Works Approval Report.pdf - the National Capital Authority

Campbell Section 5 - Works Approval Project Ref: 110029Prepared for the Land Development AgencyGPT. From the GPT, the 225mm diameter pipeline will discharge the redirected low flows to a smallswale with a 1m wide, 0.3m deep deco granite lined base.It is proposed to upgrade the existing inlet structures and outlet pipes from the road low points inChowne Street and Creswell Street to eliminate overflow in storm events up to at least 5 year ARI.The inlet on Chowne Street will be upgraded to a plantation sump, whilst the outlet from CreswellStreet will be upgraded from 600mm diameter to 750mm diameter pipe. Overland Flow PathsThe existing overland flow paths through the site will be formalised into a system of small grassedswales. The design of the swales will accord with the WSUD initiatives presented in Section 5.3.The main grassed swale will extend from the northwest corner of the site to the downstreamheadwall of the GPT outlet pipe. As mentioned above, the swale downstream of the outlet pipe fromthe proposed GPT will contain a 2m wide, 0.3m deep rock lined base. Flows in this downstreamswale will be limited to approximately 0.3m 3 /s in storm events up to 5 year.Retarding BasinA retarding basin is proposed to be excavated within the open space system immediately upstreamof Road 2-(B) and Road-(C). The basin will reduce the amount of overland flow discharged from thesite and upstream catchment to the Creswell Street/Constitution Avenue low point. Outflows from thebasin will be discharged by a new 1350mm diameter pipe down Road 3 to the existing 1200mmdiameter pipeline at Constitution Avenue. A surcharge structure will be provided at the downstreamend of the proposed Road 3, which will discharge basin outflows that exceeds the capacity of thedownstream pipeline to the Creswell Street/Constitution Avenue low point via Constitution Avenue. Itis estimated that the pit will begin to surcharge in approximately a 5 year ARI event and will have amaximum outflow of 0.75m 3 /s at 100 year ARI.Two broad crested weirs will be formed on the south eastern side of the basin to act as a secondarybasin outlet to discharge flows that exceed the basins 100 year ARI capacity or if the basin outletstructure or downstream pipeline becomes blocked enough to cause the basin to overflow.All paths within the basin storage area will be above the 2 year ARI water level in accordance withTAMS’ design standards.Proposed Road DrainageThe site’s road drainage system will be designed for a 5 year ARI capacity with sufficient inlet pits toensure that gutter flows in a 100 year ARI storm event do not exceed the maximum allowable depthof 200mm.The road drainage systems in Anzac Park East, Road 1-(A) Road 1-(B), and Road 2-(A) will bedischarged into the proposed wetland area in the retarding basin. The remainder of the roaddrainage systems for Road 2-(B) and Road 2-(C) is to be drained to the surcharge pit in thedownstream end of the Road 3 median. The carriageway of Road 3 will be graded to directstormwater into the median where WSUD measures including tree planting and a bioswale will treatthe stormwater. This drainage will be connected to the existing pipe network in Constitution Avenue.November 2012 Cardno 49