Minnegang Creek Flood Study Report - Wollongong City Council

important to note that blockage of the outlet has implications in terms of the drainingof Barina Park following a storm event. In the case of the outlet being blocked, watercan only leave the basin through infiltration until the blockage can be cleared. Thiscould affect the availability of the playing fields in Barina Park to the public. This willbe further investigated in the Floodplain Risk Management Study.6.5.3 Jane Avenue bridgeThe Jane Avenue bridge was modelled in MIKE 11 with 25% bottom up blockage.This leads to a significant increase in flood levels upstream of the structure. The affluxdue to the blockage extends upstream for a distance of approximately 250 m. Theincreases in flood levels due to blockage at this location are sensitive to the blockagefactor applied to the structure. Flood levels downstream of the bridge are slightlyincreased by blockage of the bridge over the unblocked case.Blockage of the Jane Avenue bridge does not lead to flow diversions to adjacent subcatchments.At the location of the bridge, the channel is deep with steep banks and cancontain the increased flood levels upstream of the bridge, without diversions of theflow occurring.6.5.4 Northcliffe Drive and Illawarra Yacht Club culvertsBlockage of the Illawarra Yacht Club culvert would impact flood levels forapproximately 400 m upstream. As discussed earlier, the constriction posed by thisculvert under all flood conditions creates backwater effects that significantly impactthe capacity of the Northcliffe Drive culvert. Consequently, blockage of theNorthcliffe Drive culvert has negligible impact on upstream flood levels.Blockage of the Northcliffe Drive and Illawarra Yacht culverts do not lead to flowdiversions due to the location of these culverts at the downstream end of thecatchment. It is also relevant to note that if Lake Illawarra is flooded, then the culvertsat Northcliffe Drive and in the Yacht club carpark are submerged by floodwaters fromthe lake and blockage is not an issue.6.6 FUTURE DEVELOPMENT CONDITIONSAs previously discussed, RAFTS modelling of the catchment based on futuredevelopment conditions showed that increases in flow rates compared to those basedon existing development conditions were negligible.Future development conditions were modelled by considering areas of the catchmentthat could be developed further within the current land zoning. The large undevelopedarea south east of Flagstaff Road and Hilltop Avenue could be developed in the future.All other areas within the catchment are fully developed within their respective landzone entitlements.Future development was modelled by increasing the impervious area in the RAFTSmodel for the sub-catchments covering the area identified with potential for futuredevelopment. Developed conditions led to an increase of flows in individual subcatchmentsof less than 5%, whilst the increase in the total flow in Minnegang Creekwas less than 0.2%. It was therefore assumed that the effects of future development onSV8507-D0-001 Rev 2 6-5518/10/02