Manly Lagoon Dredging Project - Manly Council - NSW Government

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Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah CouncilsImpact AssessmentCalculation of the Net Benefit due to the Removal of Contaminants in the SedimentIn order to evaluate the potential water quality benefits of the proposed dredging,calculations were undertaken to consider the net mass or load of material and associatedcontaminants removed from the system. This involved consideration of the mass of materialremoved from the lagoon in the sediments as well as the mass of material returned to thelagoon in the overburden water. The methodology, assumptions and results are shown inAppendix O.The calculation results (Appendix O) show that there is an overall net positive benefitassociated with the proposed dredging with respect to the mass of pollutants removed fromthe system. In particular, the removal of metals (approximately 180 to 210 kg), hydrocarbons(approximately 515 to 590 kg) and nutrients (approximately 595 to 995 kg) is notable.Other Water Quality ImpactsDuring dredging activities the proposed works have the potential to reduce the water qualityof the lagoon over the short term by increasing suspended solids, mobilising sediment-bornecontaminants and facilitating sedimentation processes in the water column. Elevatedturbidity has the potential to harm plant life by decreasing available light, impact on fish byclogging gill structures, or clog respiratory and feeding structures in invertebrates. However,these impacts are expected to be short term and sediments suspended in the water profileare expected to settle on the lagoon bed. Suspended solid levels of 171mg/L were observedin second flush water during the dredging trial. Data on the effects of suspended sedimentson estuarine fish suggest that levels of greater than 600mg/L may be required to cause fishmortality (Wilber and Clark 2001), and hence a short-term level of 171mg/L of suspendedsolids is unlikely to negatively impact on estuarine fish species. In the long term, it isexpected that there will be an improvement in water quality within the lagoon.Disturbing the sediments in Manly Lagoon may also release nutrients bound up in thesediments into the water column, increasing their biological availability. Elevated nutrientlevels have the potential to promote plant and algal growth.By disturbing the sediment in Manly Lagoon there is also the potential for exposure ofcontaminated soils or acid sulfate soils. These issues are discussed in Section 5.1.Water quality within the lagoon could also be affected by accidental spills during theproposed works. There is the potential for fuel to be spilled whilst refuelling machinery andequipment on site. There is also the potential for the polymer ULTIMER 7757 or lime to bespilled during dosing of the geobags on site. This risk will need to be managed appropriately.In the long term the proposed works will impact positively on water quality within ManlyLagoon. Removal of the sediment from the lagoon bed is expected to have long term waterquality benefits including:Reduction of levels of contaminants associated with the sediment (Appendix O);2 July 2010 Cardno (NSW/ACT) Pty Ltd 64H:\Doc\2010\Reports.2010\Rep2580v4.doc
Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah CouncilsImproving benthic habitat conditions as a result of an improved flushing regime.Water Depths and BathymetryThe aim of the dredging activities is to increase the depths of Manly Lagoon in the proposeddredging areas. According to the dredging plans (Appendix J), approximately 97% ofmaterial to be dredged will be removed from the 0 to 1m depth range, while only 3% ofmaterial to be dredged will be removed from the 1 to 1.2m depth range.The dredging activities will alter the bathymetry of Manly Lagoon, hence the proposal willimpact on lagoon hydrology in a positive manner in the long term by deepening certain areasof the lagoon and allowing greater tidal exchange and flushing to occur. The aim of thedredging works is to increase water depths by removing relatively recent sedimentation soas to re-establish the natural water depths. The design of the dredging works will avoid thecreation of isolated deep holes, which may potentially become anoxic.FloodingLarge rainfall and/or flood events during the works can lead to significant environmentalimpacts in a number of ways including:Exceeding the capacity of the sediment and erosion control measures;Washing loose sediment or stockpiled materials from the bank into downstreamwater bodies;Causing the silt curtains to fail if flow velocities within the lagoon are increasedsubstantially. This risk is greatest between February and June (Section 5.2);Destabilising the geobags from their position in Nolan Reserve; andMobilising potential contaminants from within the contractor’s compound.Overburden Water from Dewatering ActivitiesOverburden water from the dewatering process is proposed to be returned to the lagoon in acontrolled manner, providing testing proves it suitable. The water will be collected in bundedareas during dewatering.Results from the trial dredging (Cardno Ecology Lab, 2009c; Appendix C) indicate thatlevels of some metals, Enterococci and nutrients in the overburden water during dewateringexceed both ANZECC (2000) Guidelines values and background levels in the lagoon watersampled (Cardno Ecology Lab, 2009c). These parameters include chromium, copper, leadand zinc, ammonia as N, TP and Enterococci; however it should be noted that water qualitymonitoring by Cardno Ecology Lab (2010; Appendix H) in the lagoon in July and August2009 indicated that at that time copper and zinc exceeded ANZECC (2000) Guidelines inlagoon waters.It should also be noted that the trial dredging only removed finer surface sediments from thelagoon, which generally contain higher levels of heavy metals nutrients and microorganismsthan deeper sediments. As the Manly Lagoon Dredging Project would disturb a greatervolume of deeper sediments with a lower proportion of fine material, overburden water2 July 2010 Cardno (NSW/ACT) Pty Ltd 65H:\Doc\2010\Reports.2010\Rep2580v4.doc
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Manly Lagoon Dredging Project - Manly Council - NSW Government

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