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 CouncilsBank Vegetation and ConditionsDredging activities have the potential to destabilise banks and cause bank subsidence orerosion, if dredging works within the lagoon occur too close to unstable bank areas. If banksare undercut this could harm or destroy bank vegetation via destruction of root systems or bybank collapse into the waterway.Mitigation MeasuresA Flora and Fauna Management Plan should be prepared for the proposed works andincluded in the CEMP, containing mitigation measures to avoid harm to both aquatic andterrestrial flora. The Flora and Fauna Management Plan should include measures to protectexisting flora and fauna within the works area.SeagrassThe 2009 seagrass survey conducted by Cardno Ecology Lab (Appendix G) concluded thatno seagrass is present in the areas to be dredged, and hence no seagrass will be removedfrom the lagoon.During dredging operations primary silt curtains will be deployed across the full profile anddepth (at high tide) of the lagoon approximately 25m upstream and downstream of the areawhere dredging works are taking place. A secondary silt curtain will be deployedapproximately 15m further downstream from the primary curtain, as discussed in Section5.1. These silt curtains will ensure that any disturbed sediments mobilised into the watercolumn are contained within the works area. Hence migration of disturbed sediments ontodownstream seagrass beds will be minimised. Silt curtains should be inspected daily andmaintained to prevent the escape of suspended sediments, in accordance with consultationadvice from I&I NSW (Section 3.2). Silt curtains will not normally be removed until TSSlevels in the area have fallen to 50mg/L at the conclusion of the dredging works asprescribed by DECCW. Removal of the silt curtains, following a risk assessment may beconsidered in the event of heavy flooding.A post-dredging seagrass mapping and assessment survey has been proposed aftercompletion of dredging in order to identify if any changes to seagrass within the lagoon haveoccurred. (Cardno Ecology Lab, 2009a).Threatened FloraNo threatened fauna is known to be present in the vicinity of the works, however measureswill be implemented to prevent damage or injury to existing trees, plants and othervegetation proximal to the proposed works; in particular in geobag storage areas and alongmachinery and piping access routes. Prior to commencement of operations, appropriatefencing must be erected to provide a barrier between the works and existing vulnerablevegetation.2 July 2010 Cardno (NSW/ACT) Pty Ltd 78H:\Doc\2010\Reports.2010\Rep2580v4.doc
Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah CouncilsPest SpeciesIf Caulerpa taxifolia is encountered during the proposed works I&I NSW (Fisheries) wouldneed to be notified either by calling 4916 3877 and / or email by email toaquatic.pests@dpi.nsw.gov.au.Dredging machinery, silt curtains and other objects that may facilitate the spread of Caulerpataxifolia will be washed down with fresh water and inspected for Caulerpa taxifolia fragmentsbefore entering and leaving the site. Any fragments should be disposed of in a bin to preventcontamination of other waterways or other areas of Manly Lagoon. Further washing downand inspection of machinery will be required again before it is used for work within a differentestuary. All contractors will be made aware of these protocols and they will be recorded inthe CEMP.Endangered Ecological CommunitiesDamage or disturbance to bank vegetation in the vicinity of all identified EECs must beavoided during the proposed works. Care must also be taken during dredging works in theseareas to not undermine the bank, leading to bank instability and disruption to the rootsystems of these EECs. It should be noted that this mitigation measure will apply to the fulllength of the lagoon, regardless of whether or not an EEC is present.All contractors must be made aware of the presence and importance of the EECs, andwhere works are occurring in close proximity, fencing must be provided to segregate andprotect the EECs.Bank Vegetation and ConditionsIn order to maintain bank stability, dredging will be undertaken to give a maximum slope onthe lagoon bed of 2:1 (horizontal:vertical).It is anticipated that the use of a cutter suction dredge will minimise the areas on the lagoonbank which will be impacted since this method of dredging allows the material to beconveyed using a floating pipeline, rather than shore based machinery.5.4.2 FaunaExisting EnvironmentFish and Mobile MacroinvertebratesSeveral fish surveys have been undertaken in Manly Lagoon, the most recent in relation tothe recovery of fish and mobile macroinvertebrates (i.e. prawns, crabs) after a chemical spillin the lagoon in February 2001. Table 5.7 indicates the species caught in five surveys inManly Lagoon carried out by Cardno Ecology Lab (previously The Ecology Lab) betweenJanuary 1993 and February 2002.2 July 2010 Cardno (NSW/ACT) Pty Ltd 79H:\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>sPest SpeciesIf Caulerpa taxifolia is encountered during the proposed works I&I <strong>NSW</strong> (Fisheries) wouldneed to be notified either by calling 4916 3877 and / or email by email toaquatic.pests@dpi.nsw.gov.au.<strong>Dredging</strong> machinery, silt curtains and other objects that may facilitate the spread of Caulerpataxifolia will be washed down with fresh water and inspected for Caulerpa taxifolia fragmentsbefore entering and leaving the site. Any fragments should be disposed of in a bin to preventcontamination of other waterways or other areas of <strong>Manly</strong> <strong>Lagoon</strong>. Further washing downand inspection of machinery will be required again before it is used for work within a differentestuary. All contractors will be made aware of these protocols and they will be recorded inthe CEMP.Endangered Ecological CommunitiesDamage or disturbance to bank vegetation in the vicinity of all identified EECs must beavoided during the proposed works. Care must also be taken during dredging works in theseareas to not undermine the bank, leading to bank instability and disruption to the rootsystems of these EECs. It should be noted that this mitigation measure will apply to the fulllength of the lagoon, regardless of whether or not an EEC is present.All contractors must be made aware of the presence and importance of the EECs, andwhere works are occurring in close proximity, fencing must be provided to segregate andprotect the EECs.Bank Vegetation and ConditionsIn order to maintain bank stability, dredging will be undertaken to give a maximum slope onthe lagoon bed of 2:1 (horizontal:vertical).It is anticipated that the use of a cutter suction dredge will minimise the areas on the lagoonbank which will be impacted since this method of dredging allows the material to beconveyed using a floating pipeline, rather than shore based machinery.5.4.2 FaunaExisting EnvironmentFish and Mobile MacroinvertebratesSeveral fish surveys have been undertaken in <strong>Manly</strong> <strong>Lagoon</strong>, the most recent in relation tothe recovery of fish and mobile macroinvertebrates (i.e. prawns, crabs) after a chemical spillin the lagoon in February 2001. Table 5.7 indicates the species caught in five surveys in<strong>Manly</strong> <strong>Lagoon</strong> carried out by Cardno Ecology Lab (previously The Ecology Lab) betweenJanuary 1993 and February 2002.2 July 2010 Cardno (<strong>NSW</strong>/ACT) Pty Ltd 79H:\Doc\2010\Reports.2010\Rep2580v4.doc
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