Showing papers on "Dredging published in 2019"

Sediment deposition and coral smothering.

Abstract: Dredging in the marine environment to create and maintain safe, navigable shipping channels, and subsequent disposal of the material at sea in dredge material placement sites (spoil grounds) can generate large quantities of suspended sediment that can impact upon epibenthic marine communities. For sensitive taxa such as hard corals, understanding the mechanisms of mortality and the spatial scale over which these occur is critically important for impact prediction purposes, management of dredging using zonation schemes, and also public perception. We describe the sediment deposition field from suspended sediment falling back out of suspension created around a large (7.6 Mm3) 1.5-year capital dredging project on a reef, using data from 2 weekly repeat observations of >500 individually tagged corals at multiple locations from 0.2-25 km from the dredging. The observations were supported by concurrent in situ measurements of proxy suspended sediment concentrations, underwater light, and sediment deposition (using optical backscatter sensors), and before and after surveys of seabed particle size distributions (PSDs). The distance at which 90% of the effect (from maximum to minimum) had dissipated (ED10) was 20 km away from the dredging for suspended sediment concentrations (estimated via nephelometry), and underwater light (measured using PAR sensors) associated with turbid plumes, 14 km for sediment deposition (measured using optical backscatter sensors) and 4.6 km for changes seabed clay and silt content (PSD analysis). The ED10 for smothering of corals (the build-up of pools of loose sediment on the surface that could not be removed by self-cleaning) occurred much closer still at 3-3.3 km or (0.5-0.6 km for an ED50). Smothering was common on encrusting and foliose forms where sediments accumulated in hollows and massive hemispherical forms where surface undulations (bumps) allowed sediments to pool. Smothering was never observed on branching species, even under extreme levels of sedimentation. Sediment smothering resulted in tissue bleaching and partial mortality (lesion formation), but if sediments were removed (by currents) bleached areas regained pigmentation over weeks and there was regrowth/reparation of lesions over weeks and months even before the dredging was completed. Overall sedimentation tolerance was highly related to coral morphology and surface inclination and the ability to avoid smothering by having uninterrupted downhill pathways for sediment transport across the colony.

Assessment of Dredging Scenarios for a Tidal Inlet in a High-Energy Coast

Abstract: The high energetic wave climate of the North Atlantic Ocean causes important morphological changes at Figueira da Foz coastal system (W Portugal), which is comprised of sandy beaches and the Mondego estuary-inlet. The submerged sandbar at the inlet mouth is highly dynamic inducing short waves shoaling and breaking processes that can entail navigation problems towards the local harbor. Therefore, coastal dredging operations are performed to guarantee safe navigation. Nevertheless, these operations have a limited temporal effectiveness and require a high annual budget to be accomplished. The goal of this research is to seek long-life dredging alternatives using modeling tools (i.e., Delft3D model suite). Delft3D model is used to simulate the morphological evolution of five dredging scenarios during a three-month winter period under three wave climate scenarios. The bed level differences at the dredged area and at the inlet mouth for each scenario are analyzed in comparison with numerical solutions obtained in a reference scenario (i.e., no-dredging). Results highlight morphological changes at the dredged inlet and surrounding areas and their effectiveness in extending the operational lifetime of inlet dredged operations on dredging configuration and wave climate conditions. These findings are the basis for selecting the most suitable dredging scenario to this coastal region under current wave climate conditions.

Modelling dynamics of the estuarine turbidity maximum and local net deposition

Abstract: Net deposition in estuaries is often linked to the estuarine turbidity maximum zones, in which fine, cohesive sediments accumulate due to residual transport by the estuarine circulation and tidal asymmetries. Sediments deposit in fairways or harbours, which creates high maintenance dredging costs and the need for better prediction of dredging hotspots with process-based numerical models. In this paper, a new efficient modelling approach is presented which enables the simulation of the ETM formation, its seasonal dynamics and the local sedimentation. A 3D baroclinic large-scale estuary model with a characteristic sediment fraction with simplified sediment transport properties is used with realistic boundary conditions, but without initial sediment distribution. This approach is referred to as supply-limited, regarding the ETM formation by residual transport. A dynamic equilibrium between residual sediment import from the open boundaries, accumulation and local sedimentation establishes in the model. This is achieved by combining the large-scale supply-limited model with an extended bed exchange formulation (2-Layer-Concept). A model of the Weser estuary is used as case study to reproduce and analyse the ETM formation and the resulting sedimentation simulated with this approach. The results are compared with the equivalent sediment concentration of turbidity measurements and dredging volumes.

