Manly Lagoon Dredging Project - Manly Council - NSW Government
Manly Lagoon Dredging Project - Manly Council - NSW Government Manly Lagoon Dredging Project - Manly Council - NSW Government
Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah Councilssediments. Other contaminants were generally below ANZECC guideline values or belowlaboratory detection limits at this location (PBP, 2003a).These results indicate that the areas adjacent to the former landfill site at the end ofAddiscombe Road, and the area used for trial dredging area at the end of Riverview Paradecan be considered to be representative of the most contaminated areas which will bedredged.SalinityA Salinity Hazard Map was generated using the NSW Government’s Natural Resource Atlason 20 July 2009. This map indicated no areas of salinity hazard in the location of theproposed works. Whilst the system is estuarine, salinity hazard is primarily associated withurban soil salinity that is normally confined to the western areas of the Sydney region.Impact AssessmentThe proposed works will facilitate the movement of sediment within the lagoon through thedisturbance of sediment associated with the dredging works. This may increase the turbidityof the waters and/or result in the deposition of sediment in adjacent areas (for example,either over seagrass beds downstream of the dredge area or benthic habitats). As dredgedsediments will not be stockpiled on the foreshore but rather will be confined to the geotextilebags, there is limited potential for the dredged sediment to be transported back into thewaterway via wind action and surface flows.During the dredging phase of the proposed works, potential acid sulfate soils may beencountered on the lagoon bed. If these sediments are removed from the lagoon andexposed to air without treatment they will become actual acid sulfate soils.Disturbance of the banks of Manly Lagoon by dredgers and machinery may result in erosionof those banks and increased turbidity within the waters of the lagoon. It is noted that theconsequences of any such bank erosion can be expected to be greatest when dredgingadjacent to the Former Landfill (EPA Contaminated Site) at the end of Addiscombe Roadwhere dredging works could potentially disturb contaminated land sediments at the site if thebank sediments become unstable. It is noted that the contaminated site does not encompassany sediments on the bed of the lagoon, and is contained to the land-based site as indicatedin Figure 5.2.Any change to the hydraulic gradient in the Addiscombe Road area as a result of dredginghas the potential to increase the rate of leaching from contaminated soils into the lagoon. Itis noted however, that no change in hydraulic gradient as a result of dredging is expected.Environmental impacts of potentially beneficially reusing the dredge spoil are not assessedin this REF.Mitigation MeasuresPrior to the commencement of works a Construction Environmental Management Plan(CEMP) would need to be prepared by the contractor. This must detail how the mitigation2 July 2010 Cardno (NSW/ACT) Pty Ltd 48H:\Doc\2010\Reports.2010\Rep2580v4.doc
Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah Councilsmeasures in this document and the requirements of the technical specification (Appendix K)will be met in full.Erosion and Sediment ControlAn Erosion and Sediment Control Plan would be prepared and implemented, in accordancewith Managing Urban Stormwater: Soils and Construction (4 th Edition) (DoH, 2004) for theworks. Appropriate use of erosion control devices and structures, such as bunding, sedimentfencing, diversion drains upstream of stockpiles and sediment traps will minimise anyimpacts of the proposed works. These mitigation measures will minimise erosion ofdisturbed terrestrial areas along the foreshore, prior to stabilisation via revegetation works.Mud and dirt will be removed from the wheels and bodies of heavy vehicles leaving the sitebefore they enter public roads or other sealed pavements using an appropriate device.In order to maintain bank stability and to prevent any slumping or undercutting, three controlmeasures will be in place during the dredging activities. These are:1. Dredging at all times to give a maximum slope on the lagoon bed of 2:1 (horizontal:vertical);2. Avoidance of dredging within 1m of the bank at all times;3. It is considered that adherence to the two measures above will prevent bank erosion,however as an additional control measure, there will be a site superintendent presenton site at all times when dredging is taking place. The site superintendent will havethe authority at any time to require additional safety margins to be applied.In-Lagoon Sediment ControlAs agreed in discussion with