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