Evidence of Environmental Changes Caused by Chinese Island-Building.

Abstract: This paper quantifies environmental effects of island-building operations in the South China Sea, which result from dredging and can negatively impact marine flora, fauna, and ecosystems. The extent of the damage caused by island-creation is believed to be large, as the South China Sea reefs support the largest concentration of marine biodiversity on Earth. Through use of satellite imagery, we investigate the island-construction on Mischief Reef in the South China Sea, showing backscatter increases of up to 350% in waters surrounding the reef, with plumes of excess sediment exceeding 250 km2 at times during island-construction, and the cumulative area impacted by dredging exceeding 1,200 km2. Comparison of satellite-derived chlorophyll-a, backscatter, absorption and remote sensing reflectance at 412 nm suggest that dredging activities led to a decrease in biological health of the region resulting from the smothering of natural benthic habitats and reef complexes with sediment. We anticipate this ex post facto quantification of the connectivity between island-construction, large particulate plumes and a decrease in absorption related to marine life in the water column to establish a starting point for further study into ecosystem impact. The potential associations between these damages and a long-term reduction in ocean life and resources could serve inter-governmental bodies with a baseline metric for evaluating the level of damage caused. This may result in both forward-looking deterrent policies that limit island-building as well as backward-looking compensation.

Pollution and toxicity of sediment in potential dredging sites of the Marmara Sea, Turkey

Abstract: This study aims to assess the impact of the sediment in the potential dredging areas of the Marmara Sea. To that aim, sediments were analyzed for polycyclic aromatic hydrocarbons (PAHs), polychlori...

Numerical Study of Remote Sensed Dredging Impacts on the Suspended Sediment Transport in China’s Largest Freshwater Lake

Abstract: As the largest freshwater lake in China, Poyang Lake plays an important role in the ecosystem of the Yangtze River watershed. The high suspended sediment concentration (SSC) has been an increasingly significant problem under the influence of extensive sand dredging. In this study, a hydrodynamic model integrated with the two-dimensional sediment transport model was built for Poyang Lake, considering sand dredging activities detected from satellite images. The sediment transport model was set with point sources of sand dredging, and fully calibrated and validated by observed hydrological data and remote sensing results. Simulations under different dredging intensities were implemented to investigate the impacts of the spatiotemporal variation of the SSC. The results indicated that areas significantly affected by sand dredging were located in the north of the lake and along the waterway, with a total affected area of about 730 km2, and this was one of the main factors causing high turbidity in the northern part of the lake. The SSC in the northern area increased, showing a spatial pattern in which the SSC varied from high to low from south to north along the main channel, which indicated close agreement with the results captured by remote sensing. In summary, this study quantified the influence of human induced activities on sediment transport for the lake aquatic ecosystem, which could help us to better understand the water quality and manage water resources.

Characterization of dredged sediments: a first guide to define potentially valuable compounds – the case of Malmfjärden Bay, Sweden

Abstract: . Millions of tons of bottom sediments are dredged annually all over the world. Ports and bays need to extract the sediments to guarantee the navigation levels or remediate the aquatic ecosystem. The removed material is commonly disposed of in open oceans or landfills. These disposal methods are not in line with circular-economy goals and additionally are unsuitable due to their legal and environmental compatibility. Recovery of valuables represents a way to eliminate dumping and contributes towards the sustainable extraction of secondary raw materials. Nevertheless, the recovery varies on a case-by-case basis and depends on the sediment components. Therefore, the first step is to analyse and identify the sediment composition and properties. Malmfjarden is a shallow semi-enclosed bay located in Kalmar, Sweden. Dredging of sediments is required to recuperate the water level. This study focuses on characterizing the sediments, pore water and surface water from the bay to uncover possible sediment recovery paths and define the baseline of contamination in the water body. The results showed that the bay had high amounts of nitrogen (170–450 µ g L −1 ), leading to eutrophication problems. The sediments mainly comprised small size particle material (silt, clay and sand proportions of 62 %–79 %, 14 %–20 %, 7 %–17 %, respectively) and had a medium–high level of nitrogen (7400–11 000 mg kg −1 ). Additionally, the sediments had little presence of organic pollutants and low–medium concentration of metals or metalloids. The characterization of the sediments displays a potential use in less sensitive lands such as in industrial and commercial areas where the sediments can be employed as construction material or as plant-growing substrate (for ornamental gardens or vegetation beside roads).

