Manly Lagoon Dredging Project - Manly Council - NSW Government

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Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah Councilsreinforced concrete box culvert 3.3m wide by 1.8m high by 50m length, terminating justbeyond the seaward end of the Queenscliff Rock Pool. The low flow pipes have remainedunblocked since completion of the extension works and now allow lagoon and ocean waterexchange during each tidal cycle without interruption.2.3 Objectives of the Proposed WorksThe proposed dredging of Manly Lagoon has the following objectives which will provide bothsocio-economic and environmental benefits: Improve water quality in the Lagoon through increased tidal circulation, mixing andexchange; Improve amenity value of Manly Lagoon; Maintain an open water channel within Manly Lagoon; Progress rehabilitation of Manly Lagoon in accordance with ongoing management ofthe Manly Lagoon Estuary; Achieve a net reduction in the mass of pollutants within the lagoon.2.4 Description of the Existing EnvironmentManly Lagoon defines the boundary between the Manly LGA in the south and the WarringahLGA in the north. The majority of the lagoon foreshore is public and private open space.Open space on the foreshore within the Manly LGA, as shown on Figure 1.2, includes: Lagoon Park; Keirle Park (Council owned and Crown land); and Manly Golf Course (Private open space).Open space on the foreshore within the Warringah LGA, as shown on Figure 1.2, includes: Aitken Reserve; Hinkler Park (Crown land under management of Council); Nolan Reserve (Mostly Council owned); Passmore Reserve (Mostly Council owned); and Warringah Golf Course (Mostly Council owned, privately managed public openspace).District Park comprises Warringah Golf Course, and Nolan and Passmore Reservesadjacent to the lagoon. It also includes Miller and David Thomas Reserves which are locatedoutside the study area, west of Condamine Street, as shown on Figure 1.2. Manly Creekand Brookvale Creek flow through District Park.Land on the bed of Manly Lagoon is Crown land, managed under the Crown Lands Act1989, and hence is under the control of LPMA (formerly DoL).Historically, a great deal of land-filling and reclamation of wetlands around the Lagoon hastaken place. Manly Golf Course, Lagoon Park, Hinkler Park, Warringah Golf Course, NolanReserve and Keirle Park are all areas that were historically reclaimed and filled, mostly with2 July 2010 Cardno (NSW/ACT) Pty Ltd 8H:\Doc\2010\Reports.2010\Rep2580v4.doc
Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah Councilssediment from the Lagoon (PBP, 2003a). It is hoped that the dredge spoil from the proposedworks will be beneficially reused on open space and reserves after dosing and dewatering,Suitability for reuse will be determined by physical/chemical testing. If material is found to beunsuitable, it will be disposed of appropriately to landfill.Historically, the eastern section of Lagoon Park was used as a rubbish tip in the 1920s. Thecurrent T.S. Condamine site located at the end of Addiscombe Road, shown in Figure 1.2,was also used as a rubbish tip in the 1950s and 1960s (PBP, 2003a). Due to their proximityto the waterway, leachate from these former tip sites contributes to poor water quality inManly Lagoon (Manly Council, 2009). The open space located at the end of AddiscombeRoad is owned by the NSW Department of Housing.The three major freshwater creeks that drain the catchment area and flow into ManlyLagoon, as indicated on Figure 1.2, include: Burnt Bridge Creek; Manly Creek; and Brookvale Creek.Land use within the catchment is classified as 58% residential, 35% open space and 7%industrial (DNR, 2009). Most of the residential areas consist of medium density detacheddwellings. As discussed in Section 2.1, primary and secondary contact recreational usesincluding swimming, boating and fishing activities are currently not recommended due topolluted water and sediment (Manly Council, 2009). Specific land use zonings of thesurrounding area are discussed in Section 4.10.2.5 Proposal DescriptionThe proposed works involve dredging a portion of Manly Lagoon; from the footbridge atNolan Reserve to the end of Riverview Parade. The location of the dredging area is shown inFigure 2.1. Overall the proposed works will involve the removal of up to 6,400m 3 ofaccumulated sediment which will deepen the Lagoon to levels recorded in 1992 (based onsurvey data collected for the Manly Lagoon Flood Study (MHL, 1992). Thus the dredging willremove sediment that has accumulated over approximately a 17 year period (1992-2009).It should be noted that the 1992 levels have been selected as a target dredge depth sincethey represent the oldest accurate survey levels available. In returning to these levels, weexpect that only sediment accumulated through natural processes since 1992 will beremoved from the lagoon. Dredging to deeper depths than this is undesirable due to theadded risk of dredging older and previously undisturbed sediments. In particular, there is arisk that if earlier or deeper sediments are disturbed, historic contaminants may be mobilisedor significant heritage items may be disturbed.2 July 2010 Cardno (NSW/ACT) Pty Ltd 9H:\Doc\2010\Reports.2010\Rep2580v4.doc
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Manly Lagoon Dredging Project - Manly Council - NSW Government

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