Manly Lagoon Dredging Project - Manly Council - NSW Government

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