AGENDA - Sunshine Coast Council

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Strategy and Planning Committee Agenda 16 February 2011 A fundamental problem with the proposed stormwater quality treatment strategy proposed is that all stormwater which bypasses a rainwater tank is required to pass through a Gross Pollutant Trap before it enters a bioretention device. For this strategy to be implemented it means that stormwater needs to be treated in “at source” Gross Pollutant Trap or in “inline” Gross Pollutant Trap. Either way stormwater needs to enter the piped drainage system before being treated in a bioretention device. This approach works for sites with some grade but does not work for sites which are essentially flat and discharge underwater to a lake system. On flat sites it is not possible to treat stormwater in bioretention devices once the stormwater is in the piped system. On flat sites stormwater must be treated in “at source” bioretention devices. There are a number of errors which have been made when modelling the proposed stormwater quality treatment train in MUSIC. The saturated hydraulic conductivity of the filter media has been modelled at 90mm/hr. The saturated hydraulic conductivity of the filter media which meets the FAWB specification as required by Council is 200mm/hr. The result of using the lower hydraulic conductivity is that the device treats less stormwater but the stormwater it does treat is treated to a higher standard. Using the correct filter media saturated hydraulic conductivity the performance of the bioretention systems is improved and compliance with best practice load based reduction targets is still achieved. The second error is that the width of the overflow weir has not been increased to reflect the lumped approach taken in the modelling. The overflow weir width for a basin with a surface area of 8000m2 has been modelled with a weir width of 2m. Following the Water by Design MUSIC Modelling Guidelines the weir width for a basin with a filter media surface area of 8,000m 2 should be 800m. The third error is that Gross Pollutant Traps are included in the treatment train in locations where, due to the flat nature of the site, they cannot be incorporated. The fourth error is that the bioretention devices have been modelled with an extended detention depth of 0.3 metres. This extended detention depth is suitable for “end of line” bioretention devices, but is unsuitable for at source devices. Because “at source” devices are located within the road reserve, the level difference between the top of the filter media and the top of extended detention needs to be minimised. Previously an extended depth of 150mm has been accepted for at source bioretention devices. The fifth error is that rainwater reuse has been calculated at 700L per dwelling per day. In accordance with the Water by Design MUSIC Modelling Guidelines the daily reuse should be calculated at 205 litres per dwelling per day. A sixth error which makes the design conservative is that the storage surface area has been set as the same as the filter media surface area. This is the case where there are vertical walls associated with the bioretention devices. Generally, at source bioretention devices have 1:2 batters which means for a typical device that the storage area is 1.4 times greater in area than the filter media surface area. With all of these changes the proposed treatment train does not meet current best practice load based reduction targets for total suspended solids or nitrogen. This is expected as the bioretention filter media surface are represents only 1.1% of the total catchment area which it is treating and generally around 1.4% of the catchment area is required as bioretention filter media. Increasing the bioretention filter media surface area to the equivalent of 1.4% of the catchment area is required to meet best practice load based reduction targets. Page 54
Strategy and Planning Committee Agenda 16 February 2011 The stormwater quality treatment system for the site will be conditioned to consist of 5KL rainwater harvesting tanks collecting the roofwater from proposed dwellings with re-use for internal and external non-potable use. Overflow from the rainwater harvesting tanks and all other stormwater will then be treated in at source bioretention devices. The at source bioretention devices shall be sized at 1.4% of the contributing catchment area, have an extended detention depth of 0.15m, a filter media depth of 0.8m and maximum batter slopes of 1:2. This stormwater treatment train will result in current best practice load based reduction targets being met prior to discharge to the proposed lake. For development types such as medium and high density residential and commercial the required bioretention devices will be located within the development prior to discharging to Council’s system. For treating stormwater from individual house allotments and road reserve areas the stormwater quality treatment devices will be Council owned assets located in either the road reserve or areas of drainage reserve. At this stage of the development assessment process the strategy for stormwater quality treatment is of most importance. The exact size of required stormwater quality treatment devices needs to be determined prior to reconfiguration of lot and material change of use approvals. The development land use plan and calculated developable areas have been calculated without consideration of where these stormwater treatment devices will be located or where overland flowpaths are required in the case of Catchment R. Overland Flow The filling of the site, in order to comply with flood immunity requirements, has the potential to block overland flowpaths from existing residential development of Golden Beach and Pelican Waters. The areas designated as Overland Flow on the Development Land Use Plan are assumed to be drainage reserve and as such suitable for locating bioretention devices. These areas alone do not appear to be in the correct locations and sizes to provide all of the required stormwater quality treatment. However, this is detail which is not subject to this application and adequate room for stormwater treatment and overland flow paths can be been conditioned. It will be conditioned that the development does not result in the ponding of stormwater within existing residential areas surrounding the proposed development. In addition existing overland flow paths will be conditioned to be maintained with 100 year Average Return Interval flow capacity provided as overland flow. Piping the overland flow without any relief overland flow path will not be acceptable. Existing overland flowpaths can be modified as long as 100 year Average Return Interval capacity is maintained. In complying with these requirements the development layout proposed on the Development Land Use Plan will need to be amended. However, it is possible for the development to proceed while meeting these requirements, but it will lessen the developable area. Stormwater Quantity Stormwater quantity is dealt with through construction of the lake system and requiring sizing the major and minor system such that the system is designed for ultimate flows without detention. As such, stormwater detention will not need to be incorporated into future development applications. Page 55
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AGENDA - Sunshine Coast Council

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