AGENDA - Sunshine Coast Council

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Strategy and Planning Committee Agenda 16 February 2011 CANAL AND LAKE SYSTEMS The proposed layout includes an extension of the tidal canal off the Lamerough Canal and the construction of an additional partial tidal lake which will in effect create an island within the centre of the site. The extension of the tidal canal is proposed to include a marina basin with a base level of -5m Australian Height Datum. The proposed lake has a surface area of 14.8 hectare and a depth of approximately 5m below the mean lake surface level under current climatic conditions (-4.5 m Australian Height Datum). The two water bodies will be linked by a navigation lock and weir located on the southern arm of the loop. The northern arm of the loop will be blocked to the passage of watercraft with the construction of a bunded causeway to allow the crossing of a road into the District Business Centre. Lake flushing is proposed via a tidal exchange system which will be controlled by actuated penstocks at the lock and under the bunded causeway. This system will force one-way flow around the lake in a clockwise direction (refer Attachment 4). The primary purpose of the lake, and reason why the lake is proposed to be approximately 5m deep, is to source fill so that the proposed development has adequate freeboard to flooding both now and under future climate change conditions. The proposed tidal exchange system does not involve any pumping of water from Bells Creek or Pumicestone Passage and does not include any outlets to Bells Creek. WATER QUALITY Lake Water Quality A new lake is proposed with a surface area of 14.8 ha in size and a depth of approximately 5m below the mean lake surface level under current climatic conditions (-4.5 m Australian Height Datum). The primary purpose of the lake and reason why the lake is proposed to be approximately 5m deep is to source fill so that the proposed development has adequate freeboard to flooding both now and under future climate change conditions. It is important to note that Acceptable Solution S35.1 states that an acceptable solution for a canal, channel and small boat mooring facility is that it has a maximum depth of 6 m. The proposal is therefore in accordance with this Acceptable Solution. The proposal is also in accordance with the requirements of Section 11.6.3 of <strong>Council</strong>’s Development Design Planning Scheme Policy which deals with design requirements for Canals. The lake itself sits outside of any of the precincts so the conditions need to include a level of detail appropriate to enable the applicant to go straight to lodging an application for operational works for the lake, marina and lock and weir infrastructure. There are two key strategies proposed for maintaining the quality of stormwater within the proposed lake. Firstly a system is proposed to achieve turnover of the lake water with that of Lamerough Canal. A lake turn over time of 22 days is proposed which is proposed to result in an effective lake turnover time of 30 days. The 22 day lake turnover is required to achieve an effective 30 day lake turnover because the modelling shows that there will be some short circuiting around the marina basin. This means that rather than lake turnover occurring with the entire Lamerough Canal a portion of the lake water is recirculated through the system. Modelling has been performed showing that Lamerough Canal has a 4.5 day turnover with the Pumicestone Passage. Page 52
Strategy and Planning Committee Agenda 16 February 2011 The second strategy proposed is that all the stormwater from proposed new residential areas is treated to current best practice prior to discharge to the lake. This strategy treats the lake as the receiving waters. However, it is unavoidable that the urban stormwater discharges the remaining pollutants into the lake and the lake provides tertiary treatment of urban stormwater flows. It is important to note that untreated stormwater from existing urban areas will enter the lake without receiving any treatment. A lake water quality investigation has been undertaken by Cardno. This includes a detailed modelling of the lake system for a number of water quality parameters. In summary, the median water quality within the lake generally meets the water quality objectives. However, there are times when some water quality parameters will not meet the water quality objectives, mainly due to the inflow water quality from the existing Lamerough Canal. The modelling predicts the following: • median nutrient concentrations (TP, TN, NO3, NH4) are predicted to meet the water quality criteria specified in Environmental Protection (Water) Policy 2009 Pumicestone Passage environmental values and water quality objectives; • salinity concentrations at the lake surface on average are predicted to be fresher than bed concentrations due to stormwater inflows and the saline turnover system proposed to discharge beneath the surface layer. Predicted average salinity concentrations indicate that the lake will be a strongly brackish system the majority of the site. However following storm events in the local catchment the surface waters will be quite fresh and the entire lake will be quite fresh following a large storm event in the local catchment; • chlorophyll ‘a’ is recognised as a reliable indicator of algal activity. Concentrations of Chlorophyll ‘a’ are predicted to meet the water quality criteria specified in Environmental Protection (Water) Policy 2009 Pumicestone Passage environmental values and water quality objectives at all times; • average top and bottom water temperatures are predicted to be very similar indicating that there is not much thermal stratification expected in the system; • pH levels are predicted to be relatively neutral and slightly more acidic in the bottom waters than the top waters; and • dissolved oxygen concentrations, particularly in the bottom waters, are predicted to be depleted below the water quality criteria following large inflow events however anoxic conditions are not predicted to occur. Marina Water Quality The development involves an extension of the existing Lamerough Canal to include a proposed marina Basin with a floor level of -5m Australian Height Datum. The marina Basin is close to the entrance of Lamerough Canal and is well flushed as a result. As long as good water quality is maintained in the Pumicestone Passage and existing Lamerough Canal then the marina basin will have good water quality. Stormwater Quality Treatment MUSIC modelling has been completed to size the stormwater quality improvement devices however the location of these devices within the urban footprint has not been undertaken at this stage. The key component of the stormwater quality treatment system is bioretention devices with some limited pollutant removal through the use of rainwater tanks and Gross Pollutant Traps. Page 53
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AGENDA - Sunshine Coast Council

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