Morphological changes due to marine aggregate extraction for beach nourishment in the German Bight (SE North Sea)

Abstract: Facing the predicted rise in global sea level, sandy shorelines are under increasing pressure. In order to counteract the loss of material at eroding coastlines, beach nourishment is considered to be an environmentally friendly approach worldwide. This has resulted in a rising demand for aggregates, which are frequently extracted from the seafloor near the coast. In order to explore the long- and short-term morphological changes of such mining on the seabed, the largest extraction area in the German Bight (Westerland Dredging Area, established in 1984) was investigated in this study. Several measurement campaigns were conducted between the years 1994 and 2017 using a set of hydroacoustic techniques. The measurements revealed that up to 20-m-deep pits with diameters of more than 1 km were dredged into the seafloor. The depressions caused by this sand mining are still detectable more than 30 years later. Because of slope failures that mainly consist of fine sand, the formerly steep rims at fresh dredging pits smoothed within a few months. However, after approximately 1 year, muddy sediments dominated the deposition. Since the sedimentation rates are slow, a complete backfill of the post-dredging pits is likely to take many decades. A natural regeneration towards the former seafloor conditions is only visible at the shallow rims of the oldest dredging pits.

Targeting Remediation Dredging by Ecological Risk Assessment of Heavy Metals in Lake Sediment: A Case Study of Shitang Lake, China

Abstract: Understanding the spatial distribution and pollution characteristics of heavy metals in lake sediment is crucial for studying deposition and migration processes, assessing lake conditions, and determining the extent of remediation dredging. The present work is a case study of heavy metal pollution in Shitang Lake in Anhui province, China. Heavy metal concentrations were determined in sediment at locations across the lake to a depth of 100 cm, and pollution levels were assessed on the basis of the Geoaccumulation Index (Igeo) and Potential Ecological Risk Index (RI). Hg and Cd were the predominant heavy metals in the sediment, and the Igeo and RI indicated high pollution levels in the northern and southern zones of the lake. These findings can be used to guide the prioritization of dredging operations.

Human Health Benefits from Fish Consumption vs. Risks from Inhalation Exposures Associated with Contaminated Sediment Remediation: Dredging of the Hudson River.

Abstract: Background: Billions of dollars are spent on environmental dredging (ED) to remediate contaminated sediments, with one goal being reduced human health risks. However, ED may increase health risks i...

Valorization of dredging sediments of dam BOUHNIFIA in ceramic

Abstract: The main objective of this paper is the valorization of the dredging sediments of the BOUHNIFIA dam of the region of Mascara (western Algeria) in ceramic. The physical and the chemical analyses show that the characteristics of sediments of dam BOUHNIFIA are similar to the characteristics of clays used in the ceramic industry. The mechanical tests carried out at CERAL plant show the valorization percentage of up to 12% without any change in the manufacturing process. The study also showed that particle size and mineralogy of sediments are decisive factors influencing the quality of the ceramic tiles produced. For this reason, it is necessary to use these sediments in the field of ceramics in order to preserve the environment.

Evaluating effects of dredging-induced underwater sound on aquatic species : a literature review

Abstract: At any given moment, there may be multiple underwater sounds emanating from both natural and anthropogenic sources. Dredging activities such as the excavation, transit, and placement of material generate underwater sound. This report documents research into the biological effects of underwater sound from dredging and other anthropogenic sources to help evaluate the potential ecological risks of dredging activities. Effects data generated from exposures to anthropogenic sound sources indicate that dredging-induced sounds do not pose a significant risk of direct injury or mortality to aquatic biota. A notable exception, and much less common, is blasting activities used to remove rock and other hard substrata in navigation channels. In terms of potential non-lethal responses, low-frequency sounds produced by dredging overlap with the hearing frequency ranges of select fish and mammal species, which may pose risks for auditory temporary threshold shifts, auditory masking, and behavioral responses. To better understand the ecological risk associated with dredging sounds, a risk-based approach is needed that utilizes the available data and other site-specific information appropriate for evaluating underwater sound. The information reported herein can be used in an exposure assessment as part of a broader framework for evaluating and managing underwater sound effects on aquatic life. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. DESTROY THIS REPORT WHEN NO LONGER NEEDED. DO NOT RETURN IT TO THE ORIGINATOR. ERDC/EL TR-19-18 iii