged soil near ports and harbours. Dredging and subsequent handling of dredged materials is one of the main challenges t...

Characterization and modeling of sediment settling, consolidation, and suspension to optimize coastal Louisiana restoration

Abstract: Many research efforts have been undertaken over many decades in the field of Louisiana coastal restoration, but long-term experiments for sediment suspension and consolidation in diversion-receiving basins are still limited, despite significance of this topic to ongoing restoration strategies. Sediment samples were collected from two active diversions on the Mississippi River: West Bay, a semi-enclosed bay located on the Mississippi River Delta and fed by the West Bay Diversion, and from Big Mar pond, a receiving basin of the Caernarvon freshwater diversion from the lower Mississippi River, Louisiana, USA. A dual-core Gust Erosion Microcosm System was used to measure time-series (0.5, 1, 2, 3, 4, 5, and 6-months after initial settling) erodibility at seven shear stress regimes (0.01–0.60 Pa) using experimental cores prepared with two initial sediment concentrations (60 and 120 kg m−3). A 230-cm tall settling column with nine sampling ports was used to measure the consolidation rates for initial sediment concentrations ranging from fluid mud (10 kg m−3) to dredge effluent (120 kg m−3), in combination with two levels of salinity (1 and 5). The erodibility of West Bay sediment decreased with increasing time of consolidation. The critical shear stress for resuspension increased from 0.2 Pa after 2 months to 0.45 Pa after 4 months of consolidation. The consolidation rates were inversely and exponentially related to initial sediment concentrations. Consolidation tests in salinity of 1 generally settled faster than that in salinity of 5, and consolidation tests with low sediment concentration tests generally settled faster than high-concentration tests. An exponential coefficient was added in the Sanford (2008) model to better predict the consolidation profile of both rapid early settling and slow self-weight consolidation processes. Our study suggests that enclosed basin, low salinity, relatively low sediment concentration and minimized disturbance for 4 months all favor of mud deposition and retention, and these factors should be considered in the design of coastal restoration projects such as dredging effluent placements and sediment diversions.

Impact of Sand Dredging Activities on Ecosystem and Community Survival in Ibeshe Area of Lagos Lagoon, Nigeria

Abstract: This study investigates the impact of sand dredging activities on coastal ecosystem and community survival around Ibeshe area of the Lagos Lagoon, Nigeria. Surface water and bottom sediment samples were collected from locations partitioned into Dredged Area (DA) and Undredged Area (UA) in the lagoon for laboratory analyses. Questionnaires were administered to community dwellers and sand miners to further analyze perception of locals. Significant differences in water quality parameters and sediments properties from Dredged and Undredged areas, and highly significant relationship between sand dredging and unsustainable ecological practices in Ibeshe were observed. One way ANOVA test indicates that all water quality parameters measured with exception of turbidity and nitrate concentration were not significantly different (P > 0.05) between Dredged and Undredged area. With exception of moisture content other sediment parameters measured were not significantly different (P > 0.05) between Dredged and Undredged areas. The project also determined the impact of dredging activities on the fisheries and fishing business in the environment. Turbidity values revealed that fishes will be negatively affected due to dredging activities; moreover dwellers pay little or no attention to conservation of the area due to unawareness. Sand dredging operations negatively influence ecological resources and water composition of the area. This project is designed to randomly sample the opinion of sand mining operators and residents; and questionnaires were administered to obtain data. Strict regulation and enforcement will assist to control ecological destruction and solve problems facing the lagoon environment.

Analysis of sedimentation at the fishing harbor entrance: case study of El-Burullus, Egypt

Abstract: Sedimentation in artificial ports and harbors is a common issue throughout the world. Based on an assessment of the coastal fishing harbors in Egypt, the El-Burullus fishing harbor has been found to suffer severely from siltation. Sediment deposits are concentrated at the harbor entrance, which threatens navigation through this area. Although the harbor authority has constructed a perpendicular extension on the main breakwater to mitigate these siltation issues, fishing boats are still suffering from the siltation at the harbor entrance. Dredging is required every year to maintain continuity of navigation through the harbor; the cost of dredging is a critical element which impacts the economic feasibility of the harbor. Therefore, the need to investigate new solutions for the siltation issue is critical. In this study, a two-dimensional calibrated and validated model using the Coastal Modeling System package is used to understand the coastal processes within the study area. It is also used to investigate the effects of the proposed breakwater extension on the siltation at the harbor outlet. Field data such as hydrographic surveys, waves, tides, and lake outlet discharge data were collected and included in the model. The results obtained for the current breakwater conditions confirmed the claims of the fishermen that the sedimentation problem still occurs at the entrance. Thus, five possible solutions to mitigate the problem were proposed and simulated considering the following parameters: sediment deposition rate to the west of the main breakwater, the erosion rate in front of the seawall in the eastern part of the harbor, and the morphological changes within the El-Burullus lake outlet. According to the simulation results, reducing the length of the upper edge of the middle jetty by 90 m improves the stability of the harbor entrance. Alternatively, replacing the perpendicular extension by an inclined extension of 60 m in length and shortening the middle jetty by 60 m reduces the accretion and erosion rates around the harbor which promotes the long-term stability for the coastal area in addition to favorable conditions to the El-Burullus harbor entrance.

Drift-flux modeling of hyper-concentrated solid-liquid flows in dredging applications

Abstract: Transporting large amounts of sand is mostly done hydraulically in dredging and mining. This method of sand transport is efficient and is used in land reclamation projects or extraction of oil from tar sands. Large pieces of equipment, such as pumps and pipe line systems, dredging vessels etc., are used enabling the sand water mixtures to be transported hydraulically. Therefore, a good understanding of the hydrodynamical behavior of sand water mixtures is eminent in order to further improve these kind of systems. In this thesis a numerical model has been developed which describe the hydraulic behavior of sediment fluid mixtures. In the model the volume concentration of solids varies from 0.0 to 0.6. Moreover, the model is able to describe mixtures consisting of multiple sized sand particles.

Asia's hunger for sand takes toll on ecology.

Abstract: Across Asia, rampant extraction of sand for construction is eroding coastlines and scouring waterways. Scientists are beginning to assess the environmental toll; already, poorly regulated and often illegal sand removal has been linked to declines in seagrasses in Indonesia and in charismatic species such as the Ganges River dolphin and terrapins in India and Malaysia. In eastern China9s Poyang Lake, dredging boats are sucking up tens of millions of tons of sand a year, altering the hydrology of the country9s largest freshwater lake, a way station for migratory birds. Conservation groups are urging governments to crack down. But the political clout of developers means it will be an uphill—and perilous—battle.

Evaluating the impacts of dredging and saline water intrusion on rural livelihoods in the Volta Estuary

Abstract: Many communities around the world are directly dependent on rivers. In Ghana the damming of the Volta River for hydro-electric power led to the development of a sandbar at the estuary, which disrup...