Manly Lagoon Dredging Project - Manly Council - NSW Government

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Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah Councils Dredging plans, indicating areas and depths to be dredged with Manly Lagoon viacross-section profiles are contained in Appendix J. The dredging operation will require a shore-based site presence with a plan area ofapproximately 4,500m 2 to 5,000m 2 where the equipment, site compound, portabletoilets and removed sediment will be stored in geotextile bags prior to its reuse ordisposal. This area will be fenced, and located within Nolan Reserve. The proposedsite compound location will be near the netball courts in the upper portion of dredgingworks. The proposed location is indicated on the Site Plan in Appendix M. The dredged material will be dosed with the polymer ULTIMER 7757 (Appendix I) ata rate within the range 25ppm to 100ppm and pumped into large geotextile bags(geobags). This concentration range is considered to be an appropriate rate foreffective sediment control and is safe for mixing with lime and dredged material, asdiscussed in Section 5.3. Trials of the polymer have demonstrated that use of thispolymer effectively aids the dewatering of the sediment (Cardno Ecology Lab,2009c). The geobags will be stored in bunded areas on the lagoon banks within NolanReserve, along the northern bank of the lagoon, for a period of approximately fourweeks to allow the maximum possible dewatering to occur. The proposed sitecompound location for bunded geobag storage on Nolan Reserve is shown on theSite Plan in Appendix M. The liquid component of the dredged material will drain from the geobags into thebunded area, from where, subject to water quality testing results, it will be dischargedin a controlled manner to Manly Lagoon. All chemicals, including lime and the polymer, will be stored in a secure bunded areaon site. Consideration will be given to suitable material for storage containers(Appendix I details suitable and unsuitable construction material with regards to thestorage of ULTIMER 7757), ability to withstand potential impacts of flooding (Section5.3), vandalism and accidental spills (Section 5.10). These issues and mitigationmeasures will be addressed in the various management plans to be prepared by thecontractor. It is intended that provided the criteria for turbidity and pH defined in Section 5.3 aremet, overburden water will be returned to the lagoon (following testing). Return ofwater to the lagoon must be in a controlled manner which minimises the risk ofincreasing turbidity in the water and bank erosion. Once dewatering of the sediment is complete, chemical analysis of representativesamples of the sediment will be undertaken to determine whether the material issuitable for reuse. The potential reuse of the material is being managed andassessed by a separate process. If not suitable, the material will be disposed of tolandfill. Previous investigations and sampling studies have identified some of the sediment inManly Lagoon to have the potential for acid sulfate generation (Section 3.1). Thedredged material will be dosed with agricultural lime according to the requirements of2 July 2010 Cardno (NSW/ACT) Pty Ltd 12H:\Doc\2010\Reports.2010\Rep2580v4.doc
Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah Councilsan Acid Sulfate Soils Management Plan (ASSMP). An ASSMP has been prepared toensure that these risks are appropriately managed and is contained in Appendix E. Any disturbed vegetation along the bank will be restored at the completion ofdredging activities to its existing condition to the satisfaction of Manly and WarringahCouncils. The dredger, and any other powered equipment will be diesel driven, and is expectedto require refuelling approximately once per week. Diesel will be supplied in a selfbunded road tank, and during refuelling inflatable booms will be placed around thedredger to contain any accidental spillage. Access for plant associated with dredging activities will be via a temporary haulageaccess from either Kentwell Road or Pittwater Road through Nolan Reserve to thelagoon bank. Any damage to kerbs or grassed areas will be reinstated followingcompletion of the project.2.5.2 Proposal TimingThe proposed dredging works are expected to commence in the third quarter of 2010. Theduration of dredging works will be approximately 30 days. The geobags will need be storedon site, along the foreshore of Manly Lagoon (Appendix M), for approximately four weeks tocomplete the dewatering process. Hence overall dredging works, sediment dewatering andremoval of dewatered dredge spoil off-site are expected to take approximately eight weeks.2.5.3 Geotextile Products for DewateringHigh strength permeable geotextile products (referred to as geobags in this report) will beused to dewater the dredged sediments. Figure 2.2 shows example photographs ofgeotextile tubes (or geobags) used for dewatering. Geobags can be provided in almost anyshape and size. The general process of dewatering with geobags, will be as follows:1. Filling: The dredged sediment (in slurry form) will be pumped into the geobagcontainers after being dosed in line with a polymer and lime at a prescribed rate. Thepolymer will facilitate the dewatering process by aiding in binding the solids togetherand separating the water. The lime will mitigate any risk posed by Potential AcidSulfate Soils (PASS).2. Dewatering: Generally, over 99% of solids are captured, and relatively clearoverburden or runoff water will drain from the geobag. Overburden water will becaptured in the bunded area so testing can determine an appropriate means ofdisposal.3. Consolidation: The solids remain in the bag while the water is draining from the bag.Because sediment pore water is being expelled, a reduction in volume of the solidsgenerally occurs. This volume reduction is dependent on the nature of the sedimentand how long the material is left to dewater and consolidate in the geobags. At theend of the process the dewatered dredge spoil material can be removed and landappliedif the material is of a suitable physical and chemical composition for reuse. Ifunsuitable, the dredge spoil will be disposed of to landfill. The geobag container canbe disposed of at a landfill site or reused, for example as matting for landscapingworks.2 July 2010 Cardno (NSW/ACT) Pty Ltd 13H:\Doc\2010\Reports.2010\Rep2580v4.doc
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Manly Lagoon Dredging Project - Manly Council - NSW Government

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