Minnegang Creek Flood Study Report - Wollongong City Council

Creek and pass directly over Northcliffe Drive, re-entering the creek immediatelydownstream of the road. This results in two-dimensional flood behaviour, creating aflood surface that is not horizontal as flows diverge and either travel eastwardstowards the low point along Northcliffe Drive or continue south along MinnegangCreek.One-dimensional models such as MIKE 11 describe the water surface level at eachcross section by computation of a horizontal surface. Cross section extents shouldtherefore be minimised to help maintain an approximately horizontal water surface.However, the flat topography sloping away from Minnegang Creek would require themodel cross sections in this area to be extended into the adjacent catchment to containthe predicted flo od levels.These conflicting requirements create a problem as the horizontal surface computedby MIKE 11 to describe the water surface level at each cross section needs to be fullycontained within the extent of the cross section. If this does not occur, and theiterations begin to calculate a water level that exceeds the natural surface level at oneor both of the cross section extents, MIKE 11 will automatically enclose the crosssection with a vertical extension. The modelled cross sections have been finished atthe extent of the surveyed cross sections as a compromise between the conflictingrequirements referred to above.For the above reasons, the flood extents in the area downstream of Northcliffe Driveas presented in Section 6 should be viewed with caution. Flooding in this area can alsobe attributed to flood events in Lake Illawarra. Thus the results presented in the LakeIllawarra Flood Study (Lawson and Treloar, 2000) should also be used inunderstanding flooding mechanisms in this part of the catchment.5.12 MODELLING OF THE EXISTING PIPED DRAINAGE SYSTEMRatHGL was used to determine the capacity of the existing piped drainage system inthe catchment, as the full extent of the system was not modelled in MIKE 11. Thepiped section of Minnegang Creek was modelled in RatHGL from its upstream extent,between Lake Heights Rd and Barina Ave, to its outlet, downstream of Weringa Ave.All branches of the system joining this piped section of Minnegang Creek were alsomodelled.SV8507-D0-001 Rev 2 5-3318/10/02