Sediments supplied to an estuary undergo repeated cycles of resuspension and deposition prior to permanent accumulation or transport into the sea. A review of published literature and field observations of estuarine suspended sediment in the Chesapeake Bay region, indicate that sediment is resuspended in response to fluctuating energy over a wide range of time scales. The temporal variations of resuspension, therefore, inherit a hierarchy of scales following the scales of motion.

Effects of Fine Sediment Resuspension in Estuaries

Hopper dredging operations release suspended sediment into the environment by agitation of the bed and by discharge of overflow slurries. Monitoring of turbidity and suspended sediment concentrations in central Chesapeake Bay revealed two plumes: (1) an upper plume produced by overflow discharge and (2) a near-bottom plume produced by draghead agitation and rapid settling from the upper plume. The upper plume dispersed over 5.7 km2 extending 5,200 meters form the discharge point. Redeposited sediment accumulated on channel flanks covering an area of 6.4 km2 and reached a thickness of 19 cm. Altogether dredging redistributed into the environment an estimated 100,000 tons of sediment or 12 percent of the total material removed.

Effects of hopper dredging and sediment dispersion, chesapeake bay

Two series of laboratory experiments mimicking dredging of the uppermost phosphorus (P) rich sediment layers of the shallow eutrophic L. Muggelsee were carried out to study the extent of P release from deeper sediment layers, and changes in P mobility by means of fractional P composition of the in 10 cm steps ‘dredged’ sediment cores. In the first run over 38 days, the aerated controls reached 55%, and the ‘dredged’ cores from 18.1% (−10 cm) down-core to 0.4% (−50 cm) of the non-aerated control (50.82 mg P m−2 d−1 = 100%). In the second run over one year, the fractional P composition in the revealed sediment layers changed slightly. The water-soluble P (H2O-P) increased for the respective ‘dredged’ horizon by between 1.5 and 5.6% TP. The redox-sensitive P (BD-P) increased in each horizon from the intact core to the situation following ‘dredging’, as well as with depth in each horizon from 4.9% TP (−10 cm) to 11.4% TP (−40 cm). The organic bound P portion (NaOH-NRP) decreased least (1.7% TP) in the uppermost layer and most (15.6% TP) in the deepest horizon exposed to water after ‘dredging’. Based on the changes in P pools following dredging at the future sediment–water interface, it is predictable that dredging without reduction of the external loading may give only temporary improvement followed by a slow return to the present situation.

Assessment of the long-term effectiveness of sediment dredging to reduce benthic phosphorus release in shallow Lake Müggelsee (Germany)

Abstract : Seasonal restrictions on dredging and/or disposal operations are based upon concerns about potential dredging- or disposal-induced negative impacts to biological resources. In many cases, however, information on the degree to which either naturally occurring or dredging-induced environmental alterations directly or indirectly affect organisms is poorly quantified, in which case restrictions are based upon a reason to believe notion. This report addresses the general acceptability of seasonal restrictions through a compilation of available information on physical-chemical environmental alterations associated with dredging and disposal operations, and critical information regarding the effects of these alterations on principal biological resources. Based on this information, a method for evaluating existing or proposed seasonal restrictions on dredging and/or disposal operations is presented. This framework reflects the present understanding of effects of dredging- or disposal-induced, as well as naturally occurring, environmental alterations upon biological resources. In many cases, the magnitude of dredging- or disposal-induced alterations falls well within the range of naturally occurring phenomena and imposes little or no additional stress upon resource populations. In some cases, however, the magnitude of alterations may exceed that which occurs naturally, whereby concerns about dredging- or disposal- induced alterations are justified and should be considered when planning a project.

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A framework for assessing the need for seasonal restrictions on dredging and disposal operations

This paper synthesizes scientific information on the effects of dredging activities on marine habitats. Direct and long-term effects, dredge methods, regulatory framework, contaminated sediment issues, and a separate bibliography of contaminated sediment-related reports are also presented. Direct behavioral effects include entrainment, increased turbidity, fish injury due to suspended sediment exposure, decreased dissolved oxygen levels, and the effects of noise. A turbidity threshold of 200 mg/L could reduce dredge-induced salmonid prey-predator reaction changes. High sediment load related fish injury deserves further analysis. Gill injury thresholds specific to marine environments have not been identified. Suspended sediment size, shape, and exposure duration are likely important risk assessment factors for salmonids and other fishes. The most relevant issue is likely the fish ability to avoid plumes and dredge areas. Benthic infauna, epibenthic and demersal organisms, such as borrowing shrimp, crabs, and fish, are subject to entrainment risks. A clearer understanding of dredging effects to biota requires further synthesis of physiology, life-history strategies, water column use, and timing. In Washington State, maintenance dredging conversion of shallower subtidal to deeper subtidal habitats is much more frequent than new construction dredging conversion of intertidal to subtidal habitats, which is rarely allowed. Loss of intertidal habitats represents potential reductions in coastal habitat carrying capacity. The fish effects of channel deepening alteration to estuarine-freshwater mixing are most evident during early life history stages. Productivity recovery rates range from three months to many years, depending on the species. Lack of long-term and dredge pre- and post-project monitoring makes it difficult to conclusively identify effects.

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Dredging activities: marine issues

Sediment-Water Interaction during Dredging Operations

Studies were conducted on San Francisco Bar in June 1971 and February 1972 to monitor dredge material disposal. The purpose of the study was to determine the effects of dredging and material disposal on the physical and biological environment and to develop dredging procedures and disposal sites to mitigate possible adverse effects.. This paper presents the results of the material dispersion portion of the study including bottom deposition, current measurements and aerial photography. The biological studies and water quality monitoring are not presented.

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Richard M. Ecker

Papers

Sediment-Water Interaction during Dredging Operations

San francisco bar dredge material disposal