Manly Lagoon Dredging Project - Manly Council - NSW Government

Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah CouncilsFigure 2.2: Examples of Geotextile Tubes Dewatering Sediments (Left / Right Panel Sources: Geo-Synthetics, 2009 / US Fabrics, 2009)The first flush of overburden water from the geobag is generally higher in suspended solids(TSS) than second flush runoff water. This is because as dewatering is initiating, the largerparticles of slurry material lodge within the geotextile pores, reducing the size of theopenings. This process, which is how the products are designed to function, slows the rateof dewatering, but also reduces the amount of TSS in the overburden water. Generallygeotextile products for dewatering have pores of approximately 0.5 microns (TenCate,2007).A polymer dosing system would be required on site to add the polymer in line to the dredgedsediment slurry, prior to pumping the material into the geobags. A small diesel generatorwould be required to operate this dosing system. The dredging trials described in Section3.3 indicate that the most suitable polymer for use at the Manly lagoon is ULTIMER 7757.2.6 Calibration PeriodFull scale dredging will be preceded by a controlled ‘calibration period’, during whichdredging will be undertaken for short periods in which all impacts are carefully observed.Before full dredging can proceed, the contractor must demonstrate to the satisfaction ofManly Council that the process is operating as designed, that all mitigation measures areworking as intended, and that the resulting water and treated sediment quality is asexpected. As a minimum, the contractor will be required to demonstrate:All dredging, pumping, dosing and containment infrastructure is functioning asintended, and is free from leaks;The treated dredge spoil is free from residual acid sulfate risk (via independentchemical analysis);The chemical composition of the extracted sediment and water from the lagoon (inorder to support calculations of the efficiency of the process);The composition of the treated dredge spoil to facilitate planning for re-use ordisposal of the material (via independent chemical analysis);2 July 2010 Cardno (NSW/ACT) Pty Ltd 14H:\Doc\2010\Reports.2010\Rep2580v4.doc