Manly Lagoon Dredging Project - Manly Council - NSW Government

Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah Councils• The suspended solids level in the water to be returned is at or below the equivalentof 50mg/L, noting that In the field, turbidity will be measured and used as a surrogatefor Suspended Solids as discussed in Section 5.3.1.• Field pH testing has shown pH is also within an acceptable range of 7.0 to 8.2 asdetailed in the ASSMP (Appendix E).To reflect the comments of LPMA shown in Table 3.2, additional mitigation measures tofurther reduce the level of metals in the overburden water are being considered during thetendering process, however at this stage no practical measures to further reduce levels ofchromium, copper, lead and zinc have been identified. It should be noted however, thatoverall the loads of these metals in the lagoon is reduced by the dewatering process, assome of the metals are bound to the dredge spoil (Appendix O).To prevent contamination of downstream water bodies due to fuel spills and polymer/limespills during dosing, all refuelling of machinery on site and dosing of the geobags will becarried out in defined, bunded areas. In the event of a significant spill on site in a nonbundedarea, a spill kit is to be deployed and the Superintendent and DECCWrepresentatives are to be notified immediately. If an accidental spill of ULTIMER7757 occurs,the clean up method employed must be in accordance with Section 6 (Accidental ReleaseMeasures) in the Product Safety Data Sheet (PSDS) in Appendix I. The PSDS prescribesthat spills must be soaked up using inert absorbent materials, not water, and alkalineabsorbent materials must not be used.All materials including fuel, oil, polymer and lime are to be securely stored within a bundedarea which prevents the possibility of release following vandalism.Geobags and other structures must be securely anchored at all times to withstand floodflows. In addition, the contractor will monitor weather forecasts, and in the event of heavyrainfall or flood warnings shall implement plans to ensure the site is secured and workceases.The 5 year ARI has been selected as the design basis for all measures associated withmitigating environmental risks due to flooding. The 5 year ARI is considered appropriate tothe short term duration of the proposed works, and is consistent with the design criteriabeing employed for the ongoing landscaping works at the adjacent Manly Golf Course.In relation to bunding and containment, the contractor will be required to ensure that:All bunds will be constructed such that the minimum crest height is at 2.3m AHD(shown by the Manly Lagoon Flood Study to correspond to the 5 year ARI (MHL,1992));All bunds must be sufficiently strong to withstand flood flows;The bunds will be adequate in terms of size and strength to hold the sediment andwater if the bags break;The area inside the bunds is of a sufficient size to hold the water and the geobags;The bunds can hold the water for long enough to wait for the results of any requiredlaboratory analysis to be returned from the laboratory;2 July 2010 Cardno (NSW/ACT) Pty Ltd 67H:\Doc\2010\Reports.2010\Rep2580v4.doc