I&I NSW, primary silt curtains will be installed approximately25m upstream and downstream of the area where dredging works are taking place to ensurethat any disturbed sediment is contained within the works area. The silt curtains will beacross the full profile and full depth (at high tide) of the lagoon. In accordance with criteriaset down by the DECCW, the silt curtains will remain in place until dredging in the lagoonhas been completed and the total suspended solids (TSS) in the works area is shown to beless than 50mg/L. In addition to the primary silt curtains, a second downstream silt curtainwill be installed approximately 15m further downstream from the primary curtain to provideadditional protection in the unlikely event that the upstream or primary downstream siltcurtains fail. All silt curtains will be inspected on a daily basis.Acid Sulfate Soils ManagementIt is likely that sediment with a low to very high acid sulfate generation potential will beremoved from the lagoon during the proposed works. To manage this issue an ASSMP,which has been prepared in accordance with the Acid Sulfate Soils Manual (ASSMAC,1998), is included in Appendix E. This ASSMP should be followed by the contractor andincorporated into the CEMP. The Plan incorporates measures to test representative samplesof excavated sediments to determine appropriate liming rates. Measures are also2 July 2010 Cardno (NSW/ACT) Pty Ltd 49H:\Doc\2010\Reports.2010\Rep2580v4.doc
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<strong>Manly</strong> <strong>Lagoon</strong> <strong>Dredging</strong> <strong>Project</strong> – Review of Environmental FactorsPrepared for <strong>Manly</strong> and Warringah <strong>Council</strong>ssediments. Other contaminants were generally below ANZECC guideline values or belowlaboratory detection limits at this location (PBP, 2003a).These results indicate that the areas adjacent to the former landfill site at the end ofAddiscombe Road, and the area used for trial dredging area at the end of Riverview Paradecan be considered to be representative of the most contaminated areas which will bedredged.SalinityA Salinity Hazard Map was generated using the <strong>NSW</strong> <strong>Government</strong>’s Natural Resource Atlason 20 July 2009. This map indicated no areas of salinity hazard in the location of theproposed works. Whilst the system is estuarine, salinity hazard is primarily associated withurban soil salinity that is normally confined to the western areas of the Sydney region.Impact AssessmentThe proposed works will facilitate the movement of sediment within the lagoon through thedisturbance of sediment associated with the dredging works. This may increase the turbidityof the waters and/or result in the deposition of sediment in adjacent areas (for example,either over seagrass beds downstream of the dredge area or benthic habitats). As dredgedsediments will not be stockpiled on the foreshore but rather will be confined to the geotextilebags, there is limited potential for the dredged sediment to be transported back into thewaterway via wind action and surface flows.During the dredging phase of the proposed works, potential acid sulfate soils may beencountered on the lagoon bed. If these sediments are removed from the lagoon andexposed to air without treatment they will become actual acid sulfate soils.Disturbance of the banks of <strong>Manly</strong> <strong>Lagoon</strong> by dredgers and machinery may result in erosionof those banks and increased turbidity within the waters of the lagoon. It is noted that theconsequences of any such bank erosion can be expected to be greatest when dredgingadjacent to the Former Landfill (EPA Contaminated Site) at the end of Addiscombe Roadwhere dredging works could potentially disturb contaminated land sediments at the site if thebank sediments become unstable. It is noted that the contaminated site does not encompassany sediments on the bed of the lagoon, and is contained to the land-based site as indicatedin Figure 5.2.Any change to the hydraulic gradient in the Addiscombe Road area as a result of dredginghas the potential to increase the rate of leaching from contaminated soils into the lagoon. Itis noted however, that no change in hydraulic gradient as a result of dredging is expected.Environmental impacts of potentially beneficially reusing the dredge spoil are not assessedin this REF.Mitigation MeasuresPrior to the commencement of works a Construction Environmental Management Plan(CEMP) would need to be prepared by the contractor. This must detail how the mitigation2 July 2010 Cardno (<strong>NSW</strong>/ACT) Pty Ltd 48H:\Doc\2010\Reports.2010\Rep2580v4.doc
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